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 the pull of the moon, and 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, toward the North Star.
This means that during part of the year the North Pole tilts toward 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 toward 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.
WHAT MAKES THE WEATHER?
What is the weather anyway? It is simply what the air or atmosphere is like at any time. No matter what the air is – cold, cool, warm, hot, calm, breezy, windy, dry, moist, or wet – that’s weather. Weather may be any combination of different amounts of heat, moisture, and motion in the air. And it changes from hour to hour, day to day, season to season, and even from year to year.
The daily changes are caused by storms and fair weather moving over the earth. The seasonal changes are due to the turning of the earth around the sun. Why weather changes from year to year is still not known, however.
The most important thing to “cause” weather is the heating and cooling of the air. Heat causes the winds as well as the different way in which water vapour appears in the atmosphere.
Humidity, the amount of water vapour in the air, combined with the temperature, causes many weather conditions. Clouds are a kind of weather condition, and they are formed when water vapour condenses high above the ground.
When the cloud droplets grow larger and become too heavy to be held up by the air currents, they fall to the ground and we have the weather known as rain. If the raindrops fall through a layer of air which is below freezing, the drops freeze and our weather is snow.
One of the ways the weather forecaster studies the weather is to look at the “fronts” that exist. Fronts are boundary lines between the cold air moving southward from the north, and the warm air moving from the tropics. Most of the severe storms which cause rain, snow, and other bad weather are in some way related to these fronts.
WHAT MAKES WIND?
Sometimes when we are outdoors, a sudden and mysterious thing takes place. A wind begins to blow. We cannot see it, but we feel it, and we have no idea what started it.
A wind is simply the motion of air over the earth. What causes the air to move? All winds are caused by one thing – a change in temperature. Whenever air is heated it expands. This makes it lighter, and lighter air rises. As the warm air rises, cooler air flows in to take its place. And this movement of air is wind!
There are two kinds of winds, those that are part of a world-wide system of winds, and local winds. The major wind systems of the world begin at the equator, where the sun’s heat is greatest.
Here the heat rises to high altitudes and is pushed off toward the North and South poles. When it has journeyed about one-third of the distance to the poles, it has cooled and begins to fall back to earth. Some of this air returns to the equator to be heated again, and some continues on to the poles.
These types of winds, which tend to blow in the same general direction all year round, are called “prevailing winds.” But these world-wide winds are often broken up by local winds which blow from different directions.
Local winds may be caused by the coming of cold air masses with high pressure, or warmer air masses with low pressure. Local winds usually do not last long. After a few hours, or at most a few days, the prevailing wind pattern is present again.
Other local winds are caused by the daily heating and cooling off of the ground. Land and sea breezes are examples of this kind of wind. In the daytime, the cool air over the ocean moves inland as the sea breeze. At night, the ocean is warmer than the ground, so the cooler air moves out to sea as the land breeze.
HOW DO TORNADOES START?
Of course we are all quite accustomed to thunderstorms. These are usually local storms. But there are certain kinds of storms that may cover thousands of square miles. One such type is called a “cyclonic storm” or “cyclone.” In a cyclone, the winds blow toward the center of an area of low pressure.
A curious thing about them is that the winds blow in spiral fashion. In the northern hemisphere such storms turn counterclockwise, in the southern hemisphere they turn clockwise!
A tornado is simply a special kind of cyclone. A tornado arises when the conditions that cause ordinary thunderstorms are unusually violent. There is an updraft of air. There are winds blowing in opposite directions around this rising air. This starts a whirling effect that is narrow and very violent. When this happens, centrifugal force throws the air away from the center. And this leaves a core of low pressure at the center.
This low-pressure core acts like a powerful vacuum on everything it passes. This is one of the destructive things about a tornado. It can actually suck the walls of a house outward in such a way that the house will collapse. The other destructive thing about a tornado is the high winds that may blow around the edges of a whirl. These winds can reach 300 miles per hour and blow anything down.
WHAT CAUSES HAIL?
One of the most unusual weather conditions we can experience is a hailstorm. It is quite a thing to see and hear hailstones coming down, sometimes with such force that great damage is done. Animals, and even men, have been killed by hail! A hailstorm usually occurs during the warm weather and is accompanied in many cases by thunder, lightning, and rain. Hail is formed when raindrops freeze while passing through a belt of cold air on their way to earth.
Single raindrops form very small hailstones. But an interesting thing can happen to such a raindrop. As it falls as a hailstone, it may meet a strong rising current of air. So it is carried up again to the level where raindrops are falling. New drops begin to cling to the hailstone. And as it falls once more through the cold belt, these new drops spread into a layer around it and freeze, and now we have larger hailstones.
This rising and falling of the hailstone may be repeated time after time until it has added so many layers that its weight is heavy enough to overcome the force of the rising current of air. Now it falls to the ground.
In this way hailstones measuring three or four inches in diameter and weighing as much as a pound are sometimes built up. Snow, too, freezes around hailstones when they are carried into regions where it is forming. So the hailstones are frequently made up of layers of ice and snow.
Frozen rain is sometimes called hail, but it is really “sleet”! And soft hail which sometimes falls in winter is only a form of snow.
WHAT DON'T ALL CLOUDS PRODUCE RAIN
Have you ever flown through clouds in the airplane, or perhaps been high up on a mountain where the clouds swirled all about you? Then you must have got a pretty good idea of what a cloud is: just an accumulation of mist.
As you know, there is always water vapour in the air. During the summer there is more of this vapour in the air because the temperature is higher. When there is so much water vapour in the air that just a small reduction in temperature will make the vapour condense (form tiny droplets of water), we say the air is saturated.
It takes only a slight drop in temperature to make water vapour condense in saturated air. So when saturated warm air rises to an altitude where the temperature is lower, condensation takes place and we have a cloud. The molecules of water have come together to form countless little droplets.
What happens if all these water droplets in a cloud meet a mass of warm air? They evaporate - and the cloud disappears! This is why clouds are constantly changing shape. The water in them is changing back and forth from vapour to liquid.
The droplets of water in a cloud have weight, so gravity gradually pulls them down and they sink lower and lower. As most of them fall, they reach a warmer layer of air, and this warmer air causes them to evaporate. So here we have clouds that don’t produce rain. They evaporate before the drops can reach the earth as rain.
But suppose the air beneath a cloud is not warmer air? Suppose it’s very moist air? Naturally, the droplets won’t evaporate. Instead, the droplets will get bigger and bigger as more and more condensation takes place.
Pretty soon, each tiny droplet has become a drop and it continues falling downward and we have rain!