Dear Tom,
Could you please explain what the dew point is?
Gloria Chance

Dear Gloria,
The dew point temperature (or simple dew point) is the air temperature at which the air is totally saturated with water vapor. Let’s take it step by step.

As air is heated, its ability to hold water vapor (the gaseous form of water) increases considerably. Stated in the opposite way: As air is cooled, its ability to hold water vapor decreases. The dew point temperature is the temperature at which air becomes totally saturated with water vapor. The higher the dew point temperature, the more water vapor is present in the air; conversely, lower dew point temperatures mean air contains less water vapor.

A relative humidity value of 100 percent means the air is totally saturated with water vapor (at that temperature). When the air temperature and dew point temperature are the same, the air can hold no additional water vapor. Additional cooling requires that water vapor must begin condensing out of the air. This could be in the form of clouds, dew or frost, or fog. A relative humidity of, say, 40 percent means the air is holding only 40 percent of the water vapor it could hold at that temperature.

The relationship between air temperature, dew point and relative humidity is rather complex. It is given as:

r=10 raised to the power (4221/(t+459.4)-4221/(d+459.4)+2) where

r = relative humidity in percent
t = air temperature in degrees Fahrenheit
d = dew point in degrees Fahrenheit