Glossary

In lightning, the leader which, after the first stroke, initiates each succeeding stroke of a composite flash of lightning.

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.

The tendency for the surface wind to become much lighter than wind above it at night when the surface temperature cools.

a measure of the departure of the daily mean temperature from the normal daily temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty- five degrees Fahrenheit

Hexagonal ice crystals with complex and often fernlike branches.

A fog in which the visibility is less than one- quarter mile.

Issued when fog is expected to reduce visibility to 1/4 mile or less over a widespread are for at least 3 hours.

A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Large (one to several millimeters in diameter), cohesionless, coarse, faceted snow crystals which result from the presence of strong temperature gradients within the snowpack

A widespread and usually fast-moving windstorm associated with convection. Derechoes include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.

Moisture from water vapor in the air that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

A fall of non- branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates. (same as ice crystals)

Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.

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. It is the opposite of confluence.

Most of the time, upper- level ridges bring fairly clear weather as the storms are steered around the ridge. Sometimes, however, strong storms undercut the ridge and create precipitation. Ridges that experience this undercutting by storms are known as dirty ridges because of the unusual precipitation.

A disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.

Daily; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and falls at night).

The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence.

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

A type of weather radar that determines whether atmospheric motion is toward or away from the radar. It determines the intensity of rainfall and uses the Doppler effect to measure the velocity of droplets in the atmosphere.

A strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado.

A column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm. .

Air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions.

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

Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility.

Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels

A line of constant potential temperature on a thermodynamic chart.

A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains.

A surge of drier air; normally a synoptic scale or mesoscale process. A dry punch at the surface results in a dry line bulge.

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. A dry slot generally is seen best on satellite photographs.

A boundary which separates warm, dry air from warm, moist air. The differences in the two air masses may be significant. The dry line is usually a boundary of instability along which thunderstorms form.

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

An area where high surface winds have picked up loose dust, reducing visibility to less than one- half mile.

A rotating column of air rendered visible by dust.

Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.