Relative Humidity: The percentage that shows what share of water vapor the air could hold is actually in the air, at a constant pressure and temperature. In other words, relative humidity indicates how close the air is to becoming saturated, rather than indicating the actual quantity of water vapor in the air.
It’s best to think of a parcel of air, as an enclosed “container” of air. (See First graphic to the right.) Once the water vapor reaches 100%, the air becomes saturated, therefore filling the “container”.
It is also important to realize, that at a constant pressure, if the temperature changes, the size of the “container” changes as well. (See Second graphic to the right.) Cold air holds less water vapor, therefore the “container” is smaller than a “container” with warmer air.
For example, in the early morning hours, the air temperature is closer to the dew point temperature, therefore the humidity is higher. But as the day goes on and the air temperature heats up, while the dew point and pressure remain constant, the humidity will go down. (For more, see Dew Point explanation and graphic below.)
Dew Point: The “point” (temperature) at which dew forms. It measures the amount of water vapor in the air at a constant pressure.
For example, if the air temperature is 80Â°F and the dew point is 80Â°F, the humidity is 100%, the “container” is full, and the air is saturated. On the other hand, if the air temperature is 110Â°F, and the dew point is still 80Â°F, the humidity would be about 40% and the “container” is only about 40% full. (See Third graphic above)
Why is Dew Point Important? Dew Point is one of the many ways of measuring the water vapor content in the atmosphere. It is important to know if it is high or low because it is directly related to how well evaporative coolers work and how comfortable you are. Evaporative coolers tend to work well if the dew point is below 55Â°F. The human body usually doesn’t notice how muggy it feels outside until the dew point is above 60Â°F.