Unit D2 - The Weather


Unit D2


The weather is one of the most basic controlling influences on the environment. While we cannot (with rare exceptions) change or control the weather, we can understand it as it relates to our own local area and thus better plan the use and protection of the resources of the area.

The weather includes several things. The kinds of clouds and the amount of time they cover the sky determine the amount of sunlight there will be. Sunlight is important for the growth of plants, but too much sun under the wrong conditions can damage some plants or dry them out. Rainfall is another element of the weather that provides fresh water for us and for all living things on the land. The amount of rain that falls, how hard it falls, whether it is concentrated in a few heavy showers or spread out in many light sprinkles, and whether there are long periods without rain are all important factors. In colder countries, precipitation can also be frozen in the form of snow or hail. Another aspect of the weather is the temperature, how high and how low it gets and how much it changes from day to night and season to season. The temperature is partly determined by where a place is on the earth's surface, and partly by the weather and the temperature of the winds and air masses that pass over. The winds are themselves part of the weather, with their strength, direction and distribution during the year all being significant.

The weather in any particular place is related to the general behaviour of the air in the atmosphere, which involves patterns of circulation of warm and cold air masses around the world. The trade winds are an example of regular air movements related to global atmospheric circulation. The weather is also closely linked to the oceans. If the ocean is warmer than the air it will heat the adjacent air and if it is colder it will cool it. Since the air temperature changes more than the sea temperature, places near the ocean will have less extreme temperatures than places in the interior of continents. When the ocean is very warm, it can put so much energy into the atmosphere that it causes hurricanes or cyclones. The oceans also evaporate to provide much of the water vapour that goes to make clouds and eventually to fall as rain.

The land also has an effect on the weather. When the land heats up in the sun, it can warm the air and make it rise bringing in sea breezes. If there are mountains, the wind blowing across the land must rise to go over the mountain. Rising air gets colder, which makes the clouds and rain so frequent over mountains. The rain usually falls on the side of the mountain from which the wind comes, so that mountains often have wet and dry sides. The land can also make the air colder at night, and of course the higher a place is on a mountain the colder it gets.

As a result of all these factors, each place has its own particular weather or microclimate, with some being more sheltered and others more open, some wetter and others drier. The weather in the bottom of a valley may be quite different from that at the top, and one side may be different from the other, depending on the orientation to the sun and the direction of the prevailing winds. Even the nature of the land surface and whether it is covered by forest or other vegetation can have an influence. The weather recorded at the local weather station may thus not correspond very well to the weather in some other place. You may need to study your own weather in order to understand your particular microclimate.

Predicting the weather

One of the greatest problems with the weather is trying to predict what it will be. The weather goes through many kinds of changes, and even modern science is still far from understanding how or why they happen. We know that weather and climate can go through enormous changes over periods of tens or hundreds of thousands of years. At times in the past the world was more tropical than today; at other times there were ice ages with snow and ice over much of the land. These are not the kinds of changes that can concern us too much in our lifetimes. Then there are changes over several years or decades, some of which seem to be cyclical. A period of good years may be followed by bad years, and so on. Some changes may seem quite unusual or abnormal, as with the extreme El Niño Southern Oscillation events of 1983 and 1997 when the winds reversed across the central Pacific Ocean, the warm water went eastward, and unusual changes in weather were reported all around the world. The weather may be upset by some other factors; a severe volcanic eruption may blow so much dust and gases into the upper atmosphere that it can go around the world and affect the weather for several years. For the first time, man has also reached the stage where he can have an effect on the weather. The increasing level of carbon dioxide in the air (from burning oil and coal and cutting forests) is now causing global warming and climate change. This is changing the weather everywhere and causing the sea level to rise, which may drown many cities and island countries in the coming decades. However most changes in the weather are what might be called random variability. The weather changes because it is always changing, and whether the change in any one place is today, tomorrow or next week may be purely a matter of chance.

People who depend on the weather and who spend a lot of time outdoors learn to read the many signs in the environment that can help to tell what weather to expect. The kinds of clouds and the appearance of the sky, the waves and swells on the ocean, the behaviour of seabirds and many other things can be used to tell what weather may be coming. This knowledge depends very much on each particular locality and so is best learned from someone who knows the area well.

Today modern scientific methods for weather prediction have greatly improved, drawing on reports from many other weather stations, radar, and satellite pictures that show all the cloud patterns on a big part of the earth from out in space. If you have access to such information, use it, but there is still much you can do to understand your local weather just as your ancestors did before.

Natural disasters

Some of the most important effects of the weather come from the natural disasters caused by very severe storms or droughts. A cyclonic storm (hurricane or typhoon) can blow houses apart, flatten trees and crops, flood land, smash coral reefs, and cause more erosion that ten years of normal rains. Some tropical island and coastal regions may have cyclones, hurricanes or typhoons almost every year, and others only once in 50 or 100 years. It is often possible to collect information on what happened in previous storms, and even to look for lasting traces of storm damage such as rubble ridges along the reef or river flood crest levels. This information can be used to plan development so as to minimize the risk of storm damage.

Droughts, or long periods without rain, can be another form of natural disaster, with crops failing, livestock dying of thirst or starving for lack of forage, disease spreading through lack of hygiene, and in extreme cases people having to abandon their homes and islands. Areas subject to drought require careful advanced preparations, such as adequate (and properly maintained) water storage, emergency wells, and irrigation for crops if conditions permit.

Other natural disasters include earthquakes, volcanic eruptions and the resulting tsunamis (the giant waves sometimes caused by earthquakes or underwater landslides). If these are a risk in a local area, they can be planned for much as any other extreme environmental factor.

Average and extreme conditions

Two kinds of environmental conditions can be important for living organisms and thus for many development projects. Most things have normal requirements for the conditions they like best. A crop, for instance may grow best at a certain temperature, with a certain rainfall, weather pattern and seasonal variation. If conditions are not so ideal, it may still grow, but not quite so well. However, at some point, when the temperature is too high or too low, or it is too wet or too dry, or some combination of factors, the crop will be pushed beyond its limit and will die. Thus both the average conditions in the environment most of the time, and the occasional rare extreme factor, are important controls on the survival of plants and animals, and thus on development projects. The extreme conditions are often associated with natural disasters, so particular attention needs to be paid to them in development planning. Understanding both the normal average conditions in the environment and the occasional limiting extremes can make the difference between success and failure.


What factors are used to describe the weather?

What is the weather like in your own area?

Is it different from one place to another because of local land forms or topography?

Can you think of places near you with different microclimates?

Has the weather changed in the last few decades? How?

Is there someone in your village who can predict the weather?

What signs do you use to tell what the weather will be like?

What kinds of natural disasters is your area subject to?

Do people do anything to prepare for possible disasters?

Can you describe a natural disaster and its effects on the environment?

Why are both average and extreme conditions important?

Instructions for trainers in the use of this unit

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Last updated 25 December 2007