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Hot Air Rises
You will need:
- A large, wide mouthed jar
- A smaller glass jar (a baby food jar would be ideal) that fits through the mouth of the larger jar
- string
- water
What to do
Tie some string around the mouth of the smaller jar, leaving an end long enough for you to lower the jar to the bottom of the larger jar.
2/3 fill the larger jar with cold water and a few drops of yellow food colouring. Fill the smaller jar with a couple of drops of red food colouring, and boiling water. Very carefully, lower the smaller jar into the larger. What happens?
You should have seen the red water rise through the yellow, leaving an orange layer of water on top of a yellow layer. This is because the hot water is less dense than the cold. In the same way, warm air rises through colder air - if this didn't happen, no clouds would form. Air near the ground can get heated during the day, and rises through the colder air above it. Sometimes, it can reach an altitude where clouds can form. This is also the mechanism that hot air balloons rely on!
Now try the experiment the other way round - i.e. fill the larger jar with boiling water and the smaller with cold water. What happens? The colder, more dense water is trapped at the bottom of the jar.
Find out more:
http://www.ace.mmu.ac.uk/eae/english.html and select 'atmosphere' and
'clouds'
http://www.eballoon.org/balloon/how-it-works.html
http://en.wikipedia.org/wiki/Hot_air_balloon#Generating_lift