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Name: Patrick
Status: other
Grade: other
Location: IL
Country: USA
Date: Summer 2011


Question:
How is apparent temperature calculated when the Fahrenheit temperature is less than 80 degrees? All of the formulas I have found specify over 80. I have a table with lower apparent/temps, but it is in 5 degree increments, and I would like to break it down into 1 degree increments. I am monitoring temperatures for the safety of seniors at a non-profit nursing home near Chicago.


Replies:
Hi Patrick,

Apparent temperature can refer to either heat index or wind chill, both of which are an indication of the level of discomfort a person feels due to the weather conditions. These temperatures are not true temperature therefore, and are based subjectively on human physiology.

Give that you are concerned with the indoors, you're probably talking about heat index. The equation for that is

HI = -42.379 + 2.04901523(Tf) + 10.14333127(RH) - 0.22475541(Tf)(RH) -6.83783x10**(-3)*(Tf**(2)) - 5.481717x10**(-2)*(RH**(2)) + 1.22874x10**(-3)*(Tf**(2))*(RH) + 8.5282x10**(-4)*(Tf)*(RH**(2)) -1.99x10**(-6)*(Tf**(2))*(RH**(2))

Tf = actual temperature (fahrenheit)
RH = relative humidity%

You can find this information at
http://www.weather.gov/forecasts/graphical/definitions/defineApparentT. html

Regards,

John C Strong


For others, the "apparent temperature" is the sensed comfort feeling of the air at some temperature with the effects of humidity factored in. At a given temperature, high humidity feels warmer than low humidity. Part of it is the "comfort factor", but the reduced evaporation of sweat at higher humidity is a real effect since the body has to work harder to keep the body cooled. Like "wind chill" for very low temperatures, air speed also comes into play.

You can find "apparent temperatures" from 0 to 100 %R.H. in 10 % intervals from 60 to 75 degrees F. in 1 degree intervals at:

http://www.infoplease.com/ipa/A0001434.html

The coarser intervals of relative humidity is less of an issue because the change in "apparent temperature" is less sensitive to intervals in relative humidity.

Vince Calder


Patrick,

Below 80 deg F (and down to 57 degrees) there is little difference between air temperature and apparent temperature, so that particular temperature range is often not talked about in consideration of apparent temperature.

The formula is found toward the bottom of the Australian BOM page below:

http://www.bom.gov.au/info/thermal_stress/#wbgt

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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