Understanding the Weather

1. What is Weather?

When you wake up in the morning, you might notice that it is sunny, or cloudy, or that a cool breeze is blowing. All of these together describe the weather. Weather is the condition of the atmosphere at a particular place and time — whether it is hot or cold, wet or dry, calm or windy, clear or cloudy.

Weather affects almost everything we do. It decides what clothes we wear, whether we carry an umbrella, what farmers plant, whether aeroplanes can fly safely, and even how we feel. Because weather matters so much, people have always tried to understand and predict it.

2. The Atmosphere — The Air Around Us

All weather happens in the atmosphere, the thick blanket of air that surrounds the Earth. We cannot see air, but we can feel it when the wind blows and we need it to breathe. The atmosphere is held close to the Earth by the force of gravity.

Air is a mixture of gases. The most important are nitrogen (about 78%) and oxygen (about 21%). Small amounts of other gases such as carbon dioxide (CO₂) and water vapour are also present. Although they are present in tiny amounts, gases like carbon dioxide and water vapour play a big role in the weather.

The atmosphere acts like a warm blanket. During the day, the Sun heats the Earth; at night, the atmosphere stops too much of this heat from escaping. Without it, the Earth would be far too hot during the day and far too cold at night.

3. The Sun — The Source of All Weather

The Sun is the main source of heat and light for the Earth, and it is the engine that drives all weather. Energy from the Sun, called solar energy, travels through space and reaches the Earth. The Earth's surface absorbs this energy and warms up. The warm surface then heats the air above it.

An important point is that the Sun does not heat all parts of the Earth equally. The regions near the Equator receive the Sun's rays almost straight overhead, so they get more heat and stay hot. The regions near the Poles receive slanting rays spread over a larger area, so they get less heat and stay cold. This uneven heating of the Earth is what causes differences in temperature, sets the air in motion as wind, and ultimately produces all the variety of weather we see.

4. The Elements of Weather

To describe or measure the weather, scientists look at certain features called the elements of weather. The main ones are temperature, air pressure, wind, humidity, clouds and rainfall.

Temperature

Temperature tells us how hot or cold the air is. It is measured with a thermometer and is usually given in degrees Celsius (°C). Temperature changes during the day (warmest in the afternoon, coolest before dawn) and through the seasons.

Air Pressure

Air pressure (or atmospheric pressure) is the weight of the air pressing down on the Earth. Warm air is lighter and rises, creating low pressure; cool air is heavier and sinks, creating high pressure. Air pressure is measured using a barometer. Changes in pressure help us predict the weather — falling pressure often means rain is coming.

Wind

Wind is simply moving air. Air always moves from areas of high pressure to areas of low pressure. This movement, caused by the uneven heating of the Earth, is what we feel as wind. Winds carry heat and moisture from place to place and are very important in shaping the weather. The speed of wind is measured with an anemometer and its direction with a wind vane.

Humidity

Humidity is the amount of water vapour (moisture) present in the air. When humidity is high, the air feels sticky and damp; when it is low, the air feels dry. Humidity is important because the water vapour in the air is what eventually forms clouds and rain.

Clouds and Rainfall

Clouds are made of tiny droplets of water (or ice crystals) floating in the air. When these droplets join together and become heavy enough, they fall to the Earth as rain. Water falling from clouds — as rain, snow, sleet or hail — is called precipitation. The amount of rainfall is measured with a rain gauge.

5. The Water Cycle — How Rain is Made

One of the most important processes in the weather is the water cycle (also called the hydrological cycle). It is the continuous movement of water between the Earth's surface and the atmosphere. It has three main stages:

• Evaporation: The Sun heats water in oceans, rivers, lakes and ponds. This water turns into an invisible gas called water vapour and rises into the air. (Plants also release water vapour through a process called transpiration.)
• Condensation: As the water vapour rises, it reaches cooler air higher up. Here it cools down and turns back into tiny water droplets, forming clouds. This change from vapour to liquid is called condensation.
• Precipitation: When the water droplets in the clouds join together and grow heavy, they fall back to the Earth as rain, snow or hail. This water flows into rivers and oceans, and the cycle begins again.

The water cycle explains where rain comes from and why water never runs out on Earth. It links the Sun, the oceans, the air and the land into one connected system that produces our weather.

6. Weather and Climate — What is the Difference?

People sometimes confuse the words ‘weather’ and ‘climate’, but they mean different things. Weather is the condition of the atmosphere at a place over a short time — a day, a few hours, a week. Climate is the average weather of a place taken over a long period — usually many years.

In short, weather is what you get on a single day, while climate is what you expect over the years. A place may have a hot climate but still have a cool, rainy day now and then.

7. Measuring and Forecasting the Weather

Predicting the weather in advance is called weather forecasting. It helps farmers plan their crops, helps people prepare for storms, and keeps travel by air and sea safe. To forecast the weather, scientists first measure its elements using special instruments:

• Thermometer — measures temperature.
• Barometer — measures air pressure.
• Rain gauge — measures the amount of rainfall.
• Anemometer — measures wind speed.
• Wind vane — shows the direction of the wind.

Today, scientists also use weather satellites in space and powerful computers to study clouds, winds and storms over the whole Earth. In India, the India Meteorological Department (IMD) studies the weather and issues forecasts and warnings, for example about heavy rain, heat waves or approaching cyclones. Accurate forecasts can save lives and property, especially when a dangerous storm is on its way.