EN हिं

NCERT Solutions Exploring Society: India and Beyond Chapter 2 b) Precipitation — In-text Questions

Book page 33 Updated on2026-09-05

Q1.
If the news says that a particular place received 30 mm of rainfall in a day, what does it mean?
Answer

It means that if none of the rain had run away, soaked in or evaporated, the rain that fell in that day would have covered the whole area with a layer of water 30 millimetres (3 cm) deep.

Rainfall is measured as a depth, not as a volume
30 mm of rain on 1 square metre = 30 litres of water
30 mm of rain on 1 hectare = 3,00,000 litres
Why depth and not litres: Depth lets you compare any two places directly. A big city and a small field both get “30 mm” if the rain over them was equally heavy — the total number of litres would be wildly different, but the depth is the same. That is what makes 30 mm in Chennai comparable with 30 mm in Cherrapunji.
Tip: As a rough guide, under 10 mm in a day is light rain, 30 mm is a good soaking shower, and over 100 mm is the ‘extremely heavy rain’ the IMD marks on its warning maps.
Q2.
How is rainfall measured?
Answer

With an instrument called a rain gauge (Fig. 2.6). It has three parts, and each one has a job:

PartWhat it does
Collecting funnelCatches the rain that falls on a known, fixed area and guides it downwards
Measuring tube (cylinder)Stores the water that has been collected
Measuring scaleMarked in millimetres on the tube, so the depth of the collected water can be read off

When it rains, the water falls into the funnel and is collected in the cylinder. The scale attached to the cylinder gives the depth of the rainwater. If the height of the water collected is 5 mm, the area received 5 mm of rainfall.

Tip: The reading is always taken at the same time every day — the IMD uses 0830 hours — so that every day's figure covers exactly 24 hours. If snow falls instead of rain, it is allowed to melt first, and then measured.
Why the funnel matters: The funnel is wider than the tube, so a small depth of real rainfall becomes a taller, easier-to-read column of water inside the narrow tube. The scale is already adjusted for this, so you read the true rainfall directly.
Was this helpful? Report an error