What is freezing weather

Wet and cold, ice cold, felt cold

Charles R. Darwin describes the felt temperature

by Holger Westermann

“The climate was certainly pathetic: the summer solstice was now over, but every day snow fell on the mountains and there was rain in the valleys, accompanied by sleet. The thermometer was generally at 7 ° but dropped to 3 ° or 5 ° at night. Because of the humid and turbulent state of the air, which was not cheered up by any rays of sunshine, the climate was felt to be even worse than it actually was. ”Is the diary entry of the 24-year-old naturalist Charles Darwin on December 25, 1833 about his stay in the wigwam -Bay, "a cozy little harbor (sheltered anchorage) not far from Cape Horn", Tierra del Fuego. This vividly describes the phenomenon of the perceived temperature, the particularly grim perceived cold.

People's perception of temperature does not only determine the air temperature. Strictly speaking, people lack the ability to sense absolute temperature. This is also not important for them, because the body temperature is around 37 ° C at all times; about 0.5 ° C lower at night and morning, about 1 ° C higher in the afternoon and evening (fertile women have a body temperature of 0.5 ° C higher one to two days after ovulation). For a functional and long-term healthy body, it is important to maintain this optimal operating temperature. To do this, the body needs to know its current body temperature. They measure visceral thermoreceptors in the brain (regio praeoptica of the hypothalamus). Second, it is important to know how much body heat is being given off into the environment. This is measured by the peripheral thermoreceptors in the skin (cold sensors active <30 ° C and> 45 ° C are located below the epidermins; warm sensors active> 36 ° C are located in the dermis). The exact ambient temperature is not biologically relevant at all - only the heat loss.

Heat is created in the body through muscle work and the activity of the organs, especially the liver and intestines. With intense physical exertion, the core temperature of the body can rise to 40 ° C. Then optimal heat dissipation is vital. The body tolerates overheating much worse than excessive cooling. If the temperature is 5 ° C too high, there is a risk of death by heat, while a 5 ° C drop in core body temperature only provokes slight to moderate, reversible impairments. There is a danger to life from a temperature of 8 to 10 ° C inside the body. In contrast, extremities, especially hands and feet, can cool down to 22 ° C without being damaged, apart from a marked discomfort.

The most important medium for transporting heat is the blood flow. If the body temperature is to drop, the veins widen, which means that more blood flows under the skin and more heat is released into the environment. If the difference to the ambient temperature is small, the effect remains small even with maximally expanded cores. Then you start to sweat. The water coating evaporates, whereby the skin surface cools and thus the temperature difference increases, the blood is cooled again. When it is humid, however, the ambient air is already saturated with water vapor and sweat runs down the body without any cooling effect. This is particularly noticeable when the body temperature rises when the muscles are exerted and thick, warm clothing creates a humid microclimate on the skin. Then heat build-up and overheating can occur even in winter.

Under normal conditions and in appropriate clothing, the heat must be retained in the body when it is cold. Then the veins contract and reduce the blood flow in the skin and thus the loss of heat. If the air is not moving, when there is no wind or under multi-layered clothing that is permeable to water vapor, a warmed insulating layer is formed, which reduces further heat emission. On the other hand, high humidity and rapid exchange of air close to the skin in strong winds increase heat loss - this is also registered by the cold receptors in the skin. The physiologically effective temperature drops well below the thermometer value. In the sunshine, on the other hand, the radiant heat weakens the feeling of cold.

Charles Darwin described exactly this effect so clearly during his week-long stay at Cape Horn. The HMS Beagle stayed in this inhospitable sea area, notorious for its freezing storms and hurricanes and persistent rainfall, to explore safe sea routes and improve sea maps by surveying the coast. Nowadays, the perception of temperature is represented by the "felt temperature". It compares the influence of all weather properties (or the indoor climate conditions) on the feeling of warmth, comfort or coldness with a reference air temperature under standard conditions in the shade with a slight wind of 0.2 meters per second. The German Weather Service (DWD) has developed the Climate Michel model for this. This "Michel" is a 35-year-old man who is 1.75 m tall and 75 kg of body mass and moves with little effort (4 km / h walk). The clothing is chosen appropriately for the thermal conditions. Relevant influencing variables for a discrepancy between the sensed and measured temperature are:

  • Air temperature
  • Humidity
  • Air movement / wind speed
  • solar radiation
  • Indirect radiation; Thermal radiation from the ground, buildings, water, ...

For example, cold and wet weather makes it shiver when the wind drives the rain in gusts over the landscape at low air temperatures. A body reaction that corresponds to a temperature below 0 ° C can also be felt at + 10 ° C. If the air is really frosty cold, the influence of humidity disappears. Ice-cold air hardly contains any water vapor, it is dry. Therefore, with little air movement, sunshine is perceived as warmer at -3 ° C than + 3 ° C in the rain. But the harder the wind blows, the faster the perceived temperature falls. Compared to this, the wind chill or wind cooling effect, the other parameters lose their relevance.

The wind chill temperature is the air temperature that would have the same cooling effect without wind. If at a temperature of 0 ° C a moderate wind blows around 25 km / h, the temperature seems like -6 ° C. At -10 ° C it is already -19 ° C. When the wind is even stronger, the difference between the thermometer value and the perceived temperature increases and at 60 km / h wind (strength 7 Beaufort) it also reaches over 15 ° C in this country (0 ° C with no wind then corresponds to about -9 ° C; -10 ° C already - 23 ° C and at the currently announced -20 ° C, life-threatening -37 ° C can be reached). This cooling effect is felt to be very pleasant in summer, but the risk of sunburn often goes unnoticed.

In the open air, a perceived temperature between 0 and + 20 ° C is considered to be “comfortable” or “comfort zone”, as appropriate clothing can optimally regulate the heat balance. If it is colder or warmer, people suffer from cold stress or heat stress. If the perceived temperature is outside the comfort range, there is a risk to health. If the deviation is small, people with significant prior exposure are primarily affected, for example patients with cardiovascular diseases such as angina pectoris or cardiac insufficiency. People sweat a lot in the heat and the loss of water thickens the blood, the flow properties deteriorate, the heart has to pump harder and the risk of thrombosis increases. In contrast, when it is cold, the veins contract and if the amount of blood remains the same, the blood pressure and thus the risk of heart attack increase as a result.

For physiological effects, for the body's reaction to weather and thermal influences, only the perceived temperature is relevant. Therefore, the forecasts published by the German Weather Service (DWD) and the Central Institute for Meteorology and Geodynamics (ZAMG) from Vienna are always based on this temperature value. Even in human weather, the comfort zone between 0 and 20 ° C is a broad normal range, while the cold and warm stimuli are divided into 5 to 10 ° C levels. What is even more difficult to describe and take into account in the prognoses for the individual diseases is the influence of the weather on the mood and thus on the general feeling of health.

Charles Darwin wrote in his travel diary on July 23, 1834 in the Bay of Valparaiso (at that time the most important seaport in Chile): “When the morning came, everything looked wonderful. After Tierra del Fuego the climate seemed quite delicious - the air so dry and the sky so clear and blue in the brilliant sunshine that nature sparkled with life. (…) There are many very beautiful flowers, and the plants and bushes, as in most dry climates, have a strong and distinctive scent - even the clothes smelled through them when you passed through them. I kept marveling that day after day was as beautiful as the previous one. What a great influence the climate has on the joy of life! How contrary is the sensation when you see black mountains that are half shrouded in clouds and another chain through the light blue haze of a fine day! One can be very sublime, the other is all serenity and a happy life. "

Swell:

Darwin, C.R. (1845): The Voyage of the Beagle - Diary exploring the natural history and geology of the countries visited on the voyage of HMS Beagle under the command of Captain Robert Fitz Roy, RN. 2nd edition, translated by Eike Schönfeld for Mare Buchverlag, 2007.

M.Sc. Met. Stefan Bach: The felt temperature. Topic of the day, newsletter of the German Weather Service (DWD) from January 8th, 2018

Dipl.-Met. Christian Herold: Perceived temperature. Topic of the day, newsletter of the German Weather Service (DWD) from February 23, 2018

 

Photo: Leonhardikirche Siegertsbrunn, Monday morning, February 26th, 2018, 9 a.m., -13 ° C

Created on February 25, 2018
Last updated on March 4, 2018

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