Storm clouds have a lot of water in them in droplets that are larger than those in normal clouds. These larger droplets absorb and scatter more light than smaller ones, so the storm clouds look darker.
Q Where does the word `magnet' come from?
Ancient Greeks first found magnetic rocks in an area called Magnesia, in what is now Turkey, hence "magnet".
Q What does the term `smokeless zone' mean?
Smokeless zones were introduced in 1956 to try to solve the smog problem in Britain. So much fuel was being burnt that it was creating a smoky fog - a smog - that killed several thousand people during the winter months. Smokeless fuels do not produce particulates such as soot and ash, so they do not create a smog.
Q Why did penny farthings have such big wheels?
The larger the wheel in this early design of bike, the faster the rider could go, as the pedals were directly connected to the wheel without any gearing mechanism. The only way to make the bicycle travel further for each turn of the pedal was to enlarge the wheel to which the pedals were connected. This did make for some difficult bikes to ride, though, and the design was soon replaced by smaller models with gearing mechanisms.
Q What is the purpose of the spirals that you see around tall chimneys in factories and hospitals?
These spirals were developed after the Second World War to "shed vortices". Any tall, thin structure will have problems remaining upright because of the wind. The spirals help to stop the chimneys being blown over.
Q How many blood cells are there in an average person?
There are about 5 billion red blood cells in every millilitre of blood, which gives a total of 245,000,000,000 (245 billion) in the average-sized person. They are all sent around the body once every minute.
Q Is the ice crust on the polar seas made of falling snow? At what temperature does the sea freeze?Are icebergs made of fresh water?
Sea water is very cold and its freezing temperature depends on its salt content. Water gets less dense as it cools because of the bonding structure in the ice lattice pushing molecules further apart than they are when in liquid state. The sea freezes at about 2C due to the salt content but frozen icebergs have virtually no salt in them. They are lumps of ice from the mainland glaciers that have formed from normal snow - that is, fresh water. Because the sea is constantly moving, falling snow doesn't normally affect ice sheets. Icebergs made of salty water do exist, but they are rare.
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