Regional and local winds MR ASKEW

1. Monsoon Winds  Refer to seasonal reversal of atmospheric pressure and winds with rain.  Found over south and south east Asia and can effect northern Australia and Northern Africa.

What causes this wind shift, monsoon climates?  Heating and cooling of land and adjacent sea areas = changes in atmospheric pressure and wind systems.  ITCZ moves northward in the northern hemisphere.  The Himalayan mountain range triggers high rainfall.

The seasonal changes in the monsoon  Winter (dry monsoon): During the northern winter. ITCZ moves south with the overhead sun. The equatorial low pressure dominates the central area of the Indian Ocean. Central Asia experiences intense cooling due to less insolation.  This causes the development of a large high pressure system over Asian interior.  Under the pressure gradient force cold winds blow over the Himalayas, they become hotter as they descend over the mountain.  Avg temp in winter is 15 – 20 ⁰C.

 Summer: (Wet monsoon): During the northern summer the overhead sun moves Northwards, the ITCZ moves north as well.  The increase in insolation over the Asian interior creates a large low pressure system, at the same time the sub tropical high pressure dominates the Indian ocean with surface temps of 30 ⁰C.  The pressure gradient force is reversed, warm moist air from the Indian Ocean flows northwards and over land areas.  This produces high rainfall.

Fohn Winds  Name used for any dry, hot wind that originates from mountainous areas or in an area where there is significant changes in altitude.  The wind is caused by a low pressure over central Europe, air ascends over the Alps from the Mediterranean sea and cools at the dry adiabatic lapse rate (1 ⁰C for every 100m).  If there is enough moisture in the air condensation takes place, clouds form, rains. As air rises further it will cool at the wet adiabatic lapse rate (0.5⁰C for every 100m)

 As air descends over the leeward side, it heats up at the DALR and the relative humidity drops.  Other examples: 1. Berg wind in SA 2. Chinook on eastern side of Rocky Mountains in N USA 3. Brickfielder in AUS