Simple Mercury Barometer

What is Barometer?

A Simple Mercury barometer is a widely used weather instrument that measures atmospheric pressure (also known as air pressure or barometric pressure) – the weight of the air in the atmosphere.

A barometer is typically a tube filled with mercury sitting upside down in a container filled with mercury (called a reservoir). When the atmospheric pressure falls, it presses down on the mercury in the container thereby forcing the mercury in the tube up. When atmospheric pressure is low, the mercury in the tube is allowed to flow back into the container. The mercury barometer is made of a glass tube, sealed at the top. It contains mercury, and the base of the tube dips into a beaker, and below the surface of the mercury in the beaker.

Atmospheric pressure pushes down on the mercury in the beaker, which in turn pushes mercury up the tube. If the space above the mercury in the tube is a vacuum, then nothing is pushing down on the top of the mercury in the tube, and atmospheric pressure will push the mercury up until the pressure of the column of mercury balances the atmospheric pressure. The height h from the top of the mercury in the beaker, to the top of the mercury in the tube can be used to calculate atmospheric pressure.

This height h is approximately 760 mm of mercury, and in some countries, atmospheric pressure is still quoted in mm of mercury.

Note that mercury has a convex (curved-upwards) shape when in contact with the glass. You should measure to the top of the mercury in the tube, and to the flat surface of the mercury in the beaker.

Mercury barometers are no longer made because mercury is a highly poisonous metal with a poisonous vapor.

Using Atmospheric Pressure to Predict Weather

Weather patterns are carried out in regions of high and low atmospheric pressure. By analyzing changes in weather pressure you can also analyze changes in weather patterns. The table below is a guideline for forecasting weather based on atmospheric pressure changes.
 
Atmospheric Pressure Weather Forecast
Decreasing Storms, rain and wind together
Rising Fair, dry, and cool weather
Slow, regular decreases Strong changes in weather is likely
Small, fast decreases Short lived rain and wind
Fast drop in pressure over a short period of time Storm approaching within 5-6 hours
Fast rise in pressure during average/above average pressure Pressure will soon fall, indicates poorer weather soon
Fast rise in pressure during low pressure Predicts short-lived fair weather
Large sustained rise in pressure A long period of good weather will soon arrive

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