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Atmosphere and Air Pressure

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God uses his creation to provide for people and the other living things. Air is one of the ways that he gives us what we need. Air is all around us just like the water of a lake surrounds a fish. The air around us is where daily weather patterns and weather events take place.

What’s in the air?
The atmosphere is the layer of gases that surrounds the earth. Air is made up of gases, dust particles , pollutants, microorganisms and water vapor. In the lower atmosphere, a sample of “pure” dry air would contain 78% nitrogen, 21% oxygen and the remaining 1% is made up mostly of the gas called argon. Carbon dioxide only makes up about 0.03% of the atmosphere. Many other gases make up only one hundredth of one percent.

The Atmosphere
The earth’s atmosphere is made up of 5 layers. The five layers are (from the lowest to the highest): troposphere, stratosphere, mesosphere, thermosphere, exosphere

Click here for a diagram of the atmosphere.

It is the lowest layer. The troposphere is the layer most important to the study of meteorology. It’s thickness ranges from approximately 6 to 10 miles. Most of the ingredients for ‘making’ weather are in this layer. The temperature in the troposphere goes down as the altitude increases. The upper boundary of the troposphere is called the tropopause.

The layer just above the troposphere is called the stratosphere. The
stratosphere extends from the top of the tropopause to about 30 miles above the earth. Airline pilots prefer to fly at the lower levels of the stratosphere to avoid the turbulence of thunderstorms in the troposphere.

The mesosphere is located just above the stratosphere. The mesosphere ranges from 30 to 50 miles above the earth. Meteor trails are left in the upper parts of the mesosphere.

The upper boundary of the thermosphere is about 100 - 300 miles above the surface. The thermosphere contains less than 1% of the earth’s atmosphere. This causes the temperatures to rise rapidly. Temperatures can reach 2,700 degrees Fahrenheit. The Aurora Borealis (the Northern Lights) occur in the thermosphere.

The exosphere lies above the thermosphere. Satellites and other space craft experience very little air resistance here because there is so little air.

Air Pressure
Air has mass. Because air has mass, gravity exerts a downward force on the air particles. Air exerts a force or pressure on the surface of the earth as well as the objects and organisms found on the earth.

Air pressure is the weight per unit area that air exerts on a surface. If the air pressure is 15 pounds per square inch, the weight is 15 pounds the unit area is square inch. The average air pressure at sea level is 14.7 pounds per square inch. In meteorology, the study of weather, air pressure is important for making weather forecasts. Air pressure can be reported in millibars, inches of mercury. Millibar is the metric unit for air pressure. Scientists use millibars. The average pressure at sea level is 1013 millibars (mb). Inches of Mercury is the customary U.S. unit for air pressure. Radio and television meteorologists use inches. The average pressure at sea level is 30 inches of mercury.

Barometers are tools used to measure air pressure. A mercurial barometer is a tube filled with the liquid metal mercury. As the air pressure changes the mercury goes up and down. It is very accurate. An aneroid barometer does not use mercury. Most home barometers are aneroid barometers. They are smaller, cheaper and safer than mercurial barometers.

Air Pressure and the Weather
A rising barometric pressure usually indicates that fairer weather is approaching with a coming high pressure system. An air pressure reading that is falling will indicate the possibility of cloudy, rainy, perhaps stormy weather.

Air pressure can also partly determine which direction the wind will blow. Generally air moves from areas of high pressure to areas of lower pressure.

Low Pressure Areas
• Air moves counter-clockwise around an area of low pressure.
• Air flows toward the center of a low pressure center.
• Air rises in a low pressure area
• Warm air is less dense and rises. This creates lower pressure.

High Pressure Areas
• Air moves clockwise around an area of high pressure
• Air flows away from the center of a high pressure area.
• Air sinks in a high pressure area
• Cooler air is more dense and sinks. This creates higher pressure

The Atmosphere
from A Recipe for Weathe
r: A Teaching Module for Middle School Science - M.A.T. Project by D. Koning 1986