2.5 Earth’s Atmosphere

The atmosphere is a thin sea of air that supports life on Earth. Gases and water vapor cause the air to have weight and exert pressure. At sea level, the weight of air is 14.7 psi (pounds per square in.) and has a maximum pressure of 1 atmosphere.

The lift on the top airplane wing and the suction on a straw or vacuum cleaner is caused by decreased air pressure.

Atmospheric Regions Mesosphere UV & Visible light Stratosphere 30 miles 50 km Ozone layer Troposphere Up to 11 miles Snow from higher clouds Rain from low altitude clouds 5 miles

- clouds, wind, weather fronts There are 4 layers in the atmosphere: 1. Troposphere (0-10 miles up) is where weather occurs. - temperature decreases with altitude - clouds, wind, weather fronts Air in the troposphere is: 78% nitrogen 21% oxygen 0.93% trace gases – most of which is Argon and other noble gases, with some CO2, methane, nitrous oxide, water vapor, and ozone.

There is more water vapor at the equator; less at the poles There is more water vapor at the equator; less at the poles. This is because colder air holds less moisture than warm air. Equal amounts of cold air are also heavier than warm air. The increased weight creates pressure differences that cause winds to develop. Weather changes as cool, heavy air moves toward warm, light air.

With a fixed amount of gas, when the pressure is increased, the volume is decreased. Gases and water vapor cause the air to have weight and exert pressure. The atmosphere is like a sea of air surrounding earth. At the top of the atmosphere, there is less air and less pressure. Atmospheric pressure and the air’s density always decreases with height. A barometer measures small changes in air pressure. There are 2 types - aneroid and mercury.

If the barometric reading indicates rising pressure, fair weather is expected. A decrease in the barometric reading indicates storm fronts. If the barometer is steady, the weather won’t change. At sea level, one atmosphere (1atm.) is the pressure required to support 760 mm of Hg on a mercury barometer at 25 oC. (The SI unit of pressure is the Pascal.)

On top of Mt. Everest (at 9000 meters altitude) the air exerts only enough pressure to push a column of Hg to 253 mm high. Humidity refers to the amount of moisture or water vapor in the air. As the temperature of the air increases, its capacity to hold more water vapor increases. When the air is filled to capacity with water vapor, it is saturated.

2. Stratosphere (11 - 30 miles up) has similar gases to troposphere. The ozone layer here protects Earth from the sun’s deadly UV radiation. It traps heat and warms this layer. NOAA NOAA - calm, no winds - temperature increases with altitude Launch of an ozonesonde attached to a high-altitude balloon from South Pole.

3. Mesosphere (30-50 miles up) Temperature decreases with altitude. Sunclimate.NASA 4. Thermosphere (50-75 miles up) Temperature increase with altitude. These temperature reversals separate the layers of the atmosphere. The Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission is studying the influences of the Sun and humans on these regions. NOAA