A warm, moist, and generally stable air mass that originates in the subtropical anticyclones over the oceans. The principal source regions are the North Atlantic and North Pacific in the northern hemisphere, and the South Atlantic, South Pacific, and south Indian Ocean in the southern. The last three together cover practically half of the southern hemisphere in a broad belt down to about latitude 40° S. (In the southern summer, mT air covers most of southern Africa and South America, with only very small source regions of continental tropical air in both continents.) Because of its warmth and humidity, stratiform cloud frequently develops as mT air moves towards the poles. When the sea-surface temperature is low, it may produce extensive advection fog in the South-Western Approaches, the South China Sea (see crachin), off Newfoundland, and off northern California.
Subjects: Meteorology and Climatology.