NORFOLK, Va. — The old saying goes “it’s not the heat, it’s the humidity!” And while that may be true, it’s only half of the story. It’s the “dew point” that really matters.
But what is the dew point? In simple terms, the dew point is a measurement of how much moisture is present in the air.
Meteorologically, the dew point is the temperature at which the air becomes saturated with moisture, leading to the formation of dew, fog, or clouds. Think of a cold glass of water on a warm day — droplets form on its surface as the temperature of the glass reaches the dew point of the surrounding air. Similarly, when the air temperature cools to the dew point, moisture condenses out of the air, forming dew on surfaces like grass, cars, and even your windows.
The dew point is important for several reasons:
- Weather Forecasting: We use the dew point to forecast fog, frost, or dew forming overnight. It also helps predict the potential for thunderstorms or heavy rainfall.
- Comfort: High dew points mean high humidity levels, which can make hot temperatures feel even worse.
- Health: High humidity levels can exacerbate respiratory issues for some people. By monitoring the dew point, you can take precautions to stay hydrated and cool during hot, humid weather.
While most people are familiar with relative humidity, it’s really the dew point that matters. Both measurements tell us the amount of moisture in the air but they do it in different ways.
Relative humidity is a measure of how much moisture the air currently holds compared to the maximum amount it can hold at its current temperature. It's expressed as a percentage. For example, if the relative humidity is 50%, it means the air is holding half of the amount of moisture it could hold at that temperature. Relative humidity truly is relative, relative to the air temperature. Warmer air can hold more moisture than cooler air, so the same amount of water vapor will result in lower relative humidity at higher temperatures and higher relative humidity at lower temperatures.
Unlike relative humidity, the dew point is not affected by changes in air temperature. If the air temperature remains constant, the dew point will also remain constant. However, if the moisture content in the air changes, the dew point will change accordingly. It’s a more exact measurement of just how humid the air actually is.