-is the pressure exerted by the weight of air in the atmosphere of Earth. It is measured in millibars. It is measured using an instrument known as a barometer. There are two different types of barometers: the mercury and aneroid barometers.
A diagram showing how a Mercury barometer works.
It is very large, the glass tube is typically almost 1m in height.
It gives very accurate readings.
It has a bulb that is dipped in a container of mercury.
When the atmospheric pressure rises, it exerts force its force on the mercury which becomes compressed.
The mercury is pushed up the bulb to a height of 760 mm.
When pressure decreases the mercury contracts and moves down the thermometer.
It gives accurate readings.
It is simple to construct and maintain.
It is very large and cumbersome.
Mercury must be at least a meter long.
They break easily.
They cannot produce a continuous reading, so readings have to be taken regularly.
Mercury is a dangerous poison.
A diagram showing an Aneroid barometer.
These are portable and can be safely used at homes and schools as they do not contain mercury.
They are made up of an air-tight metal box.
If pressure increases the box is squashed inwards.
Conversely if pressure decreases the box expands outwards.
A complex series of springs is attached to the box.
The other end of the spring is connected to a point which has a calibrated dial on the other end to record the various pressure readings.
The spring amplifies the expansions and contractions of the box in accordance to atmospheric pressure.
It is portable.
It can be safely used in the home or at school since it does not rely on mercury which is poisonous.
It is easier to read since it comes with a calibrated dial.
Can make continuous readings.
Can be attached to a computer to make automated continuous readings.