Air masses, fronts and extra tropical cyclones

Air masses. Air that stays long enough over a uniform surface (a vast ocean or plain) takes on its character. An air mass is a large body of air with little horizontal variation in temperature and moisture. The surfaces where they form are source regions:

• warm tropical and subtropical oceans: maritime tropical (mT);
• subtropical hot deserts: continental tropical (cT);
• cold high-latitude oceans: maritime polar (mP);
• very cold, snow-covered high-latitude continents (such as the Siberian plain in winter): continental polar (cP);
• permanently ice-covered Arctic and Antarctica: continental arctic (cA).
• Tropical air masses are warm; polar ones are cold.

Fronts. Where two different air masses meet, the boundary is a front; its formation is frontogenesis. Fronts occur in middle latitudes, with steep changes in temperature and pressure; they force air to rise, forming clouds and precipitation.

• Cold front: cold air advances into warm air.
• Warm front: warm air advances onto cold air.
• Stationary front: the front does not move.
• Occluded front: the warm air mass is fully lifted off the ground.

Extra-tropical (mid-latitude) cyclones form along the polar front:

• At first the front is stationary. In the northern hemisphere, warm air blows from the south and cold air from the north. When pressure falls along the front, warm air moves north and cold air south, starting an anticlockwise circulation with a warm front and a cold front, and a warm sector between.
• Warm air glides up over cold air ahead of the warm front, bringing a sequence of clouds and rain. The cold front pushes the warm air up from behind, forming cumulus clouds.
• The cold front moves faster, catches the warm front, the warm air is lifted completely (occlusion), and the cyclone dies out.

Compared with tropical cyclones: extra-tropical cyclones have a clear frontal system, cover a larger area, can form over land and sea, have lower wind speeds, and move west to east (tropical cyclones move east to west).

Example (new): the winter 'western disturbances' that bring rain to Punjab and snow to Kashmir are mid-latitude systems travelling from the west, which is why they reach Pakistan before they reach north-west India.