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Cyclones of middle and high latitudes depend for their development and structure upon the coming together of large bodies of air of contrasting physical properties. A body of air in which the upward gradients of 'temperature and moisture are fairly uniform over a large area is known as an air mass. In areal extent, a single air mass may be of subcontinental proportions; in vertical dimension it may extend through the troposphere. A given air mass is characterized by a distinctive combination of temperature, environmental lapse rate, and specific humidity. Thus we find air masses differing widely in temperature-from very warm to very cold-and in moisture content-from very dry to very moist. A given air mass may have a rather sharply defined boundary between itself and a neighboring air mass. This discontinuity is termed a front. We found an example of a front in the contact between polar and tropical air masses below the axis of the jet in upper airwaves. This feature is called the polar front; it represents the highest degree of global generalization. Fronts may be nearly vertical, as in the case of air masses having little motion relative to one another, or they may be inclined at an angle not far from the horizontal, in cases where one air mass is sliding over another. A front may be almost stationary with respect to the earth's surface, but nevertheless the adjacent air masses may be in relatively rapid motion with respect to each other along the front.

The properties of an air mass are derived in part from the regions over which it passes. Because the entire troposphere is in more or less continuous motion, the particular air-mass properties at a given place reflect the composite influence of trajectories covering thousands of miles and passing alternately over oceans and continents. This complexity of influences is particularly important in middle and high latitudes in the northern hemisphere, within the flow of the global westerlies. However, there are vast tropical and equatorial areas over which air masses reflect quite simply the properties of the ocean and land surfaces over which they move slowly or tend to stagnate. Thus, over a warm equatorial ocean surface the lower levels of the overlying air mass may develop a high water vapour content combined with a steep environmental temperature lapse rate. Over a large tropical desert, slowly subsiding air forms an air mass of high temperatures and low relative humidities. Over cold, snow-covered land surfaces in arctic latitudes in winter, the lower layer of the air mass remains very cold with a very low water vapour content. Meteorologists have designated as source regions those land or ocean surfaces that strongly impress their temperature and moisture characteristics upon overlying air masses moving over them.

Air masses move from one region to another following the patterns of barometric pressure. During such migration, lower levels of the air mass undergo gradual modification, taking up or losing heat to the surface beneath, and perhaps taking up or losing water vapour as well.

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