A cloud is a mass of water drops or ice crystals suspended in the atmosphere. Clouds form when water condenses in the sky. The condensation lets us see the water vapor. There are many different types of clouds.13
The range of ways in which clouds can be formed and the variable nature of the atmosphere results in an enormous variety of shapes, sizes and textures of clouds.
Many people believe that clouds are just made of water vapour (a gas). However, this is not strictly true. Water vapour is invisible, and it is around us all the time in the air. Sometimes there is more water vapour in the air and it feels humid or muggy. Other times, the air has less water vapour and it feels drier and fresher.
Clouds appear when there is too much water vapour for the air to hold. The water vapour (gas) then condenses to form tiny water droplets (liquid), and it is the water that makes the cloud visible. These droplets are so small that they stay suspended in the air.
What causes the air to rise?
1. The sun – The sun heats the ground, which then heats the air just above it, causing it to rise upwards in the sky (warm air rises). This tends to produce cumulus clouds.
2. Hills and mountains - When air is travelling towards a mountain or hill, it cannot go into the hill and so it rises upwards along the terrain. Stratus clouds are often produced this way.
3. Weather fronts - A weather 'front' is where warm air meets cold air. The warm air rises up and over the cold air (warm air rises). This produces nimbostratus clouds, amongst others.
4. Convergence - Streams of air flowing towards each other from different directions are forced to rise when they meet, or converge. This can cause cumulus cloud and showery conditions.
5. Turbulence - A sudden change in wind speed high up can create circulations in the air which can bring the air at the surface high up into the sky.
Nacreous clouds
Nacreous clouds form in the lower stratosphere over polar regions when the Sun is just below the horizon. The clouds are illuminated from below and often glow in vivid colours.
Height of base: 68,500 - 100,000 ft
Shape: large thin discs, often reflecting vivid colours
Latin: no direct Latin translation, but the old English word “Nacre” means “mother of Pearl”
Precipitation: none
