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A barometer is an instrument that is used to measure the atmospheric pressure pressing down onto the surface of the planet. If you fly in an aircraft the higher you go into the earth’s atmosphere the lower this pressure becomes, conversely if you travel down into a deep part of the earth the pressure becomes lower. For convenience the level of the sea is regarded as the zero point for measuring pressure.
Air pressure is affected by the temperature of the air at the time of measurement and it is therefore necessary to ascertain the temperature if more than one barometric pressure reading is to be compared with another; the height above or below sea level must also be allowed for.
There are two main types of barometer, the aneroid and the mercury. The latter instrument’s glass tube is filled with mercury and the bottom end of the tube is curved upwards and is open to the air; the top end is closed with a glass bubble containing no air. The air pressing on the mercury at the open end supports the mercury column which moves according to the atmospheric pressure, allowing measurement. An aneroid barometer has no mercury but contains a small metal box which has had all the air removed from it; this box expands and contracts according to the air pressure on its surface. This expansion and contraction operates an indicator mechanism that displays the measurement on a dial.
Barometers are vital in forecasting the weather as the changes in air pressure can indicate the weather trends we can expect at different times of the year. Put simply, high pressure is usually good news in the summer and you may need shorts and a sunhat, if pressure is low you will probably require an umbrella. |
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