# Why does blowing air over hot wet surface makes it dry faster?

After removing the boiled water from a hot kettle, which is wet, the water evaporates rapidly if air is blown over it. What is the science behind this phenomenon?

This is basically the same thing that happens with convective heat transfer. The main mechanism for water escaping from the surface is diffusion. Removing moist air and replacing it with drier air increases the concentration gradient of water in the air near the surface and, by Fick's first law, increases the diffusive flux (basically, @Martin's mechanism). In the case of convective heat transfer, removing hot air and replacing it with cooler air increases the temperature gradient in the air near the surface, and, by Fourier's law of heat conduction, increases the heat flux.

We know that moving air has a lower pressure than static air. So, when you blow over a hot wet surface, you create an area of lower pressure than usual(static air). This creates an ease for the water, or any liquid on the surface, to readily convert to gaseous form. But keep in mind, this blowing action not only lowers the pressure, but also cools the hot surface, so if incase the hot surface has a means to keep hot, this process will eventually stop the liquid from evaporation all together.

Also, the use of pressure cookers work in the same way, but vise versa. By creating a high pressure environment and not allowing the water to easily convert into a gas, a higher over all temperature is attained, more than the water's boiling point. this allows the food to cook quicker.

• Another process that might add to this: the plate will have a high humidity. When it is nearly saturated, evaporation becomes more difficult. By blowing on the plate, you exchange the moist air above the plate by dry air, allowing a better evaporation rate. Dec 11, 2016 at 11:08
• Moving air has NOT always a lower air pressure. The air pressure drops, if the air is accelerated due to pressure changes. The answer looks wrong to me. What Martin suggested seems much more reasonable. Dec 11, 2016 at 12:33
• When you blow air in open atmosphere the pressure is always atmospheric. Bernoulli equation is inapplicable here since viscous effects on the flow are significant.
– Deep
Dec 12, 2016 at 5:47
• I'm skeptical that the mechanisms you describe dominate over the simple fact that the newly arriving air is drier. Can you provide any references? Dec 12, 2018 at 7:31

This is because your breath is warmer and causes the water to heat up and evaporate quicker.

It can also be because moving air has more kinetic energy which it passes on to the water, causing it to evaporate.

• Hi, welcome to Physics! I have combined your two answers together; if you would like to clarify further, use the edit link under the question to make more changes. Note that most folks in this community like answers with a little more justification than what you have here; if you keep hanging out here and reading good answers, you'll start to see what I mean.
– rob
Dec 12, 2018 at 7:12