03. Relative Humidity

Relative humidity - the amount of water vapour in the air as a percentage of the total amount of water vapour the air can hold at a particular temperature.

Instrument used to measure relative humidity - wet and dry bulb thermometers or sling psychrometers and a relative humidity chart.

Sling Pyschrometer

Using a relative humidity chart to read the relative humidity

Graph showing how relative humidity changes with temperature throughout the day

· Temperature rise/increase from around 18°C at 6 am to around 31 °C at 6 pm and

· Relative humidity fall/decrease from around 62% to 8%

· Temperature fall/decrease from around 31°C at 6 pm to around 18 °C at 6 am and

· Relative humidity rise/increase from around 8% to 62%

· At higher temperatures, the air will be able to hold more water vapour.

As air molecules are more spread out, more water vapour can fill the spaces. Thus the relative humidity is low.

· Therefore, assuming the actual water vapour in air is constant, relative humidity is lower at higher temperatures and higher at lower temperatures.. There is an inverse relationship between temperature and relative humidity.

If you are asked to describe from the diagram or graph, you must state the temperature and relative humidity`at the different time of the day to show that you are making reference to it with the data.