Class 7 Social Science Chapter 2: Understanding the Weather

A body that reads the weather

You wake on a winter morning and shiver, so you reach for something thick to wear. When summer comes you pick something that keeps you cool instead.

Nobody taught you to do that. Your body is reading the weather and you are answering it.

What weather actually is

Weather is the state of the air around you at one particular time and one particular place. Both halves of that matter.

Change the time and the weather changes. Change the place and it changes again.

The blanket of gases

That air is part of the atmosphere, the layer of gases wrapped round some planets. On Earth we simply call those gases air.

It helps to picture the atmosphere as a cake built in layers, each one sitting on the one below.

The layer we live in

The layer touching the ground is the troposphere. Every land plant and animal lives and breathes inside it, ourselves included.

Nearly all weather happens there too. So when this chapter talks about weather, it is talking about the bottom layer of the cake.

Thicker in some places

The troposphere reaches somewhere between 6 and 18 kilometres above the ground, and the figure is not the same everywhere.

It is thinnest over the poles, where cold air contracts, and thickest over the tropics, where warmer air expands. The air itself decides how far up the layer goes.

Five things to measure

We describe weather with a pile of words: hot, cold, rainy, cloudy, humid, snowy, windy. Behind all of them sit five measurable elements.

They are temperature, precipitation, atmospheric pressure, wind and humidity. Learn those five and every weather word slots into one of them.

Temperature and precipitation

Temperature is simply how hot or cold the air is.

Precipitation is water in any form falling from the sky. Rain is the obvious one, but snow counts, and so do sleet, which is rain frozen or part frozen, and hail, which falls as small hard balls of ice.

Pressure, wind and humidity

Atmospheric pressure is what all the air overhead weighs, pressing down and felt at ground level.

Wind is air on the move, described by both its speed and its direction. Humidity is how much water vapour the air holds, vapour being water in its gaseous form rather than liquid.

Why a feeling is not enough

Imagine Krishnan in Chennai telling Amir in Kashmir that it has turned chilly after last night’s rain. Amir asks how cold it is.

Krishnan is stuck. What counts as chilly in Chennai might feel pleasant in Kashmir, so the word carries no useful information between them.

What fixes it

The problem is not that Krishnan lacks words. It is that the two of them have no shared scale.

Say twenty degrees instead and both know exactly what is meant. That is the whole reason for measuring the weather rather than describing it, and every instrument in this chapter exists to do it.

Reading Nature instead

Long before instruments, people forecast weather by watching closely. Forecasting simply means working something out in advance.

Birds flying low, ants carrying their eggs, squirrels gathering nuts and frogs croaking hard were all read as signs of rain or storm coming.

The pine cone

One of those signs still works, and the reason is understandable. A pine cone closes up when the air is humid and opens when it is dry.

Closing protects the seeds in weather that would not suit them. Opening releases them when conditions are good, so they travel in dry air and land somewhere useful.

Passed down, and still used

That knowledge moved from one generation to the next without being written in any book.

In many parts of India people still read such signs today, especially when judging whether the monsoon is about to arrive.

Where the science came in

Over the last few centuries, instruments have let us measure the same elements very precisely indeed.

The systematic study of weather and how it changes is called meteorology, and the people who do it are meteorologists. Their forecasts may run a few hours ahead, a few days, or even weeks.

What carries forward

Weather is the state of the air at one time and one place, and nearly all of it happens in the troposphere, the bottom layer of the atmosphere, which runs 6 to 18 kilometres up and is thinnest over the poles.

Five elements describe it: temperature, precipitation, atmospheric pressure, wind and humidity. Describing weather in words fails the moment two people compare notes, because chilly means different things in Chennai and Kashmir. A shared scale fixes that, which is why meteorology measures rather than describes.

Scenario:

Krishnan in Chennai rings Amir in Kashmir after a night of rain, and says it has turned chilly. Amir asks exactly how cold it is, and Krishnan finds he cannot answer in a way that helps. Later the same week Amir plans to visit, and asks what the weather will be like. Krishnan says he has noticed birds flying low and the ants busy with their eggs, so he expects rain. Amir laughs and says that is just an old story with nothing behind it.

Analysis:

One call, two problems: a word that will not travel, and a sign that is dismissed too quickly.

Activity:

Keep a weather diary for a week, writing both a describing word and a measured temperature each day, and compare how useful each is.

Title:

Describe it, then measure it

Safety:

No hazards here.

What You Notice:

The words disagree surprisingly often, because warm and cool are judged against what each person is used to. The numbers agree every time, which is the point of measuring rather than describing.

The troposphere is thinner over the poles because cold air contracts, and thicker over the tropics because warm air expands.

A pine cone closes in damp air to protect its seeds and opens in dry air so that they scatter in good conditions.

People in many parts of India still read natural signs to judge when the monsoon will arrive.

Weather is air, here and now.

Five elements: heat, water falling, weight, movement, vapour.

From a feeling to a number

The last topic ended with a problem. A describing word will not travel between two people, so the weather has to be measured.

This topic takes the first three elements and shows the instrument that measures each one.

Thermometers

Temperature comes first. Some thermometers simply read the air around them, which is called the ambient temperature.

Others are built to record the highest and lowest readings reached during a day, which is more useful than a single glance.

How a simple one works

Many thermometers hold a coloured liquid in a narrow tube. The liquid expands when it gets warmer and so climbs the tube.

Digital thermometers are steadily replacing them, because they read more precisely and can store far more readings.

Two useful numbers

Once a day’s highest and lowest are known, two statistics follow. The range is simply the highest minus the lowest, usually over 24 hours.

The mean daily temperature is the highest plus the lowest, divided by two. Both say something a single reading cannot.

A week of readings

Suppose a city in Madhya Pradesh records highs of 29, 30, 31, 32, 30, 28 and 29 degrees Celsius over a week, with lows of 16, 15, 17, 18, 17, 14 and 15.

The highest reading that week is 32 and the lowest is 14, so the range across the week is 18 degrees.

Working one day

Take the day with the high of 32. Its low was 18.

The range for that day is 32 minus 18, which is 14 degrees. The mean daily temperature is 32 plus 18 divided by two, which is 25 degrees.

More than one scale

A number only helps if you know which scale it is on. Celsius is the one used here, written with a small circle before the C.

The Fahrenheit scale also exists. A cool 15 degrees Celsius is the same air as 59 degrees Fahrenheit, which shows how badly a number needs its scale attached.

Who does the measuring

India has had a national weather service for a very long time. India has had a national weather service since 1875, the India Meteorological Department.

Its motto, adityat jayate vrishti, means that rain arises from the sun. The line comes from the Manusmriti, and continues that food comes from rain, and living beings from food.

Measuring the rain

Precipitation is next, and the instrument is a rain gauge. Rain falls into a funnel and collects in a cylinder below it.

A scale on the cylinder reads off the depth collected. If the water stands 5 millimetres deep, the area has had 5 millimetres of rainfall.

Why depth, not volume

That is worth pausing on. Rainfall is reported as a depth rather than as a quantity of water.

A depth works for any size of container and any size of field, so the figure means the same thing everywhere. Thirty millimetres in the news is thirty millimetres on every roof.

Air that feels heavy

The third element is atmospheric pressure. That is what the air overhead and all round us weighs, pressing down.

You may have felt the air turn heavy before a thunderstorm. That feeling is this element changing.

Pressure and height

Pressure is greater near the sea coast and falls as you climb. High up, the air is thinner than in the plains below.

Thinner air holds less oxygen, so less reaches the blood and the body has to work harder. That is why people feel breathless, dizzy or tired at altitude.

Giving the body time

Travellers going high are advised to stop on the way and let the body acclimatise, which means adjusting to a new condition.

Indian army personnel serve at Khardung la in Ladakh, over 5,600 metres above sea level, where the pressure is about 650 millibars.

Reading the pressure

The instrument is a barometer, and its usual unit is the millibar, written mb. Beside the sea the usual figure runs close to 1013 mb.

A reading below 1000 mb means a depression, also called a low-pressure system. Those can grow into a storm or even a cyclone, so the number is a warning as well as a measurement.

What carries forward

Thermometers measure temperature, and from a day’s highest and lowest come two statistics: the range, which is one subtracted from the other, and the mean daily temperature, which is their total halved. A number needs its scale, since 15 degrees Celsius is 59 degrees Fahrenheit.

A rain gauge collects rain in a cylinder and reports a DEPTH, which means the same on any roof. A barometer reads atmospheric pressure in millibars, around 1013 at the coast, and below 1000 signals a depression that may become a storm. India has measured all this since the India Meteorological Department was founded in 1875.

Scenario:

A school starts a weather board in its entrance hall. On Monday a pupil writes that the highest was 32 degrees and the lowest 18, and then records the range as 25. Another pupil writes the temperature as simply 15, with no scale named beside it. A third reports that the playground received two litres of rain during the night. On Friday the barometer reads 994 millibars. A fourth pupil looks at it and says that a number is only ever a measurement, and that no measurement can tell anybody anything about tomorrow.

Analysis:

Four entries on one board, and every one of them needs correcting.

Activity:

Keep a daily high and low for two weeks, and plot both the range and the mean to see which varies more.

Title:

Build a rain gauge and prove the depth rule

Safety:

Mop up spills; water on a floor is slippery.

What You Notice:

The depths agree closely between the two containers, while the quantities do not, because a wider container catches more water from the same shower. That is exactly why rainfall is reported as a depth.

The India Meteorological Department was founded in 1875, and its motto says that rain arises from the sun.

At Khardung la in Ladakh, over 5,600 metres up, the pressure is about 650 millibars against roughly 1013 at the coast.

Rainfall is reported as a depth so that the same figure applies to a rooftop and to a whole field.

Range subtracts, mean adds and halves.

Rainfall is a depth, not a bucketful.

Air that moves

Two elements are left, and the first is wind. Wind is simply air moving from somewhere with high pressure towards somewhere with low pressure.

That connects it straight back to the barometer. Pressure differences are what make the air move at all.

Two things to know about it

A wind is never described by one number. You need its speed and you need its direction.

Leave either out and the description is useless. A strong wind from the sea and a strong wind off the desert are not the same news.

Who needs to know

Pilots and sailors watch wind data closely, because wind pushes aircraft and boats about.

Farmers use the direction to judge where rain is likely to come from. They also know that a faster wind dries the soil out more quickly.

The wind vane

The simplest instrument for direction is a wind vane, sometimes called a weather vane. An arm turns on it, carrying a pointer at the front and a broad tail behind.

The wind pushes the broad tail round, which swings the pointer to face into the wind. It will move for even a light breeze.

The wind sock

At an airfield you will see a different version, a fabric tube called a wind sock. It tells pilots which way the air is moving as they leave the ground and as they return to it.

Similar socks are used at industrial sites that release ash or gases, for the same reason: everybody needs to know which way the air is carrying things.

The anemometer

For speed, the usual instrument is an anemometer. Three or four metal cups sit on a vertical shaft and spin when the wind catches them.

A stronger wind spins them faster. A meter at the bottom counts the spins over a set time and works the speed out in kilometres per hour.

Humidity, the last element

Humidity is how much water vapour the air is carrying. It depends on temperature, on wind, on pressure and on where you are.

Kochi sits by the sea and Jaipur does not, so you would expect Kochi to be the more humid. Expecting is not knowing, which is why it is measured.

Measured against what the air could hold

Humidity is given as relative humidity, and the word relative is doing real work. Air with no water vapour at all would be 0%, though that never happens in nature.

Air holding as much as it possibly can is 100%. So the figure compares what the air holds with what it could hold.

Reading the percentages

In practice, dry weather falls somewhere between 20% and 40%. Humid weather usually sits between 60% and 80%.

Say Delhi reads 52% while Kochi reads 84%. Washing dries faster in Delhi, because drier air takes up evaporating water more readily, and you sweat more in Kochi for the same reason reversed.

The instrument

Humidity is read with a hygrometer, and there are several kinds depending on the principle each uses.

The measurement matters well beyond the weather report. Food processing depends on it, and museums watch it constantly, because a dry environment protects what they have on display.

Putting the instruments together

Five elements need several instruments, and keeping them scattered would be awkward. A weather station gathers them in one place.

Readings are taken at regular intervals, and that regularity is what makes mapping and forecasting possible. One reading is a fact; a series is a pattern.

Stations that run themselves

An Automated Weather Station, or AWS, is self-operating. Its sensors take temperature and humidity, the wind by speed and by direction, the rainfall and the pressure, and record every one with nobody present.

That matters for farming, aviation, navigation and environmental monitoring, and especially for places nobody can easily reach.

A station on a glacial lake

A glacial lake in Sikkim, more than 4,800 metres up, was fitted with one in 2023 by the National Disaster Management Authority.

It gives early information about weather coming in. Nobody could sit there taking readings by hand, which is exactly the point of an automatic station.

Why the forecast matters

Meteorologists gather data over long periods and study it to predict what the weather will do. This matters more than it used to, because climate change is making droughts, floods and cyclones more frequent.

A good forecast lets governments move resources before an event. Fishermen can be warned not to put to sea, and a coastal area can be cleared if a cyclone is coming.

What carries forward

Wind is air moving from high pressure to low, described by speed and direction. A wind vane and a wind sock give direction, and an anemometer counts cup spins to give speed in kilometres per hour.

Relative humidity compares what the air holds with what it could hold, 20 to 40% being dry and 60 to 80% humid, read with a hygrometer. A weather station brings the instruments together, an automated one needs nobody present, and the forecasts they allow let people act before a storm arrives rather than after.

Scenario:

A coastal village relies on its boats. The local school has set up a weather board, and a pupil reports that the wind is strong, without saying anything else. Another pupil says the air feels very wet, so washing will dry quickly today. A visitor asks why the district has put an automatic station on a hill nobody climbs, when a person with instruments would be cheaper. Then the forecast warns of a cyclone building offshore, and the village has to decide what to do.

Analysis:

Four questions from one village, and the chapter answers all of them.

Activity:

Watch a week of local forecasts and note every warning issued, then check afterwards what actually happened.

Title:

Make a vane and count the spins

Safety:

Mind the pin, and keep it away from your eyes.

What You Notice:

The pointer settles facing into the wind, because the larger tail catches more air and is pushed round. The spinner turns far more often on the windy day, which is the principle an anemometer uses to measure speed.

A wind sock is used not only at airfields but at industrial sites that release ash or gases, so everybody can see which way the air carries them.

Museums watch humidity closely, because a dry environment protects the objects they have on display.

In 2023 an automated station was placed at a glacial lake in Sikkim, more than 4,800 metres above sea level.

Wind runs downhill in pressure.

Relative humidity: what it holds against what it could.