A cool, moist air mass, which in the northern hemisphere often originates as continental polar air over the North American or Asian land masses and is modified as it moves out over the Atlantic and Pacific Oceans. In the southern hemisphere, there is a continuous zone of mP air between approximately 40° and 60° S that tends to expand towards Antarctica in summer. Heating by the relatively warm oceans leads to instability, with convective clouds and showers over the sea and on windward coasts. Convection is greatest in the rear of depressions and tends to be suppressed towards lower latitudes, when subsidence may limit cumulus growth and promote extensive altocumulus or stratocumulus.
Subjects: Meteorology and Climatology.