Air Mass

By definition, air masses are vast individual bodies of air within which the horizontal distribution of temperature and moisture is fairly uniform and stable. Covering expansive tracts of Earth’s surface in a single stretch, they are formed when the air remains stationary over a particular geographical location for an extended period of time, thus deriving a uniform temperature from that of the land or the sea below.

In practice, however, air masses are the purveyors of the world’s varied weather systems, including those unique elements required for tropical cyclone generation. Because the atmosphere is in constant motion, an air mass will not remain stationary indefinitely; in time, other air masses will move to displace it, creating in the convergence of different temperatures an atmospheric collision that yields the highs and lows associated with clement and stormy weather, respectively.

A front is the line where two air masses come together. The line where a warm air mass overtakes a cold air mass is a warm front; a cold air mass displacing a warm one is a cold front, and the situation where, because of similarities in temperature, neither air mass is able to overcome the other is a stationary front. Either way, the result is generally vicious weather, be it rain, snow, sleet, or thunderstorms.

Although hurricanes, typhoons, and cyclones are not extratropical cyclones and so do not contain the sort of frontal systems generally associated with stormy weather, they are nevertheless subject to the influence of air masses in motion. First, tropical cyclones begin as a revolving collection of thunderstorms over warm ocean waters—thunderstorms that are in themselves the direct result of a collision between air masses of contrasting temperature and moisture content.

Second, a tropical system develops as its central barometric pressure begins to fall and curving cloud bands start to gather around what will eventually become its eye. Rising air masses within the cyclonic system establish a crucial temperature difference between the storm’s core and its external environment. Following the laws of advection and convection, this temperature differential allows cooler air masses to descend toward the ocean’s surface and warmer air masses to ascend to what is known as an outflow layer. In time this relentless mechanical process determines the central pressure of the tropical cyclone and the intensity of its winds.

Furthermore, air masses often determine what course, or track, a particular hurricane will take across Earth’s surface. Because most tropical cyclones contain weak internal steering currents and so are not always able to choose where or when—if ever—they will make landfall, the presence of powerful external air masses becomes the primary means of locomotion for these storms. By deciding in what direction and at what speed a tropical cyclone will travel, air masses determine whether that system will make a potentially destructive landfall or will harmlessly pass out to sea.