Daily March of Temperature: The daily cycle of air temperature changes. Commonly, the minimum temperature occurs just before sunrise and the maximum temperature occurs during mid-afternoon.

Daily Mean Temperature: The mean temperature for a day that is determined by averaging the maximum and minimum temperatures for that particular day.

Daily Range of Temperature (Daily Temperature Range): The difference between the maximum and minimum temperatures for any given day.

Dalton's Law of Partial Pressures: For a fixed temperature and volume (container), the total pressure of a mixture of gases is the sum of the partial pressures that each of the individual component gases would have if placed alone in the same volume (container).

Dart Leader (Leader): The discharge of electrons that proceeds intermittently toward the ground along the same ionized channel taken by the initial lightning stroke. See also Leader.

Deepening: A term used when the pressure at the center of a low decreases with time. (The opposite is termed filling.)

Dendrochronology: The analysis of the annual growth rings of trees as a means of interpreting past climatic conditions.

Density: Mass per unit volume usually expressed as grams per cubic centimeter or kilograms per cubic meter.

Density Current: An almost horizontal pattern of flow due to differences of density in air or liquids. (See also Gravity Current.)

Deposition: A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first. Water vapor phase change directly to a solid. The reverse of Sublimation.

Depression: A region of low pressure surrounded by a series of closed isobars.

Desert: One of the two types of dry climate--the driest of the dry climates.

Desertification: A general increase in the desert conditions of a region.

Deterministic: Later states evolve from earlier ones according to a fixed law usually expressed as a mathematical equation.

Dew: Condensation of water vapor on a cooled surface.

Dew Cell: An instrument used to determine the dew-point temperature.

Dew Point (Dew Point Temperature): The temperature to which air must be cooled to become saturated with water vapor. Further cooling leads to the formation of fog or cloud droplets in the atmosphere or the deposit of dew on the surface.

Diffluence: The moving apart of adjacent streamlines. The reverse of Confluence.

Diffraction: The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.

Diffused Light: Solar energy is scattered and reflected in the atmosphere and reaches Earth's surface in the form of diffuse blue light from the sky.

Directional Divergence: The spreading out of an airstream that typically occurs slightly downwind of the axis of an upper-air trough.

Discontinuity: A zone characterized by a comparatively rapid transition of meteorological elements.

Dispersion: The separation of white light into its different component wavelengths.

Dissipating Stage: The final stage in the "development" of an air mass thunderstorm when downdrafts exist throughout the cumulonimbus cloud.

Dissipation Trail: (often shortened to Distrail): The opposite of Contrail, occasionally seen when an aircraft flies through a very thin cloud layer. The cloud dissolves where the wing-tip vortices produce downward motion.

Diurnal Variation (Diurnal): Changes which occur during the day such as the rise and fall of temperature or the regular variations of pressure.

Divergence: The moving apart of air currents to cause a depletion of the air. (The reverse of Convergence.) Divergence at one level is often almost balanced by convergence at another. When such balance is not achieved, the surface pressure rises or falls.

Doldrums: The region near the equator that is characterized by low pressure and light, shifting winds.

Doppler Lidar: The use of light beams to determine the velocity of objects such as dust and falling rain by taking into account the Doppler Shift.

Doppler RADAR: RADAR which detects motion towards or away from the set by measuring the change in frequency between outgoing and reflected signals.

Doppler Shift (Doppler Effect): The change in the frequency of waves that occurs when the emitter or the observer is moving toward or away from the other.

Downburst: A very powerful descent of air under or near a Cb. Vertical velocity may exceed 60 knots. Damage may be caused when the downburst reaches the ground and spreads out horizontally. (See also Microburst, which is a small downburst.)

Drizzle: Drops of water with a diameter between 0.2 and 0.5mm. It is usually caused by coalescence of droplets of cloud. It evaporates if it falls through dry air; dirzzle at ground level indicates moist air below cloud and suggests a very low cloud base.

Drought: A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.

Dry Climate: A climate deficient in precipitation where annual potential evaporation and transpiration exceed precipitation.

Dry Line: A term used chiefly in the USA for the division between warm moist air from the Gulf of Mexico and dry desert air. Severe thunderstorms may develop along this line.

Dry-Summer Subtropical Climate: A climate characterized by mild, wet winters and warm to hot, dry summers. Typically located between 30 and 45 degrees latitude on the western side of continents. Also called Mediterranean Climate. This is the climate type our station operates in.

Duplicatus: Coinciding cloud patches, sheets, or layers, sometimes partly merged at slightly different levels. Applies mainly to cirrus, cirrostratus, altocumulus, altostratus, and stratocumulus.