Local winds

Exceptions exist to all the generalisations made in the preceding section. One important local effect occurs when, at coastlines, land and sea breezes result from the unequal heating. In the daytime, air rises over the hot land, and cool air comes in from the sea to displace it as the day advances. At night, the rapid cooling of the land chills the air in the vicinity of its surface which then moves out to sea to displace warmer air, which rises. This effect results in an averaging out of coastal air temperatures. The diurnal variation in dry — bulb temperature is less near the coast than it is further inland.

The seasonal variation of temperature (as well as the diurnal variation) is greatest over the middle of large land masses. This results in seasonal winds, called monsoons, blowing in from the sea to the land in the summer and outwards from land to sea in winter. India, Asia and China have distinct monsoon effects. Tornadoes, the origin of which is obscure, are instances of the highest wind speeds encountered. They are vortices, a few hundred metres across, which move in a well-defined path. They can occur all over the world but, fortunately are common only in certain areas. Their lives are short but violent. Wind speeds of 480 km If1 or more are likely and pressures at their centre have reached values as low as about 800 millibars. (The lowest recorded barometric pressure in the United Kingdom is 925 mbar and the highest is 1055 mbar. In London the values are 950 mbar and 1049 mbar, respectively.)

At higher altitudes, the comparative absence of dust particles and the reduction in the amount of water vapour in the atmosphere mean that radiation outwards to space is less impeded at night. The consequence is that the surface of the earth on high ground cools more rapidly at night than it does at lower altitudes. The air in contact with the colder surface becomes chilled (and hence denser) and slides down mountain sides to low-lying ground where it is apparent as a gentle wind, termed a katabatic or gravity wind. Examples are common and a typical case is the Mistral, a katabatic wind into the Mediterranean from the high plateau of south and eastern France.

Other local winds result from the passage of air in front of an advancing depression, across a hot desert—the Sahara, for example, which gives a dry wind along the coast of north Africa. (The same wind changes its character in crossing the Mediterranean, becoming a warm moist wind to southern Europe.)

There is another local wind which results from a process of adiabatic expansion and cooling as air rises. Air striking the windward side of rising slopes expands as it ascends, cooling occurring meanwhile. A temperature below the dew point is eventually reached, clouds form and, in due course, descend. The same air then flows over the highest point of the slope and rain falls on the lee side, suffering adiabatic compression and an increase in temperature as it does so. It appears as a dry, warm wind on the lee side of the high ground. An example of this is the Chinook wind, blowing down the eastern slopes of the Rocky mountains.

One last instance of a local wind of importance occurs in West Africa during winter in the northern hemisphere. The Sahara desert cools considerably at night in the winter, causing a progressive drop in air temperature as the season advances. This cool, dry air flows westwards to the African coast, displacing the warmer and lighter coastal atmosphere.

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