Continental tropical air is found over the United States only in the summer. Its source region is the relatively small area over the northern portion of Mexico, western Texas, New Mexico, and eastern Arizona. High surface temperatures and very low humidities are the main air mass characteristics. Large diurnal temperature ranges and the absence of precipita-tion are additional properties of cT air. Flying conditions are excellent. However, during the daytime turbulence sometimes extends from the surface throughout the average flying levels.

Superior air usually exists over the southwestern states and is believed to be the result of strong subsiding motions. Most frequently this air is observed above an inversion layer at high levels; it rarely descends to the surface. Above the inversion layer, this superior air is the warmest air mass observed in the United States at its altitude; but, because of its steep lapse rate, its temperature at higher levels is less than that of tropical air. Relative humidities are usually less than 30 percent. Quite often they are too low to measure accurately.

Superior air is observed in both summer and winter. Flying conditions are excellent in this air mass, since no cloud forms are present and risibilities are usually very good because of the dryness.

This type of air mass is very important because superior air frequently stops all convective activity caused by intruding maritime tropical air. This generally prevents the formation of showers and thunderstorms unless the mT air mass is deep. NOTE: Views A and C of figure 4-1-19 show the properties of significant North American air masses during the winter and summer seasons from the standpoint of flying.

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