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Apparent Temperature

Apparent Temperature

The concept of apparent temperature (AT) was initially put forward by Robert Steadman in 1971 and was intended as an assessment of what exposed body surfaces feel like in cold, windy conditions. Subsequently he (Steadman, 1979) extended his work on the thermal resistance of an unclothed body to warm, humid conditions. A further paper (Steadman, 1984) combined, and bridged the gap between the temperature ranges into a universal scale.

Regression equations of this universal scale are formulated for indoors, outdoors in shade but exposed to wind, and outdoors exposed to wind and solar radiation. Of these, outdoors in shade but exposed to wind, has been chosen as most informative:

		AT = -2.7 + 1.04*T + 2.0*e -0.65*v

		where AT and T (air temperature) are deg-C,
		e is vapor pressure in kPa,
		and v is 10m wind speed in m/sec.
	

Of course there can be no accurate formula for what a person feels like. There are many variables that cannot be put in to the equation because they are subjective, such as area of skin exposed, metabolism and sweat rate. In fact, there are numerous other formulations of "heat index", for example Rothfusz (1990), and Masterton and Richardson (1979), but they are all non-linear and can give spurious results if used with temperatures or humidities outside of their designated regression limits. They are, therefore, unsuitable for building a comprehensive database, albeit for summer months only, which would make the calculation of mean and threshold values possible. Steadman's universal AT is a linear regression that is not restricted, and is more appropriate to outside conditions because it includes wind.

Excessive heat can be a killer, in fact, it is the number one weather-related killer in the United States, resulting in hundreds of fatalities each year (National Weather Service). The National Weather Service NWS) is charged with issuing forecasts, advisories and warnings. They use the complex, non-linear formula given in Rothfusz (1990) for this purpose, so values may be slightly different from those given by the AT equation above especially as wind is not included by NWS).

Hourly station data are obtained from the National Climatic Data Center's (NCDC's) Integrated Surface Data (ISD) dataset (ftp://ftp.ncdc.noaa.gov/pub/data/noaa/isd-lite/). In this dataset observation times are UTC, but maximum, minimum and average apparent temperatures are calculated for local standard time days.

The mean base period is 1981-2010, and the "feel hot" threshold is defined as the 85th percentile of daily July and August values in this same period.

  • Masterton, J.M., and F.A. Richardson, 1979: Humidex: A method of quantifying human discomfort due to excessive heat and humidity. Atmospheric Environment Service, Environment Canada, pp45
  • National Weather Service, http://www.nws.noaa.gov/os/heat/index.shtm
  • Rothfusz, L.P., 1990: The Heat Index "Equation" (or, more than you wanted to know about Heat Index). Scientific Services Division, NWS Southern Region HQ, Fort Worth, TX.
  • Steadman, R.G., 1971: Indices of windchill of clothed persons. J. Appl. Meteor., 10, 674-683.
  • Steadman, R.G., 1979: The assessment of sultriness. Part 1: A temperature-humidity index based on human physiology and clothing science. J. Appl. Meteor., 18, 861-873.
  • Steadman, R.G., 1984: A universal scale of apparent temperature. J. Appl. Meteor., 23, 1674-1687.

Wind Chill

The National Weather Service (NWS) Wind Chill Temperature (WCT) is based on the rate of heat loss from skin that is exposed to wind and cold temperatures. As the wind increases, the body is cooled faster, causing skin temperature to drop below that of the ambient air. In short, WCT is what it "feels like". WCT only applies to exposed skin. There is no impact on inanimate objects.

WCT is based on a "Human face model", with wind speeds from observations taken at the standard anemometer height of 33 feet reduced to typical face level, five feet above ground, and assumes no solar radiation impact.

		WCT = 35.74 + 0.6215*T - 35.75*(V**0.16) + 0.4275*T*(V**0.16)

		where WCT and T (air temperature) are deg-F and
		V is anemometer wind speed in mph
		http://www.nws.noaa.gov/os/windchill/index.shtml
	

WCT values in tabular format can be viewed at the above world wide web site. It is clear from this table that the possibility of frostbite or hypothermia are greatly enhanced if a person is exposed to cold and windy conditions, and the NWS is charged with issuing wind chill advisories and warnings at critical thresholds.

It should be noted that the regression is non-linear and is only valid for air temperatures at or below 50deg-F and wind speeds above 3mph. However, to build a comprehensive database of WCT values, albeit for extended winter months only, these limits have been stretched, but only to the point where calculated WCT is less than observed air temperature. If WCT should exceed air temperature then the unadjusted observed value is used in the database.

Hourly station data are obtained from the National Climatic Data Center's (NCDC's) Integrated Surface Data (ISD) dataset (ftp://ftp.ncdc.noaa.gov/pub/data/noaa/isd-lite/). In this dataset observation times are UTC, but maximum, minimum and average wind chill temperatures are calculated for local standard time days.

The mean base period is 1981-2010, and the "feel cold" threshold is defined as the 15th percentile of daily January and February values in this same period.