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Water in the Sky

Evaporation and Condensation: Changing Water

There are three types of water.

  • When it is a solid we call it ice.
  • When it is a liquid we call it water.
  • When it is a gas we call it vapor.

These three different types of water are called states.

Water can change its state. When ice melts, changing into water, or when water evaporates, changing into vapor, a little heat is needed. When water freezes into ice or condenses from vapor to a liquid, a little bit of heat is released. The heat that is needed for or released from these changes in state is called latent heat.

Have you ever noticed that your skin feels cold when you get out of a swimming pool, lake or ocean on a windy day? It feels much colder than if you were dry. That’s because the wind helps the liquid water on your skin to evaporate becoming water vapor, and a little latent heat is needed for this change. In this case, the heat comes from you!


How Clouds Are Formed

Cooling air that contains water vapor will cause the vapor molecules in the air slow down. As they slow down, some molecules are not able to maintain their vapor form. They cluster (condense) in the air to form tiny liquid droplets. Typically they need some sort of particle, such as dust, to condense upon. These particles are called condensation nuclei.

A cloud is composed of tiny water droplets (or ice crystals) that are suspended in the air. If the droplets become large enough,they may be visible as a cloud or fog.They may also fall to Earth in the form of rain (or snow).