Air Pressure:
How to measure the air around us.

Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. Since there's a lot of "empty" space between air molecules, air can be compressed to fit in a smaller volume.

Air Pressure:
At sea level, the air pressure is about 14.7 pounds per square inch. As your altitude increases (for example, if you climb a mountain), the air pressure decreases. At an altitude of 10,000 feet, the air pressure is 10 pound per square inch (and there is less oxygen to breathe).

Measurement of air pressure:
Millibar:

Small units of pressure commonly found on surface weather maps.

  • 1013.25mb   = is the average sea level air pressure.
  • 34 millibars = 1 inch of   mercury.
  • 1 millibar = 1/1000 of a bar
  • 1 bar = 100,000 newtons
  • 1 newton is forced it takec to move an object of 1kg at a rate of 1 meter per second.

Isobars:
The lines that join points on a map having the same air pressure at a given time.

Millibar Pressure Williams Class

Measuring Air Pressure:

Barometer:

•  Instrument used to measure pressure.

A barometer is a device that measures air (barometric) pressure. It measures the weight of the column of air that extends from the instrument to the top of the atmosphere. There are two types of barometers commonly used today, mercury and aneroid (meaning "fluidless"). Earlier water barometers (also known as "storm glasses") date from the 17th century. The mercury barometer was invented by the Italian physicist Evangelista Torricelli (1608 - 1647), a pupil of Galileo, in 1643. Torricelli inverted a glass tube filled with mercury into another container of mercury; the mercury in the tube "weighs" the air in the atmosphere above the tube. The aneroid barometer (using a spring balance instead of a liquid) was invented by the French scientist Lucien Vidie in 1843.

2 types of Barometers used  

•  Aneroid Barometer

•  Toricelli Mercury Barometer

Aneroid Barometer:

An aneroid barometer is a flexible metal chamber that has been tightly sealed after having some air removed. As the higher atmospheric pressures pushes the metal chamber the attached needle is moved. When the atmosphere has lower pressures it allows the chamber to expand which moves the needel in the opposite direction.

Aneroid Barometer Williamsclass
Air Pressure Mr. Williams

Toricelli Mercury Barometer

A glass tube from which the air has been removed is inserted into a dish of mercury. The air pressing down on the mercury in the dish forces some of the mercury up into the glass tube.

The height that the mercury rises in the glass tube is directly related to the atmospheric pressure. This pressure is usually measured in inches of mercury. A standard mercury barometer has a glass column about 30 inches long. A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars.

Barometer Williamsclass Mercury Toricelli Barometer Williamsclass

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