if we have same tempature water and air, why does the water feel colder to the air when we go for a swim for example?
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Our nerves don’t actually sense the temperature of our surroundings. They are embedded in our flesh so technically they measure the temperature of our flesh, or more precisely the *change* in temperature of our flesh.
The speed of heat transfer varies depending on the difference in temperature between two objects; touching a hot stove for example will transfer heat into you much more quickly than one which is merely slightly warm. So our bodies recognize the temperature outside our flesh by gauging how quickly heat is transferred to or from the flesh in which the nerves reside.
Water has a much higher thermal conductivity than air (0.6 vs 0.025 W/(m·K)) which is to say that heat transfers to and from water much more easily than via air. Air is a great insulator which is why we use fluffy things as insulation, as they trap air which doesn’t conduct the heat very well.
So when you touch water heat leaves your body much more quickly than with air at the same temperature and your nerves will register this precipitous heat loss as implying a colder temperature. This phenomenon is the same as with touching metals as they tend to have high thermal conductivity, meaning metals will feel cooler despite being the same temperature as ambient.
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