Hadley Cell: A thermal circulation proposed by George Hadley to explain the movement of the trade winds. It consists of rising air near the equator and sinking air near 30° latitude.

Hail (Hailstones): Small pellets or balls of ice with diameters between 5 and 50mm (or even more) formed in Cb when raindrops are carried above the freezing level. Large hailstones consists of several layers of ice deposited during successive ascents above the freezing level. If the particles are smaller than 5mm they are called Ice Pellets or Graupel. The costliest hailstorm in the USA pounded parts of Colorado with hail the size of golfballs and baseballs, causing over $600,000,000 in damage on July 11, 1990!

Halo: The ring of light around the Sun or our moon caused by refraction and reflection of light by ice crystals, typically in a layer of cirrostratus. the commonest form is a circle of radius 22 degrees.

9, 18, 20, 22, 24, 46 degree halos can be seen at Harald E. Edens' site:
www.weather-photography.com

Haze: Reduction of visibility caused by microscopic dry particles suspended in the atmosphere, usually below an inversion. Some particles are hygroscopic (attract water) and grow by taking up moisture; this makes the visibility worse. [a side note: Brake fluid, especially DOT 3 (Department Of Transportation -- DOT approved) is hygroscopic, therefore if you open a bottle to put in your vehicle be sure to close the lid tightly before storing the leftover fluid in the bottle so it does not attract moisture. Any water in your brake lines will compress translating to brake failure! DOT 4 fluid is far less hygroscopic. DOT 5 is synthetic and is made ONLY for certain vehicles. Though DOT 5.1 isn't and is used in motorcycles... like my KTM! :)

Heat: A form of energy transferred between systems (flux) by virtue of their temperature differences.

Heat Budget: The balance of incoming and outgoing radiation. If more comes into a system than goes out the result is net heating. If more goes out than comes into a system than out the result is net cooling.

Heat Index (HI): An index that combines air temperature and relative humidity to determine an apparent temperature--how hot it feels. See my FAQ page for a truncated chart!

Heating Degree-Day: A form of the degree-day used as an index for fuel consumption. Each degree of temperature of the daily mean below 65°F is counted as one heating degree-day. The amount of heat requried to maintain a certain temperature in a building is proportional to the heating degree-days total.

Heat Lightning: Distant lightning that illuminates the sky but is too far away for its thunder to be heard. It is common on hot summer nights when the overhead sky is clear. As light from distant electrical storms is refracted through the atmosphere, air molecules and fine dust scatter the shorter wavelengths of visible light, often causing heat lighting to appear orange to a distant observer. Common in Rose Valley, CA. during the summer.

Heat Low: A depression formed over land which has been heated during the day. It is commonly associated with sea-breezes along coasts and may precede the development of thundery showers.

Heatstroke: A physical condition induced by a person's overexposure to high air temperatures, especially when accompanied by high humidity.

Heiligenschein: Heiligenschein is a bright spot of light around the shadow of the observer's head, cast on dewy grass land. The dew droplets on the grass act as lenses, focusing sunlight on the grass leaves, strongly illuminating them, and working again as a lens for the backscattered light. This effect is enhanced by the fact that in the antisolar point (the point opposite the sun, i.e. the shadow of one's head), no other shadows are visible, and thus the region looks much brighter because the light and dark details of the surface are integrated by the eye. Look for the Heiligenschein early in the morning, when the sun is low and thus your shadow long, and move yourself to let the shadow be cast on wet dewy grass. The effect is very common, so you should be likely to see it.

Helm Wind: Lee wave phenomenon produced when strong easterly winds descend the western slopes of the Crossfell range in Cumbria.

Hertz (Hz): Unit of frequency of one cycle per second.

Heterosphere: The region of the atmosphere above about 85km where the composition of the air varies with height.

High: Commonly used to refer to an anticyclone.

High Cloud: A cloud that normally has its base above 6,000m; the base may be lower in winter and at high-latitude locations.

High Inversion Fog: A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland Climate: Complex pattern of climate conditions associated with mountains. highland climates are characterized by large differences that occur over short distances.

Hill Fog: Occurs when high ground becomes covered by low cloud.

Hoar Frost: A deposit of ice crystals in the form of scales, needles, feathers or fans.

Homosphere: The region of the atmosphere below about 85km where the composition of air remains fairly constant.

Hook Gage Micrometer: Measuring tool used to determine water loss from an evaporation pan. It is placed on the rim of a stilling well. See our Instrumentation page for pictures and more details!

Hook-Shape Echo: The shape of an echo on a RADAR screen that indicates the possible presence of a tornado.

Horse Latitudes: The belt of latitude at about 30° to 35° where winds are predominantly light and weather is hot and dry. In centuries past, ships carrying heavy cargo would get "stuck" in these latitudes (since all trans-oceanic ships then were wind driven). Captains would order their crew to throw off heavy cargo to make the ship lighter and easier for light winds to move. Unfortunately for horses onboard these ships, they were often the ones to be tossed overboard. Hence the name.

Humid Continental Climate: A climate characterized by severe winters and mild to warm summers with adequate annual precipitation. Typically located over large continental areas in the Northern Hemisphere Between about 40° and 70° latitude.

Humidity: Relative Humidity is the ratio of the actual vapor pressure to the saturated vapor pressure, expressed as a percentage. Specific Humidity is the ratio of the mass of water vapor to the total mass of moist air. It is almost the same as the Humidity Mixing Ratio which is the ratio of the mass of water vapor to the mass of dry air. This is usually given in grams of water vapor per kg of dry air.

Humid Subtropical Climate: A climate characterized by hot, muggy summers, cool to cold winters and abundant precipitation throughout the year. (Muggy is in reference to relatively high humidity.)

Humilis (Hum.): Applied to small or shallow fair-weather cumulus; Slightly vertical cumulus clouds that generally appear flattened.

Hurricane: An intense tropical cyclone with winds of 64 knots or more. This name is used in the West Indies. In the western Pacific it is called a Typhoon.

Hurricane Warning: A warning given when it is likely that a hurricane will strike an area within 24 hours.

Hurricane Watch: A hurricane watch indicates that a hurricane poses a threat to an area (often within several days) and residents of the watch area should be prepared.

Hydrologic Cycle: A model that illustrates the movement and exchange of water among Earth, atmosphere and oceans.

Hydrosphere: The water portion of our planet; one of the traditional subdivisions of Earth's physical environment.

Hydrostatic Approximation: This has the effect of filtering out vertically propagating sound waves (not wanted in most meteorological equations!)

Hydrostatic Equation: Expresses the variation of pressure with height. It is a very good approximation when the air is still or moving horizontally.

Hydrostatic Equilibrium: The state of the atmosphere when there is a balance between the vertical pressure gradient force and the downward pull of gravity.

Hygrograph: See Hygrometer.

Hygrometer: An instrument for measuring humidity.

Hygroscopic: Of a substance which absorbs water.

Hygroscopic Nuclei: Condensation nuclei having a high affinity for water, such as salt particles.

Hypothermia: The deterioration in one's mental and physical condition brought on by rapid lowering of human body temperature.

Hypoxia: Lack of oxygen to the brain. Occurs in people exposed to heavily-polluted, confined areas, such as a poorly-ventilated car garage.