Barometer
Also called: Mercury barometer, Aneroid barometer · Topic: Atmosphere: Composition, Structure and Elements of Weather · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"
Meaning
A barometer is an instrument that measures atmospheric pressure, the weight of the air pressing down on a place. Pressure is given in millibars (mb). Normal sea-level pressure is about 1,013 mb, and a reading below 1,000 mb points to a depression (a low-pressure system that brings bad weather).
- The mercury barometer (Torricelli, 1643) shows pressure as the height of a mercury column held up by the air. It is about 760 mm at sea level.
- The aneroid barometer uses a sealed metal box that flexes as pressure rises or falls. It has no liquid, so it is easy to carry.
- A barograph records pressure all the time.
Example
Air pressure falls with height. At sea level near Chennai the barometer reads about 1,013 mb. At Khardung La in Ladakh, a high Himalayan pass, it reads only about 650 mb.
Don't confuse with
- Anemometer: it measures wind speed, not air pressure.