Weather and climate
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What are clouds made of?
Clouds are made of tiny ice crystals or drops of water that form around dust particles in the atmosphere. The droplets are minute—measuring just a hundredth of a millimetre across. Whether a cloud is made up of ice or water droplets depends on the height of the cloud above the ground and the temperature of the atmosphere around it. Being so tiny, water droplets can remain in liquid form at temperatures of down to –30°C (–20°F), when normally they would have turned to ice. At extremely low temperatures found at high altitude, clouds are made up of ice crystals only.
Clouds form when water vapour in the air (which is invisible) condenses into water droplets or ice crystals (which are visible). Tiny particles, such as salt and dust, which are known as aerosols, float around in the air along with water vapour molecules—water that has evaporated from the Earth’s surface. The particles and molecules are constantly bumping into one another. When the air cools, some of the water vapour sticks to the aerosols: this is called condensation. The water droplets around the aerosol particles may get bigger and bigger as more and more water condenses. They then may start to stick together with other droplets. This is how clouds form. The water droplets grow until they become so heavy they start to fall as rain or snow.
The tiny cloud droplets scatter all the colours on the sunlight spectrum (the colours of the rainbow) equally, so we see the light reflected from them as white—which is why many clouds look white. As the cloud gathers more and more water droplets, it becomes thicker and denser. The droplets now absorb more light than they scatter, and so the cloud appears darker—which is why the clouds in the sky just before it rains or snows look grey.
The warmer the air is, the more water vapour it can hold. But as the air rises, it will cool. The reduced temperature of the air decreases its ability to hold water vapour, so condensation occurs. The temperature at which this happens is called the dew point. The height above ground at which the dew point is reached and clouds form is called the condensation level.
What causes the air to rise? When the sun heats the ground, the air in contact with it warms too, and rises. The rising columns of air are often called thermals. Air also can be forced to rise as it passes over hills or mountains, or when a mass of warm air meets a mass of cold, dense air along what are known as weather fronts—the boundaries between the two air masses. The different ways in which clouds can form results in the variety of shapes and sizes of clouds.
Consultant: Ian Fairchild
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