Why does blowing on hot soup cool it faster?

A thin layer of fresh steam and warmed air sits over hot soup. While that layer stays put, water at the surface has a hard time escaping as vapor. When you blow, the layer is pushed aside and less humid air moves in.

Because the breeze keeps replacing the air, warmed air also carries the broth's heat away faster. It's the same reason standing in front of a fan feels cool.

Turning water into vapor takes a lot of heat. The heat 1 g of water takes when it becomes vapor at its boiling point is more than five times what it takes to warm that same 1 g from 0°C to 100°C. That heat comes from the broth's surface, so the surface cools fast. It's the same thing that makes you feel cool when sweat dries.

The hotter the soup, the more steam it gives off, and the bigger the share of cooling evaporation takes. Put a lid on, by contrast, and the steam is trapped, so the soup stays warm longer.

Your breath is in the mid-30s °C, cooler than soup above 70°C, but that temperature gap alone wouldn't cool it this fast. On top of that, your breath mixes with surrounding air before it reaches the soup and gets even cooler.

Breathing 「hah」 on the back of your hand feels warm while blowing 「hoo」 feels cool, and that's not a difference in breath temperature either. Blowing fast through pursed lips pulls in surrounding air to mix in, and it dries the moisture on your skin faster.

The breeze can't reach the bottom of the bowl. So blowing on the whole bowl cools only the top for a moment, and as soon as you stir, the hot broth below rises again. Blow on a small spoonful instead, and with so little liquid it soon reaches a good temperature. If the soup is shared, this is also more hygienic.

To cool a whole bowl fast, ladle it into a wide, shallow bowl and stir now and then. The larger surface lets steam escape and it cools much faster. Ice is quicker still, but you'll have to accept a blander soup.