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.

dBZ - 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.

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

Density - the mass of a substance per unit volume

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

Desertification - a tendency toward more prominent desert conditions in a region

Dew - 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) - 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

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

Difluence - 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 reflected, refracted or scattered)

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

Dispersion - the process of separating radiation into various wavelengths

Diurnal - 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)

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

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 - a 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

Downburst - an intense localized downdraft which 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 - a relatively small-scale current of air with marked downward motion

Downstream - 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

Drizzle - 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 drops or cloud droplets of specified sizes

Drought - 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 Microburst - a 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 which 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.

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).

Dry-Line Bulge - a bulge in the dry line, 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 dry line

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