(figure available in print form)
Note how the angle at which the sun’s rays strike the earth changes during the year. When the North Pole points toward the sun, the rays fall more directly on the Northern Hemisphere. The more direct the sun’s rays focus on an area the hotter the area. The sun’s energy plays a major role in the various life styles of different people around the world. Four factors control the amount of the sun’s energy that falls on different parts of the globe. These four factors are the earth’s shape, rotation, tilt and revolution.
If the earth were not spinning on its axis, tilted and revolving around the sun, it would still be unequally heated by the sun’s energy. This is because the earth is a sphere and more of the sun rays will fall on the equator than on an identical area near the poles. Because the earth is tilted more of the sun’s energy will fall on the equator. Consequently, these areas will be warmer.
The earth does rotate and the sun’s rays can only strike areas that face it. Obviously, if the earth did not rotate only that part of the earth facing the sun would get any heat. There would be enormous differences in temperature between the two surfaces, from very hot to very cold. The earth’s rotation makes it possible for all the earth’s surface to be exposed for an equal amount of time to the warming light of day to the cooling dark of night. The earth rotates on its axis once every twenty-four hours.
If the earth’s axis pointed straight up and down every day would be the same. There would be twelve hours of daylight and twelve hours of darkness. This is not the case, the earth’s axis is tilted. Summer days are longer and nights shorter and in winter the days are shorter and nights surface during certain times of the year than others. We call these times seasons. The difference in length of daylight causes the seasons.
There are reasons for changes in the length of day. The earth’s axis is tilted and if you refer to the diagram, you will see the reason.
The earth also revolves around the sun every 365 1/4 days. This is called an earth year. Leap year accounts for the “quarter day.” The earth, when it revolves around the sun, stays tilted in the same direction, always pointed toward the North Star. For this reason the North Pole is pointed toward the sun, while at other times it is tilted away from the sun.
The earth’s tilt and revolution also mean that the part of the earth receiving more of the sun’s energy also changes. In either hemisphere the days are longer and the nights are shorter in the summer, the opposite is true of the winter.
The times when the earth’s axis is tilted to extremes have special names. The time when the poles are not tilted away from the sun is called equinox. One day a year the North Pole is tilted away from the sun more than at any other time of the year. We call this winter solstice. It generally occurs on December 21st. At this time the sun’s closest and direct rays strike the earth along a line 23 1/2° south of the equator in the Southern Hemisphere. This line is called the Tropic of Capricorn. In the Northern Hemisphere, however, there would be no sunlight at all above the 23 1/2° line south of the North Pole. This area is called the Arctic Circle. After the winter solstice the days begin to get longer in the Northern Hemisphere. Around March 21st is the time of the spring equinox. During this time both hemispheres receive an equal amount of sunlight. The days begin to get longer in the Northern Hemisphere until we reach the summer solstice. This occurs around June 21st. On this day the North Pole is tilted as close to the sun as it will get. The sun’s direct rays strike a line 23 1/2° north of the equator. This line is called the Tropic of Cancer. During this time the sun does not set at all in the Arctic Circle. The fall equinox occurs around September 21st and the poles are once again the same distance from the sun.Since the sun always shines and is sometimes overhead at noon between the Tropics of Cancer and Capricorn, this part of the earth is always warm. Because the tropics are heated constantly, their temperatures do not vary greatly during the year.
Climate is the resultant energy from the sun. Because the tropics are always warm, an air flow pattern develops in which the warm air rises over the equator and flows toward the poles. As it travels toward the poles it is being cooled. Cold heavier air travels from the poles to the equator and a cyclical pattern develops. The rotation of the earth causes global winds and pressure belts result from the need to exchange or balance energy.
The Greater Antilles are in the climatic belt called the low latitudes, which extends from the equator to about 30°. Trade winds are the dominate surface winds in this region and blow mostly from the east. Between the humid tropics and the tropical and subtropical deserts are the
wet and dry tropical climates
. These climates are produced bythe seasonal movement of the sun’s direct rays north and south of the equator. When the sun’s direct rays move north of the equator, they increase temperatures in that region.
This increase of temperature shifts the area of warm, rising air northward towards the Tropic of Cancer. It also brings summer rainfall into areas north of the equator between the tropics and the desert. In the winter when the sun’s direct rays move southward, warm rising air moves southward as well. A subtropical high pressure zone dominates the area and little rain falls in this region. There is a humid tropical climate pattern on this northern portion of the island of Hispaniola. This is because the northeast trade winds (cooler air) are meeting up with warm ocean currents causing heavier rainfall and because it is primarily coastal plain.
The climate of the West Indies is pleasant. Steady northeast trade winds moderate the tropical heat and humidity. The sun shines much of the year, and the islands receive 40 to 60 inches of rainfall. Crops can be grown throughout the year because of the favorable climate. The soil is fertile because of limestone and volcanic deposits. Sugar cane is the leading crop.