Can’t Stand the Heat?

On with this trend of summer-related posts. If you have been in a dark-coloured car on a hot summer’s day, you may find that you’re roasting inside, whereas you may feel slightly cooler in a light-coloured car. Physics can help explain why.

Heat can be transferred in three ways: conduction, convection, and radiation. All objects continually emit and absorb heat by infrared radiation. If an object is hotter than its surroundings, it will emit more heat than it absorbs, so it will gradually cool down. Likewise, if an object is cooler than its surroundings, it will absorb more heat than it emits, so it will gradually warm up. The greater the difference in temperature between the object and its surroundings, the more quickly heat will be transferred. For example, a hot object will cool down more quickly in a refrigerator than at room temperature.

As a general rule, dark surfaces are more efficient at emitting and also at absorbing radiation, whilst light surfaces are poor at emitting radiation and more efficient at reflecting radiation away.

Heat from the Sun is transferred by radiation. Because a car is usually cooler than its surroundings in direct sunlight, how efficiently the car absorbs radiation will determine how quickly it will heat up. A dark-coloured car will generally absorb radiation more efficiently than a light-coloured car, and the difference in temperature can be up to several degrees Celsius. Therefore energy can also be saved from not needing to use your air conditioning system as often.

This reasoning also explains why you see few dark-coloured parasols.

There is a myth that white clothing is generally keeps you cooler in the summer. But then surely the Bedouins who pace through the desert every day will roast in their black clothing?

There is some truth to this myth, but it is also incomplete. With clothing, it is a little more complicated because many factors are involved. These include how the intensity of the radiation from the Sun compares with that from your body (which will change if you step into shade); whether there is wind or not; the material the clothing is made out of, etc.

It is true that light colours will reflect radiation more efficiently, but this can also work against you on the inside by reflecting the radiation emitted by your body back to yourself. While dark colours are more efficient at absorbing radiation, they are also more efficient at emitting radiation, including that from your body. However, some general results can be concluded.

Firstly: it is almost certain that tight, thin, black clothing is a terrible idea unless you want to be cooked. The black clothing will absorb radiation very efficiently, heat up and stick to your skin. If you do wear black, it needs to be loose.

Secondly: loose, black clothing is not as bad an idea as you think. It can be better at keeping you cool than white if there is wind, because the wind is able to convect away the radiation emitted towards your body. This may be a reason why some desert dwellers wear loose, black clothing.

Thirdly: if there is no wind and you are mostly in direct, intense sunlight, then loose, white clothing is usually better since the benefit of being able to reflect away radiation outweighs the trapping of heat on the inside.

Otherwise, the differences between light and dark clothing is insignificant and often vary.

Theoretically, if the clothing had white on the outside and black on the inside, it would be very efficient at keeping you cool. But I don’t see any being sold out there unfortunately (I’m sure it would be the new fashion trend).



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