Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

How does humidity work?

ScienceLesson on requestFree to read

Quick answer

Humidity is the amount of water vapor in the air. It comes from evaporation from oceans, lakes, and plants. Warm air holds more vapor than cold air. Relative humidity compares the actual vapor to the maximum possible at a given temperature. When air cools to its dew point, vapor condenses into dew, fog, or clouds.

Humidity measures the amount of water vapor in the air, driven by evaporation and temperature. Warm air holds more vapor than cold air, so when air cools, vapor condenses into dew, clouds, or precipitation.

Step by step

  1. Water evaporates from oceans, lakes, and soil, turning liquid into invisible vapor.
  2. The vapor mixes into the air, and warm air can hold more of it than cold air.
  3. Relative humidity shows how close the air is to holding its maximum vapor at that temperature.
  4. As air cools, it can hold less vapor, so the relative humidity rises toward 100%.
  5. At the dew point, vapor condenses onto tiny particles, forming dew, fog, or cloud droplets.
  6. These droplets grow and may fall as rain or snow, returning water to the surface.

Key things to understand

  1. Water evaporates from oceans, lakes, and plants, adding vapor to the air.
  2. Temperature determines how much vapor the air can hold; warmer air holds more.
  3. Relative humidity compares the actual vapor to the maximum possible at that temperature.
  4. When air cools to its dew point, vapor condenses into liquid water.
  5. Condensation forms dew, fog, clouds, and precipitation, completing the water cycle.

Labelled diagram

  1. Evaporation

    Liquid water from oceans, lakes, and plants turns into invisible water vapor.

  2. Vapor in air

    Water vapor mixes with air; warm air can hold more vapor than cold air.

  3. Relative humidity

    The percentage of vapor in air compared to the maximum it can hold.

  4. Dew point

    The temperature at which air must cool for vapor to condense.

  5. Condensation

    Vapor turns into liquid water on surfaces or tiny particles, forming dew or clouds.

How humidity forms and condenses

Real-world examples

  • On a hot day in Mumbai, high humidity makes 32°C feel like 40°C because sweat evaporates slowly.
  • In a desert like the Sahara, low humidity lets temperatures drop sharply at night because water vapor traps less heat.
  • A cold glass of water on a humid day gets wet on the outside as vapor condenses on the cold surface.
  • In a bathroom after a hot shower, humidity rises so much that water vapor fogs the mirror and windows.

Common mistake

✗ Myth: Humidity is the same as heat.

✓ Fact: Humidity is the amount of water vapor in the air, while heat is thermal energy. High humidity can make hot weather feel hotter, but humidity and temperature are separate measurements.

People also ask

What causes high humidity?
High humidity occurs when air contains a large amount of water vapor, often from evaporation from nearby oceans, lakes, or wet ground. Warm air holds more vapor, so tropical regions and summer months tend to have higher humidity. Weather systems that bring moist air from the sea also raise humidity.
Why does humidity make it feel hotter?
When humidity is high, sweat on your skin evaporates more slowly. Sweating cools the body only when it evaporates, so less evaporation means you feel hotter and stickier. This is why a humid 30°C day can feel more uncomfortable than a dry 35°C day.
What is the difference between humidity and dew point?
Humidity is the amount of water vapor in the air, often expressed as relative humidity. The dew point is the temperature at which air must cool for that vapor to condense into liquid water. A high dew point means the air is very moist and feels muggy.
What is relative humidity?
Relative humidity is the percentage of water vapor in the air compared to the maximum it could hold at that temperature. Warm air can hold more vapor, so the same amount of vapor gives a lower relative humidity as temperature rises.
What is the dew point?
The dew point is the temperature at which air must cool for water vapor to condense into liquid water. When the dew point is close to the actual temperature, the air is near saturation and fog or dew is likely.
How does humidity affect weather?
Humidity supplies the water vapor that forms clouds and precipitation. Rising warm, moist air cools and condenses, building clouds; if enough vapor is present, rain, snow, or storms develop.

Part of the guide: Water & Weather Phenomena →