What it shows
The dew point is the temperature at which air becomes saturated and water starts to condense. Cooling a shiny can slowly until it mists over gives the dew point, and from dew point and air temperature the relative humidity can be read from a table.
The science
Air at a given temperature can hold only so much water vapour. As the can cools, the thin layer of air touching it cools too, and at the dew point that layer becomes saturated and vapour condenses as a mist on the metal. The closer the dew point is to the air temperature, the more humid the air. Relative humidity is the ratio of the vapour present to the vapour needed for saturation, and standard tables convert the two temperatures into a percentage.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A clean shiny steel or aluminium can
- A thermometer that reads to 1 degree or better
- Ice cubes and water
- A stirring rod
- A relative humidity table printed from a textbook
- A notebook
Safety
For this project: Handle the thermometer gently; a digital probe is safer than glass.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Fill the can half full with water at room temperature and record the air temperature in the shade.
Add a single ice cube and stir steadily, watching the outside of the can against the light.
Keep the thermometer in the water and read it every few seconds.
The instant a faint mist appears on the metal, note the water temperature; this is the dew point.
Pour the water out, dry the can, and repeat twice; average the three readings.
Read relative humidity from the table using the air temperature and the dew point.
Repeat at the same time on a clear day, a humid day and a rainy day.
Plot dew point against relative humidity and comment on the pattern.
What you should see
Three dew point readings per day within a degree of each other. On a monsoon afternoon the dew point sits only 2 or 3 degrees below the air temperature with humidity above 80 per cent; on a dry winter afternoon the gap can be 15 degrees or more.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Define dew point.
The temperature to which air must be cooled, at constant pressure, for it to become saturated.
2Why must the can be stirred?
So the water and the metal surface are at the same temperature as the thermometer reads.
3What does a small gap between air temperature and dew point mean?
High relative humidity.
4Why is dew common on clear nights?
The ground cools fast by radiation and the air next to it reaches its dew point.
Take it further
Build a wet-and-dry bulb hygrometer from two thermometers and compare its humidity values with the dew point method.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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