ENVIRONMENTAL SCIENCE PUBLISHED FOR EVERYBODY ROUND THE EARTH
One of the principal modes of heat transfer from an object is the convection of heat to the surrounding air. Convective heat transfer increases significantly with increasing air velocity. As a result, a person is cooled at a faster rate under windy conditions than under calm conditions, given equal air temperature.
1. source: http://www.erh.noaa.gov/er/
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. So the wind chill temperature is not actually a temperature, but rather an index that helps relate the cooling effect of the wind to the air temperature under calm conditions.
It is important to remember that the wind does not cause an exposed object to become colder than the ambient air temperature. Higher wind speeds simply cause the object to cool to the ambient air temperature more quickly.
2. Parched Yugoslav Lake
An extreme heat event or heat wave is a period of excessive daytime and nighttime heat and high humidity relative to the geographic location and time of year.
Human bodies loose heat by changing the rate and depth at which the blood is circulated and by water loss through the skin and sweat glands. To cool the body, the heart begins to pump more blood, blood vessels dilate (expand) 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 is responsible for about 90 percent of the body’s heat loss function. Sweating alone does nothing to cool the body. For cooling to occur, the sweat must be lost by evaporation from the surface of the skin. High humidity conditions retard this process of evaporation.
Under conditions of high sultriness, i. e. high temperature and high relative humidity together, the body aims to maintain an internal temperature of 37°C. To do this, the heart pumps more blood through dilated blood vessels and sweat glands pour liquid, which includes essential dissolved chemicals like sodium and chloride, onto the surface of the skin.
When more heat enters the body than 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 illnesses 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. Illnesses such as heat stroke and heat exhaustion can occur in healthy people who are overexposed to, or are overactive in the heat. However, the majority of excess deaths that occur during heat waves are actually the result of other illnesses, which are exacerbated by heat stress. Children, the elderly and people who are already ill, particularly those with circulatory problems, are most at risk during excessive heat.
Many indices have been developed to measure the influence of meteorological parameters on the human body. A comparison of formulations used to determine the effect of wind speed on wind chill is shown here.
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. To convert temperatures in Fahrenheit (TF) to temperatures on the Celsius scale (TC) use the following equation:
TC = (5/9)*(TF-32)
4. Temperature (F) versus Relative Humidity (%)
5. Possible heat disorder
6. This table compares Temperature and Dewpoint. The different colours show the disorders which are possible (see above).
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