AIR PRESSURE: ALL AROUND US

To understand pressure, consider a sponge. 

When left alone, a sponge stretches out to its full size. As you squeeze down on the sponge, what happens? How small can you make it? Can you hide the sponge entirely within your hand? When the sponge is squished up as tightly as you can make it, how does it feel? The sponge should feel hard and tight. Now, as you let go of the sponge and it once again takes its original size and shape, how does it feel? In this state, it should feel soft again.

The atmosphere is similar to a sponge in that it can be compressed, or squished up. The weight of the air above compresses the air below tighter and tighter. The lower we travel down into the atmosphere, the more weight there is above, and so the tighter the air is squished or compacted. The higher we travel into the atmosphere, the less air there is above and so the less weight there is pushing down, and as a result, the less squished up, or the less compact the air.

Half of the air making up our atmosphere is squished down into the first three layers, and 90% of the atmosphere is squished into the first 10 miles (16km). Above this altitude the air is so thin that the pressure is even less than that of the best vacuums on the surface of the Earth.

When your friend squeezes your arm, you feel pressure! That's because molecules collide with each other and things like your arm, the ground, or a tree. They exert a force on those surfaces.

Molecules in Earth’s atmosphere constantly bounce off each other and everything else around them. The force exerted by these air molecules is called air pressure.

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. Units of Pressure meteorologists use millibars In aviation and television weather reports, pressure is given in inches of mercury ("Hg), while meteorologists use millibars (mb), the unit of pressure found on weather maps. As an example, consider a "unit area" of 1 square inch. At sea level, the weight of the air above this unit area would (on average) weigh 14.7 pounds! That means pressure applied by this air on the unit area would be 14.7 pounds per square inch. Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars.

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.

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.

ATMOSPHERE PRESSURE - VIDEO

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