Class 7 Social Science Chapter 2: Understanding the Weather Notes

What is this chapter about?

On a winter morning you shiver and pull on a sweater; in summer you pick light clothes. Your body is sensing the weather. This chapter turns that feeling into numbers. It asks two big questions: how can we measure and monitor the weather around us, and how do weather predictions help us prepare for heavy rain, storms, drought and heat waves?

Weather and the atmosphere

Weather is the state of the atmosphere at a particular time and place. The atmosphere is the layer of gases, which we call air, around the Earth. Like a cake, it has layers. The lowest is the troposphere, where plants, animals and people live and where almost all weather happens. It is about 6 km thick over the poles, where cold air contracts, and about 18 km over the tropics, where warm air expands.

The five elements of weather

Words such as hot, rainy, windy and humid describe five elements of weather: temperature (how hot or cold the air is), precipitation (rain, snow, sleet or hail), atmospheric pressure (how hard the air overhead presses down), wind (moving air, with its speed and direction) and humidity (the water vapour in the air). Saying “it is chilly” means different things in Chennai and in Kashmir, so we need agreed units to share the weather. Read more in elements of weather.

Nature’s clues

Long before instruments, people read signs in Nature. Ants shifting their eggs to higher ground, frogs croaking loudly, birds flying low and squirrels gathering nuts hinted at coming rain or storms, and pine cones close in damp air and open in dry air. In many parts of India such traditional knowledge is still used, especially to guess when the monsoon will arrive.

Measuring the weather

Today meteorologists, who study the weather, use instruments. Thermometers measure temperature, often recording the day’s maximum and minimum; from these come the range (maximum − minimum) and the mean daily temperature ((maximum + minimum) ÷ 2). A rain gauge measures rainfall as the depth of water collected: 5 mm of water means 5 mm of rain. A barometer measures pressure in millibars, about 1013 mb at the coast. Pressure falls as you climb, which is why people feel breathless high up; at Khardung La in Ladakh, over 5,600 m above sea level, it is about 650 mb. A sudden fall near the coast, below about 1000 mb, is a depression, which can grow into a storm or cyclone.

Wind blows from high to low pressure. A wind vane or windsock shows its direction and an anemometer, with spinning cups, its speed in km/h. A hygrometer measures relative humidity: 20–40% is dry weather and 60–80% is humid. See weather instruments.

Weather stations

A weather station keeps all these instruments in one place, and readings are taken at regular intervals. An Automated Weather Station (AWS) records and sends the data by itself. In 2023 an AWS was installed by the National Disaster Management Authority beside a glacial lake in Sikkim, more than 4,800 m above sea level, to give early news of the weather there.

Predicting the weather

Meteorologists study data collected over long periods and use scientific methods to forecast the weather. Forecasts matter more as climate change makes droughts, floods and cyclones more frequent. The India Meteorological Department (IMD), set up in 1875, issues warning maps in four colours: green for no warning, yellow to watch, orange to be prepared and red to take action. Warnings let governments get ready, keep fishermen ashore before a storm and move people away from a coast before a cyclone. Read weather forecasts and warnings.

Before we move on

Together the five elements describe the weather of a place, and instruments let us monitor and predict them. At times one element dominates: rainfall in July, temperature in May and December, pressure when a cyclone moves in, and wind when the hot, dusty loo blows across north India or a forest fire spreads. Weather is closely linked to climate, the subject of the next chapter. Answers to every question in the chapter are in the NCERT solutions for Chapter 2.