In summer, is it cooler to wear black or white clothes?
I have been asked this many times and the answer is that, as almost everybody actually knows, it barely makes any difference. But why? We all know that black objects absorb sunlight more quickly than white objects and so heat up more quickly. So surely black clothes must be a bad idea? Well, No. But the reasons are complicated.
Firstly, the ‘object’ that we care about here is not usually an ‘object’ but a human body. And human bodies actually produce heat all the time (see here for amazing fact about how much heat we produce!). Typically, a human being who is not asleep produces roughly 100 watts of heat as they sit around. And this must be removed in order to keep the person alert and healthy, and prevent their internal temperature rising above the magic 37 °C at which all mammals flourish.
How do people cool themselves? Well, we sweat, and the evaporation of water from our skin cools us:
- Evaporating 100 millilitres of water (≈ a glass of water) in one hour cools a person by around 50 watts – wow!
You haven’t mentioned clothes yet! No. Well, clothes thermally isolate our bodies from the external air temperature.
In winter, when the air temperature is low (say 10 °C), we wear extra clothes to keep us warm i.e. in order to keep a big difference in temperature between the inside of our clothes, and the outside. Clothes do this by trapping the air which would otherwise carry away our body heat trapped in humid air.
In summer, when the air temperature is high (say 25 °C), things are more complicated and we need to distinguish several quite distinct situations
- No sunshine. Now the colour of clothes doesn’t matter (obviously!). Air in contact with our skin cools us, but not by much. If the air moves over our skin it removes heat and moisture much more effectively, which is why even a gentle breeze on a warm evening feels so pleasant.
- Full sunshine. No Clothes (e.g. on the beach) Full sunshine amounts to a heat input in excess of 1000 watts per square metre. So, over the surface area of a human body the heat input is enormous, perhaps 300 watts. Your body begins to sweat profusely to remove the heat – it requires around 600 millilitres per hour (roughly 1 English pint) to achieve this.
- Full sunshine. Close-Fitting Clothes. Now the sunshine heats your clothes, which in turn heat you. So, in terms of direct heating, black clothes (think – for example – of a close-fitting leather catsuit as worn by Diana Rigg ) then black clothes would definitely be warmer than white clothes.
- Full sunshine. Loose-Fitting Clothes Now things are not so straightforward. Black objects will become hotter that white objects, but if the clothes are not in direct contact with your skin, this does not directly translate into more heating. Why not? Well, hotter objects heat the air next to them, and so cause stronger convection and so cause more air to pass over the skin. And this increased air flow can be more effective at removing heat and humid (sweaty) air. Additionally many white clothes are often not completely opaque, and so let (say) 10% of the sunlight through which then directly heats your skin.
So to summarise, the main thermal effect of clothing is to alter the rate of air flow over our skin. If the clothes are loosely fitting then the colour barely matters. And just in case the English summer develops into something substantially warmer than it has been recently, please do bear in mind the advice of that great scientist Noel Coward – only Mad Dogs and Englishmen go out in the midday sun.