Understanding the Weather

Why This Matters

You wake up one winter morning and shiver. You quickly reach for a thick sweater. In summer, you do the opposite. You pick light, cool clothes so you do not sweat all day.

Did you notice something? Your body already knows what the air outside is like. Hot, cold, wet, dry, windy — your skin feels it. That feeling is the weather.

But “it feels hot” is not enough. Think about this. A boy named Krishnan in Chennai says, “It got chilly here after last night’s rain.” His friend Amir lives in Kashmir, where it is much colder. Amir asks, “How chilly? How cold exactly?”

Krishnan is stuck. What feels cold to him might feel pleasant to Amir. They need numbers. They need a way to measure the weather that means the same thing to both of them.

That is what this chapter is about. You will learn what the weather is really made of, how we measure each part with simple instruments, how rain forms, how to read a weather forecast on a phone or TV, and how weather is different from climate. By the end, you will be able to read a weather report like a scientist.

The Big Idea

Weather is what the air is like at one place at one moment — hot or cold, wet or dry, calm or windy. It is built from five parts called the elements of weather: temperature, humidity, air pressure, wind and rainfall. Each part can be measured with its own instrument and written as a number. When scientists called meteorologists measure these numbers and watch how they change, they can predict the weather for the coming hours or days. And when we take the average weather of a place over many, many years, we get its climate — the usual pattern, not just today.

Let’s Break It Down

We will go slowly. First, what weather actually is. Then each of its five elements, one by one, with the instrument that measures it. Then how rain forms, how to read a forecast, and finally how weather is different from climate.

What is weather?

Weather is the state of the air around us at a particular time and place. In short, weather is what the air is doing right now, right here.

The “air around us” has a proper name: the atmosphere. The atmosphere is the blanket of gases that wraps all around the Earth. We simply call these gases air.

The atmosphere is not one even blanket. It is made of layers stacked on top of each other, like the layers of a cake. The layer right at the bottom, the one touching the ground where we live and breathe, is called the troposphere. Almost all the weather you will ever feel — every cloud, every shower, every gust of wind — happens in this bottom layer.

Figure 2.1 below shows these layers and points out where the weather lives.

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We use many everyday words to describe the weather: hot, cold, rainy, cloudy, humid, snowy, windy. Each of these words is really describing one of the elements of weather. Let us meet them now.

The elements of weather

The weather is made of five main parts. Scientists call them the elements of weather. (Here “element” just means a building block — one of the pieces that make up the whole.) The five elements are temperature, humidity, air pressure, wind and rainfall.

The clever thing is that each element has its own instrument that measures it and turns it into a number. Figure 2.2 shows all five together with the instrument for each. We will then take them one at a time.

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Temperature — measured by a thermometer

Temperature tells us how hot or cold the air is. That shiver on a winter morning? Low temperature. The sweat on a summer afternoon? High temperature.

We measure temperature with an instrument called a thermometer. A simple thermometer has a thin tube with a coloured liquid inside. When the air gets warmer, the liquid expands (spreads out and takes up more room) and climbs up the tube. When the air gets cooler, the liquid shrinks back down. We read the number next to the top of the liquid. Today, many thermometers are digital — they simply show the number on a screen.

You learned about thermometers in your Grade 6 Science book. Here is a quick reminder so you do not need to go back.

Now Krishnan’s problem is solved. Instead of saying “it feels chilly,” he can tell Amir, “it is 20 °C here.” That number means exactly the same thing in Chennai and in Kashmir.

Humidity — measured by a hygrometer

Humidity is the amount of water vapour in the air. Water vapour is water in the form of an invisible gas. It is water that has spread into the air and you cannot see it.

When there is a lot of water vapour in the air, we say humidity is high. The air feels sticky and heavy, and your sweat does not dry quickly. When there is little water vapour, humidity is low. The air feels dry, and washed clothes dry fast.

Humidity is usually written as relative humidity, a percentage from 0% to 100%. Air with no water vapour at all would be 0% (this never really happens). Air that is completely full of water vapour and can hold no more is 100%. Dry weather is usually about 20% to 40%. Humid weather is usually about 60% to 80%.

We measure humidity with an instrument called a hygrometer.

Here is a quick “why” check before we move on.

Air pressure — measured by a barometer

Air is light, but it is not weightless. There is a tall column of air stretching above your head, all the way up to space. All that air has weight, and it presses down on everything below. This push is called air pressure (or atmospheric pressure). It is the weight of the air above us, felt on the Earth’s surface.

You sometimes feel air pressure without knowing it. Before a thunderstorm, the air can feel “heavy”. That is air pressure changing.

Air pressure is higher near the sea coast and lower high up on a mountain. Why? Figure 2.3 shows the reason.

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Think of it like a stack of books. If you are the bottom book, the whole pile presses on you — high pressure. If you are near the top of the pile, only a book or two presses on you — low pressure. At the sea coast you are at the bottom of the air. High on a mountain you are near the top, so less air presses down.

That is also why people often feel breathless or dizzy high up a mountain. The air there is thinner (more spread out), so each breath gives the body less oxygen, and the body has to work harder.

We measure air pressure with an instrument called a barometer. Air pressure is usually written in a unit called the millibar (mb). Normal pressure at the sea coast is about 1013 mb. When the pressure drops well below 1000 mb, meteorologists call it a depression or low-pressure system. A deep low-pressure system can grow into a storm or even a cyclone — so watching air pressure helps warn people early.

Wind — measured by an anemometer

Wind is simply moving air. Two things matter about wind: its speed (how fast it moves) and its direction (which way it comes from).

But why does air move at all? This connects to what we just learned. Air always moves from a place of high pressure to a place of low pressure — just like air rushes out of a balloon (high pressure inside) into the room (lower pressure outside) when you let it go. That rushing air, on a big scale across the land, is wind.

Wind matters to many people. Pilots and sailors must know wind speed and direction before they fly or sail. Farmers watch the wind to guess where rain might come from. Strong wind also dries the soil faster.

We measure wind with an instrument called an anemometer. An anemometer has three or four small cups on arms that spin around a pole. The stronger the wind, the faster the cups spin. A meter at the bottom counts the spins and works out the wind speed in kilometres per hour (km/h). (A simpler tool called a wind vane has a pointer that turns to show which direction the wind is coming from.)

Rainfall — measured by a rain gauge

Rainfall is the water that falls from the sky as rain. The bigger word is precipitation, which means any water that falls from the sky — not just rain, but also snow (frozen white flakes), sleet (partly frozen rain) and hail (small hard balls of ice).

When the news says “Mumbai got 30 mm of rainfall today,” what does 30 mm mean? It means that if the rain had collected on flat ground without draining away, it would have made a layer 30 millimetres deep.

We measure rainfall with an instrument called a rain gauge. A rain gauge is a funnel sitting on top of a measuring cylinder. Rain falls into the funnel, runs down, and collects in the cylinder. A scale on the cylinder shows the depth of water collected. If the water reaches the 5 mm mark, that area received 5 mm of rain.

How rain forms (and why)

We measure rainfall with a rain gauge — but where does rain actually come from? Why does water fall out of the sky at all? The answer is a journey that water makes over and over, called the water cycle. And it all starts with the Sun.

The India Meteorological Department (our country’s official weather office) has a lovely old motto: “From the sun arises rain.” That sounds strange at first. How can the hot, dry Sun make rain? Figure 2.4 shows the whole story.

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Let us walk through the three steps, and the why behind each.

1. Evaporation. The Sun’s heat warms the water in seas, lakes and rivers. When water gets warm enough, some of it changes from liquid into an invisible gas — water vapour. This change is called evaporation. The warm, light vapour rises up into the air. (This is the “from the sun” part — without the Sun’s heat, there would be no vapour and no rain.)

2. Cooling and condensation. The higher you go, the colder the air gets. As the vapour rises, it cools down. Cold air cannot hold as much vapour, so the vapour turns back into tiny liquid water drops. This change, from gas back to liquid, is called condensation. Millions of these tiny drops floating together are what we see as a cloud.

3. Rain. Inside the cloud, the tiny drops bump into each other and join up. They grow bigger and heavier. When a drop becomes too heavy for the air to hold up, it falls — and that falling water is rain. If the air on the way down is very cold, the water freezes and falls as snow or hail instead.

So the Sun’s heat lifts water up, cold air turns it into clouds, and gravity pulls it back down as rain. That is why the old motto is exactly right: from the sun arises rain.

Reading a weather forecast

Now you know the five elements and how rain forms. So you can read a real weather forecast. A forecast is a prediction of what the weather will be like in the coming hours or days. Meteorologists make it by measuring all the elements at many places, then using science to work out what comes next.

A forecast usually shows you the same few numbers each time. Figure 2.5 is a sample forecast card, like one you might see on a phone app or a news channel.

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Let us read this forecast together to make a real decision, just like a meteorologist would help a traveller.

Weather vs climate

There is one last idea, and it is the one students mix up the most. The difference between weather and climate.

Weather is what the air is doing right now, at one place, on one day. It changes quickly — it can be sunny in the morning and rainy by evening.

Climate is the average weather of a place taken over a very long time — usually 25 years or more. It tells us the usual pattern of a place, not what is happening today.

Here is a simple way to remember it. Weather is your mood today; climate is your personality. Your mood can change in an hour — happy now, grumpy later (that is weather). But your personality is your usual nature over the years (that is climate).

Figure 2.6 shows the difference as a picture.

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Let us put the two side by side so the difference is crystal clear.

Common Mistakes

Let us clear up the three ideas students most often get wrong about weather.

Quick Check

Time to test yourself. Pick one option in each question.

Practice Problems

Try each one yourself first. Then tap to reveal the full answer.

Easy

Medium

Challenge

Summary

• Weather is the state of the air at one particular place and time. Almost all weather happens in the troposphere, the lowest layer of the atmosphere.
• The weather is built from five elements: temperature, humidity, air pressure, wind and rainfall.
• Each element has its own instrument: thermometer (temperature, in °C), hygrometer (humidity, in %), barometer (air pressure, in millibars), anemometer (wind, in km/h) and rain gauge (rainfall, in mm).
• Air pressure is the weight of the air above us. It is high at the sea coast and low high up a mountain, because there is less air above you up high.
• Rain forms through the water cycle: the Sun evaporates water into vapour, the vapour cools and condenses into clouds, and heavy cloud drops fall as rain.
• A weather forecast predicts the coming weather. Reading it means reading the numbers for each element and understanding what they mean for your day.
• Weather is one place on one day and changes fast. Climate is the average weather of a place over many years and stays steady for a long time.

What’s Next

You now understand the weather — what it is, how we measure it, and how it is different from climate. But what is the climate of India? Why does Rajasthan get so little rain while Cherrapunji gets oceans of it? Why does most of India wait every year for the monsoon?

In the next chapter, Climates of India, you will explore how climate changes from one part of our huge country to another, and what gives India its very own weather pattern through the seasons.