Dew Point

In winter's morning, you can find small droplets of water (dew) on the leaves. As the temperatures dips in the early morning, the amount of water vapour present in the air saturates and further cooling results in condensation. The dew point is the temperature at which the air becomes saturated with water vapor, and condensation begins to form. It is the temperature at which the air must be cooled (at constant pressure and water content) in order for the water vapor in the air to condense into water droplets (dew).

The dew point is a useful measure of the amount of moisture in the air. Once the dew point is known the relative humidity (RH) is calculated by taking ratio of "saturation vapour pressure at dew point" and "saturation vapour pressure at the room temperature". (Here saturation vapour pressure is the pressure exerted by the vapour when the air is saturated by the vapour. Saturation vapour pressure has been measured for various temperatures and tables are available which can give us its value at the required temperature.)

Procedure

You need two new steel glasses, cold water bottle and thermometer. Put two glasses side by side.

1. Take out a cold water bottle from the refrigerator. See that water has condensed on the surface of the bottle after you take it out of the refrigerator.
2. Now fill one fourth of one glass with normal tap water and put a thermometer in the glass.
3. Gradually pour some cold water in the glass and stir. See if there is a difference in the shine of the surfaces of the two glasses.
4. If yes, note the temperature of water in the thermometer.
5. If not, pour some more cold water till you start noticing a difference in the shine of the surface of the glasses. Note the temperature of the water.
6. This is the "Dew point" of your room.

Discussion

Steel is a good conductor of heat. So the temperature of the water inside the glass and the outer surface of the thin steel glass up to the height of water quickly become same. The air in contact with the surface cools down as it also attains the temperature of the surface. When the temperature falls just below the dew point, vapour starts condensing and small droplets of water collect on the glass surface making it look a little foggy. So a careful comparison of the shininess of the two glasses, one with the cold water and the other empty, gives a fair measure of the dew point.

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