(a) With the aid of labelled diagrams, describe the instruments used to measure rainfall. (b) Explain how the instruments are used to obtain total annual ra...

(a) Instruments used to measure rainfall

Rainfall is measured with a rain gauge and a graduated measuring cylinder. The rain gauge has a metal outer cylinder sunk firmly in the ground. At its top is a funnel with a receiving rim of known area. The funnel directs rainwater into a glass collecting bottle placed inside the outer cylinder. The collected water is poured into a graduated measuring cylinder and its depth is read in millimetres.

(b) How total annual rainfall is obtained

1. The rain gauge is inspected at the same fixed time every day, commonly 09:00 hours.
2. The funnel is removed and the water in the collecting bottle is poured into the graduated measuring cylinder.
3. The water level is read at eye level from the graduation in millimetres. This gives the daily rainfall.
4. The daily rainfall readings are added to obtain each month's total rainfall.
5. The totals for the twelve months are added to obtain the total annual rainfall:

\[\text{Total annual rainfall} = \sum_{m=1}^{12}\text{monthly rainfall total}\]

(c) Precautions

1. The rain gauge should be sited in an open space, well away from trees, buildings and walls that may intercept rain or cause dripping.
2. It should be fixed firmly with the rim level and about 25 to 30 cm above the ground, to prevent splashing into the gauge.
3. Readings should be taken daily at the same time, and the collecting bottle should be emptied after each reading.