11.2: Humidity

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Would you rather spend a summer day in Phoenix or in Houston?

People who live in Phoenix, Arizona, are told that summer isn't so bad because "it's a dry heat." What does that mean? Imagine that both Phoenix and Houston have a temperature of 90°F. In Phoenix, the relative humidity is 20%. In Houston, the relative humidity is 90%. So in Phoenix it feels like it's 90°F. But in Houston it feels like it's 122°F! Of course in Phoenix in July, the average high temperature is 106°F. That's hot, dry or not!

Humidity

Humidity is the amount of water vapor in the air. High humidity increases the chances of clouds and precipitation.

Relative Humidity

Humidity usually refers to relative humidity. This is the percent of water vapor in the air relative to the total amount the air can hold. How much water vapor can the air hold? That depends on temperature. Warm air can hold more water vapor than cool air (Figure below).

Graph showing water vapor capacity and temperature

How much water vapor can the air hold when its temperature is 40° C?

Humidity and Heat

People often say, “it’s not the heat but the humidity.” Humidity can make a hot day feel even hotter. When sweat evaporates, it cools your body. But when the air contains as much water vapor as it can hold, water condenses on your skin just as fast as it evaporates, providing no cooling benefit. The heat index (Figure below) is a measure of what the temperature feels like because of the humidity.

Graph of apparent temperature based on air temperature and humidity

How hot does it feel when the air temperature is 90°F? It depends on the humidity.

Dew Point

You’ve probably noticed dew on the grass on a summer morning. Why does dew form? Remember that the land heats up and cools down fairly readily. So when night comes, the land cools. Air that was warm and humid in the daytime also cools over night. As the air cools, it can hold less water vapor. Some of the water vapor condenses on the cool surfaces, such as blades of grass. The temperature at which water vapor condenses is called the dew point. If this temperature is below freezing, ice crystals of frost form instead of dew (Figure below). The dew point occurs at 100 percent relative humidity. Can you explain why?

Dew and frost form due to the dew point

The grass on the left is covered with dew. The grass on the right is covered with frost. The difference is the temperature of the grass.

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Summary

Air reaches its dew point when humidity increases or temperature decreases.
Water droplets form when the air reaches 100% humidity. If the temperature is cold enough, frost will form.
Relative humidity is how the air feels at its temperature and humidity.