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!

Why do Volcanoes appear only in certain places?

There are no volcanoes near New York City or London or Paris- nor are there likely to be any in the future. Yet there are parts of the world where there are several volcanoes quite near each other.

Central America, bordering the Pacific Ocean, is one of the most active volcano areas of the world. In fact, more than two-thrids of the active volcanoes and a lrage numer of those which have been extinct for only a short time are found around the borders of the Pacific Ocean!

The reason is simply this: The earth's crust in these areas must be "weak" or have weak spots in realtion to the earth's crust in other parts of the world. For without a weak spot in the crust of the earth, a volcano couldn't come into being.

Here is how a volcano is born: As you know, the center of the earth is hot. The deeper you go under the surface of the earth, the higher the temperature. At a depth of about 20 miles, it is so hot (1,000 to 1,100 degrees centigrade) that most rocks found there simply melts.

When rock melts, it expands and needs more space. In certain areas of the world, new mountain ranges have recently been formed (new in terms of thousands of years). Under and near these new mountain ranges, the pressure in less than elsewhere. It is a kind of "weak spot" in the earth's solid crust.

So the molten rock, which is called "magma", expands into these parts and a local reservoir of the molten rock is formed. This material rises along the cracks formed by the uplift. When the pressure in the reservoir of molten rock becomes greater than the strengthof the roof over it, it burst forth as a volcano. The eruption lasts until the gas is gone.

The material that comes out of volcano is mainly gaseous, but large quantities of molten rock (which we call "lava") and solid particles that look like cinders and ash are also thrown out. The eruption is really a gas explosion, but some of the lava becomes finely p[owdered and makes the eruption look like black smoke.

Why do we still have Glaciers today?

The great ice mass that began the Ice Age in North America has been called "a continental glacier" it may have been about 4,500 metres thick in its center. This great glacier probably formed and then melted away at least four times during the Ice Age.

The Ice Age or glacial that took place in other parts of the world still had not had a chance to melt away! For instance, the big island of Greenland is still covered with a continental glacier, expect for a narrow fringe around its edge. In the interior, this glacier often reaches heights of more than 3,000 metres. Antarctica is also covered by a vast contiental glacier which is nearby 4,000 metres high in places!

So the reason we still have glaciers in certain parts of the world is that they have not had a chance a melt away since the Ice Age. But most of the glacier that exist today have been formed in recent times. These glaciers are usually the valley type of glacier.

It starts in a broad, steep-walled valley shaped like a great amphitheatre. Snow is blown into this area or slides in from avalanches from the slopes above. This snow doesn't melt during the summer but gets deeper year by year. Eventually, the increasing pressure from above, together with some melting and refreezing, forces the air out of the lower part of the mass and changes it into solid ice. Further pressure from the weight of ice and snow above eventually squeezes this mass of ice is the valley glacier.

There are more than 1,200 such glaciers in the Alps of Europe! Galciers are also found in the Pyrenees, Carpathian, and Caucasus Mountains of Europe, and in southern Asia. In Southern Alaska, there are tens of thousands of such glaciers, some from 25 to 50 miles long!