Why is my breath hot when i open my mouth all the way, but cold if I blow like a whistle?

Question

I observed this as when I want to cool my soup I blow like a whistle and when I want to warm my hands I open my mouth more as my breath is warmer now.

Answer 1

Your breath is the same temperature either way. The difference is how much ambient air is brought along with the breath by the time it reaches the object.

Emitting a thin and fast stream of air will cause a lot of other air to follow along with it. When you are blowing on the soup to cool it, what you're really doing is using your breath to move a lot of ambient air, which is what cools the soup.

When you breath on your hands to warm them, you are emitting a wide and slow stream and hold your hands close to it. The result is that your hands get mostly that original wide and slow stream without a lot of ambient air mixed in yet.