


Dry Adiabatic Lapse Rate (see Dry Adiabat)

Dalton's Law

for "perfect gases", a mixture of gases will have a pressure equal to the sum of the pressures of the individual gases, assuming no chemical reaction has taken place between the gases. Named for John Dalton (1766 - 1844), a British chemist who formulated the concept.

Dart Leader

a faint, negatively charged channel that travels more or less directly and continuously from cloud to ground after the initial lightning stroke has traveled through the channel.

Data Logger

an electronic device that records data over time. Instruments such as thermometers and anemometers can be connected to a data logger to collect and store the data.


the nondimensional "unit" of radar reflectivity. It represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z. The value of Z is a function of the amount of radar beam energy that is backscattered by a target and detected as a signal (or echo). Higher values of Z (and dBZ) thus indicate more energy being backscattered by a target. The amount of backscattered energy generally is related to precipitation intensity, such that higher values of dBZ that are detected from precipitation areas generally indicate higher precipitation rates.


District of Columbia



Debris Cloud

a rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado. A debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.

Deepening (DPNG)

a decrease in the central pressure of a surface low-pressure system. The storm is intensifying.

Degree Day

a measure of the difference between the mean daily temperature and some given base temperature: one degree day is given for each degree (degree Celsius or degree Fahrenheit) of departure above (or below) the base temperature during one day




the mass of a substance per unit volume



Department of Commerce (DoC)

a federal department that is made up of several divisions, including the Census Bureau, International Trade Administration, Patent and Trademark Office, National Institute of Standards and Technology, and the National Oceanic and Atmospheric Administration (NOAA)

Department of Defense (DoD)

a federal department that coordinates and supervises all agencies and functions of the government relating directly to national security and the military. Its three main components are the Department of the Army, the Department of the Navy, and the Department of the Air Force.


when a gas turns directly into a solid, without first condensing into a liquid. An example is frost: when the temperature cools down to the frost pointwater vapor can change directly to a solid (ice).


any family of downburst clusters produced by an extratropical mesoscale convective system


a tendency toward more prominent desert conditions in a region


water condensed upon the surfaces of objects near the ground when temperatures of the surface air have fallen below the dew point, due to cooling during the night, but are still above freezing

Dew Point (Dewpoint Temperature, DP or DWPT)

a measure of atmospheric moisture; the temperature to which air must be cooled for saturation to occur (given a constant pressure and constant water vapor content)

Dewpoint Depression

 the difference in degrees between the air temperature and the dewpoint temperature.




a process that occurs with the addition or loss of heat. The opposite of adiabatic. An example of a diabatic process is an air parcel warming due to the absorption of infrared radiation or release of latent heat.

Differential Motion

specifically, cloud motion that appears to differ relative to other nearby cloud elements; cloud rotation is one example of differential motion, horizontal wind shear along a gust front is another example




a pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow; opposite of confluence; difluence is not the same as divergence



Direct Solar Radiation

the component of solar radiation received by the earth’s surface only from the direction of the sun’s disk (i.e. it has not been reflectedrefracted or scattered)

Directional Shear

the component of wind shear that is a result of a change in wind direction, e.g., southeasterly winds at the surface and southwesterly winds aloft


an instrument that measures the drop-size distribution (lots of little drops, lots of big drops, or a mix) and speed of falling precipitation. By measuring these parameters, precipitation rates can be determined. Some types of disdrometers can distinguish between raingraupel, and hail.


the process of separating radiation into various wavelengths


daily; related to actions which are completed during a single calendar day, and which typically recur every calendar day (e.g., diurnal temperature cycle of temperature increase and decrease)


the net outflow of air from a region, typically caused by horizontal wind motion; the opposite of convergence








see Downstream


the regions on either side of the equator where air pressure is low and winds are light.

Doppler Dilemma

a limitation with a pulsed Doppler radar (like the WSR-88D) that involves a trade-off between a wide range of observable radial velocities and the detection of echoes at a long range from the radar. Having a wide range of velocities (desired for detection of severe weather) limits the range from the radar that echoes can be detected. When more areal coverage (e.g., a long range) is desired, a narrower range of radial velocities must be computed. The Doppler Dilemma is related to the time between successive transmitted pulses of energy. A "long" amount of time (in milliseconds) between successive pulses allows the radar to detect echoes at a far range from the radar. However, a short amount of time between successive pulses allows for more accurate and higher Doppler velocities to be calculated.

Doppler Radar

radar system that utilizes the Doppler effect for measuring the radial velocity of the wind (i.e., the motion toward or away from the radar)

Doppler Shift (or Doppler Effect)

the change in frequency with which energy from a given source reaches an observer when the source and the observer are in motion relative to each other


an intense localized downdraft that may be experienced beneath a thunderstorm, typically a severe thunderstorm; it results in an outward burst of damaging winds on or near the ground

Downdraft (DWNDFT)

a relatively small-scale current of air with marked downward motion

Downslope Flow (DWNSLP)

wind that flows down a mountain slope, sometimes at night (e.g., Mountain breeze). Other examples of downslope flow are katabatic winds and Chinooks.

Downstream (DNSTRM)

in the same direction as a stream or other flow, or toward the direction in which the flow is moving

Downwelling Radiation

the component of radiation directed toward the earth’s surface from the sun or the atmosphere, opposite of upwelling radiation




see Dew Point


see Deepening











Drizzle (DZ)

very small, numerous, and uniformly dispersed water drops between 0.2 and 0.5 millimeters in diameter that generally follow air currents

Drop-size Distribution

the distribution of rain droplets or cloud droplets of specified sizes


a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious shortages of water for agriculture and other needs in the affected area

Dry Adiabat

a line of constant potential temperature on a thermodynamic chart

Dry Air

in meteorology, air that contains no water vapor

Dry Line Bulge

a bulge in the dryline, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge.

Dry Line Storm

any thunderstorm that develops on or near a dryline

Dry Microburst

microburst with little or no precipitation reaching the ground; most common in semi-arid regions; at the ground, the only visible sign might be a dust plume or a ring of blowing dust beneath a local area of virga

Dry Slot

a zone of dry (and relatively cloud-free) air that wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system; generally is seen best on satellite photographs. The dry slot should not be confused with the clear slot, which is a storm-scale phenomenon.


an adiabatic process in a hypothetical atmosphere in which no moisture is present OR an adiabatic process in which no condensation of its water vapor occurs and no liquid water is present.

Dryline (or Dry Line)

a boundary separating warm, dry air from warm, moist air, typically across parts of New Mexico, Texas, Oklahoma, or Kansas. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).











Dust Devil

a small whirlwind, usually of short duration, that is not associated with a thunderstorm and contains dust, sand, and debris picked up from the ground

Dust Plume

a non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front.

Dust Storm

a severe weather condition characterized by strong winds and dust-filled air over an extensive area. Also known as a haboob.




downward vertical velocity (sinking air)


see Downdraft


downslope (see Downslope Flow)


see Dew Point

Dynamic Lifting

the forced uplifting of air from various atmospheric processes, such as weather fronts, and cyclones.


generally, any force that produces motion or causes change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.


the force required to accelerate a mass of 1 gram at a rate of 1 centimeter per second squared.


see Drizzle