Regional storm and monsoon

In meteorological terms, regional and monsoon storms are mainly classed as orographic storms. What they have in common is that they are formed by air masses rising on the leeward side of mountains. The air cools down in the process, condenses when humidity has passed saturation point – which sometimes results in heavy rain – and rushes down into the valleys from mountain ridges or pass summits.

In the case of regional storms, too, wind speeds increase with the difference in temperature and height of fall. If orographic winds additionally combine with a large-scale stream of air moving in the same direction, speeds of up to 200 km/h are possible.

The best-known examples of regional storms are the Bora on the Adriatic Coast of Dalmatia, warm winds like the foehn in the Alps, the Mistral in the lower Rhône Valley, and the Chinook in the Rocky Mountains. But such orographic winds may occur in all mountains regions of the world, particularly on the edge of temperate climate zones. Their formation is so closely linked to the respective topography that it is common for them to occur repeatedly at the same place and with the same wind direction.

These wind systems are most intensive on the extremities of the Antarctic and Greenland, where the extremely cold air of the central plateaus plunges to sea level – sometimes by more than 3,000 m – through narrow glacier valleys. In the process, it frequently reaches and maintains hurricane force for long periods of time.

The monsoon is a separate windstorm phenomenon of regional expanse. When the great land surface of Asia heats up under the almost vertical rays of the sun in early and mid-summer, it draws in warm and moist air masses from the Indian and Pacific Oceans. Incidentally, without this circulation, the entire Indian subcontinent and adjacent regions would be uninhabitable deserts.