Chapter 2 — Understanding the Weather

Weather is the state of the Earth's atmosphere at a particular time and place. We use words like hot, cold, rainy, cloudy, humid, snowy, and windy to describe the different ways we experience it. Almost all weather phenomena take place in the troposphere, the layer of the atmosphere closest to the Earth's surface.

The five elements of weather are temperature (how hot or cold the atmosphere is), atmospheric pressure (the weight of air above the Earth's surface), wind (the movement of air, including its speed and direction), precipitation (any form of water — rain, snow, sleet, or hail — that falls from the sky), and humidity (the amount of water vapour in the air).

The troposphere is the layer of the atmosphere closest to the Earth's surface, where all land-based plants and animals live. It is also where almost all weather phenomena take place. It extends 6 to 18 kilometres from the ground — thinner at the poles (where cold air contracts) and thicker in the tropical zone (where warmer air expands).

A rain gauge has a collecting funnel that channels rainwater into a measuring cylinder. A scale attached to the cylinder shows the depth of water collected. When the height of collected water is, for example, 5 mm, we say that the area received 5 mm of rainfall.

Atmospheric pressure is the pressure exerted by the weight of the air above and around us. It is measured with a barometer, which displays readings in millibars (mb). The normal atmospheric pressure at the sea coast is around 1013 mb; a pressure below 1000 mb indicates a depression, which can sometimes develop into a storm or cyclone.

An anemometer has three or four metal cups that rotate on a vertical shaft when the wind blows — the stronger the wind, the faster the rotation. A meter attached at the bottom counts how many times the anemometer spins in a certain period of time and calculates the wind speed in kilometres per hour (km/h).

Humidity refers to the amount of water vapour present in the air. It is measured as relative humidity using an instrument called a hygrometer. Air with no water vapour at all would be rated 0%, while air fully saturated with water vapour is 100%. In practice, dry weather has a relative humidity of 20%–40%, while humid weather usually falls between 60% and 80%.

The India Meteorological Department (IMD) was set up in 1875. Its motto is ādityāt jāyate vriṣhti, meaning 'From the sun arises rain', a phrase drawn from the ancient text Manusmṛiti. The IMD collects weather data and issues forecasts and warnings — for example, it issued the weather warning map for India on 19 May 2024 shown in the chapter.

An Automated Weather Station (AWS) is a self-operating system that uses sensors to measure and record temperature, humidity, wind speed and direction, precipitation, and atmospheric pressure without human intervention. AWS are used in agriculture, aviation, navigation, and environmental monitoring. In 2023, the National Disaster Management Authority set up an AWS at a glacial lake in Sikkim at more than 4800 metres above sea level to provide early warning about upcoming weather conditions.

From early times, humans observed Nature's signals to forecast weather. Signs like birds flying low, ants carrying their eggs, squirrels gathering nuts, frogs croaking loudly, and pine cones opening or closing gave valuable information about coming rain or storms. This knowledge was passed down from generation to generation, and even today many people in India use such traditional methods, especially to predict the arrival of the monsoon.

Accurate weather predictions help people and governments prepare for extreme events. For example, if stormy weather is expected at sea, fishermen are warned not to venture out. If a cyclone is approaching, entire coastal areas may be evacuated. Predictions also allow local governments to mobilise resources for disasters. The chapter also notes that climate change is making extreme weather events such as droughts, floods, and cyclones more frequent, making accurate forecasting even more important.

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