DAILY MEAN
The average temperature for a day computed by averaging either the hourly readings or, more commonly, the maximum and minimum temperatures.
DALTON'S LAW
States that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures of the gases. Formulated by John Dalton, an English physicist.
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 atmosphere prior to the sun's rise to the observer's horizon.
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 is approximately equal to 23 hours, 56 minutes, and 4.09 seconds.
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 action of a pressure system on a constant pressure chart, it also means a surface low is increasing in cyclonic circulation and acquiring more energy. The opposite of filling.
DEGREE
A measure of temperature difference representing a single division on a temperature scale. Some common scales are Celsius, Fahrentheit, and Kelvin etc.
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 one day.
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 that of an equal volume of distilled water at 4.0°C or 39.2°F.
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 altitude under consideration.
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 distinguish it from other synoptic features.
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.
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.
DISTURBANCE
This has several applications. It can apply to a low or cyclone that is small in size and influence. It can also apply to an area that is exhibiting signs of cyclonic development. It may also apply to a stage of tropical cyclone development and is known as a tropical disturbance to distinguish it from other synoptic features.
DIURNAL
Pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-four hours. Meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature, and wind.
DIVERGENCE
Wind movement that results in a horizontal net outflow of air from a particular region. Divergence at lower levels is associated with a downward movement of air from aloft. Contrast with convergence.
DOG DAYS
The name given to the very hot summer weather that may persists for four to six weeks between mid-July through early September in the United States. In western Europe, this period may exist from the first week in July to mid-August and is often the period of the greatest frequency of thunder. Named for Sirius, the Dog Star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months.
DOLDRUMS
Located between 30 degrees North and 30 degrees South latitudes in the vicinity of the equator, this area typically has calm or light and variable winds. Also a nautical term for the equatorial trough.
DOPPLER RADAR
Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.
DOWNBURST
A severe localized downdraft from a thunderstorm or shower. This outward burst of cool or colder air creates damaging winds at or near the surface. Sometimes the damage resembles tornadic damage.
DOWNDRAFT
A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with a thunderstorm or shower.
DOWNPOUR
A heavy rain.
DOWNSLOPE EFFECT
The warming of an air flow as it descends a hill or mountain slope.
DRAINAGE WIND
A katabatic wind, it is caused by the cooling of air along the slopes of a mountain.
DRIFTING SNOW
Snow particles blown from the ground by the wind to a height of less than six feet.
DRIFTS
Normally used when referring to snow or sand particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind.
DRIZZLE
Slowly falling precipitation in the form of tiny water droplets with diameters less than 0.02 inches or 0.5 millimeters. It falls from stratus clouds and is often associated with low visibility and fog.
DROPSONDE
A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon to measure the atmosphere below.
DROUGHT
Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water to cause serious hydrological imbalance.
DRY ADIABAT
The line on a Skew T-Log P chart that depicts the lifting of dry air, or air that is unsaturated. As a parcel rises adiabatically, its pressure decreases and its temperature falls due to the expansion of the air parcel. When an air parcel is unsaturated and rises, then the temperature decreases at a rate of 1°C per 100 meters (5.5°F per 1,000 feet).
DRY BULB THERMOMETER
A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.
DRY SLOT
An area of dry, and usually cloud-free, air that wraps into the southern and eastern sections of a synoptic scale or mesoscale low pressure system. Best seen on a satellite picture, such as a water vapor image.
DUSK
The period of waning light from the time of sunset to dark.
DUST
Small particles of earth or other matter suspended in the air. It is reported as "DU" in an observation and for wide spread dust on the METAR.
DUST DEVIL
A small, rapidly rotating column of wind, made visible by the dust, dirt or debris it picks up. It usually occurs in arid or semi-arid areas and is most likely to develop on clear, dry, hot afternoons in response to surface heating.
DUSTSTORM
A severe weather condition characterized by strong winds and dust-filled air over a large area. Visibility is reduced to between 5/8ths and 5/16ths statute mile. It is reported as "DS" in an observation and on the METAR.
D-VALUE
The deviation of actual altitude along a constant pressure surface from the standard atmosphere altitude of that surface.
DYNAMICS
A branch of mechanics that deals with forces and their relations to patterns of motion. In metorology, this relates especially to wind and water patterns.