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-Wind chill
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More about the wind chill






       One of the principal modes of heat transfer from an object is convection to the surrounding air. Convective heat transfer increases significantly with increasing air velocity. Thus a person is cooled at a faster rate under windy conditions than under calm conditions, given equal air temperature. Wind chill is a concept that relates the rate of heat loss from humans under windy conditions to an equivalent air temperature for calm conditions. The wind chill temperature (WCT) is an equivalent air temperature equal to the air temperature needed to produce the same cooling effect under calm conditions. Thus, it is not actually a temperature, but rather an index that helps relate the cooling effect of the wind to the air temperature under calm air conditions.
       It is important to remember that the wind will not cause an exposed object to become colder than the ambient air. Higher wind speeds will only cause the object to cool to the ambient temperature more quickly.


1. source:

  2. Parched Yugoslav Lake

       An extreme heat event or heat wave is a period of excessive daytime and nighttime heat in association with high humidity relative to geographic location and time of year.


       Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands. For cooling, the heart begins to pump more blood, blood vessels dilate to accomodate the increased flow, and the bundles of tiny capillaries threading through the upper layers of skin are put into operation. The blood is circulated closer to the skin’s surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90 percent of the body’s heat dissipating function.

       Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation, and high humidity retards evaporation. Under conditions of high temperature and high relative humidity, the body wants to maintain the 37°C inside. The heart is pumping a torrent of blood through dilated circulatory vessels; the sweat glands are pouring liquid, including essential dissolved chemicals, like sodium and chloride onto the surface of the skin. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body’s inner core begins to rise and heat-related illness may develop. Death rates can increase markedly as a result of heat waves, and the peaks correlate with maximum daily temperature 1-2 days before death; that is, there is a 1-2 day lag between the hottest temperatures and the peak in death rate. Illness (heat stroke, heat exhaustion, etc.) may occur in healthy people who are overexposed to, or overactive in, the heat. However, the majority of excess deaths that occur during heat waves are primarily from other illnesses in which heat stress accelerates death. Infants, the elderly, and people already ill, in particular those with circulatory problems, are most at risk during excessive heat.

       Many indices were developed to measure the influence of meteorological parameters on the human body. The comparison of some of them is shown below.

3. Differences in the various wind chill equivalent formulations at an air temperature of 0°F (adapted from Quayle et al. 2000).

       The Heat Index (HI) is the temperature the body feels when heat and humidity are combined. The chart below shows the HI that corresponds to the actual air temperature and relative humidity. (This chart is based upon shady, light wind conditions. Exposure to direct sunlight can increase the HI by up to 15°F.) (Help for calculation between the Fahrenheit and Celsius scales: TC = (5/9)*(TF-32) )

Temperature (F) versus Relative Humidity (%)

This table is comparing Temperature and Dewpoint, with the same disorders possible

About this pages:
author: Sándor Szalai - Hungarian Meteorological Service
scientific reviewing: Dr. Ildikó Dobi Wantuch / Dr. Elena Kalmár - Hungarian Meteorological Service, Budapest
last updated: 2004 - 02 - 03

Report on Wind Chill Temperature and Extreme Heat Indices: Evaluation and Improvement Projects. NOAA, Washington D.C. 2003.



last updated 09.07.2005 19:04:38 | © ESPERE-ENC 2003 - 2013