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Weather Terms

Temperature: The current air temperature, measured in degrees Fahrenheit.  Air temperature is actually a measure of sensible heat.  When air molecules have high energy levels, they bump into your skin and transfer heat energy into your skin (so the air feels warm).  When air molecules have low energy levels, heat energy from your skin is transferred to them (and the air feels cold).  The thermometer measures the amount of energy being transferred into or taken from it through collisions with air molecules, and uses this information to determine air temperature.


Humidity: The amount of water vapor in the air.  Humidity here is reported in relative humidity.  Relative humidity is based on the air's ability to hold water vapor.  (Warmer air can hold more water vapor than colder air can.)  Relative humidity reports the percentage of the air's capacity to hold water vapor that is currently being used.  For example, if the relative humidity is 50%, then the air is only holding 50%, or half, of the water vapor it could be holding.  If the relative humidity is 100%, then the air is holding all it can hold and is said to be saturated.


Dew point: The temperature at which air is saturated.  If the current air temperature were reduced to the dew point, the air would be saturated.  When the temperature and dew point are very close together, the relative humidity is high and water vapor is likely to start condensing and forming liquid droplets (like those that make up clouds and fog).    


Wind: The horizontal flow of air molecules above Earth's surface.  Wind flows from areas of high pressure to areas of low pressure.  The greater the difference between the two, the faster the wind blows. 


Barometer: The current barometric air pressure (sometimes called atmospheric pressure). Air pressure is generated by the weight of air molecules being pulled towards the Earth's surface.  The units "inches of mercury" come from an old-fashioned mercury barometer.  When the atmosphere pressed down on the mercury in an open dish, the mercury was forced to rise higher into a glass tube.  The higher the mercury went, the greater the atmospheric pressure.  Modern barometers no longer use mercury, but the mercury units are still in use in the United States.  A rising barometer usually indicates the approach of a high pressure system and clear skies, although in the winter time high pressure often brings lake-effect snowfall to the Great Lakes region.  A falling barometer usually accompanies the approach of a low-pressure system or front, which often bring clouds and precipitation.


Rain: The amount of rain received since midnight of the current day.  This total resets to zero every night at midnight.


Rain Rate: The rate at which rain is currently falling, measured in inches per hour. 


Storm Total: The amount of rain received during the current storm.  This total does not reset at midnight, but continues compiling until the storm is over.


Monthly Rain: The total amount of rain received during the current calendar month.


Yearly Rain: The total amount of rain received during the current calendar year.


Wind Chill: The apparent temperature that results from the combination of air temperature and the effects of wind.  When wind blows over exposed skin, it disperses body heat away from the skin and speeds evaporation.  Both of these effects cause the skin to feel colder.  Therefore, when wind is blowing, the air feels colder than it actually is.


THW Index: Temperature Humidity Wind index.  A combination of wind chill and the heat index, this apparent temperature is calculated by factoring in both the higher temperature perception felt during periods of high humidity and the cooling effect of wind.


Heat Index: The heat index combines the current air temperature with the effects of humidity.  The body's natural cooling method is to evaporate sweat from the skin.  (Evaporation is a cooling process.)  When the humidity of the atmosphere is high, it's more difficult for your sweat to evaporate.  You end up feeling sticky and much hotter.  The apparent temperature is therefore much higher on days with high relative humidty.


UV:   The UV (ultraviolet) index.  A rating of 0 to 2 indicates a low risk of damage from UV exposure, 3 to 5 means a moderate risk, 6 to 7 means a high risk, and 8, 9, or 10 means a very high risk of sun damage from UV exposure. An index of 11 is considered extreme risk, and skin can sunburn very quickly (only a few minutes).  Our UV index is calculated based on a Type II individual, which is a very conservative estimate of risk.  Type II individuals have fair skin that burns easily and tans minimally.  If your skin burns quickly and never tans, you should take extra precaution.  If you have darker skin that tans easily, your risk is somewhat lower.  It's always wise to use sunscreen and other appropriate means of sun protection.  For more on the UV index, visit


Solar Radiation: Radiant energy emitted by the sun from a nuclear fusion reaction that created electromagnetic energy. The spectrum of solar radiation is close to that of a black body with a temperature of about 5800 K. About half of the radiation is in the visible short wave part of electromagnetic spectrum. The other half is mostly in the near infrared part, with some in the ultraviolet part of the spectrum. When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause a change in the skin color of humans.


The definitions on this page were compiled by Dr. Jennifer Johnson, Assistant Professor of Geography at Ferris State University, and Kristin Deur, a student in the Weather and Climate class at Ferris in Fall 2007.


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