Science / Weather Glossary

Dawn: The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning twilight. The visual display is created by the scattering of light reaching the upper atmos . . . View Full Definition

Day: Considered a basic unit of time as defined by the earth's motion. It represents the time needed for one complete revolution of the earth about its own axis. Also know as a sidereal day, it i . . . View Full Definition

Debris Cloud: Considered a rotating cloud of debris or dust that is on the ground or near the ground. The debris cloud appearing beneath a thunderstorm will most likely confirm the presence of a tornado.

Deepening: Used in describing the history of a low pressure system or an area of cyclonic circulation, it means a decrease in the central pressure of the system. Although it usually describes the actio . . . View Full Definition

Degree: A measure of temperature difference representing a single division on a temperature scale.

Degree Day: A measure of the departure of the mean daily temperature from a given standard. That is one degree day for each degree (Fahrenheit or Celsius) of departure above or below the standard during . . . View Full Definition

Dense Fog Advisory: Advisory issued when fog reduces visibility to 1/8 mile or less, creating possible hazardous conditions.

Density: The ratio of the mass of a substance to the volume it occupies. In oceanography, it is equivalent to specific gravity and represents the ratio of the weight of a given volume of sea water to . . . View Full Definition

Density Altitude: The altitude at which a given density is found in the standard atmosphere. Used in aviation, it is computed from the station pressure at takeoff and the virtual temperature at the particular . . . View Full Definition

Depression: In meteorology, it is another name for an area of low pressure, a low, or trough. It also applies to a stage of tropical cyclone development and is known as a tropical depression to distingu . . . View Full Definition

Derecho: A line of intense, widespread, and fast-moving thunderstorms that moves across a great distance. They are characterized by damaging straight-line winds over hundreds of miles. Spanish for straight.

Dew: Condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours.

Dew Point: The temperature to which air must be cooled at a constant pressure to become saturated.

Diablo Winds: Dry winds in the Diablo mountain range in central California that can exceed 60 miles per hour. Similar to the Santa Ana winds, they develop as the wind flows from high pressure over Nevada . . . View Full Definition

Diffluence: A rate at which wind flow spreads apart along an axis oriented normal to the flow in question. The opposite of confluence.

Diffraction: The result of light waves interfering with other after passing through a narrow aperture, causing them to bend or spread.

Directional Shear: The shear created by a rapid change in wind direction with height.

Discontinuity: Comparatively large contrast in meteorological elements over a relatively small distance or period of time. In oceanography, it is the abrupt change or jump of a variable at a line or surface.

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