Question

Why does the air we blow/exhale out from our mouths change from hot to cold depending on the size of the opening we make with our mouth?​

Answer 1

Answer:

It is mainly due to air entrainment.

If you blow through a tight mouth, there is smaller volume of air but a higher velocity. This pulls in and mixes with a lot of ambient air (Venturi or Bernoulli effect) - in fact typically the air stream is only 40% body warmth and 60% ambient so it will be markedly colder. As an experiment you pucker and blow through a tube held to your mouth, this excludes the ambient air and you will get reduced airflow but at the higher temperature again.

With a wide mouth there is hardly any air entrainment. Its temperature will be almost same as its temperature in your lungs which is higher than the ambient temperature.

Reference:

https://physics.stackexchange.com/questions/7868/why-does-the-air-we-blow-exhale-out-from-our-mouths-change-from-hot-to-cold-depe