Joule Thomson Effect
Air under pressure expands suddenly and lowers the temperature.
Bring your palm in front of your face. Blow air from your mouth on the palm, keeping your mouth wide open. Your palm will feel warm. Now, bring your lips close to each other leaving a small gap between them. Blow air again. This time palm will feel cold.
When the mouth is wide open, the warm air from your body touches your palm and it feels warm. But in the second case the air under pressure inside your mouth suddenly expands. While expanding, the warm air molecules have to do work against inter-molecular forces of attraction between the molecules. For doing work, the air needs energy. It gets energy from the heat of the air. When heat is taken out, the air cools and when this cold air strike the palm, you feel cooler. This cooling by expansion of a gas is known as Joule Thomson effect. Joule-Thomson effect provides a practical method of liquefying gases like nitrogen, oxygen etc.