Exploring Society I · Chapter 2 — Understanding the Weather

On a winter morning you shiver and pull on a sweater; on a May afternoon you look for shade and cold water. Your body is sensing the weather, and you change what you wear and do because of it.

Weather. Weather is the state of the Earth's atmosphere at a particular time and place. Both parts matter. The weather in Shimla this morning can be very different from the weather in Chennai this morning, and even Shimla's weather can change between morning and evening.

The atmosphere. The atmosphere is the layer of gases that surrounds a planet. On Earth we call these gases air. Think of the atmosphere as a cake with several layers stacked one on another.

The troposphere. The layer nearest the ground is the troposphere. All land plants and animals, including people, live and breathe in it, and almost all weather happens in it: clouds, rain, storms and wind. It reaches a height of about 6 to 18 kilometres. It is thinner over the poles, where cold air contracts, and thicker over the tropics, where warm air expands. Above it lie other layers such as the stratosphere, which holds the ozone layer; you will study them in Science.

Words for weather. We describe weather with words such as hot, cold, chilly, pleasant, rainy, cloudy, humid, snowy and windy. Every Indian language has its own rich set of such words. Each word tells us how we are experiencing one or more elements of weather, which you will meet in the next topic.

Weather is not climate. Weather is about a short time: today, this week. Climate, which you will study in the next chapter, is the usual pattern of weather in a place over many years. 'It is raining in Mumbai today' is weather. 'Mumbai gets heavy rain every June to September' describes climate.

Why care? Weather affects what we wear and eat, when farmers sow, whether fishermen go to sea, whether planes fly and even whether schools open. Learning to measure it helps us plan and stay safe.

Krishnan in Chennai tells his friend Amir in Kashmir, 'It has become chilly here after last night's rain.' Amir asks, 'How cold?' What is chilly for Krishnan may feel pleasant to Amir. Without a shared way of measuring, they cannot really compare.

The elements of weather. Weather is made of five measurable elements:

Why measure precisely? Numbers give everyone a common language. If Krishnan says '20 degrees Celsius', Amir, used to much colder winters, will understand at once that Chennai's 'chilly' is mild for him. Precise measurements also let us forecast, that is, predict the weather in advance, so we can plan a trip, a harvest or a cricket match.

Nature's clues. Long before instruments existed, people watched nature closely and passed their knowledge down the generations. Signs that rain may be coming include:

• Birds flying low.
• Ants carrying their eggs to higher ground.
• Frogs croaking loudly, as in the Western Ghats before rain.
• Squirrels gathering nuts.
• Pine cones closing: they close in humid air to protect their seeds and open in dry air to release them.

In many parts of India people still use such traditional signs, especially to guess when the monsoon will arrive.

Meteorology. In the last few centuries scientists have built instruments to measure each element with great accuracy. The scientific study of the weather and how it changes is called meteorology, and the scientists are meteorologists. They collect readings, study patterns and predict weather hours, days or even weeks ahead.

Which element matters most? At different times one element takes the lead: rainfall in July, temperature in May and December, pressure when a cyclone is moving, and wind when the loo, the hot, dusty summer wind of north India, is blowing.

Snow melts quickly on a warm day, coconut oil turns solid in winter and curd takes longer to set when it is cold. All of these tell us about temperature, but a thermometer tells us exactly.

Scales. Temperature is measured in degrees on a scale. India uses the Celsius scale (written °C). Some countries use the Fahrenheit scale (°F). A cool 15°C is the same as 59°F.

Thermometers. A thermometer measures temperature. Some show the ambient temperature, that of the immediate surroundings. Others record the maximum and minimum temperatures of the day. Many use a coloured liquid that expands and rises as the temperature goes up. Today digital thermometers are increasingly preferred, because they are more precise and can store many readings.

Using the data. Readings become useful statistics, that is, organised data from which we can find patterns and make predictions. Two simple ones are:

• Range of temperature = maximum temperature minus minimum temperature over a period, usually 24 hours.
• Mean daily temperature = (maximum + minimum) divided by 2.

Practice with a week's data. Here are readings from a city in Madhya Pradesh, late February to early March:

The highest temperature of the week was 32°C (3 March) and the lowest 14°C (5 March), so the range for the whole week was 32 minus 14 = 18°C.

India's weather office. The India Meteorological Department (IMD) was set up in 1875. Its motto, from an ancient Sanskrit text, means 'From the sun arises rain'. The idea is sound: the sun's heat evaporates water, which rises, forms clouds and falls again as rain.

The evening news says, 'Kolkata received 30 mm of rain today.' What does 30 mm actually mean, and how did anyone measure it?

The rain gauge. Rainfall is measured with a rain gauge. It has three main parts:

• A funnel at the top that catches the rain.
• A measuring tube or cylinder below it where the water collects.
• A scale marked in millimetres to read the depth of water.

What the number means. Rainfall is reported as the depth of water that would cover flat ground if none of it soaked in, ran off or evaporated. If the water in the gauge stands 5 mm high, the place received 5 mm of rain. So 30 mm of rain means the rain would have formed a layer 3 centimetres deep.

Snow too. If snow falls into the gauge, it is allowed to melt first and the water is then measured.

Placing a rain gauge correctly. A reading is only useful if the gauge catches rain the same way every time. Good practice:

• Put it in an open area, away from trees, walls and roofs that could block rain or splash extra water in.
• Keep it level on a firm, flat surface so it does not tilt or blow over.
• Read it at the same time every day.

For example, a verandah or a spot beside a compound wall would be poor choices, because the roof or wall blocks or splashes rain. Open ground with a firm raised platform is a much better site.

Making sense of readings. With daily readings you can add up totals and work out averages. Suppose your gauge records 0, 12, 4, 0, 20, 6 and 0 mm over one week. The weekly total is 42 mm and the average is 42 ÷ 7 = 6 mm per day. Comparing weeks shows how rainfall varies, for instance as the monsoon arrives or weakens.

Who uses rainfall data? Farmers decide when to sow; city officials prepare drains before heavy rain; dam managers decide how much water to release; and scientists track whether a monsoon is normal, weak or strong. You can even make a simple rain gauge at school with a bottle, a funnel and a ruler taped to the side.

Before a thunderstorm the air sometimes feels 'heavy'. High on a mountain you may feel breathless after a few steps. Both experiences are linked to atmospheric pressure.

What it is. Atmospheric pressure is the push of the weight of all the air above and around us. We do not feel it crushing us because our bodies push back equally, but it is always there.

Pressure and height. Pressure is highest near sea level and falls as you go higher, because there is less air above you. Higher up, the air is also thinner, so each breath brings in less oxygen. Your body must work harder, so people at high altitude may feel breathless, dizzy or tired. Travellers are advised to stop and rest on the way up so the body can acclimatise, that is, adjust to new conditions.

The barometer. Pressure is measured with a barometer. Barometers usually show pressure in millibars (mb).

Indian soldiers serve at places like Khardung La, where the pressure is only about two-thirds of that at the coast and oxygen is scarce. Imagine working hard in such thin air.

Low pressure in the plains too. Pressure is not always high on the coast or plains. Sometimes it drops sharply over an area. Meteorologists call this a depression or low-pressure system. A depression can grow into a storm or even a cyclone. That is why a falling barometer is a warning sign.

Pressure and temperature. Warm air expands and rises, leaving lower pressure near the ground; cooler air sinks, giving higher pressure. These differences set air moving, as you will see in the next topic on wind.

Who needs pressure readings? Weather forecasters use them to spot storms early. Pilots need them to set their altimeters and fly safely. Sailors and fishermen watch for falling pressure as a sign of rough seas. Mountaineers use them to understand the risk of altitude sickness.

Have you watched a fluffy seed float away on a breeze? Without wind, many plants could not spread their seeds, kites would not fly and sailing boats would sit still.

What wind is. Wind is the movement of air from areas of high pressure to areas of low pressure. The bigger the difference in pressure, the stronger the wind. Two things describe any wind: its direction and its speed. A wind is named after the direction it comes from: a west wind blows from the west towards the east.

Why wind data matters.

• Forecasters use wind direction and speed to predict where clouds, rain and storms will move.
• Pilots and sailors need wind data, because wind strongly affects flying and sailing.
• Farmers use wind direction to judge where rain may come from.
• Strong winds dry the soil faster, which matters for crops.

Measuring direction: the wind vane. A wind vane (weather vane) has a rotating arm with a pointer at one end and a broad tail at the other. The wind pushes the tail, and the pointer swings round to face into the wind, showing where it is coming from. It responds even to a light breeze.

The windsock. At airports you may see a cloth tube on a pole, called a windsock. It fills with air and points away from the wind, telling pilots the wind's direction during take-off and landing. Factories that release ash or gases use similar socks.

Measuring speed: the anemometer. An anemometer has three or four cups fixed on arms that spin around a vertical shaft. The stronger the wind, the faster they spin. A meter counts the turns in a set time and works out the speed in kilometres per hour (km/h).

A wind to know: the loo. In north India in summer, a strong, hot, dusty wind called the loo blows in the afternoons. On such days wind becomes the most important weather element, and people are warned to stay indoors and drink plenty of water. Wind also spreads forest fires quickly.

Wet clothes dry in an hour in Jaipur but can stay damp all day in Kochi during the monsoon. The difference is the amount of water vapour already in the air.

Humidity. Humidity is the amount of water vapour in the air. Water vapour is water as an invisible gas. Humidity depends on temperature, wind, pressure and location; places near the sea, like Kochi or Mumbai, are often more humid than inland places like Jaipur. But to compare places properly, we must measure it.

Two facts from Science. First, evaporation causes cooling: when sweat evaporates from your skin, you feel cooler. Second, when the air already holds a lot of water vapour, water evaporates slowly, as on a rainy day.

Relative humidity. Humidity is usually given as relative humidity, as a percentage. Air with no water vapour at all would be 0% (this never happens in nature). Air holding all the water vapour it can, called saturated air, is 100%.

Applying it. If Delhi has 52% humidity and Kochi 84% at the same temperature, clothes will dry faster in Delhi. In Kochi, sweat evaporates slowly, so it stays on the skin and you feel hotter and stickier, even though the thermometer shows the same number.

The hygrometer. Humidity is measured with a hygrometer. There are several kinds, working on different principles. One common idea uses two thermometers, one dry and one with its bulb kept wet. Water evaporating from the wet bulb cools it. In dry air evaporation is fast, so the wet bulb cools a lot; in humid air it cools only a little. The difference between the two readings tells us the humidity.

Who needs it? Food-processing factories control humidity so food does not spoil. Museums keep their air dry to protect old paintings, manuscripts and objects from damp and mould. Weather forecasters use humidity to predict rain, fog and how hot a day will feel.

Packing for a trip. If the forecast for Mumbai in June shows 29°C and 84% humidity, light, loose cotton clothes that dry quickly, plus an umbrella or raincoat, would be wise.

A cyclone is moving across the Bay of Bengal. Two days before it reaches the coast, fishermen are told to stay ashore and villages are moved to safety. How did anyone know in time?

Weather stations. Measuring the weather needs many instruments at once. A weather station brings them together: thermometer, rain gauge, barometer, wind vane, anemometer and hygrometer. Readings are taken at regular times, so the data from many stations can be compared and mapped.

Automated Weather Stations. An Automated Weather Station (AWS) uses electronic sensors to measure and record temperature, humidity, wind, rainfall and pressure by itself, without a person reading the instruments. AWS are used in farming, aviation, shipping and environmental monitoring. In 2023 the National Disaster Management Authority set up an AWS beside a glacial lake in Sikkim, more than 4,800 metres above sea level, to give early warning of dangerous weather.

From data to forecast. Meteorologists collect readings over long periods, study the patterns and use scientific methods to predict what the weather will do next. Forecasts matter more and more because climate change is making extreme events such as droughts, floods, heat waves and cyclones more frequent.

Warnings in colour. The IMD issues maps that mark each region with a colour showing how serious the expected weather is, together with icons for heavy rain, thunderstorms, heat waves, cold waves, fog, dust storms, hail and more:

• No warning: no special action needed.
• Watch: be updated.
• Alert: be prepared.
• Warning: take action.

How forecasts help. Fishermen are told not to sail when the sea will be stormy. Whole coastal areas can be evacuated before a cyclone. Local governments move food, boats and rescue teams in advance. Farmers time sowing and harvesting. Families plan travel and protect themselves from heat waves.

Remember. Temperature, humidity, precipitation, wind and pressure together define the weather of a place at a moment. Measuring them carefully, everywhere and regularly, is what turns guesses into reliable forecasts.