Dry Adiabatic Lapse Rate (see Dry Adiabat)
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.
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.
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.
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.
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
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)
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 point, water vapor can change directly to a solid (ice).
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.
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
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 rain, graupel, and hail.
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)
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.
a relatively small-scale current of air with marked downward motion
in the same direction as a stream or other flow, or toward the direction in which the flow is moving
the component of radiation directed toward the earth’s surface from the sun or the atmosphere, opposite of upwelling radiation
see Dew Point
very small, numerous, and uniformly dispersed water drops between 0.2 and 0.5 millimeters in diameter that generally follow air currents
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
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
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.
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).
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
downslope (see Downslope Flow)
see Dew Point
the force required to accelerate a mass of 1 gram at a rate of 1 centimeter per second squared.