The season
The earth’s rotation produces day and night. Everyone knows that in most parts of the world the days are warmer than night. Sometimes the difference is hardly noticeable. The atmosphere keeps out some heat during the day and holds in some heat during the night. When the relative humidity is high this effect is increased. In some place on windward coasts and in calms near equator, the difference between night and day temperatures may be only 5 to 7 f. When the atmosphere is clear and dry, as in tropical deserts, the range may be 50 or more.
The length of the day at given point is detemined by latitude and season. The revolution of the earth gives us the seasons, because of the tilt of the earth’s axist ( chapter 1 ). Seasional variations in temperature result from charging anglr of the sun’ rays san the changing length of the day. Both of these are determaind by the latitude. In the low latitudes that is the regions nearst the equator the seasonal changes affect rainfall particully. In middle latitudes, the seasons vary mainly in temperature in high latitudes, nearst the poles, summer and winter are alomst the same as day and night.
In the tropics the days and nights are alomst equal and the sun’s rays are never far from vertical. Seasonal changes in the temperature are not great. In some places the warmest mounths are only a few degrees warmer than cloudest. However, the rainfall is effected by slight change in temperature so that these climates have seasonal pattern humidity is high and rains are frequent and heavy. During the winter the air fairly clear and dry.
In the middle latitudes the year can be divided into spring, summer, autumn, and winter. In midsummer the sun’s rays are most nearly vertical, in midwinter, they are most slanting. The different in temperature are more strongly marked in some areas than in others. The lengthand time of the seasons varies also. Winters may be cold or mid; summers may be hot and cool. Spring may be early and long, or late and short. However, seasonal variations in temperature can always be observed. These affect the growth of crops and the care of livestock. Therefore the largely decide the type of agriculture or other economic activity in region.
Many tropical and subtropical areas livable that would otherwise be too hot for comfort, however, they also give rise to hurricanes and typhoons. Many semiarid and desert regions are in the tradewind belts. Warm air increases temperatures and lowers the relative humidity. The prevailing westerlies often bring stoms coulds and rain. In the ocean of the southern hemisphere, the atlantic sometimes went around south america and acroos the pasific past asia to get europe, rather than go throught the belt of westerlies in the open ocean. In other parts of the world, westerlies warm cooler regions.
Seasonal changes in the pressure belt
As the sun’s vertical rays north and south of the equator with the seasons, positions of the pressure and wind belt also shift. The change does not follow the movement of the sun’s rays directly; the wind and pressure belt change only about one fourth as much as the sun’s rays do. Nevertheles in some parth of the world a place may be in one wind belt during the summer and another during the winter. Southern california westerly winds prevail in winter, trade winds in the summer
High altitude winds
The surface winds such as the trades and westerlies blow at fairly low altitudes. At high altitudes, the raising air from the equatorial region meets cold polar air flowing southward. In the region about 40 latitude, strong winds from as the typeas air meet and mix. The band of winds thus formed is called the circumpolar whirl.
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