Category Our World

People have been able to forecast the weather for hundreds of years. Meteorologists have used simple instruments like thermometers, rain gauges, barometers and wind gauges for many years, but with the arrival of satellite photography, weather forecasting has been transformed. With the use of computers, increasingly accurate forecasting is now possible.

Traditional forecasters have been known to use pine cones, which open and close according to the humidity of the air. An open cone is supposed to mean dry weather. Seaweed also responds to changes in humidity. Dry seaweed indicates dry weather.

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A barometer is used to measure the changes in air pressure. Air pressure varies across different parts of the Earth’s surface, and these differences cause winds. Air moves from an area of high pressure, or an anticyclone, to an area of low pressure, or a depression. Depressions are usually associated with worsening weather conditions and rain. In a mercury barometer the air pressure pushes down on the mercury, which is forced up the barometer to give an accurate reading.

Air pressure is greatest at sea level, where it amounts to 14.7 pounds a square inch. It is greatest there because that is the bottom of the atmosphere. At highest altitudes the pressure is less.

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It is tiny water droplets condensing from moist air that cause fog and mist to rise. The water droplets can occur at ground level. The air can only hold a limited amount of water. If the air suddenly cools, its capacity to hold water is reduced, which results in a mist or fog.

When fog develops, visibility can be affected quite badly. Mist is less dense. It often occurs on calm, clear nights when heat rises, forming a thin layer of mist close to the ground. Mist often forms over water because a mass of warm air passes over a cold stretch of water.

Sometimes visibility is affected in built up areas due to mist and fog, but this is sometimes mistaken for smog. Smog is a buildup of exhaust fumes and factory smoke which hangs over the area until a huge amount of air movement can blow the smog away.

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A monsoon is a season of very heavy rain falling is tropical countries. Monsoons are most common near the Equator. They occur when seasonal winds spring up carrying moist air in from the sea. Important crops such as rice depend entirely on the monsoon season to provide the right growing conditions.

If the monsoon rains fail, famine often follows. There are also some special winds called monsoon winds. These are winds that change their direction with the season. In India, the monsoons blow south as hot, dry winds in the wintertime, and blow north in the summer, bringing heavy rainfall.

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Mount Everest, which is in the Himalayas, is 8,863 metres high. At these altitudes, mountains are always covered in snow and ice, and there is little oxygen to breathe. Mount Everest was finally conquered on May 29, 1953, when a Nepalese guide, Tenzing Norgay, and a New Zealander, Edmund Hillary, reached the highest point on the Earth’s surface. Since then, many people have climbed Everest, and all the world’s major peaks have now been conquered.

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Mountains can be formed in three different ways. Volcanoes form mountains when lava from deep inside the Earth cools and hardens on the surface.

Other mountains are formed when two plates move towards each other under pressure or where an oceanic plate is pushed under a continental crust. The pressure causes the ground near the joining plate margins to fault and fold. The ground is forced upwards to form mountains. The Rockies, Alps, Andes, Urals and Himalayas were all formed in this way.

Finally the Earth’s crust can fracture and create faults which mean that large blocks of land can be moved upwards or downwards. Faults in the rocks normally occur when there is a lot of pressure on the rocks. Mountain building is a slow process and happens over centuries.

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