Science 1604
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Read the following and highlight or circle all the terms used in this lesson.
The air or the atmosphere around the Earth extends upwards up to about 300 km from the sea level.
The density of air is at a maximum at sea level and becomes less dense as we go higher up (due to
gravitational pull of the Earth, air is more dense close to the surface of the Earth). The air exerts
pressure on surfaces that it is in contact with. At sea level, the air pressure is 101325 N/m2 or 101.3 kPa.
Atmospheric pressure decreases as we go to higher and higher altitudes, since the density of air also
decreases. Atmospheric pressure at a given point is same in all directions. This is similar to the case of
liquids, as stated in Pascal’s Law. The entire weather system of the Earth depends on the movement of
air. Air moves from high-pressure regions to low pressure regions in order to equalize pressure. A man
named Pascal discovered that changes in air pressure related to changes in weather. Pascal created the
first barometer. They became extremely popular in Europe. Sailors, to predict weather, used them.
Barometric pressure is the pressure of the air that is pressing in all directions. In regular weather
forecasts, the meteorologists are almost always talking about the air's pressure at the ground. The air's
pressure is caused by the weight of all the air above the ground pressing down (gravity pulls air toward
the Earth just as it pulls everything else). It's called "barometric" pressure because an instrument called
a barometer is used to measure air pressure. A barometer measures the pressure of the air pushing on
it. On the average, at sea level, the air has a pressure of 14.7 pounds per square inch. This means a
square one inch on each side has 14.7 pounds of air pressure on it! We do not feel it because the
pressure is pushing with equal force in all directions. Instead of using pounds per square inch,
barometers usually measure the pressure in inches of mercury or kPa.
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