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How does dew form?

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Quick answer

Dew forms when surfaces cool below the dew point, the temperature at which air becomes saturated with water vapour. On clear, calm nights, grass and other objects radiate heat away, chilling below that threshold. Water vapour touching them condenses into tiny liquid droplets, creating the dew seen at dawn.

Dew forms when water vapour in the air condenses into liquid droplets on surfaces that have cooled below the air's dew point. This cooling usually happens on clear, calm nights as the ground and objects radiate heat away into space.

Step by step

  1. During the day, the ground absorbs solar energy and becomes warmer than the air above it.
  2. After sunset, surfaces radiate heat upward and cool faster than the surrounding air.
  3. When a surface drops below the dew point, the air layer touching it reaches saturation.
  4. Excess water vapour condenses as tiny droplets on the cooled surface, forming dew.
  5. Dew remains until morning warmth evaporates the droplets back into water vapour.

Key things to understand

  1. During the day, the ground absorbs heat from sunlight and stays warmer than the surrounding air.
  2. After sunset, surfaces like grass, leaves and metal radiate their heat upward and cool faster than the air.
  3. When a surface's temperature drops below the dew point, the air layer touching it becomes saturated with water vapour.
  4. Further cooling forces excess water vapour to condense into tiny liquid droplets on the surface.
  5. Calm air and clear skies help dew form because they let surfaces cool efficiently without mixing in warmer air.

Labelled diagram

  1. Daytime heating

    Sun warms the ground, which stores heat and stays warmer than the air.

  2. Sunset cooling

    Surfaces radiate heat away and begin cooling faster than the air.

  3. Dew point reached

    Surface temperature drops below the dew point, saturating the air layer.

  4. Condensation

    Water vapour turns into tiny liquid droplets on the cooled surface.

  5. Morning evaporation

    Sunrise warms the surface and the dew droplets evaporate back into vapour.

How dew forms on a clear night

Real-world examples

  • On a clear autumn morning, blades of grass in a garden are often coated with dew droplets.
  • A cold metal car roof can be wet with dew even when the air feels dry, because metal cools quickly.
  • In desert regions like the Atacama, dew is a vital water source for some plants and insects.
  • A glass of iced drink sweats in summer because its surface is below the dew point, just like dew-covered grass.

Common mistake

✗ Myth: Dew falls from the sky like rain.

✓ Fact: Dew does not fall; it condenses directly on surfaces that have cooled below the dew point. The water vapour comes from the air touching those surfaces, not from above.

People also ask

What causes dew to form?
Dew forms when surfaces cool below the dew point, the temperature at which air becomes saturated. On clear, calm nights, grass and other objects radiate heat away and drop below that threshold. Water vapour in the air touching them condenses into liquid droplets, creating dew by morning.
Why is dew on grass in the morning?
Grass blades are thin and exposed, so they radiate heat quickly and cool faster than the surrounding air. When their temperature falls below the dew point, water vapour condenses on them. This is why dew is most noticeable on grass at dawn.
Does dew form on windy nights?
Wind mixes warmer air down to the surface, preventing objects from cooling below the dew point. Strong wind also evaporates droplets as they form. Dew therefore favours calm nights, when a thin layer of still air sits against the cooling surface.
Why does dew form mostly at night?
Dew forms at night because surfaces cool by radiating heat into space after the sun sets. Without incoming solar energy, the ground and objects can drop below the dew point, letting water vapour condense on them.
What is the difference between dew and frost?
Both form by condensation on cooled surfaces, but frost appears when the surface temperature is below freezing. Instead of liquid droplets, water vapour changes directly into ice crystals, a process called deposition.
Does dew fall from the sky?
No, dew does not fall. It forms directly on surfaces that have cooled below the dew point, as water vapour in the air touching those surfaces condenses into droplets. The sky supplies the vapour, not the droplets.
Why is dew less common on cloudy or windy nights?
Clouds trap heat by reflecting infrared radiation back to the ground, keeping surfaces warmer. Wind mixes warmer air down to the surface. Both effects stop surfaces from cooling below the dew point, so dew is less likely.

Part of the guide: Water & Weather Phenomena →