Why Dew Point is Better Than Relative Humidity

If you have ever stepped outside in Florida during the summer and felt like you could drink the air, you have experienced a high Dew Point. While most weather apps prominently display Relative Humidity (RH), meteorologists actually prefer using the Dew Point to measure human comfort.

Why? Because Relative Humidity is highly misleading. RH measures how close the air is to being completely saturated with water, but this capacity changes with temperature. Hot air can hold vastly more water than cold air. If the temperature is 95°F with 50% humidity, the air feels violently oppressive. But if the temperature is 35°F with 50% humidity, the air feels crisp and dry. The RH is the exact same, but the physical amount of water floating around you is entirely different.

Dew Point, on the other hand, is an absolute measurement. It tells you exactly how much water vapor is physically in the air, regardless of the temperature. A dew point of 70°F feels identically sticky and gross whether the air temperature is 75°F or 95°F.

What is Dew Point Physically?

Physically, the dew point is the temperature the air must drop to in order for water to condense out of it. Imagine holding a glass of ice water outside on a hot day. The glass cools the thin layer of air immediately surrounding it. If that layer of air cools down to the dew point, the air can no longer hold its water vapor, and the water condenses into physical drops on the outside of your glass.

This is exactly how morning dew forms on grass, or why fog rolls in when the sun goes down and temperatures drop.

Why High Dew Points are Dangerous

The human body's primary method of cooling itself is sweating. However, sweating only works if the sweat can evaporate off your skin into the air, taking heat energy with it. When the dew point is exceptionally high (above 70°F), the air is so heavily loaded with water vapor that there is simply no "room" for your sweat to evaporate. Your sweat stays trapped on your skin, your core temperature rises, and you become highly susceptible to heat exhaustion or heat stroke.