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The Dew Point
The dew point is the temperature at which, with a given amount of water vapour in the air ('humidity'), water vapour will condense to form cloud droplets, if particles are present for the droplets to form on (see 'clouds in a fizzy drink' experiment)
You will need:
- a tin can (e.g. soup or baked bean tin)
- a thermometer that can get wet and fits inside the can
- some crushed ice
What to do:
Make sure the can is clean and has the paper removed.
Put the thermometer in the can and add enough tap water to fill the can
half full.
Cool the water slowly by adding a little ice at a time, and waiting for
the temperature to fall. Stir and wait for the ice to melt before adding more
ice. When you first see condensation forming on the outside of the can, read
the thermometer. If your thermometer reaches 0ºC and no condensation
has formed, add salt and ice to get the water to cool below freezing. The
temperature at which condensation first forms is the dew point of the air.
How much cooler is the dew point than the air temperature outside?
The air temperature falls by about 6ºC for every 1000m you go up in the
atmosphere. You can use your dew point to calculate how high up you would
expect clouds to form.
If you have also tried the 'DIY hygrometer' experiment, you can use the converter
at http://www.srh.noaa.gov/elp/wxcalc/wetbulb.shtml to
check your dew point temperature.
Find out more:
Measuring humidity in the atmosphere
http://www.ace.mmu.ac.uk/eae/english.html and
select 'atmosphere' and 'clouds'
Make a Rainbow
http://www.weatherwizkids.com/rainbow.htm