Summer has arrived with an abundance of severe weather events all across the country. Thunderstorms, rising heat indices, and soaring temperatures are things you hear a lot during the summer season. We also hear the phrase, “It’s not the heat, it’s the humidity” quite often. This is because while heat certainly takes center stage during the summer months, humidity and dew point readings are really what play a key role in your comfort during the warm weather.
But what’s the difference between the two?
Humidity and dew point are two terms used regularly by your meteorologist during your evening forecast, but they are not interchangeable. They each refer to different things when it comes to measuring moisture in the air. Humidity, or “relative humidity,” measures how close to saturation the air is with water vapor. So if it’s 97% relative humidity outside, that means the air only needs 3% more water vapor to achieve complete saturation of 100% (total air saturation = fog. Many people believe 100% saturation = rain, but fog is what forms in 100% relative humidity).
However, knowing the humidity percentage doesn’t tell the whole story on these summer days. The dew point is a better indicator of how sticky it feels outside. While we may say, “boy, it’s humid out!” chances are, what we’re really feeling is a high dew point. Even if you enjoy the warm temperatures of summer weather, a high dew point temperature –usually anything over 60º Fahrenheit — feels downright uncomfortable for just about everyone.
Meteorologists like to use dew point because it shows a better picture of the relationship of relative humidity to the air temperature. Dew point’s definition is, literally, the temperature at which dew forms, or the temperature the air has to cool at constant pressure to reach total saturation (“fog”).
So that glass of ice cold lemonade that’s leaving a messy ring of moisture on your patio table tells us a lot. It means the ice has actually cooled the glass and the liquid inside down to or below the dew point, so any moisture that’s in the air condenses on the glass. When dew points are low, that glass of lemonade isn’t as sweaty and you’ll notice your towels drying much faster on the clothesline.
While it’s true that the dew point is not a directly measurable parameter, your home weather station can now give dew point readings.
40-50 – Nice and comfortable.
60s – Getting uncomfortable.
70s – Oppressive!
Here’s a temperature, humidity and dew point calculator to play with. Have fun!
Your best defense in the oppressive heat is to ratchet up the fan to high and wait for a thunderstorm to usher in some drier air.