Wind chill

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Perceived temperature difference (wind chill factor) at 0 ° C depending on the wind

The wind chill (Engl.) And the wind chill (also Windfrösteln ) describes the difference between the measured air temperature and the sensed temperature as a function of wind speed . It is defined for temperatures below approx. 10 ° C.

The wind chill effect is caused by the convective removal of air close to the skin and thus relatively warm air and the associated increase in the rate of evaporation . The energy necessary for the phase transition of the water is drawn from the body surface by conduction and cools it down accordingly. The wind therefore has the effect of accelerating the adjustment of the surface temperature of the body to the ambient temperature of the air, which people perceive as cooling.

While wind chill is mainly used for temperatures below comfort , the heat index is more meaningful for temperatures above it .

Quantification - sizes of wind chill

There are various ways of quantifying the wind chill effect , for example via the heat loss per skin area concerned or via the temperature of the skin itself. However, these have not yet prevailed against the currently prevailing wind chill temperature (WCT) or have been replaced by it repressed.

The WCT is defined by the air temperature that would cause the same rate of heat loss per skin area exposed to the wind in a weak wind as the actual air temperature with wind influence. This definition was chosen for reasons of clarity, since a temperature is better understood by the general public than, say, the specification in watts per square meter . In the strict sense, it is not a temperature, but a measure of the rate of heat loss that is only given in units of temperature.

It is assumed that the air is dry and does not take into account the low effect of humidity at low temperatures. The "weak wind" used here is usually referred to as "calm". However, this can lead to misunderstandings as it is usually a wind speed of 1.34 m / s (previously 1.79 m / s) when walking. As long as a reference is assumed to be total calm, the wind chill temperature is always lower than the actual measurable temperature.

The general understanding of the WCT through its specification in temperature units easily leads to a wrong understanding of what the WCT actually expresses. It is precisely not the temperature that a body assumes due to the wind. At a measured temperature of 5 ° C and a wind speed of 55 km / h, the result is a WCT of −1.6 ° C (see below), but the skin will never show frostbite regardless of the period of action. The WCT is just an expression of how much faster the temperature of the skin approaches 5 ° C than it would be without wind. This also explains why the wind chill does not show itself at high temperatures, because a faster temperature adjustment hardly plays a role with small temperature differences.

Current calculation and table

The empirical formula in the form of a numerical equation for calculating wind chill with metric units and a wind speed measured at a height of 10 meters above the ground is as follows:

With units of the Anglo-American system of measurement (for 33 feet above the ground):

It should be noted that the formulas do not relate to a complete calm and that at wind speeds below 1.34 m / s, a value is obtained that can be higher than the air temperature. This is due to the insulating effect of the layer of air close to the skin, which warms up when there is no wind without being carried away by the wind. The air temperature then perceived is higher than the actual ambient temperature at some distance from the skin surface due to this warmer body shell. However, the formula is not designed for such low wind speeds and the corresponding results are unreliable. As a rule, the range of validity of the formula is therefore only estimated for wind speeds above 5 km / h.

The origins of these equations, the problems in their implementation and accuracy, and alternative approaches are presented in the following sections.

Wind chill temperature
Wind speed Air temperature
00 km / h 10 ° C 5 ° C 0 ° C −5 ° C −10 ° C −15 ° C −20 ° C −30 ° C −40 ° C −50 ° C
05 km / h 9.8 +4.1 −1.6 0−7.3 −12.9 −18.6 −24.3 −35.6 −47.0 −58.3
10 km / h 8.6 +2.7 −3.3 0−9.3 −15.3 −21.2 −27.2 −39.2 −51.1 −63.0
15 km / h 7.9 +1.7 −4.4 −10.6 −16.7 −22.9 −29.1 −41.4 −53.7 −66.1
20 km / h 7.4 +1.1 −5.2 −11.6 −17.9 −24.2 −30.5 −43.1 −55.7 −68.3
25 km / h 6.9 +0.5 −5.9 −12.3 −18.8 −25.2 −31.6 −44.5 −57.3 −70.2
30 km / h 6.6 +0.1 −6.5 −13.0 −19.5 −26.0 −32.6 −45.6 −58.7 −71.7
40 km / h 6.0 −0.7 −7.4 −14.1 −20.8 −27.4 −34.1 −47.5 −60.9 −74.2
50 km / h 5.5 −1.3 −8.1 −15.0 −21.8 −28.6 −35.4 −49.0 −62.7 −76.3
60 km / h 5.1 −1.8 −8.8 −15.7 −22.6 −29.5 −36.5 −50.3 −64.2 −78.0
Note: If the boxes are blue, there is a chance that you will freeze to death within 30 minutes or less . There is a risk of frostbite when the skin temperature reaches -4.8 ° C, below which frostbite occurs for about 5% of people.

Importance and application

Wind chill is of particular importance in cold and windy regions of the world, especially in the Arctic , Antarctic and in the high mountains , i.e. for mountaineers . Fast human movement also corresponds to a high wind speed, which affects certain winter sports . Wind chill is therefore of great economic importance and therefore politically explosive in weather reports from winter sports areas, insofar as it is used there (which is usually not the case in Europe). The effect can impair the usability of machines, especially vehicles. It is of great importance for all life in extreme climates and influences the distribution of biological species in open winds.

The main area of ​​application of the windchill in the form of the WCT is the USA and Canada , which is why most of the definitions come from there or from the National Weather Service and Environment and Climate Change Canada . Both currently use ready-made tables to evaluate the measurement data. In addition to efforts in European countries and Israel, this diversity results in sometimes considerable differences, both in the basic approach and in the result, depending on the specialist literature or calculation methods used, their topicality and the possible adaptation to specific conditions.

history

Introduction of Windchill

The development of the first empirical formulas and tables goes back to the efforts of the armed forces of the United States to adequately equip their soldiers for the hardships of the European winters of World War II . She commissioned the US polar explorers Paul Siple and Charles F. Passel , who carried out an experiment during Richard E. Byrd's second Antarctic expedition (1939–1941) in the winter of 1941. However, your measurements were not based on a person, but on a plastic cylinder filled with 250 g of water. This consisted of cellulose acetate , was 14.9 cm long, 5.7 cm in diameter and had a thickness of 0.3 cm. They used two resistance thermometers to measure the air and water temperature and a cup cross anemometer to record the wind speed. Over the duration of the freezing process, using the known heat of fusion of the water , they were able to determine the heat loss of the cylinder in kilocalories per hour (see heat flow ) and thus ultimately also the heat transfer coefficient in kilocalories per hour, square meters and degrees Celsius. Influencing variables were the area of ​​the cylinder exposed to the wind and the difference between its temperature (estimated at 0 ° C) and the air temperature. The air temperatures ranged between −56 ° C and −9 ° C during their measurements, the wind speeds from absolute calm to 12 m / s. Siple and Passel obtained their first real results by graphically interpolating a diagram in which their measured wind speeds were plotted against the determined heat transfer coefficients. The resulting straight line of best fit for this interpolation (1.1) could now easily be linked with the heat loss of the plastic cylinder (1.2).

1.1)
1.2)

Based on this relationship, the heat loss was equated with the conditions of the wind chill temperature.

1.3)

The individual symbols stand for the following quantities :

  • α - coefficient of heat transfer with wind
  • α 0 - heat transfer coefficient without wind
  • v - wind speed
  • Φ - heat flow (amount of heat per time)
  • A - area of ​​the surface
  • ϑ O - surface temperature
  • ϑ L - air temperature
  • ϑ WCT - wind chill temperature

For ϑ O they estimated a skin temperature of 33 ° C and a wind speed of 1.79 m / s was used to determine α according to formula 1.1. The empirical equation resulting from transformations for the dry temperature in degrees Celsius and the wind speed in km / h is:

1.4)

Only actual temperature and wind speed are used as variables in the formula. In the real sense, it is only valid under extreme conditions, such as mountain peaks with strong winds with a low air temperature, because in addition to the wind, other parameters also influence the perceived temperature, such as humidity (see Humidex , sultriness ), body size and weight , clothing, the Sun exposure (degree of shade, position of the sun ) and skin moisture .

In Canada, only the left side of equation 1.3 was used with otherwise identical assumptions. This gives the so-called wind chill index (WCI) in watts per square meter, i.e. the actual heat loss, through transformations .

The NWS estimated a threshold value of −29 ° C for the WCT, above which one speaks of a danger of wind chill. This rather arbitrary value is influenced by a large number of factors and should therefore only be viewed as a rough guide. As an example, exposure to strong sunlight would degrade him.

Criticism of the measurements by Siple and Passel

The original measurements have many weak points. For example, only a thermometer was used to record the water temperature, although the water in the plastic cylinder only freezes very unevenly. This leads to a deviation of the measured values ​​that cannot be compensated for afterwards, which were very widely scattered on the diagram used by Siple and Passel for their interpolation . The estimate of 0 ° C for the plastic cylinder also ignores its thermal resistance , which actually causes a lower temperature. In addition, the problems related to extrapolation from a small plastic cylinder to the human body were not considered. This, in turn, applies particularly to the thermal resistance , which is considerably greater on a skin surface.

The biggest problem, however, is probably the use of 33 ° C for the temperature of the skin, because it can be below this value very quickly in a cool environment. A problem that is still partly current is related to the measurement of wind speed. This is required as an input variable for the equations, standardized meteorologically, but recorded at a height of ten meters above the ground. At a person's height, i.e. between the ground and around two meters, the wind speed is usually much lower due to the effects of friction on obstacles.

All of these factors act in the direction of an overestimation of wind chill by the equation of Siple and Passel. Nevertheless, their results made it possible for the first time to make the general public aware of the wind chill.

Further development until 2001

In the 1970s, this data was finally made available to the National Weather Service , and through his work in 1971 and 1984, Australian researcher Robert G. Steadman adapted this formula to an "average person" wearing clothes. The result was adopted as the official formula by the National Bureau of Standards and used by the National Weather Service in the USA from 1973. However, it is rather a reaction by the responsible authorities to the use of the Windchill by some media representatives, which began in the USA in the 1960s and 1970s.

It was Steadman who took on the problem of measuring wind speeds. He created a formula which, in the case of an open area, determined the wind speeds at face height to be around two thirds of the wind speeds at ten meters. Even higher values, all linked to an increase in the WCT, show up in forests or urban surroundings. Since such obstacles are difficult to calculate in terms of their influence on the wind speed, they continue to pose a problem. If the influence of the environment cannot be determined exactly, the standardized measured wind speed should still be roughly corrected (as long as the correction is not already included in the equation) .

The work of Randall Osczevski in 1995 made a significant advance in the accuracy of WCT / WCI possible. He developed a model of the human head and was able to carry out measurements in the wind tunnel , with results similar to those of a cylinder. Maurice Bluestein and Zecher used a similar approach in 1999, but only with a theoretical analysis. Cylinders are chosen because they are dealt with intensively in the specialist literature on heat conduction and are therefore easier to model mathematically. As a result, Osczevski no longer looked at the head as a whole, but only at the face, since it is most exposed to the wind. The heat loss in light wind at this point, the front of the test cylinder, turned out to be higher compared to its sides. In equation 1.3. this results in a higher α 0 , which increases the WCT. Together with a realistic temperature for the skin surface, the temperatures were higher than with Siple and Passle, Bluestein and Zecher and, especially for high wind speeds, also Steadman. A comparison of the various calculation methods was made by Quayle, among others.

Efforts were made by Steadman in particular to incorporate other factors such as the radiation intensity and the associated cloud cover. However, this leads to an increasing complexity of the calculation basis, whereby a factor for solar radiation dependent on the season and latitude could still be implemented reasonably well. Developing a full-body model that takes metabolism, clothing, and various other factors into account has been suggested by Steadman, among others, but is proving difficult. An attempt in this direction and thus the indication of a perceived temperature over large intervals without changing the basis of calculation is represented by the Climate Michel model of the German Weather Service.

Today's calculation from 2001

An Internet conference organized by the Canadian authorities in 2000 pushed efforts to carry out a fundamental reform of the Windchill calculations. A group of specialists, the Joint Action Group for Temperature Indices (JAG / TI), was tasked with developing a recommendation for the WCT. The International Society of Biometeorology (ISB) also formed a group of experts to examine the international transferability of various solution concepts. In 2001 two international conferences were held to discuss the problems and suggested solutions with regard to wind chill. The main questions were whether a temperature index could be developed for the entire range of heat transfer, whether this index could be applied to all climates and all seasons, whether it would be useful for weather forecasting and other uses, and whether this index would be independent of individual characteristics how about the respective clothing.

Ultimately, Osczevski and Bluestein were commissioned to find a compromise between their original work. For this purpose, test measurements were carried out in June 2001 at the Defense and Civil Institute of Environmental Medicine in Toronto . Twelve people were exposed to different temperatures and wind speeds in a wind tunnel, resulting in values ​​that better considered the thermal resistance of the face. Thereafter, the methods of heat transfer were applied to a half cylinder, which was supposed to represent the face turned towards the wind. The conditions for calm were reduced to a value of 1.34 m / s, which should allow for a realistic walking speed of 4.8 km / h. By iterating with progressive estimates of the skin temperature, the heat loss was determined on this basis and converted into a WCT by referring back to the windless conditions. The corrections for the wind speed measurement were included in these regression formulas, which is why, as stated, they work with wind speeds measured at a height of 10 m.

Future developments are primarily aimed at taking solar radiation into account, but improvements are expected with regard to the risk of frostbite. Another field of research is the development of a wind chill taking into account the influence of moisture, which is particularly important for marine environments.

criticism

There are several ways to quantify the effect of wind chill. Usually, simple approximation formulas with severely restricted validity, ready-made tables or nomograms are used. What all methods have in common, however, is that the value determined by them should ideally only be used taking into account how it came about, because the various calculation methods do not produce uniform results, nor does the calculated value have to have much to do with the reality of the specific individual case.

Despite all efforts, the wind chill cannot make generally valid statements about the subjective temperature perception of an individual, since this is by definition beyond general validity. What statements the wind chill can ultimately make about this temperature perception depends on a large number of factors. The assumptions that are used in the calculations represent good mean values, but are incorrect if one deviates from these mean values ​​in a specific situation. The speed at which you walk or drive and the wind-influencing environment in which you move are generally different from those estimated for wind chill. Wearing large glasses, having a large beard or using an insulating cream have a very strong influence on the skin's reaction to the wind, as does the peculiarities of the body's own thermoregulation , which differ greatly from person to person. A person with a large body weight compared to the body surface has lower skin temperatures than a slim person, who also cools down more easily ( hypothermia ). In addition, there is the different acclimatization and also genetic adaptation , which illustrates a comparison between the temperature perception of a Central European and an Eskimo at an outside temperature of −20 ° C.

Another problem is that tacitly sea level is assumed, although in high mountains at high altitudes, e.g. B. on the summit of Kibo , the air is not even half the density at sea level. This reduces the heat capacity of the air and the heat conduction through convection and the wind chill is much weaker.

The wind chill effect is often incorrectly equated with the term “ sensed temperature ”. This is misleading in that even at high summer temperatures, wind means that the temperature is perceived as lower. High humidity, on the other hand, at low temperatures means that the temperature is perceived as cooler than the actual temperature; In a very warm environment, on the other hand, high humidity makes the temperature perceive as even higher.

These factors strongly limit the informative value of the Windchill, but if they are known, their reliability can also be assessed and adapted to the individual case. However, this is only possible if you have precise knowledge of the WCT and how it came about, which is usually not the case. The benefit of the wind chill in the context of a weather report for the general population is therefore often assessed as low and in ignorance of how it has come about, the often very low "temperatures" can have a deterrent effect. Because of these factors, the wind chill is often viewed as a useless variable, which, especially for laypeople, has no added value for specifying real air temperature and wind speed. This is also one of the reasons why wind chill is rarely used outside of North America.

See also

literature

  • Maurice Bluestein, Jack Zecher: A New Approach to an Accurate Wind Chill Factor . In: Bulletin of the American Meteorological Society . tape 80 , 2010, p. 1893-1899 , doi : 10.1175 / 1520-0477 (1999) 080 <1893: ANATAA> 2.0.CO; 2 .
  • JC Dixon, MJ Prior: Wind-chill indices, a review . In: Meteorological Magazine . tape 116 , no. 1374 , 1987, pp. 1-17 .
  • James R. Holton (Ed.), John Pyle (Ed.), Judith Curry (Ed.): Encyclopedia of Atmospheric Sciences. Academic Press, San Diego / London 2002, ISBN 0-12-227090-8
  • Randall J. Osczevski: Windward Cooling: An Overlooked Factor in the Calculation of Wind Chill . In: Bulletin of the American Meteorological Society . tape 81 , 2010, p. 2975-2978 , doi : 10.1175 / 1520-0477 (2000) 081 <2975: WCAOFI> 2.3.CO; 2 .
  • PA Siple, CF Passel: Measurements of dry atmospheric cooling in sub-freezing temperatures. Reports on Scientific Results of the United States Antarctic Service Expedition , 1939–1941. In Proceedings of the American Philosophical Society 89, 1945, pp. 177-199, Australian Geographer 5, 1946, doi: 10.1080 / 00049184608702259
  • RG Steadmen: Indices of wind chill of clothed persons. In Journal of Applied Meteorology. 10, 1971, pp. 674-683, doi : 10.1175 / 1520-0450 (1971) 010 <0674: IOWOCP> 2.0.CO; 2
  • RG Steadmen: Comments on "Wind chill errors". In Bulletin of the American Meteorological Society. 9, 1995, pp. 1628-1630
  • RG Quayle, RG Steadmen: The Steadmen wind chill: an improvement over present scales. In Weather and Forecasting , 13, 1998, pp. 1187-1193
  • DP Wyon: Wind-chill equations predicting whole-body heat loss for a range of typical civilian outdoor clothing ensembles. In Scandinavian Journal of Work, Environment and Health. 15 (supplement 1), pp. 76-83

Web links

Commons : Windchill  - collection of images, videos and audio files
Wiktionary: Windchill  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ Perceived temperature ( memento from March 16, 2016 in the Internet Archive )
This version was added to the list of articles worth reading on August 7, 2006 .