Air Pressure

We cannot see air but it is all around us. The atmosphere is pulled down onto the earth by Gravity. This pressure cannot be felt normally because an equal pressure from inside our bodies pushes outwards. The pressure is greatest at ground level because there is a large weight of air overhead which pushes down. The higher one goes, the less air there is and therefore less pressure is exerted.

Did you know that aeroplanes flying high in the sky have pressurized cabins so that there is enough air to breathe ?

High and Low Pressure

Pressure is not the same everywhere. If the air is cold, it sinks and pushes down to create a high pressure on the earth. When the air gets squashed together, it warms up and brings good weather. Warm air rises to create a low pressure on the earth. Warm air also helps in evaporation of water bodies to form clouds. This is why low pressure brings rainfall. In a low pressure area, warm air rises and condenses into clouds. In a high pressure area, air sinks to the ground and spreads. It absorbs moisture and usually brings good weather.

Pressure is measured in millibars or mb. On weather maps, all areas having equal pressure are joined together to make a curving line known as an Isobar. This helps in identifying easily the areas of high and low pressure.

Air pressure is measured with a Barometer. An aneroid barometer resembles a clock. It consists of a sealed metal box with no air inside. The pointer is joined to the box. When air pressure rises, the box gets squashed inwards and the pointer shows the change on a dial. The changing air pressure is a good indicator of forthcoming weather.

Simple Experiment

Take an open dish with mercury. Be very careful because mercury is poisonous. Place a glass tube to stand inside the mercury. As air pressure rises and falls, the level of mercury in the tube changes.

How does this happen ? Air presses down on the mercury and forces it down the tube.

Objective

Experiment to show that there is air pressure pushing on us from all directions on the earth.

You Will Need

Balloons, Kite string, Plastic straws, Cello Tape, Two chairs.

1. Cut the plastic straw into two and tie a length of string between the two chairs.
2. Slip the straw onto the string and tie the other end.
3. Make the string fairly tight between the chairs.
4. Blow up a balloon and tape it to the straw.
5. Let go off the balloon. You will find that it shoots off towards the second chair like a rocket.

Why does this happen ?

The Earth is covered by a layer of air which is nearly 80 miles thick. At sea level it exerts or ‘pushes’ almost 15 pounds of pressure per square inch.

The Rocket Balloon illustrates that air pressure is used to move something. When you blow up a balloon, it increases in size. This is because of the air pressure which you can see as the size increases. When that pressure is released, it shoots forward.