In general, terms used in fire-weather discussion and summaries are plain language 'dictionary' words. There are, however, a few terms which have a meteorological connotation not covered by the standard dictionary definition. These are defined below:

Advection: The transfer of atmospheric properties by horizontal movement of air. Most commonly used in reference to transfer of warmer or colder air.

Dry Thunderstorm: A lightning storm accompanied by less than a wetting rain, 0.10 inch precipitation or less, often with very gusty winds.

Front: (cold, warm, or stationary) A zone of temperature and density discontinuity between two air masses.

Gradient: (pressure gradient) Change of value of the atmospheric pressure per unit of distance. The greater the change per unit of distance, the stronger the gradient, and the stronger the winds.

High: An area of high-atmospheric pressure delineated by closed isobars.

Instability: (unstable air mass) A state in which the vertical distribution of temperature is such that an air particle, if given either an upward or downward impulse, will tend to move away with increasing speed from its original level. Thunderstorm development would be an example of an unstable air mass.

Low: (depression, cell, disturbance) An area of low atmospheric pressure delineated by closed isobars (lines of equal pressure).

Low Aloft: (cold low, cold low aloft, upper-level low) Same as low above, except occurring in the upper atmosphere and characterized by moist, unstable and abnormally cooler temperatures aloft.

Ridge: (high-pressure ridge) An elongated area of relatively high atmospheric pressure.

Ridge Aloft: The same as ridge but occurring in the upper atmosphere. When a ridge is strong and persistent, it is often associated with warm and dry subsiding air.

Stability: (stable air mass) A state in which the vertical distribution of temperature is such that an air particle will resist displacement from its level. An inversion is an example of a very stable condition.

Subsidence: (subsiding air) A descending motion of air in the atmosphere.

Temperature Inversion: (inversion) A layer in which the temperature increases with altitude.

Thermal low: (heat low) A low pressure system caused by intensive heating at the earth's surface. Not associated with frontal systems. Occurs under high-pressure aloft and remains stationary.

Trough (Trof): An elongated area of relatively low atmospheric pressure. The axis of a trough is the trough line. Fronts are often located in the trough line at the surface.

Upper-level Trough: (upper trough, trough aloft) A pressure trough existing in the upper atmosphere.

Updated April 2008