## 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 wet-bulb.

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.