Air Masses and Fronts
When air stagnates in a region where surface characteristics are uniform, it acquires those characteristics and becomes an air mass. Warm, moist air masses are formed over tropical waters; cold, moist air masses over the northern oceans; cold, dry air masses over the northern continent; and warm, dry air masses over arid regions. The weather within an air mass--whether cool or warm, humid or dry, clear or cloudy--depends on the temperature and humidity structure of the air mass. Within horizontal layers, the temperature and humidity properties of an air mass are fairly uniform. The depth of the region in which this horizontal uniformity exists may vary from a few thousand feet in cold, winter air masses to several miles in warm, tropical air masses. Weather within an air mass will vary locally from day to day due to heating, cooling, precipitation, and other processes. These variations, however, usually follow a sequence that may be quite unlike the weather events in an adjacent air mass.
Air masses have characteristic weather in their source regions. But, as air masses leave their source regions, they are modified according to the surface over which they travel, and the air-mass weather changes. The day-to-day fire weather in a given area depends, to a large extent, on either the character of the prevailing air mass, or the interaction of two or more air masses. In frontal zones, where differing air masses meet, considerable weather is concentrated. Cloudiness, precipitation, and strong and shifting winds are characteristic of frontal passages; but, occasionally, frontal passages are dry and adversely affect fire behavior.
Encyclopedia ID: p361
