The Atmosphere
Movement in the Atmosphere
Atmospheric Stability
Localised Winds
Clouds
Fronts
Visibility and Fog
Aircraft Icing
Thunderstorms
Altimetry
Weather Reports and Forecasts
Practice Exam

The States of Water

Water exists in three different states in the atmosphere – vapour, liquid, and ice. There is a different amount of energy contained within the molecules in each state.

Vapour
Water vapour is a gas that exists when liquid water evaporates. It is the highest energy state of water.
Liquid
When we think of water, liquid is what comes to mind. Liquid water forms when water vapour condenses or when ice melts. Liquid water is the second highest energy state of water.
Ice
Ice is the solid state of water, created when liquid water freezes. Ice is the lowest energy state of water.

Changing States

When water changes from one state to another, energy must be either released or absorbed. This energy comes in the form of heat, so heat is released when water moves to a lower energy state and heat is absorbed when moving to a higher energy state.

When water vapour condenses to form liquid water, or when liquid water freezes to form ice, heat is released into the atmosphere. Ice melting to create liquid water or liquid water evaporating to create vapour causes heat to be absorbed by the water.

Think of when you step out of the shower and the water on your body begins to evaporate. The evaporation requires heat to be absorbed by the water and your body is a good supplier of this heat. So the evaporation absorbs heat from your skin and you begin to feel cold after a warm shower!

I find it easiest to remember this by remembering ice is cold, so ice must have the least heat. To move ‘up’ from ice, heat must be absorbed by the water. To move in the other direction, heat must be released by the water into the atmosphere.