Common Meteorological Variables
	Principle	Air is a compressible mixture of gases. It has properties such as temperature, pressure and density. It obeys the laws of thermodynamics and fluid mechanics, including the ideal gas law.
	Table 1 - Composition of the Atmosphere	Gas % by Volume* Nitrogen ~78% Oxygen ~21% Water Vapor ~0 to 4% Argon ~0.93% Carbon Dioxide ~0.035% Neon ~0.0018 Helium ~0.0005% Methane ~0.00017% Hydrogen ~0.00005% Nitrous Oxide ~0.00003% Xenon ~0.000009% Ozone ~0.000004% Particles (dust, salt, pollens, etc.) ~0.000001% Chlorofluorocarbons (CFCs) ~0.00000001% * Does not add to 100% because of the variability of water vapor.
	Figure 1 - Temperature Scales
	Temperature	Temperature is a measure of the hotness or coldness of a substance. The faster a group of molecules move (on average), the higher is its measured temperature. Temperature typically is measured in either Fahrenheit, Celsius, or Kelvin.   Fahrenheit (1714, Germany): Freezing of water at sea level = 32 degrees Fahrenheit Boiling = 212 degrees Fahrenheit The scale is linear.   Celsius (1742, Sweden): Freezing = 0 degrees Celsius Boiling = 100 degrees Celsius The scale is linear.   Kelvin (1800's, Lord Kelvin) Freezing = 273 Kelvin Boiling = 373 Kelvin The scale is linear. K = degrees Celsius + 273 One Kelvin equals one degree Celsius, but the scale is offset by 273 degrees. At 0 K there is absolutely no molecular motion of any kind (absolute zero). The hottest temperature measured on Earth was 136 deg F (Azizia, Libya).   The coldest temperature measured on Earth was -127 deg F (Vostok, Antarctica).
	Figure 2- Dewpoint Temperature and Relative Humidity
	Dewpoint Temperature and Relative Humidity	Dew point (or dewpoint temperature) is a measure of atmospheric moisture. The dew point is the temperature to which air must be cooled for saturation to occur. The best way to understand dew point (and the existence of water vapor in the air) is to take a cold glass of ice water outside on a warm day and watch the beads of water condense on the outside of the glass. When the glass cools the air to its dewpoint temperature, the moisture in the air condenses on the outside of the glass. Relative humidity is the ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor the air can hold at that particular temperature and pressure.   Dewpoint temperature is a better measure of moisture in the air than is relative humidity.
	Figure 3- Relationship Between Air Pressure and Air Temperature
	Air Pressure	Air pressure is the weight of the column of air above a designated area. Air pressure results from the gravitational pull of the earth on the molecules. Atmospheric pressure is about 15 pounds per square inch, or what would be about 600 pounds on an average-sized head. Meteorologists generally measure air pressure in millibars (mb). Average sea level pressure = 1013 mb The highest measured sea level pressure on Earth was 1084 mb (Agata, Siberia). The lowest measured sea level pressure on Earth was 870 mb (within a typhoon).   As a gas, air obeys the ideal gas law. Hence, when the pressure of a given volume of air (called an air parcel) decreases, the volume (or parcel) cools. Likewise, when the pressure of an air parcel increases, the parcel warms. As a result, an ascending air parcel always cools. A descending air parcel always warms. (Pressure always decreases with height in the atmosphere.) The rate at which the parcel cools or warms as it ascends or descends is dependent on the moisture content of the parcel. An air parcel with no water vapor in it will cool or warm at a rate of is about 10 degrees Celsius for every kilometer change in altitude (or 5.4 deg F per 1000 feet). A moist, unsaturated air parcel (e.g., relative humidity below 100%) will cool or warm at the same 10 deg C per kilometer. A saturated air parcel (e.g., condensation has occurred) cools or warms at a rate generally between 4 deg C and 7 deg C per kilometer (2 deg F to 3.5 deg F per 1000 feet). (These "lapse rates" are different because condensation adds heat to the air and evaporation removes heat from the air.)
	Wind, Air Density, and Water Vapor Mixing Ratio	Wind is simply the velocity of the air with respect to the earth. In meteorology, we refer to the wind direction as the direction from which the wind is blowing. Thus, a north wind is air blowing from the north toward the south. Wind results from physical forces acting on the air. These forces result from gravity, buoyancy, friction, differences in pressure (pressure gradients), the rotation of the earth, and centrifugal force, to name a few. The density of air at 0 deg Celsius at sea level is 1.3 kilograms per cubic meter.   Warm air is less dense than cold air; hence, warm air tends to rise.   Moist air is less dense than dry air; hence, moist air tends to rise.   Water vapor mixing ratio is simply the ratio of the mass of water vapor to the mass of dry air in a given volume The water vapor mixing ratio is usually between 0 and 20 grams per kilogram in the atmosphere.
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