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Mesoscale
Objective Analysis Parameters
Surface Temperature - Air
temperature at 10-meter height or 30 feet above ground-level, in
degrees Fahrenheit.
Surface Dew Point - Dew
point temperature at 10-meter height or 30 feet above ground-level, in
degrees Fahrenheit. The dew point is a relationship between relative
humidity (see below) and the air temperature (see above). It is the
temperature at which condensation (dew or frost) occurs. The dew point
temperature is useful to predict overnight low temperature and
temperature changes during precipitation. A large difference between
the dew point and air temperature indicates low humidity. If the dew
point is equal to the air temperature the humidity is 100%. The dew
point temperature can never be greater than the air temperature and it
does not vary with temperature, as does relative humidity. Since dew
point is a direct measure of the actual moisture content in the air, it
is the preferred unit of moisture measurement in meteorology.
Relative Humidity -
Relative humidity (RH), expresses a measure of the amount of water in
the air compared with the amount of water the air can hold at the
current temperature. RH is depicted in percent (%) at 10-meter height
or 30 feet above ground-level. Since RH changes with the temperature,
it's difficult to compare over a period of time. For example, as
temperature rises during the day, RH falls, and as temperature falls at
night, RH rises. For this reason, meteorologists typically use dew
point to provide a better measure of atmospheric moisture.
Pressure - Pressure is
the weight of the atmospheric at mean sea level (either directly
measured by stations at sea level or derived from the station pressure
and temperature for stations not at sea level). Mean sea level pressure
is used as a common reference for analyses of surface pressure
patterns, and is measured in millibars (mb). To convert mb to inches of
mercury, multiple by 0.0295.
Wind - Winds (at 10-meter
height or 30 feet above ground-level) are depicted with cyan 'flags',
where one full barb on a flag staff indicates a 10 knot (12 mph) wind,
one half barb indicates a 5 knot (6 mph) wind. The flag staff indicates
the direction the wind is coming from, for example, an east wind of 15
knots (17 mph) would be indicated by: A calm wind is
indicated by a circle. To convert knots to mph, multiply by 1.15.
CAPE - Convective Available
Potential Energy (or CAPE) represents the potential energy
available to an air parcel to be converted to kinetic energy in a
buoyant updraft (i.e. a measure of the amount of positive (upward)
buoyancy present or forecast). CAPE is directly related to the
maximum potential vertical speed within an updraft; thus, higher values
indicate greater potential for severe weather. The units of CAPE are
Joules per kilogram (J/kg; units of energy). This graphic can be
especially useful for convection initiation and severe weather forecast
purposes. In general, the following relative CAPE classifications can
be useful:
CAPE = 0 to 1000 > marginally unstable
CAPE = 1000 to 2500 > moderately unstable
CAPE = 2500 to 3500 > very unstable
CAPE = 3500 or greater > extremely unstable
Sky - The percentage of
cloud cover; 0-30% = Mostly Clear skies, 30-60% = Partly Cloudy skies,
60-70% = Partly Cloudy to Mostly Cloudy skies; 70-90% = Mostly Cloudy
skies, 90%+ = Overcast skies. |
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