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Air-Mass Weather 

Flying weather is usually best in tropical maritime air, at its source, within the sub-tropical highs. Scattered cumulus and patches of stratocumulus clouds may develop, but the sky is almost never overcast. Scant precipitation falls in scattered showers and variable, mild winds prevail.

The excellent flying weather in these mT source regions commonly extends through the moving air masses some distance from the sources. Cloudiness in the mT air increases with an increase in distance from the source. On flights from Hawaii or from the Azores northward, through northward-moving mT air, stratiform clouds increase. On flights from Hawaii or the Azores southward, through southward-moving mT air (or the northeast trades), cumuliform clouds increase. Here we are considering only Northern Hemisphere situations; however, a comparable pattern exists in the Southern Hemisphere.

A typical breakdown of the weather condi-tions you may encounter in air masses around the subtropical highs (fig. 6-7-2) is as fol-lows:

1. North of a subtropical high. Any mT air that moves northward becomes cooled over the cool ocean surface. A stratus overcast may form, and drizzle may fall. Farther north, low ceilings (usually below 1,000 feet) may reach the surface, producing fog. The mT air surges farthest north in summer because subtropical highs are best developed and polar fronts lie farthest north. This mT air brings most of the summer fogginess to northern seas and coasts. It brings the greatest fogginess in the Atlantic where it blows from the warm Gulf Stream over the cold Labrador current (near Newfoundland), and in the Pacific where it blows from the warm Kuroshio current over the cold Oyashio current (near the Kamchatka peninsula).

Figure 6-7-2.óWeather, winds, and stability conditions around the subtropical high.

2. East of a subtropical high. Along the California coast, and along the Atlantic coast of North Africa, the mT air blows from the west and the northwest. This air tends to remain stable for the following reasons:

a. It is coming from the northern, cooler portion of the source region.

b. Its surface layers remain cool because it moves over cold ocean currents.

c. Its upper portions warm adiabati-cally because of subsidence.

Throughout the year, airways are smooth. The skies are clear to partly cloudy. Clouds are generally patches of stratocumulus, and rain is rare. The chief flight hazard in this air is coastal fog, which often hides the California or European coastal land. Stratus and stratocumulus clouds may cause the sky to be overcast, develop low ceilings, and produce drizzle that reduces visibility.

3. South of a subtropical high. Where the mT air moves southward or southwestward (as trade winds), its lower layers are warmed by the tropical ocean surface. This produces scat-tered cumulus. Near the equator, after absorb-ing much moisture and being heated, this air may develop cumulonimbus.

4. West of a subtropical high. This mT air blows from the east and the southeast. Since it flows over warm water all of the way, the air neither cools or warms. Over the ocean near the Philippines (and near Florida and the West Indies), this trade wind brings good flying weatheróclear or scattered cumulus clouds. When it is moving over land, this warm, moist air becomes unstable and turbulent and is a source of thunderstorms. When it moves over cold land (for example, southeastern United States in the winter), it becomes stable and produces stratus clouds or fog. Over cold ocean surfaces, such as the Sea of Japan and the Kamchatka and Labrador currents, it develops the persistent low stratus and fogs characteristic of these areas.

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