Windchill

There is also a published in 2007 horror movie called Wind Chill

The wind chill (English ) or the wind chill (also Windfrösteln ) describes the difference between the measured air temperature and wind chill temperature as a function of wind speed. Defined for temperatures below about 10 ° C.

The wind chill effect ( engl. wind chill factor) is caused by the convective removal skin close and thus relatively warm air and the concomitant increase in the evaporation rate. The time required for the phase transition of the water energy is extracted through heat conduction from the body surface and cools it accordingly from. The wind therefore has the effect of speeding up the approximation of the surface temperature of the body with the ambient temperature of the air, which people perceive as a cooling effect.

While the wind chill is used primarily for temperatures below the comfort that is meaningful for temperatures above the heat index.

Quantification - sizes of Windchill

There are several ways to quantify the wind -chill effect, for example via the heat loss per of respective skin surface or the temperature of the skin itself These have so far but not against the currently prevailing wind chill temperature ( WCT) enforced or have been displaced from their.

The WCT is defined by the air temperature in light winds, which has the same rate of heat loss per area of ​​skin exposed to the wind as it would be at the measured temperature with wind the case. This definition was chosen for reasons of clarity, since a temperature will be better understood by the general public, such as the indication as in watts per square meters. Thus it is as such not a temperature, but a measure of the rate of heat loss, which is only shown in units of temperature.

The starting materials of dry air, therefore does not consider the effect of the low temperatures at low humidity. The related here "weak wind " is usually referred to as " calm ", but this can lead to misunderstandings, since it is usually s (previously 1.79 m / s ) is a wind speed of 1.34 m / in walking. As long as you estimated as a reference a total calm, the wind chill temperature is always lower than the actual measurable temperature.

The comprehensibility of the WCT through their specification in temperature units can easily lead to a false understanding of what the WCT actually expresses. It is not just the temperature that takes a body due to the wind. At a measured temperature of 5 ° C and a wind speed of 55 km / h, a WCT results of -1.6 ° C ( see below), yet the skin will never have completely independent of the effect period frostbite. The WCT is just an expression of how much faster the temperature of the skin approximates to 5 ° C, as would be the case without wind. This also explains why the wind chill does not show at high temperatures, because a faster temperature equilibration hardly plays a role in low temperature differences.

Current calculation and Table

The November 2001 valid empirical formula expressed as a numerical value equation for calculating the windchill with metric units and a measured height of 10 meters above the ground wind speed is:

• WCT - wind chill temperature in degrees Celsius
• Ta - air temperature in degrees Celsius
• V - wind speed in kilometers per hour

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

• WCT - wind chill temperature in degrees Fahrenheit
• Ta - air temperature in degrees Fahrenheit
• V - wind speed in miles per hour ( country mile )

It should be noted that the formulas do not refer to a complete calm and one under 1.34 m / s is given a value in wind speeds, located above the air temperature can be located. This is due to the insulating effect of skin-tight layer of air that heats up with complete calm, without being carried away by the wind. Then the sensed air temperature is higher than the actual ambient temperature in some distance from the skin surface due to the warmer body shell. For such low wind speeds, the formula is, however, not designed and the results are unreliable. In general, the scope of the formula is therefore estimated only for wind speeds above 5 km / h.

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

Meaning and application

A special importance has the wind chill in very cold and windy regions of the world, particularly in the Arctic, Antarctic and in the high mountains, so for climbers. Even a quick movement of the people corresponds to a high wind speed, of which certain winter sports are concerned. A great economic importance and therefore political sensitivity comes the wind chill to, especially in weather reports of the winter sports areas, insofar as it is used there (which in Europe is not usually the case ). The effect may also affect the usability of the machine, in particular of vehicles. It has a high importance for all life in appropriate Extremklimaten and thus also influences the proliferation of biological species in open terrain wind.

The main application of Windchill in the form of the WCT are the U.S. and Canada, which is why most definitions have come from here and the National Weather Service and Environment Canada. Both currently use this pre-made tables for evaluating the measurement data. In addition to efforts in European countries and Israel resulting from this diversity depending on the used literature or calculation procedures, the timeliness and the eventual adaptation to specific conditions in some major differences, both in the basic approach as well as in terms of the result.

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 equip their soldiers adequately for the rigors of the European winter of the Second World War. It instructed the U.S. polar explorer Paul Siple and Charles F. Passel, which during the second Antarctic Expedition Richard E. Byrd (1939-1941) conducted an experiment with this in the winter of 1941. However, their measurements were not based this on a man, but to a container filled with 250 g of water plastic cylinder. It consisted of cellulose acetate, was 14.9 cm long, 5.7 cm in diameter and had a thickness of 0.3 cm. They took advantage of two resistance thermometers to measure air and water temperature, and a cup anemometer for measuring the wind speed. Over the duration of the freezing process they could use the known heat of fusion of water heat loss from the cylinder in kilocalories per hour ( see heat flow ) and thus ultimately also determine the heat transfer coefficient in kilocalories per hour, square meters and degrees Celsius. Effect sizes were there exposed to the wind surface of the cylinder and the difference between its temperature ( estimated at 0 ° C ) and the air temperature. The air temperatures were moving during their measurements in the range between -56 ° C and -9 ° C, wind speeds of absolute calm to 12 m / s Your first real results Siple and Passel received by a graphical interpolation of a diagram in which their measured wind speeds were plotted against the specific heat transfer coefficient. The resulting fit line of this interpolation ( 1.1) could be easily linked to this with the heat loss of the plastic cylinder (1.2).

Based on this relationship, the heat loss to the terms of wind chill temperature was equated.

The individual symbols stand for the following sizes:

• α - heat transfer coefficient with wind
• α0 - heat transfer coefficient without wind
• V - wind speed
• Φ - heat flux (heat flow per unit time )
• A - area of ​​the surface
• TO - surface temperature
• TL - air temperature
• WCT - Windchill

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

In the formula, only the actual temperature and the wind speed are used as variables. It is in the true sense only under extreme conditions, such as windy hilltops with low air temperature valid, because in addition to the wind also affect other parameters the sensed temperature, such as the humidity ( see humidex, sultriness ), body height and weight, the clothing, the sunlight (degree of shading, sun) and skin moisture.

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

The NWS estimated for the WCT a threshold of -29 ° C, from which one speaks of a danger of the wind chill. This rather arbitrary value is influenced by a variety of factors and should therefore be regarded only as a general reference. As an example, it would demean a strong sunlight.

Criticism of the measurements from Siple and Passel

The original measurements have many vulnerabilities. Thus, for example, has only a thermometer for measuring the temperature of water used even though the water in the plastic cylinder freezes very uneven. This leads to a not compensable in retrospect deviation of the measured values, which were very widely distributed in the Siple and Passel used by their interpolation graph. The estimate of 0 ° C for the plastic cylinder also ignored the thermal resistance, the one actually lower temperature results. In addition, the problems have not been considered with respect to an extrapolation from a small plastic cylinder on the body of the person. This applies, in turn, especially for the thermal resistance, which is considerably greater for a skin surface.

The biggest problem, however, is probably the use of 33 ° C for the temperature of the skin, as this may be located below this value very quickly in a cool environment. A still partly current problem is related to the measurement of the wind speed. This is required as input for the equations, meteorologically standardized, however, recorded in ten meters above the ground. In the height of a man, that is, between the ground and about two meters, but the wind speed by friction effects of obstacles is much less usually.

All these factors act together towards an overestimation of the Wind Chills by the equation of Siple and Passel. Nevertheless, it has enabled its results for the first time, to make a wider public aware of the wind chill.

Further development to 2001

In the seventies, these data were finally made ​​available to the National Weather Service and its 1971 and 1984 published works fit the Australian researcher Robert G. Steadman this formula to a clad " average person " to. The result has been taken over by the National Bureau of Standards as an official formula and applied by the National Weather Service in the USA from 1973. It is, however, rather a reaction of the competent authorities at the onset as early as the 60s and 70s in the United States use of Windchill by some media representatives.

Steadman it was who took care of the problem of wind speed measurement. He created a formula that determined for the case of an open surface wind speeds in facial height to about two thirds of the wind speeds in ten meters. Even larger values ​​, all linked to an increase in the WCT, thereby showing in forests or urban environment. Since such obstacles are to be calculated badly in their influence on the wind speed, they also continue to represent a problem is the influence of the environment is not exactly determined, should the standardized wind speed measured nevertheless be roughly corrected ( as long as the correction is not already included in the equation ).

A major step forward for the accuracy of WCT / WCI enabled the work of Randall Osczevski in 1995. He developed a model of the human head and was able to perform measurements in the wind tunnel to yield very similar results to those of a cylinder. Maurice Bluestein and Zecher 1999 use a similar approach, but only with a theoretical analysis. Cylinder choose this because they are dealt with very extensively in the literature of heat conduction and are therefore better to model mathematically. Osczevski went into the episode about to be no longer the head as a whole, but only the face, because it is exposed to the wind the most. The heat loss in light wind at this point, the front of the cylinder test, proved to be higher in comparison to its pages. In Equation 1.3. this has a higher α0 result, what the WCT increased. Along with a realistic temperature for the skin surface, there were higher temperatures than at Siple and Passle, Bluestein and Zecher and particularly for high wind speeds also Steadman. A comparison of different methods of calculation has been made, among others, of Quayle.

Especially from Steadman effort has been made to allow other factors such as the intensity of radiation and the associated flow in the cloud. However, this also leads to a growing complexity of the calculation basis, where a dependent on season and latitude factor in the sunlight could still be reasonably well implemented. The development of a model for the entire body with consideration of metabolism, clothing, and various other factors was even proposed by Steadman, however, is difficult. An attempt in this direction and thus the indication of a sensed temperature over long intervals, without changing the basis of calculation, represents the Klima-Michel model of the German Weather Service dar.

Today's calculation from 2001

The effort was accelerated by a, scheduled in 2000 on the part of Canadian managers Internet conference to conduct a fundamental reform of the wind chill calculations. A group of specialists, the Joint Action Group for Temperature Indices ( JAG / TI ) was established to make a recommendation for the WCT. The International Society of Biometeorology ( ISB) formed an expert group to examine the international transferability of various solution concepts. In 2001, funds were made available two international conferences conducted in order to address the problems and solutions with respect to the wind chill. The questions were the focus of whether a temperature index could be developed for the entire span of the heat transfer whether this index would be applicable to all climates and seasons, whether he would be useful for weather forecasting and other uses and also whether this index is independent of individual characteristics would like the particular clothes be.

Finally Osczevski and Bluestein were commissioned to realize a compromise between their original work. For this purpose, test measurements were carried out in Toronto in June 2001 at the Defence and Civil Institute of Environmental Medicine. In this case, twelve people were subjected to different temperatures in a wind tunnel, and the wind speeds to obtain values ​​that are better taken into account the thermal resistance of the face. Hereinafter, the methods of heat transfer to a half- cylinder have been used, which should represent the wind facing face. The conditions for calm have been reduced to a value of 1.34 m / s, which should take a realistic walking speed of 4.8 km / h invoice. By an iteration with progressive assessments of the skin temperature of the heat loss was on this basis determined and converted via a reference back to the windless conditions in a WCT. The corrections for wind speed measurement, it was concluded in this regression formulas, which is why they, as indicated by measured at 10 m height wind speeds work.

Future developments aim mainly to the consideration of the sunlight, but improvements in the Frostbite it is expected. Another field of research is the development of a wind chill considering the influence of moisture, which is especially important for marine environments.

Criticism

There are different ways to determine the effect of wind chill quantitatively. Mostly this simple approximation formulas with severely impaired validity, prefabricated tables or nomograms are used. All these methods, however, have in common is that the particular through them value should ideally be used only considering its coming, because neither yields the various calculation methods consistent results, yet the calculated value must have a great deal to do with the reality of the specific individual case.

Allen spite of great efforts can not make general statements about something that is almost by definition the generality cut the Windchill: the subjective sensation of temperature of an individual. What the wind chill can ultimately make statements at this temperature sensitivity depends on a variety of factors. The assumptions used in the calculations, namely represent good averages, however, are also incorrect when departing in a concrete situation of these averages. The speed at which you go or even moves, and the wind affecting environment in which to move, are usually different than those that have been estimated for the wind chill. Wearing big glasses, an extensive facial hair or use an insulating cream the skin reaction to the wind strongly influence, as well as the peculiarities of the body's thermoregulation, which differ greatly from person to person. A man with a large body weight compared to body surface area in this case has lower skin temperatures than a slender person, but it does it even easier cools (hypothermia ). Add to that the different acclimation and genetic adaptation, which is a comparison between the temperature sensitivity of a Central European and one Inuk at -20 ° C illustrates outdoor temperature.

Another problem is that tacitly assumes sea level, although in the high mountains at high altitudes, such as on the summit of Kibo, the air has less than half the density at sea level. Thereby, the heat capacity of air per volume and the heat conduction is reduced by convection and accordingly the wind chill much weaker.

The windchill effect is often mistakenly equated with the term "perceived temperature". This is misleading, since even with high summer temperatures, wind causes the temperature is felt lower. High humidity, however, effected at low temperatures, that the temperature is cooler than the actual perceived temperature; in high-temperature environment has high humidity, however, the opposite effect: It makes us feel the temperature even higher.

These factors limit the explanatory power of the wind chill though strong, but can also be assessed their reliability in their knowledge and adapted to the individual case. However, this is only possible if accurate knowledge of the WCT and the existence of which are present, but this usually just is not the case. The benefits of Windchill as part of a weather report is for the general population therefore rather low and in ignorance of the size, the often very low " temperatures " deterrent. Due to these factors, the wind chill is often deemed useless size, which has no added value for specifying real air temperature and wind speed just for amateurs. This is also one of the reasons why the wind chill outside North America are rarely used.