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The amount of water vapor in air is its humidity; the water condenses when the temperature reaches its dew point.

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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°. But in Houston it feels like it's 122! Of course in Phoenix in July, the average high temperature is 106°. That's hot, dry or not!


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 sweat can’t evaporate when the air already contains as much water vapor as it can hold. 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.

Science Friday: Snowflake Safari

Next snowstorm, grab a magnifying glass and look carefully at snowflakes. Bullet rosettes, stellar plates and capped columns are just a few of the many varieties of snow crystals. In this video by Science Friday, physicist Kenneth Libbrecht shares secrets about snowflakes.


  • 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.


  1. What is humidity? What is relative humidity?
  2. Explain what heat index is.
  3. Why does water come out of the air at its dew point?

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Use the resource below to answer the questions that follow.

  1. What is humidity?
  2. What is water vapor?
  3. What is absolute humidity?
  4. What two factors does relative humidity consider?
  5. What are you likely to see when the humidity is 100%?

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dew point Temperature at which air is saturated with water vapor at 100% humidity.
heat index What the temperature feels like, taking into account the humidity.
humidity Amount of water vapor in the air.
relative humidity Amount of water vapor in the air relative to the maximum amount of water vapor that the air could contain at that temperature.

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