Air Pressure

The weight of the air pressing down on the Earth, the ocean and on the air below causes air pressure. Earth's gravity, of course, causes the downward force that we know as "weight." Since the pressure depends upon the amount of air above the point where you're measuring the pressure, the pressure decreases as you go higher.

Air pressure is related to its density, which is related to the air's temperature and height above the Earth's surface.

Air pressure changes with the weather. In fact, it's one of the most important factors that determines what the weather is like.

Air pressure is also called barometric pressure because barometers are used to measure it.

Measurements are usually given in hectopascals, although you'll sometimes see them in centimeters of mercury on older barometers.

The use of direct pressure measurements goes back to the late 19th century. This was when the Norwegian meteorologist Vilhelm Bjerknes, a leader in making meteorology a mathematical science, urged weather services to use direct pressure measurements because they can be used in the formulas that describe the weather, unlike measures of the height of the mercury in a barometer.

What's a high pressure system?

Sometimes, high in the sky, air slows its forward motion and piles up. This is called convergence. The piled-up air sinks. When the sinking air reaches the Earth’s surface, it spreads out. This is called divergence. In the Northern Hemisphere it spreads out with clockwise surface rotation and is called

a high pressure system

What's a low pressure system?

High in the sky, air moves away from an area faster than air enters an area. When air spreads out (divergence) at high altitudes, air from below flows upward to fill the space, making a low pressure system. The system has surface convergence and anti-clockwise rotation in the Northern


How Does Weather Affect Air Pressure?

Heavier gases weigh more than lighter gases.

Different chemical elements, have different atomic weights. Those which form gases often combine two atoms at a time to form a gaseous molecule - like N2 or O2.

The atomic weight of nitrogen is 14 and of oxygen it's 16. The molecules N2 and O2 have molecular weights of 28 and 32, respectively. At room temperature and normal atmospheric pressure, 28 grams of nitrogen occupies a volume of 22.4 liters and 32 grams of oxygen occupies the same volume.

Therefore, under STP the weight of 22.4 liters of a gas in grams equals the molecular weight of the gas.

Air is about 80% nitrogen and 20% oxygen.

Gaseous water weighs 18/22.4 grams or 0.8 grams per liter.

You can see that air normally weighs 1.286 grams per liter but, if we substitute water for some of the air, the mixture becomes lighter. So, if there's water in the air, the air mixure becomes lighter and it doesn't push down so hard on the mercury and the barometer's lower.

Why rising pressure indicates fair weather:

When the atmospheric pressure is fluctuating very little, it means the weather is apt to remain stable. A pronounced rise in pressure indicates fair weather is soon upon you.

Conversely, a slow but steady drop in pressure signals the oncoming of a storm. Storms are at the center of extreme low pressure systems as wind rushes toward the center of the storm to replace the air that through convection is rising into the upper atmosphere.

Weather forecasting instruments (Using Pressure):

Barometer: This device measures air pressure with a glass tube and a small quantity of mercury, much like a thermometer. An airless tube is inserted into a plate of mercury, the top of the tube is sealed, while the bottom is open. Air pressure pushes down on the mercury, which in turn pushes it up the tube against the vacuum in the tube. In this regard, a basic barometer works like a scale. Air pushes down on one side, mercury rises on the other. By measuring the position of the mercury in the tube, we can determine current atmospheric pressure.
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