Dew point is a term used by the meteorologist who is, of course, the weatherman. As a rule he does not mention the dew point, but: he has it in mind when he talks about the relative humidity for the day. On a hot, muggy day, the relative humidity may be one hundred percent  and this very uncomfortable weather event is caused by the dew point.

The day's weather report includes the humidity, whieh is the amount of water vapor in the air. This vapor is an invisible gas and the air can hold just so much of it and no more. When the day's air has its full quota of water vapor, it has reached its dew point and the weatherman reports that the relative humidity is one hundred percent,

The word relative suggests that two things have been compared. In this case, the two items compared are the amount of vapor in a sample of the days air and the temperature of the days air. For warm air can hold more vapor than cool air. Air at 50 degrees Fahrenheit needs almost twice as much vapor to reach its dew point as air at 32 degrees.

Relative humidity is given as a percentage which tells how much vapor the air actually contains and how much it would contain if it had its full quota. When the relative humidity is 50%, the air has half its full quota of vapor, at 25% it has only a quarter of its quota. It has its full quota at 100% and reaches its dew point.

Lets check a cubic meter sample of the day's air. If the temperature is 68 and the sample contains 17.31 grams of water vapor, the air has reached dew point.

Now suppose a cold wind blows in or the temperature drops after sunset to 50 degrees. At this temperature, our cubic meter of air can hold only 9.41 grams of vapor,

You can guess what happens when air which has reached its dew point is lowered in temperature. It now has a surplus of vapor., more than its quota allows. This surplus can no longer be held as a gas in the air   so it is turned back again into liquid water. It may become raindrops or it may become dew. For a short time in rare conditions, the air can keep some of its surplus vapor. But as a rule, the surplus vapor gas is turned to liquid water as soon as t e air reaches its dew point.

Checking the relative humidity of the air is one of the regular chores of the weatherman. As a rule, he uses a cubic meter sample of air to find the vapor content. He checks his findings with a chart which tells exactly how much his sample of air can hold at any given temperature.