Drafts under clothing. Why you can catch a cold while lying under a warm blanket

Continuing the drafts topic.
Few people know this, but drafts UNDER clothing (or under blankets) can be just as harmful as
regular drafts. The physical mechanism of the phenomenon is simple: warm air is lighter than cold air, so it always tends
to rise. Including from under clothing (blankets).

Accordingly, cold air penetrates from below to replace it, creating a classic draft (the draft in a chimney works on the same principle).
Within reasonable limits, this is only beneficial, as such ventilation is needed to remove excess water vapor from the body.
However, it is not always possible to stay within reasonable limits. Let’s estimate on our fingers the speed of the air flow in such a draft under clothing.

I won’t bore you with the formula, those who remember their school physics course can easily derive it themselves You need to calculate the work of the Archimedes force for a small volume of air along the path of the air flow and equate it to the kinetic energy of the same volume. Taking into account that the difference in density between the heated and cold gas is proportional to the absolute temperature. Friction losses at such gas speeds can be neglected in a first approximation., for the rest I will simply provide the result:

V = SQR(2g∆TH/T)
Where g — acceleration due to gravity, T — absolute body temperature ∆T — difference in temperatures between outside air and air under clothing, H — height to which the gas rises passing under clothing the full path.

Substituting T=310, g=9.8, H = 0.5 ∆T = from 5°C to 2°C (in our model, the air under clothing passes through
quickly enough and does not have time to heat up to body temperature), we have:
V= from 40 to 20 cm/sec

The obtained numbers are more of an upper estimate, we did not consider friction, and the path of the air under clothing may not be straight. Nevertheless, from the estimate provided, it is clear that under certain circumstances, a draft under clothing can make a significant contribution to the local cooling of your skin. After all, the intensity of moisture evaporation from the surface at zero air speed and at 0.2 m/s can differ by 2-3 times.And therefore, the rate of skin cooling will also differ. If drafts in your clothing are a constant occurrence, and if you lead a sedentary lifestyle, this may be sufficient to develop local inflammation with a not very strong immune system.

For more details on how and why drafts affect health, see a separate article: “Thermal comfort. Why even healthy people are better off avoiding drafts”.

How to deal with drafts under clothing?

Nothing complicated, all the remedies have been known for hundreds of years:

• tight belt
• buttoned collar and cuffs
• warm, well-fitting underwear, warm socks

By the way, precisely because of possible drafts under clothing, a sleeping bag is preferable for camping than a regular blanket, and for people with weakened immunity, it is better to go to bed in pajamas, even if they plan to bundle up warmly.