What could be the possible method to compile this data?
Answer
Rain gauges in each city measure the rainfall every day. The daily figures are added to give a monthly total, and this is repeated for several years. The totals for the same month across those years are then averaged — so ‘average rainfall in June at Kovalam’ means the mean of the June totals over all the years recorded.
Why an average is used: a single year’s rainfall can be freakish — one cloudburst, or one failed monsoon. Averaging over years smooths out those accidents and leaves the underlying pattern of the seasons, which is what the graph is trying to show. The price is that the graph can no longer tell you about any particular year.
Q2.
Mark these cities on a map of India. What is common to how they are grouped in the graphs? Share your observations and inferences about the graphs.
Answer
The six cities split neatly into two coasts, and that is exactly how the two graphs are grouped.
Group
Cities
Where they are
First graph
Kovalam, Udupi, Mumbai
Along the west coast (Kerala, Karnataka, Maharashtra)
Second graph
Rameswaram, Chennai, Puri
Along the east coast (Tamil Nadu, Tamil Nadu, Odisha)
Observations: the west-coast cities show one tall, narrow peak in the middle of the year, and their rainfall is much heavier — Udupi’s peak runs to several hundred millimetres a month. The east-coast cities have flatter, later peaks and receive less rain overall.
The inference: the west coast faces the Arabian Sea and the Western Ghats stand right behind it, so the monsoon winds are forced up and drop their moisture at once — a lot of rain, in a short season. The east coast lies in the rain shadow of those hills for the same winds and has to wait for the returning winds later in the year.
Q3.
Identify the peak months and low months of rainfall for each city.
Answer
City
Coast
Peak months
Low months
Kovalam
West
June – August
January – March
Udupi
West
June – August
January – March
Mumbai
West
June – August
January – March
Puri
East
July – September
January – March
Chennai
East
Rain from June, peaking in November, continuing into December
January – March
Rameswaram
East
October – December
January – September (very little rain)
January to March are dry for every one of the six cities. Rameswaram is the extreme case — it gets almost nothing for nine months and then nearly all of its rain in the last three.
Q4.
Read about the south-west monsoon and north-east monsoon and which regions come under the influence of these and when.
Answer
India has two monsoons, and the graphs are a picture of both.
South-west monsoon
North-east monsoon
When
June to September
October to December
Wind direction
From the south-west, off the Arabian Sea and Bay of Bengal
From the north-east, back across the Bay of Bengal
Who gets the rain
Most of India — especially the west coast, the Western Ghats and the north-east
Tamil Nadu, coastal Andhra Pradesh, south Karnataka, Kerala’s coast, Puducherry
How the graphs show it: Kovalam, Udupi, Mumbai and Puri all peak between June and September — the south-west monsoon. Rameswaram and Chennai peak between October and December — the north-east monsoon, sometimes called the retreating monsoon. Chennai shows both: some rain from June onwards and its real peak in November. This is why a ‘monsoon holiday’ falls at very different times in Mumbai and in Chennai.
Did you know? The north-east monsoon carries winds that have crossed the Bay of Bengal and picked up moisture there, which is why they bring rain to the eastern coast rather than to the dry interior they started from.