Definitions of Atmospheric Variables

Temperature: The temperature is a measure of the internal energy that a substance contains. This measure of the internal energy that a substance contains. This is the most measured quantity in the atmosphere.
Dewpoint: The dewpoint is the point at which air, when cooled at a specific pressure and moisture content, reaches the saturation point of water vapor. In other words, when the temperature is lowered, this is the point at which condensation occurs or dew forms.
Dewpoint depression: The dewpoint depression is the difference in degrees between the temperature and the dewpoint.
Wind Direction: This is the direction from which the air is moving. The directions are in terms of degrees from true north (0 degrees), and the angle increases in a clockwise direction. In other words, if the wind direction is 45 degrees, the wind is from the northeast.
Wind Speed: The wind speed is a measure of the average speed of movement of the wind at a specific point. When measured, the value represents an average taken over a couple of minutes.
Wind Gust: The wind gust is the maximum wind speed recorded over a specified time period. When wind speeds are measured and the peak wind speed during the measuring period is roughly 10 knots more than the average wind speed, a wind gust is reported.
Station pressure: This is the absolute air pressure at a given reporting station. The air pressure is the combined weight of all air located in a column directly above the reporting site. Consequently, the station pressure may vary tremendous in mountainous regions due to the strong variation of atmospheric pressure with height. Vertical variations of pressure range up to 150 mb per mile whereas horizontal variations are usually less than .1 mb per mile.
Sea Level Pressure: The sea level pressure is a correction of the station pressure to sea level. This correction takes into account the standard variation of pressure with height and the influence of temperature variations with height on the pressure. The temperature used in the sea level correction is a twelve hour mean, eliminating diurnal effects. Once calculated, horizontal variations of sea level pressure may be compared for location of high and low pressure areas and fronts.
Altimeter Setting: The altimeter setting is a correction of the station pressure to sea level used by aviation. This correction takes into account the standard variation of pressure with height and the influence of temperature variation with height on the pressure. The temperatures used correspond to the standard atmosphere temperatures between the surface and sea level.
Height or Geopotential Height: The geopotential height is roughly the height above sea level of a pressure level. This is an estimated height based on temperature and pressure data.
Present Weather: This is the type of weather observed at the reporting time. These conditions may include types of precipitation like light rain or heavy snow, as well as the condition of the air environment such as foggy, hazy or blowing dust.
Visibility: The visibility is the maximum distance an object may be seen considering air conditions. Precipitation, fog, haze, pollutants, and suspended dust all contribute to lowering visibility.
Cloud Cover: This is the amount of total sky coverage by clouds.
Clear = less than 10% cloud coverage.; Scattered = more than 10% and less less than 60% cloud coverage.; Broken = more than 60% and less than 90% cloud coverage.
Overcast = more than 90% cloud coverage.; Obscured = cannot determine cloud information due to obscuration by precipitation, fog, dust, etc. Sometimes referred to as vertical visibility.
Cloud Ceiling: This is the height of the cloud base for the lowest broken or overcast cloud layer.
Pressure Tendency: This is the change of sea level pressure with time. This is usually reported as a three hour pressure change in millibars.
Precipitation: This is the amount of liquid equivalent precipitation measured over a particular range of time. Snow is melted down and measured. Common time periods include 1, 3, 6, 12 and 24 hours.
Maximum Temperature: This is the highest temperature recorded during a specified period of time. Common time periods include 6, 12 and 24 hours.
Minimum Temperature: This is the lowest temperature recorded during a specified period of time. The time period can be 6, 12 or 24 hours.
Snow Cover: This is the depth of snow in inches measured at the reporting time. Since snow may drift, this value represents an average snow depth measured at several locations.
Probability of Precipitation: The probability based on climatology that measurable precipitation will fall at that location during a specified period of time. For example, if the probability is 30%, your location will recieve .01" or more of precipitation in 3 out of 10 similar situations.
Probability of Thunderstorms: The probability based on climatology that a thunderstorm will be reported at that location during a specified period of time.
Quantitative Precipitation: The forecasted amount of precipitation that will fall at a particular location over a given amount of time usually either for 6 or 12 hours.
Vertical Velocity: The rate of upward or downward motion of air passing through a given pressure level.
Potential Temperature: The potential temperature is the temperature an air parcel at a specific pressure level and temperature would have if it were lowered or raised adiabatically to 1000 mb. This is defined by Poisson's equation.
Equivalent Potential Temperature: The equivalent potential temperature is the temperature a parcel at a specific pressure level and temperature would have if it were raised to 0 mb, condensing all moisture from the parcel, and then lowered to 1000 mb.
Virtual Temperature: The virtual temperature is the temperature a parcel which contains no moisture would have to equal the density of a parcel at a specific temperature and humidity.
Virtual Potential Temperature: The virtual potential temperature is the temperature a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb. This is defined by Poisson's equation.
Vapor Pressure: This the partial pressure that water vapor exerts on the total air pressure.
Mixing Ratio: This the ratio of the weight of water vapor in a specified volume to weight of dry air in that same volume.
Relative Humidity: This is the ratio of actual vapor pressure to the saturation vapor pressure at a specific temperature.
Specific Humidity: This the ratio of the weight of water vapor in a specified volume to weight of the air in that same volume.
Lifted Condensation Level (LCL): The lifted condensation level represents the pressure level at which a parcel would first reach saturation if lifted adiabatically from a specific level in the atmosphere. As the parcel rises, its temperature decreases and moisture content remains constant. Therefore, the parcel will reach a point at which it is saturated with water vapor and condensation begins.
Wind Chill: The apparent temperature of exposed skin with a four knot wind that has the same heat loss of exposed skin at a certain certain temperature and wind speed. The higher the wind speed, the higher the heat loss from the skin's surface due to sensible heat transfer. This is calculated from heat transfer algorithms.
Heat Index: The apparent temperature of exposed skin when exposed to air with a high moisture content. Dry air allows evaporative cooling of the skin's surface. The higher the humidity, the less the skin can cool itself with evaporative cooling.
Convergence: The convergence is a measure of the rate at which air is converging along a horizontal plane. Convergence near the surface and divergence aloft can be correlated with upward vertical motion.
Vorticity: The vorticity is a measure of the rotation of air in a horizontal plane. Positive (counter-clockwise or cyclonic) vorticity can be correlated with surface low development and upward vertical motion (in areas of positive vorticity advection).
Convective Condensation Level (CCL): The CCL is the pressure level at which a convectively mixed parcel reaches saturation when lifted. The initial parcel has the moisture content equivalent to the mean of the lowest 100 mb and retains the temperature of the surrounding atmosphere when lifted. This is usually found by determining the mean mixing ratio of the lowest 100 mb and findingtermining the mean mixing ratio of the lowest 100 mb and finding where that mixing ratio line crosses the sounding.
Convective Temperature (CT): This is the surface temperature a parcel would have so that its temperature when lifted adiabatically to the CCL would equal the environmental temperature. This is generally referred to as a critical surface temperature for the development of convective coulds.
Precipitable Water: This is a measure of the total amount of water vapor in a column of air and can be used to infer preciptation amounts.


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