NORTH AMERICAN AIR MASSES, TRAJECTORIES, AND WEATHER (WINTER)

The shape and location of the North American continent make it an ideal source region and also permit the invasion of maritime air masses. You must be able to identify these air masses and trace their trajectories to develop and present an in-depth weather briefing.

Within an air mass, weather is controlled primarily by the moisture content of the air, the relationship between surface temperature and air mass temperature, and terrain (upslope or or downslope). Rising air is cooled; descending air is warmed. Condensation takes place when the air is cooled to its dew point. A cloud warmed above the dew point temperature evaporates and dissipates. Stability tends to increase if the sur-face temperature is lowered or if the temperature of the air at higher levels is increased while the surface temperature remains the same. Stability tends to be reduced if the temperature aloft is lowered.

Figure 4-1-4.-Air mass changes.

Smooth stratiform clouds are associated with stable air, whereas turbulence, convective clouds, and thunderstorms are associated with unstable air.

The weather conditions with cPk and cAk air over the United States depend primarily on the trajectory of the air mass after it leaves its source region. Trajectories, as observed on a surface chart, are indicated as one of the tra-jectories (A, B, C, D, E, F, G) shown in figure 4-1-5.

In the mid-latitudes, for an air mass to be classified as arctic, the surface temperature is generally 0 degrees Fahrenheit ( 18 degrees Celsius) or below.

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