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.
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.
The dewpoint depression is the difference in degrees between the temperature and
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.
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.
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 12 mph more than the average wind speed, a wind gust is reported.
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.
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.
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.
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
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.
This is the height of the cloud base for the lowest broken or overcast cloud layer.
This is the change of sea level pressure with time. This is usually reported as
a three hour pressure change in millibars.
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.
This is the highest temperature recorded during a specified period of time. Common
time periods include 6, 12 and 24 hours.
This is the lowest temperature recorded during a specified period of time. The
time period can be 6, 12 or 24 hours.
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
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported at that
location during a specified period of time.
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.
The rate of upward or downward motion of air passing through a given pressure level.
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.
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.
This the partial pressure that water vapor exerts on the total air pressure.
This the ratio of the weight of water vapor in a specified volume to weight of dry
air in that same volume.
This is the ratio of actual vapor pressure to the saturation vapor pressure at a
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.
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.
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.
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.
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
This is a measure of the total amount of water vapor in a column of air and can
be used to infer preciptation amounts