Wet and dry bulb thermometer. Image by TradeIndia

Wet and dry bulb thermometer. Image by TradeIndia

ZIMSEC O Level Geography Notes: Humidity

-a quantity representing the amount of water vapor in the atmosphere.

  • It is measured using a hydrometer.
  • The wet and dry bulb thermometer is also known as a psychrometer.
  • Weather satellites can also measure the amount of humidity in the atmosphere.
  • A hygrometer measures the humidity in the atmosphere using two thermometers: dry and wet bulb thermometers.
  • The wet bulb thermometer is wrapped in thin muslin and dipped in a container of water to keep the muslin cloth wet.
  • If the air is dry water evaporates from the muslin and cools such that the mercury contracts.
  • The dry bulb is not affected hence the two show different readings.
  • The wet bulb shows a low reading whilst the dry bulb shows a higher reading.
  • Depression/Difference=the reading of the dry bulb less the reading of the wet bulb thermometer.
  • The humidity is then found by reading humidity tables. For example if there is no difference between the two readings the humidity is 100%
  • Electronic hygrometers are also used. These are easier to read.
  • Relative humidity is measured in percentage terms.

Types of humidity
There are several ways to measure and express humidity:

  1. Absolute Humidity-the total mass of water vapor present in a given volume of air.
  2. Relative Humidity-the amount of water vapor in a given volume of air at a given temperature.
  3. Specific Humidity-the ration of the mass of  water vapor to the mass of the parcel of air.

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