National Weather Service United States Department of Commerce


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.