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The Basic Properties of the Atmosphere


Pressure

Atmospheric pressure is the force exerted by air on a unit area. It can be thought of simply as the weight of the air above a given point. Simply, the fewer molecules above you, the lower the pressure exerted on you and vice versa (more molecules above = higher pressure). Since there are fewer molecules above you as you move up in the atmosphere, pressure always decreases with increasing altitude.



In the United States, pressure is commonly expressed in millibars (mb) or inches of mercury (Hg). Meteorologists use millibars (the unit shown on weather maps), while aviation and television weather reports use inches of mercury. Atmospheric pressure is measured with a barometer, which is why it is sometimes called barometric pressure. The average sea level pressure is 1013.25 mb or 29.92 Hg.

1 millibar (mb) = 0.02953 inches of mercury (Hg)



Temperature

Temperature is a measure of the degree of hotness or coldness of an object. It is actually a measure of the average kinetic energy or speed of the molecules in a substance (air). The more kinetic energy (speed) the molecules have, the higher their temperature and vice versa. Air temperature is measured with a thermometer and is expressed using the Kelvin scale, Fahrenheit scale (°F) or the Celsius scale (°C). The Kelvin scale is convenient for scientific calculations, but is not used to report the air temperature. In most of the world, air temperature is expressed in °C, but in the United States, only temperatures above the surface are expressed in °C. Temperatures at the surface are usually expressed in °F.

°C = 5/9(°F-32)
K = °C + 273

Temperature is used to define the layers of the atmosphere.


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The layer closest to the earth's surface is the troposphere and it is a very important layer to meteorologists because it is the layer that contains all of our weather. Sunlight warms the earth's surface and then the surface warms the air above it. As one moves away from the earth's surface (the heat source), the air becomes cooler. This is why temperature usually decreases with height in the troposphere. Sometimes the air temperature may increase with height in a narrow layer. This is referred to as a temperature inversion. Air temperature may also stay the same with increasing height. This is called an isothermal layer. At about the altitude where jet aircraft fly (˜30,000 ft), the air temperature becomes isothermal. The bottom of this isothermal layer marks the end of the troposphere and the beginning of the stratosphere. The boundary separating the troposphere from the stratosphere is called the tropopause. The air temperature begins to increase with increasing height (temperature inversion) in the stratosphere. The reason for this warming is that ozone in the stratosphere absorbs ultraviolet (UV) radiation. The ozone also protects life on earth from this dangerous radiation. Above the stratosphere is the mesosphere, where air temperature again decreases with height. The boundary separating these two layers is called the stratopause. The air temperature decreases with height because there is little ozone at those altitudes to absorb the UV radiation. The final layer is the thermosphere, which is separated from the mesosphere by a boundary called the mesopause. Air temperature increases again in this layer, due to the absorption of solar radiation by oxygen molecules.



Dewpoint Temperature

Dewpoint temperature is a measure of the moisture content in the atmosphere and is the temperature to which air must be cooled (at constant pressure, with no change in water vapor content) for saturation to occur. When saturation is reached, condensation occurs and such things as dew, frost or fog may occur. The dewpoint temperature is a good indicator of the actual amount of water vapor in the air. High dewpoint temperatures indicate there is high water vapor content, which indicates the air is moist. Low dewpoint temperatures indicate there is low water vapor content, which indicates the air is dry.

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