Heat index - apparent temperature in high humidity and heat

Air temperature

Outdoor sports don’t always take place in the cool mountain regions of Central Europe. Anyone who has travelled in the tropics could tell you a thing or two about how much the extreme humidity and heat can really wear you down.

But even in the lowlands of Europe, you can experience heat and humidity so bad that sweat pours off your body and the heat weighs you down. This increase in the apparent temperature compared to the actual temperature is represented by the so-called Heat Index.

What is behind the heat index?

The Heat Index is effectively the inverse of the wind chill, which gives an indication of the influence of the wind on the apparent temperature. However, the determining factor on the Heat Index is not the wind, but rather the relative humidity.

The Heat Index is used as a measure of the expected stress on the body, resulting from high humidity in combination with high temperature. This is of course only a guide, since the subjective experiential temperature can be influenced by many factors, including background, age, weight, size, gender and health.

How is the heat index calculated?

For temperatures under 20°C the humidity has no influence on the apparent temperature. A noticeable shift in temperature perception only happens from around 27°C, which is why the Heat Index is only applied from this point. The minimum relative humidity is 40%.

The temperature, which is calculated using a relatively complex equation, is then used as a reference for the apparent temperature and has been converted into a warning table, which makes it easy to see at a glance how great the danger of heat stroke is for the human body:

Warning Table for Heat Illness
Heat Index Health Danger
27 - 32 °C Signs of exhaustion possible with prolonged exposure and physical activity.
33 - 40 °C Sun stroke, heat cramps and fainting possible.
41 - 54 °C Sun stroke, heat cramps and fainting likely. Heat stroke possible.
> 54 °C High risk - high likelihood of heat stroke and sun stroke!

The formula for calculating the heat index (HI):

HI = c₁ + c₂T + c₃φ + c₄Tφ + c₅T² + c₆φ² + c₇T²φ + c₈Tφ² + c₉T²φ²

Where T represents temperature in °C or °F and φ is the relative humidity as a %. The c values are constant parameters dependant on the temperature inputs:

Parameters for HI calculation
Parameter in °C in °F
c₁ -8,785 -42,379
c₂ 1,611 2,049
c₃ 2,339 10,143
c₄ -0,146 -0,225
c₅ -1,231 ⋅ 10⁻² -6,838 ⋅ 10⁻³
c₆ -1,642 ⋅ 10⁻² -5,482 ⋅ 10⁻²
c₇ 2,212 ⋅ 10⁻³ 1,229 ⋅ 10⁻³
c₈ 7,255 ⋅ 10⁻⁴ 8,528 ⋅ 10⁻⁴
c₉ -3,582 ⋅ 10⁻⁶ -1,990 ⋅ 10⁻⁶

How can I protect myself from heat?

hat for heat protection

Hats with wide brims, Sunglasses and a cloth on the neck to protect the head; you should of course also ensure that you drink enough water – and preferably isotonic liquids with salt, to replenish the minerals lost through perspiration, Drinking low-sodium water can in the long run lead to so-called water intoxication. Individual fluid requirements can also be determined using our water requirement calculator.

Heat illnesses: Types, symptoms and procedures

Sun stroke - too much time in the sun

Most of us have had this experience at some time or other: After a long day in the sun you start to feel dizzy, nauseous and headachy. The diagnosis is simple - it must be sun stroke. Most of the time this diagnosis is correct. Sunstroke always occurs when the head and neck are exposed to hot sun for extended periods of time.

This causes irritation of the cerebral membrane and brain tissue. In severe cases it can also lead to cerebral oedema. As well as the symptoms mentioned above, you may also experience vomiting, tinnitus, light-headedness, increased pulse, neck stiffness, and in extreme cases a loss of consciousness and cardiovascular failure are possible. Fatalities have been known in some cases.

In mild cases, the first thing to do is to get out of the sun. The symptoms generally subside after a few hours. Affected persons should lie on their back with their head slightly elevated. The head and neck can be cooled using a damp cloth or cold pack (always wrap a cold pack with a layer of fabric, to prevent frost bite). If the symptoms do not resolve, it is advisable for affected persons to go to hospital.

Heat stroke - overloading of the body's heat regulation

Heatstroke is most likely to occur when the body is exposed to high temperatures for long periods of time, or after excessive physical exertion in hot conditions. It can also be caused by severe dehydration, if fluid levels are not replenished. Dehydration causes sweat production to fail, meaning that the body can not get rid of enough heat. This leads to a build up of heat, causing the core temperature of the body to rise.

Typical symptoms are increased pulse rate, dizziness and nausea. The skin is dry, hot and sometimes also reddened. The blood pressure is initially normal, but drops over time. Shock can cause the circulation to slow, which means that the organs are not provided with enough blood. If the brain is affected, consciousness can be impaired and in extreme cases death may occur.

Affected persons should be taken to a cool place as quickly as possible. Often a shady spot under a tree or sun-shade will suffice. Active cooling can be effected using cold packs (not applied directly onto the skin) and damp wrappings on the arms, legs, neck and groin. If the temperature is very high, an ice-water bath can even be life saving. In any case removing the clothes will help.

Moving the person into the shock position is recommended: lie the affected person on their back with their legs elevated. If the person is conscious, hydration should be given slowly, in order to balance the loss of fluids. The ideal fluid temperature is lukewarm, so that it can be quickly absorbed by the body. Alcohol is of course strictly forbidden. Tea, water and weak fruit juices are ideal.

Drinking to compensate dehydration
Dehydration can be life-threatening

Heat cramps - electrolyte deficiency

Along with sun stroke and heat stroke, there are other possible complications, for example heat cramps, which are caused by an excessive loss of electrolytes through sweat, especially sodium chloride aka table salt. The symptoms are cramps and circulation problems. Taking strongly seasoned broth or electrolyte drinks is recommended, since the consumption of tap or mineral water can lead to water intoxication.

Heat syncope (fainting) - hypotension

Fainting or heat syncope occurs when the peripheral blood vessels of the arms and legs dilate due to heat. Blood then collects in these regions, causing the blood pressure to drop. This leads to a feeling of weakness, which is usually short lasting. Warning signs are dizziness, nausea and feelings of weakness. Fainting most often occurs under the influence of alcohol or in large crowds. Affected persons should remove themselves from the situation and get to a shaded place.

Heat exhaustion - dehydration

When extreme dehydration and electrolyte deficiency are not compensated by adequate fluid intake, this can often lead to heat exhaustion. The blood volume decreases and the body cannot be supplied with enough blood. Headaches, dizziness and loss of consciousness can follow. Further symptoms are a racing pulse and low blood pressure, quick and shallow breathing and skin which is at first reddened and later pale and clammy.

Affected persons should seek out a cool and shaded position. Unconscious persons should be placed in a stable position on their side and persons who are still conscious can be placed in the shock position. Lukewarm, electrolyte enriched liquid can be given to compensate for the fluid loss. However, liquid should be given slowly, to prevent choking.

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