Weather Calculator Definitions
Have you ever wondered what the heck is "virtual temperature" or what is the
difference between "pressure altitude" and "density altitude"? Well
wonder no more. I have put together some of the more commonly asked for definitions that
the weather calculator computes. All the definitions came from the Glossary of
Meteorology. I added some of my own comments in an effort to make the definitions a
little more clear. Where I add something to a definition it will be italicized.
- Altimeter setting -
- The value of atmospheric pressure to which the scale of a pressure altimeter is set. In
the United States the setting represents the pressure required to make the altimeter
indicate the elevation of the airfield. This helps a pilot to know during his/her
flight how high the plane is above the ground, by reading the altitude from the altimeter
and subtracting the terrain elevation from the altimeter reading.
- Density altitude -
- The pressure altitude corrected for temperature changes from the standard atmosphere. This
is used by pilots to help them judge how much runway it will take to land or take off on.
- Dew point temperature -
- The temperature to which a given parcel of air must be cooled at constant pressure and
constant water-vapor content in order for saturation to occur (when the relative
humidity equals 100 percent).
- Heat index -
- A combination of the temperature and the relative humidity to give a temperature of
what it feels like to the human body on a hot muggy day.
- Mixing ratio -
- In a system of moist air, the dimensionless ratio of the mass of water vapor to the mass
of dry air. The amount of water vapor in the air divided by the amount of dry air.
- Pressure altitude -
- The altitude which corresponds to a given value of atmospheric pressure according to the
standard atmosphere. It is the indicated altitude of a pressure altimeter set at 1013.2
millibars; therefore, it is the indicated altitude above 1013.2 millibars. Once again
this is used by pilots to determine how high above the ground they are.
- Relative humidity -
- The ratio of the actual vapor pressure of the air to the saturation vapor pressure of
the air. In other words, how much water vapor is in the air divided by how much water
vapor the air could possibly hold.
- Sea level pressure -
- The atmospheric pressure at mean sea level, either directly measured or most commonly
mathematically determined from the observed station pressure. Sea level pressure is
calculated so that a surface pressure chart won't show giant areas of low pressure over
all of the mountain ranges.
- Station pressure -
- The atmospheric pressure computed for the level of the station elevation. It is usually
used as the base value for calculating sea level pressure and the altimeter setting.
- Vapor pressure -
- The pressure exerted by the molecules of a given vapor. In meteorology it is almost
exclusively used to show the partial pressure exerted by the water vapor in the air.
- Virtual temperature -
- In a system of moist air, the temperature of dry air having the same density and
pressure as the moist air. It is the temperature that a dry parcel of air would have
to raised for that parcel to have the same density and pressure as a moist parcel. The
virtual temperature is always higher than the actual temperature.
- Wet-bulb temperature -
- The temperature an air parcel would have if you cooled it adiabatically to saturation at
constant pressure by evaporation of water into it, all latent heat being supplied by the
parcel. This is different from the dew point in that you are adding moisture to the
parcel as you cool. Thus the virtual temperature will always be equal to or greater than
the dew point and the actual temperature will always be equal to or greater than the
- Wind chill -
- The cooling effect of any combination of temperature and wind. In the U.S. this is
translated into a temperature that a person might feel when he or she went outside.
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