The air we exhale on a cold day is visible because of the formation of dew. The main constituents of the air exhaled from the lungs are water vapor and carbon dioxide. The temperature of the air is about 40 degree centigrade. While that of the outside temperature Is much less than that.

Cold air cannot hold as much water vapor as warm air. Hence dew is formed. The moisture in the air begins to condense forming tiny water droplets. The temperature, to condence forming tiny water droplets. The temprature, at which the moisture in the air begins to condense is called dew point. If relative humidity is 100 percent, then dew point is nearly equal to the atmospheric temperature.

On a cold day, When we breath out air with water vapour, it is cold from 40 degree centigrade.That it is cooled to below the dew point,but above its freezing point.Hence the tiny water droplets float in the air, and are visible.

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