Question submitted by John Birk
John, sometimes the seemingly simplest of questions are the best!
The dew point is the temperature at which the air would become saturated if it were cooled sufficiently. If the air is cooled below this point, spontaneous condensaton of the water vapour in the air will occur. The dew point is actually calculated from two readings on two different thermometers - a dry bulb and a wet bulb thermometer.
The temperature/dew point spread measures the same thing as relative humidity,
but relative humidity is given as a percent. As the relative humidity goes up, the temperature/dew point spread decreases.
So, answering the second part of your question first (!), the measurement of the dew point of the air, essentially gives us the relative humidity of the air.Dew Point values provide useful insights into the characteristics of an air mass at the surface. Generally, higher Dew-Point Temperatures during periods of warm or hot weather will indicate a greater likelihood of discomfort because the air is nearer saturation and therefore less capable of absorbing moisture from the surface of one’s skin. Likewise, low Dew-Point Temperatures will indicate greater comfort during warm weather because moisture on exposed skin evaporates more rapidly, therefore increasing heat loss from the body, making us feel cooler.
Dew Point values can also give insights into the amount of energy available at the surface for the development of thunderstorms. Higher Dew-Point values can indicate a greater supply of heat and moisture at the surface. Greater potential energy in the form of heat and moisture provides better "fuel" for the growth of thunderstorm updrafts.
So the dew point is a crucial piece of the puzzle when creating a forecast.