CHARLOTTE, NC (WBTV) - Want to hear a forecast secret? The dewpoint can help you figure out what the overnight low will be. You don't even need special equipment!
The dewpoint is a measure of how much moisture is in the atmosphere. It's a better (and more absolute) measure of the water vapor in the atmosphere than relative humidity. That's because it's, well – not relative.
With relative humidity, warmer air can hold more water than cooler air. If you have a dewpoint of 60°, the humidity might be 100% first thing in the morning when the temperature is also 60°. If the dewpoint is still 60° and the temperature is 90° in the afternoon, the relative humidity would be around 36%. The dewpoint hasn't changed.
Back to the dewpoint and the low temp… If you're forecasting the overnight low, the temperature won't go much lower than the dewpoint the evening before. (Obviously, if a cold front moves through, that changes everything. It brings in a whole new air mass and both temperatures and dewpoints can fall drastically. This is assuming that doesn't happen.)
When the temperature falls to the dewpoint, latent heat must be released for condensation to take place. That offsets more cooling and prevents the temperature from going lower.
If the dewpoint is considerably lower than the temperature, the low may not fall all the way to the dewpoint but you can still use it as a forecast tool. You know that the drier the air is, the temperature will fall faster and farther. On the other hand, if the dewpoint is high, temperatures will fall much more slowly.
There you go! You're on your way to being a meteorologist.