Air Pressure — Reading Comprehension Lesson

Reading Passage

Air Pressure

Air pressure, also called atmospheric pressure, is the force exerted by the weight of the air above a given point. Although air seems weightless, the atmosphere has mass, and gravity pulls it toward Earth’s surface. At sea level, the atmosphere exerts a pressure of about 14.7 pounds per square inch — meaning the air pushes down on every square inch of your body with about 14.7 pounds of force. You do not feel this pressure because the fluids inside your body push outward with equal force. Air pressure is measured with an instrument called a barometer, invented by Evangelista Torricelli in 1643.

Air pressure decreases with altitude because there is less air above you as you go higher. At the top of Mount Everest (about 29,000 feet), air pressure is only about one-third of what it is at sea level, which is why climbers need supplemental oxygen. This reduction in pressure also means that the air is less dense — there are fewer oxygen molecules per breath — which is why people feel short of breath at high altitude. Aircraft cabins are pressurized to maintain comfortable breathing conditions even though the plane flies at altitudes where outside pressure is very low.

Differences in air pressure across Earth’s surface drive wind and weather. Areas where air is rising (due to surface heating or converging air masses) have lower pressure at the surface — called low pressure systems or cyclones. Areas where air is sinking have higher pressure — called high pressure systems or anticyclones. Air flows from high to low pressure, creating wind. In the Northern Hemisphere, winds spiral counterclockwise around low pressure centers and clockwise around high pressure centers, due to the Coriolis effect. Low pressure systems bring cloudy, stormy weather; high pressure systems bring clear, calm conditions.

Forecasters use barometric pressure to predict weather. A rapidly falling barometer indicates an approaching storm, as low pressure moves in. A rising barometer suggests improving weather, as high pressure builds. The standard sea-level pressure is 1013.25 millibars (or 29.92 inches of mercury) — the unit meteorologists use to measure pressure. Pressure patterns shown on weather maps with lines called isobars — lines connecting areas of equal pressure — reveal the location and intensity of pressure systems. Closely spaced isobars indicate steep pressure gradients and strong winds; widely spaced isobars indicate gentle pressure gradients and light winds.

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