Wind chill

Wind-chill or windchill (popularly wind chill factor) is the lowering of body temperature due to the passing-flow of lower-temperature air.

Wind chill numbers are always lower than the air temperature for values where the formula is valid. When the apparent temperature is higher than the air temperature, the heat index is used instead.

Wind chill
A chart of wind chill values for given air temperatures and wind speeds

Explanation

A surface loses heat through conduction, evaporation, convection, and radiation.[1] The rate of convection depends on both the difference in temperature between the surface and the fluid surrounding it and the velocity of that fluid with respect to the surface. As convection from a warm surface heats the air around it, an insulating boundary layer of warm air forms against the surface. Moving air disrupts this boundary layer, or epiclimate, allowing for cooler air to replace the warm air against the surface. The faster the wind speed, the more readily the surface cools.

Alternative approaches

Many formulas exist for wind chill because, unlike temperature, wind chill has no universally agreed upon standard definition or measurement. All the formulas attempt to qualitatively predict the effect of wind on the temperature humans perceive. Weather services in different countries use standards unique to their country or region; for example, the U.S. and Canadian weather services use a model accepted by the National Weather Service. That model has evolved over time.

The first wind chill formulas and tables were developed by Paul Allman Siple and Charles F. Passel working in the Antarctic before the Second World War, and were made available by the National Weather Service by the 1970s. They were based on the cooling rate of a small plastic bottle as its contents turned to ice while suspended in the wind on the expedition hut roof, at the same level as the anemometer. The so-called Windchill Index provided a pretty good indication of the severity of the weather.

In the 1960s, wind chill began to be reported as a wind chill equivalent temperature (WCET), which is theoretically less useful. The author of this change is unknown, but it was not Siple or Passel as is generally believed. At first, it was defined as the temperature at which the windchill index would be the same in the complete absence of wind. This led to equivalent temperatures that exaggerated the severity of the weather. Charles Eagan[2] realized that people are rarely still and that even when it was calm, there was some air movement. He redefined the absence of wind to be an air speed of 1.8 metres per second (6.5 km/h; 4.0 mph), which was about as low a wind speed as a cup anemometer could measure. This led to more realistic (warmer-sounding) values of equivalent temperature.

Original model

Equivalent temperature was not universally used in North America until the 21st century. Until the 1970s, the coldest parts of Canada reported the original Wind Chill Index, a three or four digit number with units of kilocalories/hour per square metre. Each individual calibrated the scale of numbers personally, through experience. The chart also provided general guidance to comfort and hazard through threshold values of the index, such as 1400, which was the threshold for frostbite.

The original formula for the index was:[3][4]

where:

  • WCI = wind chill index, kcal/m2/h
  • v = wind velocity, m/s
  • Ta = air temperature, °C

North American and United Kingdom wind chill index

In November 2001, Canada, the United States, and the United Kingdom implemented a new wind chill index developed by scientists and medical experts on the Joint Action Group for Temperature Indices (JAG/TI).[5][6][7] It is determined by iterating a model of skin temperature under various wind speeds and temperatures using standard engineering correlations of wind speed and heat transfer rate. Heat transfer was calculated for a bare face in wind, facing the wind, while walking into it at 1.4 metres per second (5.0 km/h; 3.1 mph). The model corrects the officially measured wind speed to the wind speed at face height, assuming the person is in an open field.[8] The results of this model may be approximated, to within one degree, from the following formula:

The standard wind chill formula for Environment Canada is:

where Twc is the wind chill index, based on the Celsius temperature scale; Ta is the air temperature in degrees Celsius; and v is the wind speed at 10 m (33 ft) standard anemometer height, in kilometres per hour.[9]

When the temperature is −20 °C (−4 °F) and the wind speed is 5 km/h (3.1 mph), the wind chill index is −24. If the temperature remains at −20 °C and the wind speed increases to 30 km/h (19 mph), the wind chill index falls to −33.

The equivalent formula in US customary units is:[10]

where Twc is the wind chill index, based on the Fahrenheit scale; Ta is the air temperature in degrees Fahrenheit, and v is the wind speed in miles per hour.[11]

Windchill temperature is defined only for temperatures at or below 10 °C (50 °F) and wind speeds above 4.8 kilometres per hour (3.0 mph).[10]

As the air temperature falls, the chilling effect of any wind that is present increases. For example, a 16 km/h (9.9 mph) wind will lower the apparent temperature by a wider margin at an air temperature of −20 °C (−4 °F), than a wind of the same speed would if the air temperature were −10 °C (14 °F).

Windchill effect en

Celsius wind chill index

WindChill Comparison

Comparison of old and new wind chill values at −15 °C (5 °F)

Windchill calculator

Wind chill calculator

The 2001 WCET is a steady state calculation (except for the time to frostbite estimates).[12] There are significant time-dependent aspects to wind chill because cooling is most rapid at the start of any exposure, when the skin is still warm.

Australian apparent temperature

The apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980s to include the effect of sun and wind. The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1994). The AT is defined as; the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity. [13]

The formula[14] is:

where:

  • Ta = dry bulb temperature (°C)
  • e = water vapour pressure (hPa)
  • v = wind speed (m/s) at an elevation of 10 m

The vapour pressure can be calculated from the temperature and relative humidity using the equation:

Where:

Ta = dry bulb temperature (°C)
RH = Relative humidity (%)
exp represents the exponential function

The Australian formula includes the important factor of humidity and is somewhat more involved than the simpler North American model. The North American formula was designed to be applied at low temperatures (as low as −46 °C or −50 °F) when humidity levels are also low. The hot weather version of the AT (1984) is used by the National Weather Service in the United States. In the United States, this simple version of the AT is known as the heat index.

References

  1. ^ Vincent J. Schaefer; John A. Day; Jay Pasachoff (1998). A Field Guide to the Atmosphere. Houghton Mifflin Harcourt. ISBN 0-395-97631-6.
  2. ^ Eagan, C. (1964). Review of research on military problems in cold regions. C. Kolb and F. Holstrom eds. TDR-64-28. Arctic Aeromed. Lab. p 147–156.
  3. ^ *Woodson, Wesley E. (1981). Human Factors Design Handbook, page 815. McGraw-Hill. ISBN 0-07-071765-6
  4. ^ https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690003109_1969003109.pdf, equation 55, page 6-113
  5. ^ "Environment Canada - Weather and Meteorology - Canada's Wind Chill Index". Ec.gc.ca. Retrieved 2013-08-09.
  6. ^ "Meteorological Tables, Wind Chill. August, 2001 Press Release:". National Weather Service. Retrieved 14 January 2013.
  7. ^ "Wind Chill". BBC Weather, Understanding weather. BBC. Archived from the original on 11 October 2010.
  8. ^ Osczevski, Randall; Bluestein, Maurice (2005). "The new wind chill equivalent temperature chart". Bulletin of the American Meteorological Society. 86 (10): 1453–1458. Bibcode:2005BAMS...86.1453O. doi:10.1175/BAMS-86-10-1453.
  9. ^ "Calculation of the 1971 to 2000 Climate Normals for Canada". Climate.weatheroffice.gc.ca. 2013-07-10. Archived from the original on 2013-06-27. Retrieved 2013-08-09.
  10. ^ a b "NWS Wind Chill Index". Weather.gov. 2009-12-17. Archived from the original on 2011-09-18. Retrieved 2013-08-09.
  11. ^ "A chart of windchills based on this formula". Weather.gov. 2009-12-17. Retrieved 2017-04-13.
  12. ^ Tikuisis, Peter; Osczevski, Randall J. (2003). "Facial Cooling During Cold Air Exposure". Bulletin of the American Meteorological Society. 84 (7): 927–933. Bibcode:2003BAMS...84..927T. doi:10.1175/BAMS-84-7-927.
  13. ^ "The Apparent Temperature (AT) - Heat Index". Bureau Of Meteorology, Australia. Bom.gov.au. 2010-02-05. Retrieved 2018-08-01.
  14. ^ "The formula for the apparent temperature". Bureau Of Meteorology, Australia. Bom.gov.au. 2010-02-05. Retrieved 2013-08-09.

External links

2019 American Ultimate Disc League season

The 2019 American Ultimate Disc League season is the eighth season for the league. It began on April 5, 2019. The Madison Radicals enter as the defending champions after winning their first league championship in 2018. The season will mark Steve Hall's first as league comissioner.

Apparent temperature

Apparent temperature is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoor temperature. However it also applies to indoor temperatures, especially saunas and when houses and workplaces are not sufficiently heated or cooled.

The heat index and humidex measure the effect of humidity on the perception of temperatures above +27 °C (81 °F). In humid conditions, the air feels much hotter because less perspiration evaporates from the skin.

The wind chill factor measures the effect of wind speed on cooling of the human body below 10 °C (50 °F). As airflow increases over the skin, more heat will be removed. Standard models and conditions are used.

The wet-bulb globe temperature (WBGT) combines the effects of radiation, humidity, temperature and wind speed on the perception of temperature. It is not often used as the resulting figure is very location specific (e.g. cloud cover and/or wind shielding).

Extreme cold warning

An extreme cold warning is issued by Environment Canada to inform the public about cold temperatures in their region that are expected to last for at least two hours.

As of April 8, 2014, Environment Canada replaced the Wind Chill Warning with an Extreme Cold Warning. In the older system a wind chill warning for Southern Ontario and Atlantic Canada was issued when the wind chill dropped to -35. Thus a temperature of −37 °C (−35 °F) with no winds would not require a warning be issued. Under the new system the extreme cold warning is issued based on either the temperature or the wind chill being a certain value for at least two hours. The values range from −30 °C (−22 °F) in the south to −55 °C (−67 °F) in parts of the Arctic.

Freezer Bowl

In National Football League (NFL) lore, the Freezer Bowl was the 1981 American Football Conference (AFC) Championship Game between the San Diego Chargers and the Cincinnati Bengals. The game, won by the Bengals, 27–7, was played in the coldest temperature in NFL history in terms of wind chill. (The coldest in terms of air temperature was the Ice Bowl.) Air temperature was −9 °F (−22.8 °C), but the wind chill, factoring in a sustained wind of 27 miles per hour (43 km/h), was −37 °F or −38.3 °C (calculated as −59 °F or −50.6 °C using the now outdated wind chill formula in place at the time). The game was played on January 10, 1982 at Cincinnati's Riverfront Stadium, and televised by NBC, with announcers Dick Enberg and Merlin Olsen.

Gregory Jacobs

Gregory "Greg" Jacobs is an American film director, assistant director, producer, and screenwriter. He has frequently collaborated with several film directors, most notably Steven Soderbergh. Jacobs has also been operating as a director himself, having overseen projects such as Criminal, Wind Chill and Magic Mike XXL.

January–February 2019 North American cold wave

In late January 2019, a severe cold wave caused by a weakened jet stream around the Arctic polar vortex hit the Midwestern United States and Eastern Canada, killing at least 22 people. It came after a winter storm brought up to 13 inches (33 cm) of snow in some regions from January 27–29, and brought the coldest temperatures in over 20 years to most locations in the affected region, including some all-time record lows. In early February, the polar vortex moved west, and became locked over Western Canada and the Western United States. As a result, February 2019 was among the coldest and snowiest on record in these regions. In early March, the cold once again shifted east, breaking records in many areas. In mid-March, the cold wave finally retreated, but combined with above-average temperatures, precipitation, and a deep snowpack, widespread flooding ensued in the Central US.

Minnesota Wind Chill

The Minnesota Wind Chill are a professional ultimate team based in Minneapolis, Minnesota. The Wind Chill are a member of the Midwest Division of the American Ultimate Disc League (AUDL). It is the only team in the AUDL to represent a state rather than a specific city.

Narrows

A narrows is a restricted land or water passage. Most commonly a narrows is a strait, though it can also be a water gap.

A narrows may form where a stream passes through a tilted bed of hard rock lying between two softer beds: "[i]f the hard beds are vertical, so that their outcrop does not shift as erosion proceeds, a narrows is developed". Like a dam, this "raises the water level for a short distance upriver". A narrows is also typically a good location for trapping migrating fish. Furthermore, a narrows is "an important topographical feature for wind mixing", an effect where a wind chill may form ice while the surrounding temperature remains above freezing.

Particularly Dangerous Situation

A Particularly Dangerous Situation (PDS) is a type of enhanced wording first used by the Storm Prediction Center (SPC), a national guidance center of the United States National Weather Service, for tornado watches and eventually expanded to use for other severe weather watches and warnings by local NWS forecast offices. It is issued at the discretion of the forecaster composing the watch or warning and implies that there is an enhanced risk of very severe and life-threatening weather, usually a major tornado outbreak or (much less often) a long-lived, extreme derecho event, but possibly another weather hazard such as an exceptional flash flood or fire.PDS watches are quite uncommon; less than 3% of watches issued by the SPC from 1996 to 2005 were PDS watches, or an average of 24 each year. When a PDS watch is issued, there are often more PDS watches issued for the same weather system, even on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower.

Pyramid Mountain (Alberta)

Pyramid Mountain is a mountain in Jasper National Park, Alberta, Canada, named for its pyramid-like shape. James Hector named the mountain in 1859 due to its appearance from the Athabasca River valley on the eastern side of the peak.Part of the Victoria Cross Ranges in the Athabasca River Valley the mountain is in the major headwater for the Athabasca River. The peak is just under 10 km (6 mi) north-west of the town of Jasper.

The mountain is a relatively easy scramble on the eastern slopes. These slopes can be reached by following a steep fire road from the parking lot at Pyramid Lake, 4.5 km (3 mi) South-East of the peak.

Based on the Köppen climate classification, Pyramid Mountain is located in a subarctic climate with cold, snowy winters, and mild summers. Temperatures can drop below -20 C with wind chill factors below -30 C.

Rangipo Desert

Rangipo Desert is a barren desert-like environment in New Zealand, located in the Ruapehu District on the North Island Volcanic Plateau; to the east of the three active peaks of Mount Tongariro, Mount Ngauruhoe, and Mount Ruapehu, and to the west of the Kaimanawa Range.

The Rangipo Desert receives 1,500–2,500 mm (59–98 in) of rainfall per year, but resembles a desert because of a poor soil quality and drying winds, and also due to the mass sterilization of seeds during a series of violent eruptions, particularly ignimbrite flows about 20,000 years ago. The vegetation is low and sparse, consisting of mainly tussock and snow grasses. The headwaters of many small streams, which later turn into large rivers, gouge deep serrated valleys through the unconsolidated ash and pumice-rich earth. The climate here is harsh and alpine, with close to 270 ground frosts per year in comparison with less than 30 in the coastal regions of Hawke's Bay, 80 km (50 mi) to the east. Heavy snowfalls - rarely seen in the rest of the island - are also a common occurrence in winter. Trampers and climbers in the area should be mindful of the extreme chill effect of the cold south wind which can produce wind chill factors lowering the temperature below zero for days on end.

Much of the desert lies at an altitude of over 600 m (2,000 ft), and a considerable proportion of it is over 1,000 m (3,300 ft) above sea level.

Due to the unproductive nature of the land and the extreme winter climate, the region is largely uninhabited. The town of Waiouru, with its army camp, lies to the south and much of the southern part of the desert is used for training purposes. To the north of the desert lies the Rangipo prison farm.

Many of the North Island's largest rivers have their headwaters in the area, particularly around the slopes of Mount Ruapehu, the North Island's highest mountain. These include the Waikato and Whangaehu Rivers, as well as major tributaries of the Rangitikei and Whanganui Rivers.

The desert is bisected by only one sealed road, a section of State Highway 1 known as the Desert Road. The road is closed for short periods most winters with barriers arms due to severe snow storms and icy road conditions. Turangi emergency services monitor the northern part of the Desert Road and the NZDF Military Police at Waiouru is responsible for the southern end.

The Lord of the Rings films were shot in New Zealand, and the Black Gate of Mordor scenes were shot in the Rangipo Desert in 2000.

Severe weather terminology (United States)

This article describes severe weather terminology used by the National Weather Service (NWS) in the United States. The NWS, a government agency operating as an arm of the National Oceanic and Atmospheric Administration (NOAA) branch of the United States Department of Commerce (DoC), defines precise meanings for nearly all of its weather terms. This article describes NWS terminology and related weather scales used by the agency. Some terms may be specific to certain cities or regions.

Wind Chill (film)

Wind Chill is a 2007 US-British horror film directed by Gregory Jacobs and starring Emily Blunt and Ashton Holmes. The film was produced by the British Blueprint Pictures company, and George Clooney and Steven Soderbergh's joint company Section Eight Productions supported the project financially. The filming began in the Vancouver area on February 1, 2006, and continued until March. The completed film opened in limited distribution in April 2007 in the US, was released in the United Kingdom and Ireland in August 2007, but went directly to DVD in most other markets.

Wind chill (disambiguation)

Wind chill is a meteorological effect.

Windchill may refer to:

Wind Chill (film), 2007 horror film

Windchill (software), PLM application

Windchill (G.I. Joe), a fictional character in the G.I. Joe universe

Wind chill advisory

A Wind Chill Advisory is issued by the National Weather Service of the United States when the wind chill is low enough that it poses a threat to human health and life if adequate protection is not taken against hypothermia and frostbite. The exact definition varies from state to state, and areas prone to colder temperatures will often require the wind chill to be lower before issuing an advisory.

Wind chill warning

A wind chill warning is issued by Environment Canada or the National Weather Service of the United States when the wind chill is low enough that it becomes life-threatening.

In the United States, the exact definition varies from state to state or between National Weather Service county warning areas, and a warning is used to express more severe conditions than a wind chill advisory. If going outside, people should make sure to take extra precaution against hypothermia and frostbite by wearing multiple layers as well as a hat and gloves.In the event that extreme wind chills are expected to quickly lead to frostbite or death, enhanced wording with the words particularly dangerous situation may be added to the text; this is rarely issued. The National Weather Service in Twin Cities/Chanhassen was the first office to do this on Sunday, January 5, 2014.As of April 8 2014, Environment Canada replaced the Wind Chill Warning with an Extreme Cold Warning. The warning is still issued based on a region's normal climate. In the older system a wind chill warning for Southern Ontario and Atlantic Canada was issued when the wind chill dropped to -35. This meant that if the temperature was −37 °C (−35 °F) with no wind a warning was not issued. Under the new system the extreme cold warning is issued based on either the temperature or the wind chill being a certain value for at least two hours. The values range from −30 °C (−22 °F) in the south to −55 °C (−67 °F) in parts of the Arctic.

Wind chill watch

A Wind Chill Watch is issued by the National Weather Service of the United States when the wind chill could reach dangerous levels within the next 12 to 48 hours. The exact definition required to issue a watch varies from state to state or from National Weather Service county warning areas to another, but if forecasters believe conditions are favorable for life-threatening wind chills meeting local criteria, a watch will be issued. People going outside should plan to protect themselves against hypothermia and frostbite.

Windbreak

A windbreak (shelterbelt) is a planting usually made up of one or more rows of trees or shrubs planted in such a manner as to provide shelter from the wind and to protect soil from erosion. They are commonly planted in hedgerows around the edges of fields on farms. If designed properly, windbreaks around a home can reduce the cost of heating and cooling and save energy. Windbreaks are also planted to help keep snow from drifting onto roadways or yards. Farmers sometimes use windbreaks to keep snow drifts on farm land that will provide water when the snow melts in the spring. Other benefits include contributing to a microclimate around crops (with slightly less drying and chilling at night), providing habitat for wildlife, and, in some regions, providing wood if the trees are harvested.

Windbreaks and intercropping can be combined in a farming practice referred to as alleycropping. Fields are planted in rows of different crops surrounded by rows of trees. These trees provide fruit, wood, or protect the crops from the wind. Alley cropping has been particularly successful in India, Africa, and Brazil, where coffee growers have combined farming and forestry.A further use for a shelterbelt is to screen a farm from a main road or motorway. This improves the farm landscape by reducing the visual incursion of the motorway, mitigating noise from the traffic and providing a safe barrier between farm animals and the road.

The term "windbreak" is also used to describe an article of clothing worn to prevent wind chill; this term is favored by Europeans whereas Americans tend to use the term "windbreaker".

Fences called "windbreaks" are also used. Normally made from cotton, nylon, canvas, and recycled sails, windbreaks tend to have three or more panels held in place with poles that slide into pockets sewn into the panel. The poles are then hammered into the ground and a windbreak is formed. Windbreaks or "wind fences" are used to reduce wind speeds over erodible areas such as open fields, industrial stockpiles, and dusty industrial operations. As erosion is proportional to wind speed cubed, a reduction of wind speed of 1/2 (for example) will reduce erosion by over 80%.

Windbreaker

A windbreaker, or a windcheater, is a thin fabric coat designed to resist wind chill and light rain, making it a lighter version of the jacket. It is usually of lightweight construction and characteristically made of a synthetic material. A windbreaker often uses elastic waistbands, and/or armbands, and a zipper to allow adjustments for the current weather conditions.

Regular jackets, coats, etc. may include a type of windbreaker as an interlining that can be removed when desired. Windbreakers sometimes include a hood that may be taken of removable and/or stowable. Many windbreakers may also include large pockets on the inside or the outside which allows belongings to be covered from weather such as light wind or rain as mentioned above. Windbreakers offer light to moderate insulating protection, more so than a sweater, but less than an overcoat.Windbreakers are primarily worn during the warmer seasons when wind or rain are expected, or as part of a layering strategy during colder seasons. Brightly colored windbreakers may also be worn by runners as protection from the weather, and as a reflective garment used for safety. A 2012 study demonstrated that adding windbreaker pants and jackets offer a lightweight but effective means of delaying hypothermia if the user is outside walking and encounters unexpected low temperatures.

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