WIND CHILL

Parameters Used: Outside Air Temperature and Wind Speed

What is it:
Wind chill takes into account how the speed of the wind affects our perception of the air temperature. Our bodies warm the surrounding air molecules by transferring heat from the skin. If there’s no air movement, this insulating layer of warm air molecules stays next to the body and offers some protection from cooler air molecules. However, wind sweeps that comfy warm air surrounding the body away. The faster the wind blows, the faster heat is carried away and the colder the environment feels.

The new formula was adopted by both Environment Canada and the U.S. National Weather Service to ensure a uniform wind chill standard in North America. The formula is supposed to more closely emulate the response of the human body when exposed to conditions of wind and cold than the old formula did.

Formulas:
Older versions of software (Versions 5.0 and earlier) and firmware (Vantage firmware revisions before Sept. 7, 2001 and all non-VantagePro products including Echo) are based on the following formula (Siple and Passel, 1945):

0.0817 * (3.71V0.5 + 5.81 – 0.25V) * (T – 91.4) + 91.4

where V is the wind speed in mph and T is the outside air temperature in °F. Wind speeds above 55 mph are set to 55 mph. For wind speeds below 5 mph or temperatures above 91.4°F, the wind chill is set equal to the air temperature.

Newer product revisions (WeatherLink version 5.1 through 5.5.1 and Vantage Pro and Vantage Pro2 consoles with Sept 7, 2001 firmware or later and Vantage Pro2 consoles with firmware before May 2005) are based on the following formula:

35.74 + 0.6215T – 35.75 * (V 0.16 ) + 0.4275T * (V 0.16 )

As with the old formula, any place where the result yields a wind chill temperature greater than the air temperature, the wind chill is set equal to the air temperature. This always occurs at wind speeds of 0 mph or temperatures above 76°F. This also occurs at lower wind speeds with temperatures between 0°F and 76°F.

The new formula takes into account the fact that wind speeds are measured “officially” at 10 meters (33 feet) above the ground, but the human is typically only 5 to 6 feet (2 meters) above the ground. So, anemometers still need to be mounted as high as possible (e.g., rooftop mast) to register comparable wind speed readings and wind chill values.

An even newer version of this formula is available in WeatherLink version 5.6 or later and Vantage Pro2 console firmware version and later. This newer version of the formula addresses the fact that the latest National Weather Service (NWS) formula was not designed for use above 40°F. The result of the straight NWS implementation was little or no chilling effect at mild temperatures. This updated version provides for reasonable chilling effect at mild temperatures based on the effects determined by Steadman (1979) (see THSW Index section), but as with the new NWS formula, no upper limit where chilling has no additional effect. The later version

28 – 2 Rev A 5/11/06

for the console table only differs in that whole degrees and less resolution in the table are used for code and memory space conservation. As with previous versions of the wind chill formula, any place where the result yields a wind chill temperature greater than the air temperature, the wind chill is set equal to the air temperature. This always occurs at wind speeds of 0 mph or temperatures at or above 93.2°F (34°C). This also occurs at lower wind speeds with temperatures between 0°F (-18°C) and 93.2°F (34°C). As per Steadman (1979), 93.2 F (34°C) is the average temperature of skin at mild temperatures, thus temperatures above this value will actually create an apparent warming effect (see THSW Index section).

The Vantage Pro and Vantage Pro2 console uses the “10-minute average wind speed” to determine wind chill, which is updated once per minute. When 10-minute of wind speed data is unavailable, it uses a running average until 10-minutes worth of data is collected. The WeatherLink® software uses the 10-minute average wind speed also. If it is unavailable, it uses the current wind speed (which updates every 2.5 to 3 seconds). All other products use the current wind speed to determine wind chill.

The reason an average wind speed is employed in the Vantage Pro and Vantage Pro2 to calculate wind chill is as follows: The human body has a high heat capacity, thus high wind speeds have no effect on the body’s thermal equilibrium. So, an average wind speed provides a more accurate representation of the body’s response than an instantaneous reading. Also, “official” weather reports (from which wind chill is calculated) provide average wind speed, so using an average wind speed more closely matches the results that are seen in weather reports.

REFERENCES

“Media Guide to NWS Products and Services”, National Weather Service Forecast Office, Monterey, CA, 1995.

“New Wind Chill Temperature Index”, Office of Climate, Water and Weather Services, Washington, DC, 2001.

Siple, P. and C. Passel, 1945. Measurements of Dry Atmospheric Cooling in Subfreezing Temperatures. Proc. Amer. Philos. Soc.

Steadman, R.G., 1979: The Assessment of Sultriness, Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. Journal of Applied Meteorology, July 1979

Rev A 5/11/06

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HEAT INDEX

Parameters Used: Outside Air Temperature and Outside Humidity

What is it:
Heat Index uses temperature and relative humidity to determine how hot the air actually “feels.” When humidity is low, the apparent temperature will be lower than the air temperature, since perspiration evaporates rapidly to cool the body. However, when humidity is high (i.e., the air is saturated with water vapor) the apparent temperature “feels” higher than the actual air temperature, because perspiration evaporates more slowly.

Formulas:
Older versions of software and the display console using the following methodology. This formula is based upon the lookup table presented by Steadman (1979). The Davis implementation simply extends the range of use of this table to make it usable at temperatures beyond the scope of the table. Some of this extension is based on the table adapted by the US National Weather Service. The GroWeather and EnviroMonitor systems do not display a value beyond the scope of the Steadman table. All other products that display this value either:
Set values at temperatures below the scope of the table to the air temperature
Extend the readings using a best-curve fit above and below the air temperature
scope of the table. The low temperature cutoff is when the heat index for the given combination of temperature and humidity is 14°C or 57.2°F or below. This corresponds to a vapor pressure of 16 hPa. Heat Indices are set equal to the air temperature or 57.2°F, whichever is less, below these values. (The 14°C cutoff corresponds to the equivalent dewpoint at average testing laboratory conditions.)
WeatherLink software versions 5.2 or later and Vantage Pro2 console firmware versions of May 2005 revision or later use the above methodology with the following exceptions for values below an air temperature of 68°F:
The values use a variable baseline to which the Heat Index is either above or below the air temperature.
The values are loosely derived from the methodology outlined by Steadman in his 1998 paper (referenced below). Thus, air temperatures below 50°F follow this 1998 procedure. Air temperatures above 68°F follow his procedure outlined in 1979 (since the US NWS continues to use this). Davis has made a smooth transition between the two methods between 50°F and 68°F.