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weather patterns and the wind

Updated on June 16, 2012

Weather is caused by differences in temperature on the surface of the Earth. The moving force behind the weather is the Sun. At the Equator the Sun is never far from being directly overhead, and its heat is concentrated. Near the Poles, however, the Sun's rays strike obliquely and its heat is spread more thinly.

It is these unequal air temperatures that cause the winds. Near the Equator, where the Sun's rays are hottest. warmed air expands, rises, and billows out at high altitudes. As it cools, most of it subsides back to the surface just beyond the tropics, about 30·N and 30·S. Its weight creates regions of high pressure. Surface winds blow from there, back into the low pressure zone around the Equator.

The Earth's rotation deflects these winds so that they blow from the northeast in the Northern Hemisphere and the southeast in the Southern Hemisphere. These are the great trade winds, named by the merchant sailors whose ships made use of them. In the polar regions, air chilled by the cold ground sinks, creating a high-pressure zone from which the winds at ground level blow outwards to warmer latitudes.

Between the tropical and polar winds in each hemisphere are the winds of the temperate latitudes. Here cold polar air meets warm tropical air, creating low-pressure eddies, known as cyclones, that bring rain, winds and gales. The pattern of these major air currents is complicated by the presence of large areas of land.

Because land surfaces heat up and cool down more quickly than water, the continents are hotter than the sea in summer, cooler in winter. In India and other parts of the tropics, air rising from the hot land creates low-pressure areas, which suck in the monsoon winds in summer. The same effect causes daily sea breezes in coastal areas during hot weather.

In the afternoon, breezes blow shorewards as cooler air from the sea replaces rising hot air above the land. During the night and early morning, breezes blow seawards from the cooler land to the warmer sea.

The warmest days of summer. usually from about July 3 until August 15 in temperate latitudes of the Northern Hemisphere, were named the dog days by the Romans. The brightest star in the sky at that time was Sirius the Dog. and the Romans associated weather patterns with the stars.

Countrymen are more often right than wrong when they repeat the weather rhyme: 'Red sky at night, shepherd's delight: red sky in the morning, shepherd's warning.' When the Sun is low in the sky, morning or evening, it tends to glow red anyway-whatever the weather conditions. Whether we see it or not, though, depends on whether there are clouds in the way.

So if the redness is visible in the evening, in a relatively cloud-free sky, it means that the air to the west-the direction from which most of the weather comes in temperate latitudes is fairly cloudless and dry, So the following day should be clear and fine. If, on the other hand, the redness is visible in the morning, it means that the clear weather is to the east- and less favourable weather is probably on the way.

Sayings about red skies are among the oldest of all weather lore, dating back at least to Biblical times. 5t Matthew's Gospel (chapter 16, verses 2 and 3) expresses the idea in these words: 'When it is evening. ye say, It will be fair weather: for the sky is red. And in the morning, It will be foul weather today: for the sky is red and lowring.'


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