Maritime Tropical (mT) Air Atlantic in Winter

Temperature and moisture content are higher in mT air masses than in any other American air mass in winter. In the southern states, along the Atlantic coast and Gulf of Mexico (fig. 4-1-11), mild temperatures, high humidities, and cloudiness are found, especially during the night and early morning. This is the characteristic weather found in mT air in the absence of frontal conditions. The stratus and stratocumulus clouds that form at night tend to dissipate during the middle of the day and fair weather prevails. Visibility is generally poor when the cloudiness is present; however, it improves rapidly because of convective activity when the stratus clouds dissipate. The ceilings associated with the stratus condition generally range from 500 to 1,500 feet, and the tops are usually not higher than 3,500 to 4,500 feet. Precipitation does not occur in the absence of frontal action. With frontal activity, the convective instability inherent in this air is released, producing copious precipitation.

If mT air is forced over mountainous terrain, as in the eastern part of the United States, the con-ditional instability of the air is released at higher levels. This might produce thunderstorms or at least large cumuliform clouds. (See fig. 4-1-12.) Pilots must be aware that these clouds may develop out of stratiform cloud systems and therefore may occur without warning. Icing may also be present. Thus, in the Great Lakes area, a combination of all three hazards (fog, thunderstorms, and icing) is possible.

Occasionally when land has been cooled along the coastal area in winter, maritime tropical air flowing inland produces an advection fog over ex-tensive areas. (See fig. 4-1-13.)

In general, flying conditions under this situa-tion are fair. Ceilings and risibilities are occa-sionally below safe operating limits; however, fly-ing conditions are relatively smooth and icing con-ditions are absent near the surface layers.

As the trajectory carries the mT air northward over progressively colder ground, the surface layers cool and become saturated. This cooling is greatly accelerated if the surface is snow or ice covered or if the trajectory carries the air over a cold-water surface. Depending on the strength of the air mass, fog with light winds or a low stratus deck with moderate to strong winds forms rapidly because of surface cooling. Occasionally drizzle falls from this cloud form; and visibility, even with moderate winds, is poor. Frontal lifting of mT air in winter, even after the surface layers have become stabilized, results in copious precipitation in the form of rain or snow.

During the winter, air resembling mT is oc-casionally observed flowing inland over the gulf and south Atlantic states. Generally the air that had a relatively short trajectory over the warm waters off the southeast coast is cP air. Clear weather usually accompanies cP air in contrast to cloudy weather accompanying a deep current of mT air. On surface synoptic charts, the ap-parent mT air can be distinguished from true mT air by the surface dew-point temperature value. True mT air always has dew-point temperature values in excess of 60F. The highly modified cP air usually has dew-point values between 50F and 60F.

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