STORM WARNING: Mammatus cloud, a form of cumulus, can precede a storm.
STORM WARNING: Mammatus cloud, a form of cumulus, can precede a storm. Terry West

Clouds full of water

MORE than 70% of the earth's surface is covered in water and as this evaporates it forms clouds.

Pressure plays a major part in cloud formation and as the warm, moist air that has evaporated rises, it experiences lower pressure, causing it to expand.

It also causes the air to cool and cooler air cannot hold the water vapour and becomes saturated.

Cloud will start to form and as it becomes more dense, may produce rain. It takes about one million cloud droplets to make a raindrop. Cloud always points to moisture in the atmosphere but it is not a guarantee it will rain.

Cloud observation is a very important tool in meteorology.

Clouds have been classified into three distinct families. A scientist named Luke Howard named them in 1804. He gave them Latin names - cirrus (hair curl), cumulus (heap) and stratus (layers). There are 10 combinations of clouds made up from these families and they are grouped by the height they are from the ground.

There are also many other formations of cloud that combine multiple structures from each family - 27 all up - but I have only listed the 10 most important.

Cirrus - high, ice-crystal clouds which look like wispy curls of hair, often the first signs of approaching weather changes.

Cirrocumulus - often called a "mackerel sky"; the ripples of cloud look like fish scales, indicating unsettled weather.

Cirrostratus - sheets of thin, milk-coloured clouds which form high up and often bring rain or snow within 24 hours.

Altostratus - layers of thin, grey clouds which can grow into rain clouds.

Altocumulus - fluffy waves of grey clouds which can bring showers or break up to give sunny periods.

Nimbostratus - thick, dark grey masses of clouds which can bring rain or snow. Nimbus means rain in Latin.

Stratus - low, grey blankets of clouds which often bring drizzle, can cover high ground and cause hill fog.

Stratocumulus - uneven rolls or patches of clouds across the sky which follow a storm and are usually a sign that drier weather is on the way.

Cumulus - clearly defined puffs of fluffy clouds that look like cauliflowers. They appear in sunny, summer skies. In the morning, they precede a storm, in the afternoon they follow a storm.

Cumulonimbus - these are towering clouds which usually bring thunderstorms with rain, snow or hail.



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