Wednesday, September 3, 2008

Why do we have different Seasons?

Since earliest times, man has been curious about the changing of the seasons. Why is it warm in summer and cold in winter? Why do the days gradually grow longer in the spring? Why are the nights so long in winter?

We all know the earth revolves around the sun, and at the same time it revolves on its own axis. As it moves around the sun, it's also spinning like a top. Now if the axis of the earth (the line from the North Pole through the South Pole) were at right angles to the path of the earth around the sun, we would have no such thing as different seasons, and all the days of the year would be of equal length.

But the axis of the earth is tilted. The reason for this is that a combination of forces is at work on the earth. One is the pull of the sun, the other is pull of the moon, the third is the spinning action of the earth itself. The result is that the earth goes around the sun in a tilted position. It keeps that same position all year, so that the earth's axis always points in the same direction, towards the North Star.

This means that during part of the year the North Pole tilts towards the sun and part of the year away from it. Because of this tilt, the direct rays of the sun sometimes fall on the earth north of the Equator, sometimes directly on the Equator, and sometimes south of the equator. These differences in the way the direct rays of the sun strike the earth cause the different seasons in different parts of the world.

When the Northern Hemisphere is turned towards the sun, the countries north of the Equator have their summer season, and the countries south of the Equator have their winter season. When the direct rays of the sun fall on the Southern Hemisphere, it is their summer and it is winter in the Northern Hemisphere. The longest and shortest days of each year are called "the summer solstice" and "winter solstice".

There are two days in the year when night and day are equal all over the world. They come in the spring and fall, just halfway between the solstices. One is the autumnal equinox, which occurs about September 23, and the other is the spring equinox, which occurs about March 21.

How does the Weatherman know what to say?

All the conditions of the atmosphere are "weather". Whether it's hot or col, dry or wet, sunny or cloudy, windy or still, its "weather". The weather is changeable from day to day, and the total effect during a year is called "the climate".

There are many complicated reasons for changes in the weather, but the most important influence is the sun. The sun's heat evaporates water and warms the air, so that rising currents of warm air carry water vapor into the sky. There the air cools and the vapor condense into rain. These things happen gently or violently. When they take place violently, we have storms.

In Great Britain there are approximately 200 weather reporting stations and roughly the same number spread over the rest of Europe. In addition, "weather ships" stationed in the Atlantic and special aircraft on regular patrol send back systematic reports on weather conditions. From this and other meteorological data weather forecasts are produced.

The maps which the weather experts study show them many things: places where the air pressure is equal, places of equal temperature, directions of local winds, cloudy or clear skies, rain or snow, the amount of rainfall, and regions where the air pressure is higher or lower than normal.

The weatherman can also tell from looking at the map what is likely to happen under the conditions that exist. He knows that low pressures indicates storms, because cold air is moving in to replace warm rising air that is laden with moisture. High pressures indicates fair weather.

In the Northern Hemisphere, the winds around a high-pressure area blow outward in a direction similar to that followed by the hands of a clock. In a low pressure area they blow inward in a counterclockwise direction. Thus the directions the winds will take can be predicted. The weatherman also knows how fast these pressure areas are moving across the country.

Knowing all these things, and having reports of the weather in most parts of the country before him, the weatherman begins to get a pretty good idea of what to say about the weather that's coming to your area!