Maritime tropical (mT) Pacific air has no direct influence on the weather over the Pacific coast. During the summer season, the Pacific anticyclone moves northward and dominates the Pacific Coast weather with mP air. Occasionally mT air reaches the West Coast; for example, tropical storms or typhoons sometimes move northerly along the Baja Coast. This synoptic condition produces a great amount of cloudiness and precipitation.

Maritime Tropical (mT) Air Atlantic in Summer 

The weather in the eastern half of the United States is dominated by mT air in summer (fig. 4-1-17). As in winter, warmth and high moisture content characterize this air. In summer, convec-tive instability extends to higher levels; there is also a tendency toward increasing instability when

Figure 4-1-17.Maritime tropical (mT) air, Atlantic, in summer.

Figure 4-1-18.mT (Gulf of Mexico or Atlantic) air in summer moving northward over warm continent.

the air moves over a warmer land mass. (See fig. 4-1-18.) This is contrary to winter conditions. Along the coastal area of the southern states, the development of stratocumulus clouds during the early morning is typical. These clouds tend to dissipate during the middle of the morning and immediately reform in the shape of scattered cumulus. The continued development of these clouds leads to scattered showers and thunderstorms during the late afternoon. Ceilings in the stratocumulus clouds are generally favorable (700 to 1,500 feet) for the operation of aircraft. Ceilings become unlimited with the development of the cumulus clouds. Flying con-ditions are generally favorable despite the shower and thunderstorm conditions, since the convective activity is scattered and can be circumnavigated. Visibility is usually good except near sunrise when the air is relatively stable over land.

When mT air moves slowly northward over the continent, ground fogs frequently form at night. Sea fogs develop whenever this air flows over a relatively cold current such as that occur-ring off the east coast. The notorious fogs over the Grand Banks of Newfoundland are usually formed by this process.

In late summer, the Bermuda high intensifies at times and seems to retrograde westward. This results in a general flow of mT air over Texas, New Mexico, Arizona, Utah, Colorado, and even southern California. The mT air reaching these areas is very unstable because of the intense sur-face heating and orographic lifting it undergoes after leaving the source region in the Caribbean and Gulf of Mexico. Shower and thunderstorm conditions, frequently of cloudburst intensity, then prevail over the southwestern states. Locally this condition is termed sonora weather.

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