Fog and Clouds

We all know that clouds and fog consist of water. But how does the water get into the air exactly, and how and why do clouds and fog form?

Pictures: Fog moves through a gorge in Norway.

The Water Cycle

During its circulation, water changes its state regularly. It evaporates, because the sun heats it up, and so the liquid water becomes water vapour. This water vapour is absorbed by the air. Hot air can absorb more water vapour than cold air. So if a warm air current cools down, its ability to retain moisture, that is water vapour, declines and the water vapour has to escape in some way. This is brought about by condensation. The water vapour condenses on small dust or soot particles, which are called condensation nuclei in this connection, and forms small water droplets. This is nothing new to people wearing glasses. Surely they know the phenomenon where their glasses fog up as soon as they enter a warm room in winter. Here, exactly the same thing happens as described before. The warm, moist air comes in contact with the cold glasses, cools down and the water vapour has to escape. It condenses visibly on the glasses.

Fog and Clouds - Same Material, Another Mode of Formation

The difference between fog and clouds is easy to explain. Clouds are formed when warm, moist air rises and afterwards cools down at high heights. The water vapour that was carried along condenses and clouds form. Depending on temperature and height, several types of clouds can form.

  Picture: Cirrus clouds

  Picture: Stratus clouds

  Picture: Cumulus clouds

Fog: A 'low-lying' cloud!

In contrast, fog forms when a warm and a cold air current, which are close to the ground, bump into each other causing the water vapour to condense. In other words, fog is nothing but a 'low-lying' cloud.

Experiment: Formation of Clouds

Material: a clear, transparent glass bottle with the cap, hot water, a metal lid, an old saucer, ice cubes, matches, paper.

• Rinse the bottle with hot water.
• Light some paper in the saucer.
• Place the bottle over the flame upside down so that smoke can rise into it.
  Extinguish the fire immediately with the metal lid!
• Close the bottle and put it into the fridge or
• place an ice cube on the neck of the bottle. Observe what happens!