### What's so relative about the humidity?

No, it’s not relativists’ plot against true humidity! Relative humidity (RH) is a way to measure the moisture content in the air around us. "It is the ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at that particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure."-Essentials of Meteorology by C. Donald Ahrens.

RH =
 (e) -------------------------- (es)
X 100

The equation used to find RH is givenabove, where “e” represents the vapor pressure in the given parcel, and “es” represents the saturation vapor pressure (pressure water vapor would have on a given parcel of air if that parcel of air’s evaporation rate no longer exceeds its condensation rate, or as some people say, cannot “hold” any more vapor, or maximum amount of water vapor that given parcel can “carry”, or think of it as “e” when RH is 100%.)

RH of course is relative humidity, and the 100 is used so that the product can be represented as a percentage rather than in decimal form. If you remove 100, then you will end up with a decimal value. For example, 0.53 would be 53% if multiplied by 100, 0.76 would be 76%, 0.10 would be 10% etc… So if the relative humidity is at 100%, then we know the equation must equal 1.00 or 100% if multiplied by 100. Remember I said you can think of “es” as “e” when RH is 100%? If both “e” and “es” are the same, then the quotient must be 1.00 or 100%!

Relative humidity can change if the air temperature changes or if more water vapor is added or subtracted to a given parcel of air (check out Dalton's Law of Partial Pressures). Obviously, if you remove water vapor from the given air parcel, the RH will drop, if you add water vapor RH will increase. Likewise, if you raise the temperature of the air, the RH will drop, if you chill the air RH will increase. To summarize: If temperature goes up, RH goes down. If temperature goes down, RH goes up (an inverse relationship). If you add water vapor, RH goes up, if you subtract water vapor, RH goes down (a direct relationship).

This is a good point to clarify a common misunderstanding about relative humidity that I see Bif Sundance, Snowy Sunshine, Windy Calm Day, and other weather dudes and dudettes on TV convey; "The air (mainly nitrogen and oxygen) no more has a holding capacity for water vapor, than, say, water vapor has for nitrogen. The atmosphere is a mixture of gases. While saturation (which involves bonds between different molecules) is a real phenomenon in liquids it does not describe the interaction of atmospheric constituents.

It is in regard to the inverse relationship of relative humidity and temperature."-Alistair B. Fraser.  In other faq questions I address the concept of temperature being a measure of energy (it is a measure of molecular motion; faster = higher temps, slower = lower temps). Thermal energy increases then temperature increases and vice versa.

We can never directly see energy, but we know it is there by the things it does, so we classify it according to what it does; Electric energy, Electromagnetic energy, Nuclear energy, Mechanical energy, Chemical energy etc… Additionally, different masses have different Heat Capacities (sometimes called Specific Heat).

The higher a substance’s Heat Capacity, the higher its potential to hold more energy is. Ifa substance can hold more energy, then it can do more work. Heat Capacity or Specific Heat (I only capitalize these words to emphasize them.They're not pronouns!) is the ratio of the change in heat energy in a unit mass of a substance (like water vapor) to the change in that substance’s temperature. Heat Capacity is a characteristic of all substances. When you measure the heat capacity of a substance you are partaking in calorimetry. We express a substance’s heat capacity using the metric system’s unit called calorie(s). Or, if you can’t get enough of that efficient and wonderful English system I promote in faq question #14, you can use British Thermal Units (BTU).

The more energy in a given air parcel, the more evaporative work can be done in that air parcel. I can measure its temperature, and that temperature tells me how much work can be done in that air parcel because it tells me the amount of energy in that air parcel! And as we know, it requires energy to do the work of evaporation.

Considertwo air parcels, one has a temperature of 40°C and a second has a temperature of 20°C, I know that the net evaporation rate in the warmer parcel is higher than the net evaporation rate in the cooler parcel. In other words, more work is being done in the warmer air parcel because it has relativelymore energy than the cooler parcel. How do I know this? Because it's warmer! Temperature and energy are directly related. When higher temperatures indicate higher energy levels. What does this mean?

Well, it doesn’t mean the warmer parcel can somehow mysteriously “hold” more water vapor, but what it does mean is that the warmer parcel evaporates at a rate much higher than the condensation rate and therefore has more energy available to evaporate more “free” liquid water molecules should they become available, while the cooler air parcel (with less energy) cannot supply as much energy to evaporation because it… well, has LESS energy, and therefore will exhaust its energy supply to evaporation work sooner than a warmer air parcel. Kinetic energy is needed to break the hydrogen bonds holding water molecules to each other. With enough kinetic energy (energy of MOTION, thermal energy, speed, momentum…) these hydrogen bonds can be broken and a tiny little water molecule is “liberated” = water vapor.

Think of it this way: If evaporation rate is higher than condensation rate, then there is energyavailable to do work should more “free” water molecules come into a given air parcel; RH will be less than 100% if this is the case.  If condensation rate is higher than evaporation rate, then there is no energyavailable to continue evaporating more “free” water molecules, all the energy in that air parcel is being used and the condensation ratewill exceed the evaporation rate…. and a cloud will form. "If more molecules areleaving a liquid surface than arriving, there isa net evaporation; if more arrive than leave, anet condensation. It is these relative flows ofmolecules which determine whether a cloud formsor evaporates, not some imaginary holdingcapacity that nitrogen or oxygen have for watervapor."-Alistair B. Fraser

From what we’ve read so far, we know that warmer air has more kinetic energy available to do more evaporation work than cooler air… with this in mind:

Let E = 1 unit of energy

Warm Air Parcel = EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE

Cold Air Parcel = EEEEEEEEEE

30% of the warm air parcel’s energy = EEEEEEEEEEEE

whereas, 30% of the cool air parcel’s energy = EEE

So even though both have a RH of 30%, that 30% doesn’t represent the same number of energy units (calories or BTUs).

Another way to put it:

If a WARM AIR PARCEL = 100 calories of energy
And if a COOL AIR PARCEL = 20 calories of energy
Then, 30% of the warm air parcel’s energy = 30 calories of energy
And, 30% of the cool air parcel’s energy = 6 calories of energy

The latter doesn’t leave much energy available to do a lot of evaporation as compared to the warmer air parcel! You can see there are 24 more units of energy available in the warm parcel of air even though it has a Relative Humidity equal to that of the cooler parcel!

As you can see, it is not a matter of warm air somehow “holding” more water vapor than cool air, or conversely, cool air somehow being unable to “hold” as much water vapor as warmer air. It is ONLY a matter of ENERGY.In fact, everything boils down to energy. Sodon't be tricked into believing the analogy of air being a sponge, or having a “temperature-dependentholding capacity”. Think in terms of energy! Now you'll know that when Bif Sundance the weather dude says it's a hot dry day with only 20% RH, remember that that 20%represents a lot more energy than it would ona colder day.

### How do I calculate relative humidity?

To findrelative humidity (RH), weather observers use a psychrometer. This device uses two mercurial thermometers side-by-side in tandem, connected to a hinge and a handle. The thermometers are identical, the only difference being that one thermometer has a wick on its bulb the other does not. The wickless thermometer simply measures air temperature; it is called the DRY BULB. The thermometer with the wick is called the "WET BULB".

The wick on the wet bulb thermometer is soaked in distilled water. Once the wick is saturated, the psychrometer is swung about through the air manually by the observer (on electricpsychrometers, an aspirating motor is used todraw air over the wet bulb). As air passes over the wet bulb thermometer, evaporation occurs. The rates ofevaporation vary depending on atmosphericconditions (see faq #1). Evaporation requires energy. This energy is acquired from the surrounding environment.

Energy is removed from the surrounding environment toenergize the evaporation process. The distilled water in the wick evaporates away removing heatenergy from the air around it in the processcausing the temperature to change around thewet bulb. The temperature becomes relatively cooler immediately around the wet bulb thermometer than it is for the dry bulb thermometer. This is exactly what happens whenwe sweat, which is why you feel cooler whenyou sweat ( evaporating smelly sweat removes thermal heat just above your skin thus cooling you down). The result is a cooler temperature recording for the wet bulb than for the dry bulb*. The difference between these temperatures is called the DEPRESSION OF THE WET BULB (or simply,"depression").

Dry Bulb Temperature - Wet Bulb Temperature = Depression.

Don't worry, the psychrometer doesn't need Prozac orlithium to feel better, the depression we aretalking about here is simply a the differencebetween the dry and wet bulbs! The depression and thedry bulb temperatures are plugged into a special chart called the Psychometric Chart which is a numbers chartwith an X and a Y axis. Across the X-axis are Depression values, across the Y-axis are the Dry Bulb Temperature values. The weather observer locates the dry bulb temperature on the Y-axis the depression value on the X-axis.

The point of intersection for thesetwo "points" on the chart will be a number. That number is the RHvalue for the given temperature anddepression. Dew point temperature is found the same way (using dry bulb temperature and depression) only with a different chart with different pre-calculated values on it.

*The wet bulb will NEVER be warmer than the dry bulb under natural conditions. The only time the wet bulb won't be cooler than the dry bulb is when it is EQUAL to the dry bulb. If this is the case, it means that the air temperature is equal to the Dew Point temperature and therefore air is at 100% RH.

### What is dew point temperature?

The dew point temperature is the temperature air has to be cooled to reach saturation. This is
why humidity levels rise dramatically at night when temperatures drop, which explains why dew
forms in the early morning hours. At night, terrestrial thermal radiationis lost (radiational cooling) which chills theair above the ground thus bringing it closerto dew point.

Moisture levels change constantly, so there are actually two ways to reach saturation (100% RH). One way is if airtemperature drops (assuming humidity staysconstant). The other way is if ambienttemperature is constant, then added moistureto an air mass can cause that mass to reachits saturation point.

### When does the coolest time of the day occur?

Generally, just before sunrise. Duringthe night, long-wave terrestrial radiation islost to space, and the longer it depletes thecolder it will become. Radiational coolingoccurs during the night and is particularlyeffective on clear nights during winter whennights are longest and radiational cooling isallowed to continue for a longer period oftime (more time for heat loss).

### When does the warmest time of the day occur?

In general, about 2-3 hours after noontime.Incoming shortwave solar radiation is absorbedby Earth. This energy is then radiated atlonger terrestrial wavelengths between 1-30 micrometers (Infrared) whichheats the air. Maximum solar radiation inputoccurs at noon. Given time, convection, emission, and absorption of radiation results in the day's extreme maximum temperature.

### What do wavelengths have to do with temperature?

First, a basic understandingof the electromagnetic spectrum: What is it? It is a
classification of all wavelengths according to their "size". Energy and wavelength are inversely
proportional to each other. Longer wavelengths are less energetic than shorter wavelengths and
vice versa.

From longest to shortest you have the following: Radio waves, Microwaves, Infrared, Visible light
(Red, Orange, Yellow, Green, Blue, Indigo, Violet), Ultraviolet, X-Ray, and Gamma Ray.
Ultraviolet to Gamma are considered to be ionizing radiation, which is another way of saying BIOHAZARDOUS WAVELENGTHS! Some folks may lump microwaves in with radio waves,
or they'll lump indigo and violet together as just violet. The wavelengths are the same, just the names include more of the spectrum.So if your friend says her favorite color is blue,then you'll know she really likes ahigh-frequency green.

Water vapor and carbon dioxide absorb IR (Infrared) wavelengths, and an abundance of these two gases are found right here in thetroposphere. IR wavelengths are what we associate with heat -- heat is actually
mechanical energy at the molecular level (motion) -- the atmosphere is
consequently heated from the ground up. If there is more water vapor and carbon dioxide present, then more IR can be absorbed resulting in Earth being heated from the lower troposphere. A large portion ofEarth's heating occurs from our own atmosphererather than directly from the Sun.

The more solar radiation received, the more Earth has to convert to IR. And the more IR and
greenhouse gases present, the more potential for heat we have.

Much of the UV radiation from the Sun is absorbed by ozone molecules in the stratosphere. The rest of the solar radiation(some UV included but mostly visible light) continues into the troposphere where approximately 20% is reflected and scattered by clouds, 6% is reflected and scattered by
atmospheric molecules, and 4% is reflected and scattered by Earth's surface (mainly oceans,
and snow covered areas), and .00000000000000000000001% is reflected by bald guys with shiny domes
(just kidding about the bald guy part). Another 19% is absorbed by the atmosphere and clouds, and the remaining 51% is absorbed by Earth's surface. **These percentages are averages**

Key Point: The energy absorbed by Earth is NOT re-radiated, or trapped by
greenhouse gases that act as "blankets" causing Earth to overheat!

When our atmosphere emits radiation, it is not the same radiation -- which ceased to exist upon being absorbed by our atmosphere -- as it received. The radiation absorbed and the radiation emitted do not have the same spectrum, nor do they have the same photons. The radiation absorbed ceases to exist by definition, thus making the term reradiate a nonsense term that some dictionaries don't bother to define for obvious reasons, and
those that do are, I believe, at fault. Ignore Bif Sundance theweather dude on TV when he tries to mystifyyou with the "blanket" in the sky that "rereradiates"heat energy causing global warming... Hisphysics are rusty at best!

The greenhouse effect is actually energy being absorbed by "greenhouse"gases, converted, emitted at longer wavelengths (IR which we associate with heat) and being absorbed again.This natural process keeps our planet from freezing over and puttingthe penguins back in power.

With each conversion, energy is lost. If it weren't we'd have a perfectly
efficient system; an impossibility as demonstrated by mycar. It would defy the laws of thermodynamics, and if possible would probably be termed re-radiation or something ridiculous like that! The temperature we feel is the result of a combination of emitted radiation, absorption, conversion, and emitted radiation at
longer IR wavelengths. It is NOT the result of trapped and reradiated heatenergy.

Furthermore, the comparison of our atmosphere to a greenhouse is misleading too. Once you've
shut the door on an actual greenhouse it becomes a closed system that ultimately suppresses
convection. Warm air just stagnates. Good forwhatever you've got growing in there, but badif you decided to live in there.

On the other hand, our atmosphere facilitates convection, and in fact, we wouldn't enjoy much
weather if it lacked the ability to be convective. Without convection life would hate life. Convection
allows for cooler air to come in, and warmer air to move out so that Earth doesn't get stagnant systems
and overheat. Precipitation would no longer occur, clouds wouldnot form, and meteorologists would be unemployed without convection!

There is no correlation between a greenhouse and our planet. The processes involved in each do not parallel each other. For simplicity the greenhouse effect is explained with terms like "trap", "reradiate", "blanket", and... well, "greenhouse". A point gets acrossbut it misses its target. So when you hearanalogous terms and descriptions trying toexplain something in weather, get suspiciousimmediately! :)

"It is like trying to reduce the fraction, 19 / 95, by imagining that you can cancel the 9s. The right
answer ensues, but for the wrong reason."
-Alistair B. Fraser

To open a new window to a great websitededicated to meteorological-relatedintellectual disasters see Alistair's Bad Meteorology Website.

### What is earthquake weather?

It's the weather we have during an earthquake... BUT it has nothing to do with Earthquake!
Literary metaphors and foreshadowing in books and on TV often use weather to depict bad things
to come (that have nothing to do with the weather.) Such things have nothing to do with real life
situations like earthquakes, and shouldn't be correlated to earthquakes.

Floods, storm surges, tornadoes, etc. can foreshadow bad things to come, but an earthquake isn't one of them. On a geological scale, the atmosphere is a super-thin eggshell compared to Earth, and doesn't have the energy needed to induce earthquakes. Contrary to popular belief, exfoliating rocks aren't enough to cause an earthquake either! Earthquake weather is a myth!

### How do you calculate the wind chill factor?

The human body heats the air immediately above the skin. On windy days, the heated air immediately above the skin is blown away and replaced with cooler air, thus making one feel cooler. On colder days, this becomes more evident and can make one feel downright cold!

The wind chill factor is not an actual temperature, but a "feels like" temperature developed by Paul Siple, a polar scientist of the 1940s. Wind chill, or WET (Windchill EquivalentTemperature) indicates that any exposed skin will lose heat at a rate equal to the rate that occurs when the temperature is lower during calm air. The faster the wind blows, the more intense WET becomes becauseheat is being removed from above your skinfaster than your poor shivering body can replaceit! Here's a partial example of the conversion chart developed by Siple and modified recently by the folks at the National Weather Service (NWS).

Though, I find charts and equations odd for windchill because it IS A FEELS LIKE TEMPERATURE. My friend thinks it feels like it's 40 degrees F outside, while I think it feels closer to a 80... in fact,sometimes my ears feel colder than my nose! Go figure. Anyway, here's the chart:

### I heard hair gets longer in humid weather. Is this true?

Yes! As humidity increases, so does the length of your terrific hair! In fact, human hair is used in
humidity measuring devices called hair hygrometers. As the humidity increases, the hair lengthens.
The hair is attached to a lever, and the lever is attached to a pen arm (the pen is attached to the penarm). As the hair lengthens or shortens, it moves the pen arm on the hygrometer, and the pen arm draws an ink line indicating this movement. The ink mark occurs on a chart that indicates the relative humidity. Our weather station uses a hair hygrometer along with a motorized psychrometer.

### Why does a 80°F day in southern states like Mississippi, Tennessee, etc. feel so much hotter than an 80°F day in Los Angeles?

This is a result of another one of those "feels like" temperatures. Here's how it works: The human
body keeps itself from overheating by sweating. This sweat is evaporated. Evaporation removes
heat which makes one feel cooler. High humidity decreases the atmosphere's ability to evaporate
(it just can't hold all that water vapor), so your sweat doesn't evaporate as readily on really humid
days, and you don't cool off as efficiently. The result is. . . you feel hotter. The temperature may be
80°, but you may feel like it's 90°. The "deep south" tends to have higher humidity on average during
the day than here in L.A. That is why the days seem so much hotter back there! There just so
happens to be a handy little chart for figuring out humidity and human discomfort : (The source for
this chart is from Frederick K. Lutgens & Edward J. Tarbuck's, "The Atmosphere 7th Edition)
Again, these are "feels like" temperature values, and should be used as a guide, but not as a literal reference.

 Temperature 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 80°F 75°F 77°F 78°F 79°F 81°F 82°F 85°F 86°F 88°F 91°F 85°F 80°F 82°F 84°F 86°F 88°F 90°F 93°F 97°F 102°F 108°F 90°F 85°F 87°F 90°F 93°F 96°F 100°F 106°F 113°F 122°F * 95°F 90°F 93°F 96°F 101°F 107°F 114°F 124°F 136°F * * 100°F 95°F 99°F 104°F 110°F 120°F 132°F 144°F * * * 105°F 100°F 105°F 113°F 123°F 135°F 149°F * * * * 110°F 105°F 112°F 123°F 137°F 150°F * * * * * 115°F 111°F 120°F 135°F 151°F * * * * * *

### Where do babies come from?

Storks.
(Source: Some cartoon I saw on TV.)

### What is meant by "normal" weather?

The World Meteorological Organization established a standard for what is normal. It uses a
30-year span from which to calculate normals. Precipitation and extreme temperatures are most
commonly derived when expressing normals. Every decade, the 30-year span is updated. Up until December 31, 2000 normals were calculated from January 1, 1961 to January 1, 1991. Now in
2001 the normals are derived from January 1, 1971 to January 1, 2001. To see the pattern used to determinenormals, see below:

1960s = Jan 01, 1931 - Jan01, 1961
1970s = Jan 01, 1941 - Jan 01, 1971
1980s = Jan 01, 1951 - Jan 01, 1981
1990s = Jan 01, 1961 - Jan 01, 1991
2000s = Jan 01, 1971 - Jan 01, 2001

We currently use theJan-1971 to Jan-2001 span to calculate normals.

Be aware that comparing along-term average with daily absolute totalscan be like comparing apples and oranges. Thetwo will almost invariably be different. It ismisleading to hear your local weather wo/mantell you that we are "above" or "below what isnormal for today" because s/he is comparing along-term average with absolute data collectedfor a single day. Let's say today's normalmaximum temperature is 75 degrees F. Yourlocal weather wo/man tells you that today wereached a high of 82 degrees which is 7degrees above what is normal for this day.

However, next year this same date may record amax temperature that is below normal, and theyear after that it will invariably be above orbelow normal again... and again, and again....until 30 years from now the max temperaturesrecorded for this date -- whether they areindividually "above" or "below" normal -- maysimply average out to be 75 degrees, plus orminus a degree. So in the larger scheme ofthings today's temperature, be it above orbelow normal, may very well average out to infact be just another normal day! It's justanother one of those misleading things many TVweather wo/men tell you... the other beingpercent chance of rain.

### Why does the United States use Fahrenheit degrees instead of Centigrade degrees?

Good question! I think it's some sort of tradition or something. Come to think of it... why don't
we use the metric system? Take a deep breath..

Well, because everyone knows distance conversions using the U.S. Customary System! It's really,
really
easy... 12 inches makes a foot, 3 feet to a yard, which is the distance from King Henry I's
nose tip to the end of his fat thumb, 5.5 yards to a rod, and 320 rods to a mile, unless of course it's
a nautical mile in which case there are 368.32 rods which isn't the same as a statute mile. If we
know this, then we also know that volume is just as easy to remember! You know, 12 cubic inches
to a cubic foot, 27 cubic feet to a cubic yard, blah, blah, blah... But wait! That was too easy, so we
have more for our learning pleasure! There is a half quart in a pint, 2 pints in a quart, 8 quarts in a
peck, and 4 pecks in a bushel. And as we know, we can only use pecks and bushels when
measuring a non-liquid volume. And since we know that, we probably know that a half quart or a
pint of sand is 33.6 cubic inches, but a half quart or pint of water is 28.875 cubic inches. To keep
things simple, we'll just apply gallons and barrels to liquid volumes only, in which case there are 4
quarts in a gallon, and 31 gallons to a barrel. Of course (as we know) a barrel could be anywhere between 31 and 42 gallons depending on usage and local laws. And this is why we leave gallons
and barrels to liquid volume measures only, because a 37-gallon barrel of flour would be 398.6
cubic inches more volume than a 37-gallon barrel of lemonade... duh! Don't forget that there are 3 teaspoons in a tablespoon, and 16 tablespoons to a cup, and 2 cups to a pint. Well, 2 cups in a pint
of vinegar, but 2.33 cups in a pint of sugar, not to be confused with a cup used in football, but that's
all common knowledge. Converting volumes in the U.S. Customary System is a cinch, so we have
a bunch of cool, efficient, easy to remember units for weight too! We all know that there are 16
ounces in a pound and 2,000 pound to a ton. Well, unless your talking about a long ton in which
case there are 2,240 pounds which isn't the same as a short ton. And come to think of it, there are
only 16 ounces in a pound as long as that pound isn't a weight value of a medicine, because if it is,
then there are actually only 13.168 ounces in a pound. But it's easier to simply remember that there
are 5,760 grains in a an apothecary pound, and a smooth 7,000 grains in an avoirdupois pound.
And everyone knows there are 60 grains in a dram, unless of course its an avoirdupois dram in
which case it would have only 27.344 grains. Now remember, this isn't much more than a scruple,
which, as we know, is 20 grains in apothecary weight! And then there is acre-feet, not to be
confused with an acre, which is 43,560 square feet. And since we're on the subject of area, we
might as well note that there are 320 square rods in 1 square mile which means that there are 2
acres for every square rod. But we knew that. And don't forget the temperature scale! Remember,
212 degrees Fahrenheit is boiling point of fresh water at sea level, and 32 degrees Fahrenheit is
freezing point. Nice even numbers that are easy to remember. This efficient system has withstood
the test of time. So the next time someone mentions the number, 368.32, the first thing that will
come to your mind is, "hey, that's how many rods are in a nautical mile!"... What a coincidence eh? You can exhale now.

Here's the MUCH MORE DIFFICULT to learn metric system: Use meters for
distance, liters for volume, and grams for weight. Add the following prefixes to the unit your using accordingly:
Yotta- (1024)
Zetta- (1021)
Exa- (1018)
Peta- (1015)
Tera- (1012)
Giga- (109)
Mega- (106),
kilo- (103)
hecto- (102)
deca- (101)
- -  UNIT (1) - - (meter, liter, gram)
deci- (10-1)
centi- (10-2)
milli- (10-3)
micro- (10-6)
nano- (10-9)
pico- (10-12)
femto- (10-15)
atto- (10-18)
zepto- (10-21)
yocto- (10-24)

*The answer to this question has been dipped in sarcasm to add to the delicious flavor of irony.

### What are the odds of lightning striking twice in the same spot?

I would say its 1:1! Lightning almost always strikes twice or more in the same spot! In fact, what
appears to us as being as single flash of lightning, is in fact several very rapid strokes between the
cloud and the ground. There is roughly 50 milliseconds between each stroke, so several strokes can occur within a few tenths of a second! You may not see it, but it happens! So the next time someone says, "The chances of you getting skin cancer from sitting nekid' in that tannin' bed er like the chances of lightning striking twice in the same spot!"...You should probably stay out of the tanning bed, and KEEP YOUR CLOTHES ON!

### What is the difference between Centigrade and Celsius

Spelling.
(Source: A dictionary)
Centigrade is actually an older term used, nowsupposedly supplanted by Celsius. I stillprefer Centigrade because it sounds so cool.

### What do weather reporters mean by a percent chance of rain?

I don't think they even know what they mean, but here's the gist of it:
Precipitation probability forecasts were developed by the National Weather Service in 1965. It
works on a scale of 0 to 1. 0 means there's no chance of precipitation (ppt) occurring, and 1 means it's
inevitably going to precipitate.

However, when we see this scale used on the news, it has been converted to percentages. 0.6 is
the same as 60%, 0.45 is 45% and so on. . . So what exactly does it mean when they say there is
a 60% chance of rain? First of all, it must be understood that this percentage refers to a specific
forecast area. i.e.) San Fernando Valley. Second, this percentage is valid only within a specific time
frame; usually within a 12-hour time frame. Third, it is a percentage that says that AT LEAST 0.01"
of ppt. will fall at ANY POINT within the specific area within the next 12 hours (or whatever the
time frame is.) Anything less than .01" of ppt is considered trace ppt, and does not fall into the
measurable ppt category.

Now, let's say Bif Sundance the weather dude says there is a 60% chance of rain for Malibu tonight. This means there is a 6 in 10 chance that at least 0.01" of ppt. will fall at any point within Malibu city limits. This doesn't mean 60% of Malibu's surface area will receive ppt. It simply means that there is a 6 in 10 chance that measurable ppt will fall somewhere in Malibu within the next 12 hours. It also means that there is a 4 in 10 chance that no ppt. will fall anywhere within Malibu, in which case Bif could say there is a 40% of no rain! (It's one of those half full/half empty cup things). A problem here is that some TV weather forecasters don't fully understand this concept and provide their audience with percentages without area and time consideration. So if your in Reseda and Bif Sundance says there is a 75% chance of rain, but the probability forecast is for Ventura county, then you may not need that umbrella after all.
(Source: Frederick K. Lutgens & Edward J. Tarbuck's, "The Atmosphere 7th Edition)

### What is El Niño?

Again, this is one of those "you gotta enroll in a Pierce Meteorology course to get the answer"
questions due to its complexity. But here's the gist of the processes behind it... El Niño is simple
physics with complex outcomes. Energy goes from where it is to where it isn't in the direction of
least resistance. With that said, let's look at the mechanics of the system. First, let's understand that
ocean currents are only little sections of a single ocean gyre. A gyre is mainly fueled by the Coriolis
Effect (it's not actually a force). In simple terms, it is a clockwise circulation of water (in the Pacific in this case) in the northern hemisphere, and a counterclockwise circulation of water in the southern hemisphere.

These two circulations meet at the equator where they both go in the same direction from east to
west. This section of the two gyres is commonly called the Equatorial Current. At the equator,
the sun strikes Earth at a right angle twice a year. At that angle the solar radiation needs only to
pass through one atmosphere, so there's a lot of energy input at the equator. Water is heated and is carried along eastward and is slowed against massive continental shelves of Australia and Asia's Malaysia, Papua New Guinea, Philippines, etc.)

The warm water continues to collect in the western Pacific because those big chunks of granite keep the warm waters from outflowing into the Indian Ocean with any efficiency (it's really just one ocean with several names). Warm water collects over there holding more energy in it relative to the eastern Pacific. It's like a spring being recoiled. Water in the western Pacific actually ends up becoming 20-50 cm higher than the eastern Pacific which is a sure sign of an energy imbalance.

E=mc2 so we know that anything with mass has energy and with conditions outlined above, there is definitely more mass building up in the western Pacific (near Australia) than there is in the eastern Pacific (near here.) In other words, there is more energy over there than here. That energy continues to build as the equatorial current continues to provide more warm water into the western Pacific. "Energy goes from where it is to where it isn't"; Eventually the energy account in the western Pacific overpowers the energy in the Equatorial
Current and the net result is an equatorial counter current. The energy (warm water) goes the other way! This affects winds, semi-permanent pressure systems (Southern Oscillation -- ENSO), jet streams, and overall weather patterns all over the planet due to the interconnectivity of energy.

Eventually the energy disperses to the point that the Equatorial Current resumes its normal east-to-west flow along the meeting point of the two massive ocean gyres in the northern and southern sections of the Pacific. Then the process starts all over again.

That's basically how it works; minus a ton of details. The important thing is that the process can now be conceptualized in simple terms. It all boils down to energy inevitably going from where it is to where it
isn't in the direction of least resistance! Yes, this means your Splitfire sparkplugs only send a spark
across only one of those two forks!

.

### What was the hottest temperature ever recorded on Earth?

September 13, 1922 the maximum temperature in Al Aziziya, Libya was 136°F. Though this is the hottest temperature ever recorded, the hottest place in the world is arguably California's Death Valley. Here's an excerpt from pg. 18 in Christopher C. Burt's book, "EXTREME WEATHER":

-----------------
Temperatures in Death Valley, located 282 feet below sea level in interior California, have been maintained since 1911 at the Greenland Ranch near Furnace Creek. With an average daily high of 115° (sic) and low of 87° (sic) during the month of July, Death Valley is far and away the hottest location in North America and perhaps the hottest place in the world.

[Death Valley's] absolute maximum temperature of 134° (sic), recorded on July 10, 1913, stands as the hottest ever observed in the Western Hemisphere and has been surpassed globally only by a reading of 136° (sic) measured in Al Aziziyah, Libya, located 20 miles south of Tripoli (not in the Sahara Desert, by the way). A 135° (sic) reading claimed by Tindoug, Algeria is of questionable veracity.

The Greenland Ranch figure of 134° (sic) has been the center of a small controversy itself because there is no documentation of the accuracy of the thermometer and condition of its shelter, and no other official reading has ever since come close to this reading. A sandstorm was raging at the time of the observation, and some speculate hot sand or dust was driven into the thermometer casing, inflating the actual temperature.

A 130° (sic) temperature recorded at Amos (Mammoth Tank) in the Mohave Desert in 1887 is also suspect for the same reasons. So, Death Valley's second hottest readings of 129° (sic) recorded in July 1960, and July 1998 may, in fact, be the highest true maximum temperatures ever recorded in the United States. As weather historian David Ludlum once put it, "Apparently, what this country needs is a good 135° (sic) reading made under standard conditions, so that the figure, like Caesar's wife, may be beyond question."

The hottest summer of record in [Death Valley] was that of 1917, when 43 consecutive days above 120° (sic) were recorded between July 6 and August 17. The average temperature for the entire month of July that year was 107.2° (sic), just shy of yet another questionable national record of 107.4° (sic) recorded at Salton, California, in August 1897.

In July of 2002, Death Valley averaged 106.0° (sic), its hottest month in modern records. On July 13 of 2002, the temperature ranged from a low of 100° (sic) to a high of 127° (sic), a daily average of 113.5° (sic) and perhaps the hottest day ([in terms of] average temperature) ever recorded anywhere in the world.

Overnight lows above 100° (sic) seem to be unique to Death Valley (although the airport at Muscat, Oman, registered a low of 100° (sic) on the night of July 30, 1989). On the night of July 31, 2003, the temperature failed to drop below 104° (sic).

The longest stretch of consecutive days with a maximum of 100° (sic) degrees (sic) or longer was 154 days in 2001. This compares favorably to the world record of such days recorded at Marble Bar, West Australia, with 161 straight days registering highs of 100° (sic) or more.

### What was the coldest temperature ever recorded on Earth?

The coldest temperature ever recorded to date was -128.6°F in Vostok, Antarctica on
July 21, 1983. Vostok holds the global record. The coldest temperature ever recorded here in North America was -81.4°F in Snag, Yukon, Canada. Prospect Creek, Alaska came in a close second with a low of -79.8°F.

### What was the most rainfall in a 12-month period ever recorded on Earth?

The most rainfall recorded in a 12-month period was 1041.78" in Cherrapungi, Assam, India from August 1860 to July 1861. The most rainfall to fall in a single minute: 1.50" in Barot, Guadeloupe, West Indies on November 26, 1970. The most rainfall to fall in a single day (24 hour period): 73.62" in Cilaos, Reunion Island March 15-16, 1952. The most rainfall to fall in a week (7 days): 183.19" in Commerson, Reunion Island during the week of January 20-27, 1980. The most rainfall to fall in a single month: 366.14" in Cherrapungi, Assam, India in July 1861. Imagine the deluge from these storms!

### There are 12 hours of day and 12 hours on the days of the equinoxes...

Although day and night are theoretically equal in length on the days of the equinoxes, that would be true only if the sun were a point, not a disk, and if Earth's atmosphere did not bend sunlight. However, the top of the sun actually rises a few minutes before the center of the sun's disk--the point used in astronomical calculations. Also, Earth's atmosphere bends sunlight, so we can see the sun for several minutes before the time sunrise would occur and after the time sunset would occur if Earth had no atmosphere.

Source: Jay M. Pasachoff, Field Memorial Professor of Astronomy and Director of the Hopkins Observatory at Williams College in Williamstown, Massachusetts.

### How will global warming affect our weather?

Considering that heat is conversely proportional to molecular predictability, and the fact that the atmosphere is a massive mix of molecules... I would venture to say that forecasters will have far more difficulty in successfully predicting weather! Climatic zones will shift towards the poles, and we won't need to use our frequent flyer miles to visit the tropics anymore... whew, because I only have 150 miles.

### What's the difference between precipitation and rain?

Rainfall refers to the liquid water that falls in droplets between the sizes of .5mm to 5mm in diameter. Precipitation refers to water falling in both solid and liquid states. Precipitation includes rain,
but is not restricted to it. Here is a list of what precipitation could be referring to:

(The source for this chart is from Frederick K. Lutgens & Edward J. Tarbuck's, "The Atmosphere 7th Edition)

 Ppt. Type Approx. Size State of H2O Mist .005mm to .05mm Liquid Drizzle Less than 0.5mm Liquid Rain 0.5mm to 5mm Liquid Sleet 0.5mm to 5mm Solid Glaze Layers 1mm to 2cm thick Solid Rime Variable accumulations Solid Snow 1mm to 2cm Solid Hail Greater than 5mm Solid Graupel 2mm to 5mm Solid

### What is the ionosphere?

A basic understanding of ions, plasma, the electromagnetic spectrum, and the relationship between electric currents and magnetic fields are assumed, so if your a little rusty on those things, here they are in a nutshell:

Ions are simply atoms that have either a positive or negative charge. Remember the proton(s) and neutron(s) in the nucleus and the electron running around them. The proton has a positive charge and the electron has a negative charge, the neutron has a neutral charge (because it is a proton and electron combined; a free neutron decays with a half-life of about 10.3 min, into a proton, and an electron... oh yea, and this little thingy called an antineutrino). When either a proton or electron is lost the neutrally charged status of the atom is lost and it becomes an ion. Generally, positive ions result from beta decay (positive ions are called cations), and negative ions result from alpha decay (negative ions are called anions not to be confused with onions that make you cry when you cut them). Remember, in beta decay an electron is lost. In alpha decay 2 protons and 2 neutrons are lost. Once it flies solo, a neutron has a half-life of 10.3 minutes, then you have more free electrons when the electron and proton break away. Oh, and by the way, you bottled water drinkers... distilled water leaches the ions out of your body which isn't a good thing. Stick with filtered water only!

In plasma you have tons of ions! Plasma is the fourth state of matter, with much higher temperatures than any gas. In fact the temperatures are so high that the electrons are excited enough to slam into other atoms and be ejected from their orbits! Atomic collisions are violent and electrons end up roaming around without a nucleus around which to swing... the result is positively charged ions (cations). Lightning is an example of a plasma here on Earth. Plasma makes up 99% of all matter in the universe!

The electromagnetic spectrum... from highest energy and shortest frequency to the lowest energy and longest frequency you have: Gamma rays, X rays, Ultra Violet (UV), [Violet, Indigo, Blue, Green, Yellow, Orange, Red]<--Visible light, Infrared (IR), Microwaves, and Radio waves. UV, X, and Gamma are referred to as ionizing radiation. Remember an ion? This radiation is energetic enough to make an ion out of a neutral atom. Now some people don't particularly care for the Indigo/Violet delineation of colors because they look the same. Since the electromagnetic spectrum is a continuum, it's often hard to say when a color becomes another color just by watching it... when is a yellow no longer yellow but orange? I bet if the color changed gradually and a classroom full of students were watching it, everyone would have a different opinion about where the color actually changed! I like to play it safe. My favorite color is a long yellow/orange/short red with a dab of hot sauce.

Electric currents and magnetic fields: It's simple. Pass electricity through a conductor and it creates a magnetic field. Move a magnetic field and it creates electricity. Pass a conductor through a magnetic field and it creates electricity. We tend to use copper as a conductor, and Earth uses Iron and Nickel, located in its core (although there are a few geophysicists who believe the core is actually a 5-mile diameter ball of uranium). Magnetic fields are measured in gauss. Earth's gauss strength is 0.5 and changes over time. In fact it reverses over time from north tight to nothing to south tight and back again in what are known as magnetic epochs. For comparisons, a typical refrigerator magnet has a gauss of about 5, and a MRI has a wopping 20,000 gauss! Simply, the rate at which electrically charged particles move determine the strength of the magnetic field they create. Faster = Stronger Gauss and vice versa. All atoms have a magnetic field because they all have spinning electrons! Electric current is directly proportional to gauss level. And now the ionosphere:

The following was taken from the HAARP official website (http://www.haarp.alaska.edu/haarp/ion1.html)

#### What is the Ionosphere?

Earth's atmosphere varies in density andcomposition as the altitude increases abovethe surface. The lowest part of the atmosphereis called the troposphere (the light blueshaded region in the figure to the left) andit extends from the surface up to about 10 km(6 miles). The gases in this region arepredominantly molecular Oxygen ( O2) and molecularNitrogen (N2 ). Allweather is confined to this lower region andit contains 90% of the Earth's atmosphere and99% of the water vapor. The highest mountainsare still within the troposphere and all ofour normal day-to-day activities occur here.The high altitude jet stream is found near thetropopause at the the upper end of thisregion.

The atmosphere above 10 km is called thestratosphere. The gas is still dense enoughthat hot air balloons can ascend to altitudesof 15 - 20 km and Helium balloons to nearly 35km, but the air thins rapidly and the gascomposition changes slightly as the altitudeincreases. Within the stratosphere, incomingsolar radiation at wavelengths below 240 nm.is able to break up (or dissociate) molecularOxygen (O2) intoindividual Oxygen atoms, each of which, inturn, may combine with an Oxygen molecule ( O2), to form ozone, amolecule of Oxygen consisting of three Oxygenatoms (O3). This gasreaches a peak density of a few parts permillion at an altitude of about 25 km (16miles). The ozone layer is shown by the yellowshaded region in the figure to the left.

The gas becomes increasingly rarefied athigher altitudes. At heights of 80 km (50miles), the gas is so thin that free electrons can exist for short periods oftime before they are captured by a nearbypositive ion. The existence of chargedparticles at this altitude and above, signalsthe beginning of the ionosphere a regionhaving the properties of a gas and of aplasma. The ionosphere is indicated by thelight green shading in the figure to the left.

#### How is the Ionosphere Formed?

At the outer reaches of the Earth'senvironment, solar radiation strikes theatmosphere with a power density of 1370 Wattsper meter2 or 0.137 Wattsper cm2, a value known asthe "solar constant." This intense level ofradiation is spread over a broad spectrumranging from radio frequencies throughinfrared (IR) radiation and visible light toX-rays. Solar radiation at ultraviolet (UV)and shorter wavelengths is considered to be"ionizing" since photons of energy at thesefrequencies are capable of dislodging anelectron from a neutral gas atom ormolecule during a collision. The conceptualdrawing below is a simplified explanation ofthis process.

Incoming solar radiation is incident on agas atom (or molecule). In the process, partof this radiation is absorbed by the atom anda free electron and a positively charged ionare produced. (Cosmic rays and solar windparticles also play a role in this process buttheir effect is minor compared with that dueto the sun's electromagnetic radiation.)

At the highest levels of the Earth's outeratmosphere, solar radiation is very strong butthere are few atoms to interact with, soionization is small. As the altitudedecreases, more gas atoms are present so theionization process increases. At the sametime, however, an opposing process calledrecombination begins to take place in which afree electron is "captured" by a positive ionif it moves close enough to it. As the gasdensity increases at lower altitudes, therecombination process accelerates since thegas molecules and ions are closer together.The point of balance between these twoprocesses determines the degree of"ionization" present at any given time.

At still lower altitudes, the number of gasatoms (and molecules) increases further andthere is more opportunity for absorption ofenergy from a photon of UV solar radiation.However, the intensity of this radiation issmaller at these lower altitudes because someof it was absorbed at the higher levels. Apoint is reached, therefore, where lowerradiation, greater gas density and greaterrecombination rates balance out and theionization rate begins to decrease withdecreasing altitude. This leads to theformation of ionization peaks or layers (alsocalled "Heaviside" layers after the scientistwho first proposed their existence).

Because the composition of the atmospherechanges with height, the ion production ratealso changes and this leads to the formationof several distinct ionization peaks, the "D,""E," "F1," and "F2" layers.

### We could solve our atmospheric pollution problem if everyone would just buy electric cars...

Blind technological optimism is the belief that technology will solve our problems. Ockham's razor is the belief that the best solution to a problem is probably the most simple one. With that in mind...

Electricity; where does it come from. Well, we can take it back pretty far, but reasonably, we need only to trace it back to a power plant. The power plant generates the electricity, but we can't convert energy without losing most of it to heat (2nd law of thermodynamics). So we have our power plant, let's say a hydroelectric plant. 65% of the energy drawn is lost to heat right off the bat, so the power plant is only 35% efficient by definition of thermodynamics. But now they have to get that energy to us folks who have our plugs ready to charge our electric cars. The energy from the power plant is sent on its way through power lines.

But energy is lost in that transfer process which is calculable in a formula that uses proportions of distance and energy loss. On average, about 10% of the original 35% of energy is lost to heat during this initial transfer. Well, the energy eventually makes it to what are called distribution power centers. You've probably seen them all over the city, they're the big ugly power plant-lookin' things with all the wires and stuff. Anyhoo, these distribution centers are necessary to properly... you guessed it... distribute the energy.

Guess what! Another 10% of the energy is lost to heat here. So now the energy is transferred again. The 90% of the 90% of the 35% of the original 100% makes its way to us in 220 form. Too much for our sockets, since most of our appliances can only accept 110. So this energy has to be converted again by being split into two 110 lines. Guess what! You get about 25% of that energy! The result? (0.35)(0.90)(0.90)(.25) =  7%

Efficiency is conversely proportional to pollution. Don't forget that the electric car has to store and convert this electricity too... to mechanical power! Chemical processes are taking place in the batteries too! So even more energy is being lost to heat in your efficient little car! The end result is an efficiency of less than 5% (dare I say less than 2%) of the original 100% initially brought in by the power plants.

Not very efficient if you ask me, and not environmentally friendly; nor is mass production of lithium carbonate envionmentally friendly. Imagine millions of Angelinos driving around in there electric cars... can you imagine the incredible demand for electricity? California can't even supply itself now, much less handle a bigger load! So if you ask me, electric cars are a nice thought but not practical or efficient.
To cut back on atmospheric pollution successfully we must consider the following...

Here is a breakdown of efficiency and its relation to transport type:
Airplanes use ~40,000 btu per ton per mile traveled. Big rigs require ~3,000 btu per ton per mile traveled. Ships spend ~750 btu per ton per mile traveled, and trains use ~700 btu per ton per mile traveled. On a ton per year basis, planes constitute ~3.8% of transport, trucks; ~56%. Ships (including rivers) account for ~5.5% of total annual transport, and rails account for ~24%*.  Pollution is proportionate to btu usage, so why is it that we use the least of the most efficient modes of transport and the most of the least efficient modes of transport?

Psychological block? The United States has one of the most expansive rail systems in the world which could be used more frequently. We should also promote small cars with small engines and multi-person capacities. Decrease the weight, decrease the size, and streamline the cars. Force of fluid friction is reduced by the square of the surface area, so what's an SUV? It is a demonstration of inefficiency. It's like spending money at the tanning salon, and those new Hummers aren't going to hold anymore groceries than a small compact. It's been proven that SUVs aren't any safer than a compact. It would be beneficial to promote comfortable, affordable rapid transit like they do in Europe. And I don't mean a subway in Los Angeles!

We have far too expansive a city for that, not to mention we live in earthquake land! Hello?! We could increase the number of buses here, make them more comfortable, air conditioned, and let them have all the plushness of a grand limousine. This will encourage their use. Increase the security on buses so passengers feel safe. AC and more buses will NOT increase the pollution if they are used in place of single passenger cars (like 4 door 2 ton dually trucks with one person and a bed that's never used). Instead of removing our carpool lanes to reduce congestion, we should ADD a carpool lane to encourage carpooling!

We ought to stop adding lanes to freeways and roads as this only encourages increased usage, and before you know it, we'll have to add another lane again. Of course, home construction into the foothills like Porter Ranch and the proposed Ahmanson's Ranch should be shelved to relieve stress on water supply and traffic. The list goes on. We have the tools necessary to improve transport efficiency, and at the same time improve air quality. There's no need to rely on a new technology to solve the problem. Just some policy changes will help. (in my opinion).

"Pollution is a measure of economic inefficiency." -Watt, UC Berkley Economist.
*(Source: W. C. Meyer, Professor of Geology, Oceanography, and Environmental Sciences)

### Plants need wavelengths in the green part of the spectrum for photosynthesis...

Actually, green is what the green-leafed plants are NOT absorbing. Since we see the green, we are seeing a wavelength that is being reflected back to our eyes (rejected by the plant). This means that green light is not necessary for green plants to grow.

### What came first, the chicken or the egg?

This is a cute little anecdote that has been passed on from generation to generation like a recessive gene. It is based on simple logic spawned from observations made in an extremely restricted temporal reference. It suggests, of the chicken and the egg, that one had to have come before the other. Such a suggestion disregards evolutionary change and is easily undermined when a much broader temporal reference is considered. Human perception of time on a geologic scale is vague at best. On the same token, to comprehend the Evolutionary Theory is equally mystifying for some people, and because of this, questions like the one above continue to be asked in spite of reason and overwhelming evidence. There is no "first" chicken or egg along a continuum of change.

### There are four states of matter...

Actually, the last I heard, there are 5 states of matter and I'm certain more will be discovered in time; at least mathematically (in theory). The five states of matter currently known are the Bose-Einstein condensation, solids, liquids, gases, and plasmas. The Bose-Einstein condensation is a gas of atoms that has been chilled to the point that their motion is virtually halted. As a consequence they lose their separate identities and merge into a single entity or "super atom".

The condensate was predicted by Albert Einstein in 1924 based on the system of quantum statistics formulated by the Indian mathematician Satyendra Nath Bose (Hence the name Bose-Einstein...). Quantum theory asserts that atoms and other elementary particles can be thought of as waves. Einstein proposed that as atoms approach absolute zero (−273.15°C;), the waves expand in inverse proportion to their momentum until they fall into the same quantum state and finally overlap, essentially behaving like a single atom.

The phenomenon could not be observed, however, until techniques were developed to reduce temperatures to within 20 billionths of a degree above absolute zero. In 1995 Eric A. Cornell and Carl E. Wieman led a team that isolated a rubidium Bose-Einstein condensate under laboratory conditions. It is believed that this state of matter could never have existed naturally anywhere in the universe, since the low temperatures required for its existence cannot be found, even in outer space. The condensate may be useful in the study of superconductivity (the ability of some materials to conduct electrical current without any resistance) and superfluidity (the ability of some materials to flow without resistance) and in refining measurements of time and distance.

Philosophically, I doubt that absolute zero will ever be achievable, furthermore, superfluidity and superconductivity are metaphysical theories that aren't supported by laws of thermodynamics, though they sound cool and get scientists the research grants they need to avoid the outdoors.

### Earth rotates once every 24 hours...

The angular speed of rotation of Earth is determined by the equation:

Ω = 7.292116 * 10-5 radians/second

First, a word on the capitalized omega... In mathematics, capital omega (Ω) is used to represent ohms (units of electrical resistance). I use the lower case omega (ω) for the above equation, however I always see it written with an upper case omega. Mathematicians I've spoken to generally prefer to write the above equation like this:

ω = 7.292116 * 10-5 radians/second

The length of a solar day is indeed 24 hours. "Solar day" is the time it takes from one noon sun (directly overhead) to the next noon sun (following day). However, the ACTUAL time it takes Earth to make one complete rotation (360 degrees) is 24 hours, 56 minutes, and 4 seconds. This latter length is termed the "sidereal day" (pronounced sigh-deer'-real). The difference between the solar day and sidereal day is due to the fact that during a day, Earth also travels nearly a degree further on its yearly trek around the Sun.

### Is it true that you can estimate the distance of a lightning strike by counting the number of seconds it takes for the sound of thunder to arrive from the time the lightning struck?

Yes. However, it is not 1 mile for every second counted. The speed of sound varies slightly depending on the medium through which it travels, and the temperature of the medium. Before we answer this question, I'd like to point out a surprisingly little known fact: Sound does not travel through a vacuum such as outer space! Star Wars and Star Trek depict huge spaceships chugging through space firing their loud laser guns at unfortunate targets, however, in reality, if ever there is going to be such space battles, they would be completely devoid of sound. Silent.

No matter how big and bad the engines are on that star destroyer, or how big and bad the photon torpedoes are on Voyager, they'd never be heard in space! Sound is a pressure wave, therefore it requires a medium through which to travel. The denser the medium (steal), the faster sound can travel. The less dense the medium (air), the slower sound can travel. No medium (space), sound cannot travel.

Medium density affects the rate at which sound travels because as a pressure wave, sound essentially travels as particles in a particular medium hit each other in the direction of propagation. If particles are spaced closer together, then propagation is more efficient, particularly due to the fact that elasticity of the medium is lower and the medium will therefore react faster to sound propagation. If sound is a pressure wave, then we can logically conclude that pressure affects the speed of sound. In addition to pressure, factors of temperature, humidity and carbon dioxide content of/in a medium will affect the speed of sound.

Considering all this, we can now go back to the original question. Yes, we can definitely estimate the distance of a lightning strike if we count the number of seconds it takes for the sound of thunder to arrive from the time the lighting struck...

We will assume standard atmospheric pressure (1013.25mb), as well as standard carbon dioxide content (~0.033% or 330 ppm) of Earth's atmosphere. First we'll look at varying temperatures at a constant humidity, then at varying humidity, at a constant temperature and their relationships to the rate at which sound travels.

If air temperature is 10ºC and relative humidity is at 90%, the speed of sound would be about 338.05 m/sec. At 20ºC and relative humidity of 90%, the speed of sound would be about 344.5 m/sec. At 30ºC and relative humidity of 90%, the speed of sound would be about 351.25 m/sec. Note that the speed of sound increases with increasing temperature of the medium through which it travels! The speed of sound in a gas (air) is proportional to the Maxwell-Boltzmann velocity distribution function; therefore it varies as the square root of the temperature of the gas (its medium). Sound travels faster in warmer air because the molecular velocity is greater. The higher the temperature is, the greater the force with which molecules will strike, and will therefore increase the pressure that the gas (air) exerts outward.

If relative humidity is 25% and air temperature is 25ºC, the speed of sound would be about 346.71 m/sec. If relative humidity is 50% and air temperature is 25ºC, the speed of sound would be about 347.13 m/sec. If relative humidity is 100% and air temperature is 25ºC, the speed of sound would be about 347.99 m/sec. Not that the speed of sound increases slightly with increasing humidity since water molecules are actually increasing the density of the air.

Let's say the average thunderstorm is witnessed in conditions where the relative humidity is about 90%, and air temperature is about 25ºC, then the speed at which the sound of thunder will travel should be about 347.82 m/sec. There are 1609.344 meters in 1 mile, therefore it takes sound about 4 1/2 seconds to travel 1 mile. So you can guestimate the distance of a lightning strike by counting the seconds it takes for the sound of its thunder to reach your ears. When you get to "4 Mississippi..." you've just figured about a mile! So the next time you count to 4, don't feel comfortable that the lightning is 4 miles away! It's actually only a mile, and as far as I'm concerned, if you can see it, it can strike you!

Below is a chart I made using an equation set up for calculating the speed of sound at various temperatures and humidity through air in the troposphere near Earth's surface. (left column is humidity (%), and across the top is temperatures (ºC). Speed of sound is given in meters/second...

 5ºC 10ºC 15ºC 20ºC 25ºC 30ºC 10% 334.52 337.53 340.52 343.49 346.45 349.39 20% 334.57 337.60 340.61 343.62 346.62 349.62 30% 334.61 337.66 340.70 343.74 346.79 349.85 40% 334.66 337.72 340.79 343.87 346.96 350.09 50% 334.70 337.79 340.88 343.99 347.13 350.32 60% 334.75 337.85 340.97 344.12 347.31 350.55 70% 334.79 337.92 341.06 344.25 347.48 350.78 80% 334.84 337.98 341.15 344.37 347.65 351.01 90% 334.88 338.05 341.24 344.50 347.82 351.25 100% 334.93 338.11 341.34 344.62 347.99 351.48

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<>Do you have a question to pose? Send it to me at pierce.wx@gmail.com and you may see your question answered here!

These are personal homepages created and maintained solely by the individual authors (in this case, Steve W. Woodruff), not by eitherthe Los Angeles Community College District("District") or Los Angeles Pierce College("College").Viewpoints/opinions/representations containedin these homepages do not necessarily reflectthe viewpoints/opinions/representations ofeither the District, the College, its Board ofTrustees, officers and/or employees. Neitherthe District nor College endorses or monitorsthe content of these homepages. While theCollege does caution authors regarding theirresponsibility to abide by applicable laws andpolicies, these pages do not constituteofficial College business, and neither the LosAngeles Community College District nor LosAngeles Pierce College accepts responsibilityfor the contents of these homepages. Allcomments regarding the contents of personalhomepages should be directed to the pageauthor.

#### STATION COORDINATES

34º 10' 55.12" North Latitude
118º 34' 28.11" West Longitude
790' (240.8 m) Above Mean Sea Level
Modified Köppen Classification Climate Type: Csa
Station established July 01, 1949
Website established July 01, 1999

#### STATION NEWS

» Solar Irradiance, Wind Vector & Air Pressure 10-minute data records have been uploaded and are now available to everyone! Follow the links to the left! 11-18-2010.

» NOAA Global Monitoring Exhibit Opened For Tours ...read more

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