Understanding the Weather – Class 7 Social Science Chapter 2 Notes & NCERT PDF

The Big Questions

"A change in the weather is sufficient to create the world and oneself anew." — Marcel Proust, French novelist

Introduction

On a winter morning you shiver and reach for thick clothes; in summer you choose clothes that keep you cool. Your body is sensing the weather.

Weather is the state of the Earth's atmosphere at a particular time and place.

The atmosphere is the layer of gases around the Earth — the air. Like a layered cake, it has several layers. The one closest to the ground is the troposphere, where plants, animals and humans live and where almost all weather happens. It extends 6 to 18 km up — thinner at the poles (cold air contracts) and thicker in the tropics (warm air expands).

Key Concepts

1. Elements of weather

Why measure? Krishnan in Chennai says it is "chilly"; Amir in Kashmir may find that pleasant! Without a common standard, feelings about weather can't be compared. Measurements also let us plan ahead.

2. Reading Nature's clues

Long before instruments, people forecast weather by watching Nature: birds flying low, ants moving eggs to higher ground (heavy rain expected), squirrels gathering nuts, frogs croaking loudly, and pine cones (they close in humid weather to protect seeds and open in dry weather to release them). Many communities still use such signs, especially for the monsoon.

Meteorology is the systematic study of weather and its evolution — the basis of weather forecasting.

The India Meteorological Department (IMD) was set up in 1875. Its motto, ādityāt jāyate vṛiṣhṭi — "From the sun arises rain" — comes from the Manusmṛiti.

3. Weather instruments

a) Temperature — thermometer. Scales: Celsius and Fahrenheit (15°C = 59°F). Some thermometers show the present (ambient) temperature; others record the day's maximum and minimum. Digital thermometers are more precise and store more data. Useful statistics:

Worked example (the book's Madhya Pradesh chart, 28 Feb – 6 Mar 2025): highest maximum = 32°C (3 March), lowest minimum = 14°C (5 March), so the week's range = 32 − 14 = 18°C. On 3 March the mean temperature = (32 + 18) ÷ 2 = 25°C.

b) Precipitation — rain gauge. Rain falls into a funnel and collects in a cylinder with a scale. If the water is 5 mm deep, the area received 5 mm of rainfall.

c) Atmospheric pressure — barometer. Pressure is higher near the sea coast and lower up in the mountains, where the air is thinner and holds less oxygen — people may feel breathless, dizzy or tired. Pressure is shown in millibars (mb): about 1013 mb is normal at the coast; below 1000 mb signals a depression (low-pressure system) that can grow into a storm or cyclone. At Khardung La (over 5600 m, Ladakh) the pressure is about 650 mb; travellers must acclimatise.

d) Wind — wind vane and anemometer. Wind blows from high pressure to low pressure. A wind vane points in the wind's direction; the windsock at airports helps pilots during take-off and landing. The anemometer has 3–4 cups that spin faster in stronger wind; a meter converts spins into speed in km/h. Pilots, sailors and farmers all need wind data, and stronger wind dries soil faster.

e) Humidity — hygrometer. Humidity is given as relative humidity: 0% (no water vapour — impossible in nature) to 100% (saturated). Dry weather: about 20–40%; humid weather: about 60–80%. Remember: evaporation cools, and in humid air water evaporates slowly. Industries (food processing) and museums monitor humidity.

4. Weather stations and prediction

A weather station brings all instruments together and takes readings at regular intervals. An Automated Weather Station (AWS) does this with sensors and no human needed. In 2023, the National Disaster Management Authority set up an AWS at a glacial lake in Sikkim, more than 4800 m above sea level, for early warnings.

Forecasts matter more as climate change makes droughts, floods and cyclones more frequent. Warnings let governments prepare, warn fishermen not to go out to sea, or evacuate coastal areas before a cyclone. IMD warning maps use colours: Watch (be updated), Alert (be prepared), Warning (take action).

Let's Explore / Think About It

Key Terms

Connect to Today

Before the monsoon, farmers watch forecasts to plan sowing; airports check wind before every flight; weather apps on phones use data from thousands of stations. In north India the loo — strong, hot, dusty summer winds — is a reminder of how one element can dominate the weather.

Common Mistakes

Questions and Activities — Hints