Heat, Cold and Wind Chill: How the Body Keeps 37 °C

Imagine you walk outside

Imagine you step out into a hot noon and, hours later, into a frosty evening. In both cases your body works hard to keep one number steady. Normal body temperature is close to 37 degrees Celsius, with a healthy range of about 36.1 to 37.2. A control centre in the brain watches it like a thermostat. If you get too warm, it opens the cooling system, and if you get too cold, it turns on heating. A change of only a few degrees in the core is already a serious matter. The figure in the picture shows both reactions.

Sweat is a cooler

In the heat blood vessels in the skin widen and you begin to sweat. The sweat does the cooling only when it evaporates, because turning a liquid into vapour takes heat from the skin. One litre of sweat that evaporates carries away roughly 2,400 kilojoules, about 580 kilocalories. Heat exhaustion shows as heavy sweating, weakness and nausea: rest in the shade and drink. Heat stroke is an emergency. The core temperature rises above 40 degrees, the person may be confused, and an ambulance must be called at once.

Shivering and cold hands

In the cold the body does the opposite. Blood vessels in the skin narrow, so the blood stays near the vital organs and your fingers and nose go pale and cold. Muscles start to shiver, and that quick shaking makes extra heat. If the core temperature falls below 35 degrees, it is hypothermia. A person becomes slow, clumsy and sleepy, and needs warmth, dry clothes and medical help. The best protection is simple: dress in dry layers and keep the wind off your skin.

Wind chill: a feeling with a formula

Wind carries away the thin layer of warm air next to your skin, so the same cold feels harsher. The wind chill formula, used in North America and the United Kingdom, is Twc = 13.12 + 0.6215 T − 11.37 v^0.16 + 0.3965 T v^0.16, with T in degrees Celsius and v in kilometres per hour. Worked example: T = −10 and v = 30 give about −20. So a frost of −10 with a 30 km/h wind steals heat like −20 in still air. The formula is meant for air at 10 degrees or below and wind above 4.8 km/h.

Humidity: when heat gets sticky

In hot weather the danger grows with moisture. Damp air takes up sweat badly, so the cooling system works worse. Weather services publish a heat index for this feeling. For example, 32 degrees with 70 per cent humidity feels like about 40 degrees to the body. Change the temperature, the wind and the humidity in the sandbox and see what the body does. Remember that wind chill and heat index describe how it feels to people, not the temperature a thermometer reads.

The record places

The coldest air temperature ever measured on Earth was −89.2 degrees Celsius at Vostok station in Antarctica on 21 July 1983. For the hottest, the record book lists 56.7 degrees at Furnace Creek in Death Valley, USA, on 10 July 1913. This value is disputed: some researchers think the old measurement was too high. Both extremes are far outside the comfortable range of the human body, and people survive there only with special clothing, shelter and care.

In heat and in cold