Air Pressure As we know, the air in the atmosphere is made up of a number of gases. These gases press down on the Earth’s surface, exerting a force that we call atmospheric pressure or air pressure. Although we are usually unaware of this pressure, it actually presses down very hard – roughly equivalent to the force of an elephant balancing on a desk! Variations... Air pressure varies over time and from place to place. If we travel up a mountain or go up in a hot air balloon, for example, the air pressure gets less the higher we go. We can explain this difference by comparing air pressure to a pile of blankets on your bed. ...with height Air pressure at sea level is the equivalent of having many blankets, which would feel very heavy. If you have only one blanket though, it would feel very light, and this is like the air pressure at the top of a mountain. Each layer of air presses down on the layers below, and so the greatest pressure is at ground level where we have the maximum amount of air above. If we go above the height of mountains and into the stratosphere, the pressure will decrease until it reaches about zero, as here there is hardly any air above it. ...with time Air pressure also varies over time, and these temporal differences are usually caused by the temperature of the air. Cool air is denser (heavier) than warm air. Warm air is less dense (lighter) than cool air and will therefore rise above it. Areas of high pressure can be caused when cool air is sinking and pressing on the ground. At this time, the weather is usually dry and clear. In contrast, when warm air rises, it causes a region of low pressure. With low pressure, the weather is often wet and cloudy. Winds... Changes in air pressure bring changes in the weather and make winds blow. Air usually moves from areas of high pressure to areas of low pressure, and this produces winds. This can be easily remembered with the phrase:"Winds blow from high to low!" Barometer... Changes in air pressure are measured on an instrument called a barometer. Most barometers give a reading in millibars (mb for short). Readings over 1010 mb indicate high pressure.
Isobars... On a weather map, lines called isobars join up areas where the pressure is the same. The closer together the isobars are, the more windy it will be. Weighing the Air: We already know that there is a blanket of air around the Earth called the atmosphere, and we have just learnt that this results in what we call air pressure. But how can we actually prove that air has weight? Isn’t it just an invisible mixture of gases that we need to breathe? Let me show you. Gravity... The air in the atmosphere is kept close to the Earth by the pull of gravity, the force that pulls everything – including you and I – down to the ground. Without gravity, we would be weightless and would float above the ground, as we see with astronauts in space. As well as giving humans weight, gravity also does the same for air. We can illustrate this with a simple experiment using two balloons. We want to test whether the balloon is heavier or lighter (or the same weight) after you blow air into it. To do this, tie a piece of string around the middle of a stick of piece of cane so that it balances. Then tie an empty balloon to each end of the cane. What happens? The two balloons should balance evenly at each end. Now remove one balloon and blow air into it. When you have done that, tie it back onto the end of the cane. Is there any change? That’s right, the end with the blown-up balloon on it should dip downwards. This is because the air in the balloon is making it heavier. Highs and Lows: We can do another experiment with balloons to look at air pressure. Think about when you blow up a balloon. When the balloon is being inflated, a lot of air is being forced inside it. This air is being compressed as it is squashed together, and so it is at high pressure. If you let go of the end of the balloon, the air inside will rush out to where the pressure is lower. If you are interested in air pressure and recording it for yourself, have a look at some related projects in the What is Weather part of the Activities. You'll be able to make a barometer of your own, and then use it to predict the weather. Let's now move on to look at some of the daily and seasonal changes that are also important factors in shaping weather and climate around the world. Source: http://www.rcn27.dial.pipex.com/cloudsrus/pressure.html