Chapter 2 Notes: Understanding the Weather

Weather is the state of the Earth's atmosphere at a particular time and place. We experience weather every day — when we choose warm clothes on a cold morning or feel rain on our skin. Weather happens mostly in the troposphere, the lowest layer of the atmosphere, which extends 6–18 km above Earth's surface.

Weather is described using five main elements:

1. Temperature – How hot or cold the atmosphere is (measured in °C or °F).
2. Precipitation – Water falling from the sky as rain, snow, sleet, or hail.
3. Atmospheric Pressure – The weight of air pressing down on Earth's surface.
4. Wind – Movement of air, described by its speed and direction.
5. Humidity – The amount of water vapour present in the air.

Each element changes from place to place and time to time, giving us different weather conditions.

Long before modern instruments, people observed nature's clues to forecast weather. These observations were passed down through generations and are still used in many parts of India today:

• Ants shifting eggs to higher ground → heavy rain expected.
• Frogs croaking loudly → rain is coming.
• Pine cones closing → humid conditions; opening → dry conditions (seeds are released in dry weather to spread better).
• Birds flying low, squirrels gathering nuts, and other animal behaviours also signal weather changes.

A thermometer measures how hot or cold the air is. There are different types — liquid thermometers (with coloured liquid that expands when warm) and digital thermometers (more precise). Two important temperature statistics are:

• Range of Temperature = Maximum Temperature − Minimum Temperature (over 24 hours).
• Mean Daily Temperature = (Maximum Temperature + Minimum Temperature) ÷ 2.

The India Meteorological Department (IMD), set up in 1875, collects temperature data across India. Its motto 'Ādityāt jāyate vrishti' means 'From the sun arises rain', reflecting the water cycle: the sun's heat evaporates water, which then falls as rain.

Precipitation (rain, snow, sleet, hail) is measured using a rain gauge. It works by:

1. Collecting rainwater through a funnel at the top.
2. Storing it in a measuring cylinder below.
3. Reading the depth of water on a scale (in millimetres).

If 5 mm of water is collected, we say the area received 5 mm of rainfall. For snow, it is allowed to melt first before measuring.

Atmospheric pressure is the weight of air pressing down on Earth's surface. Key facts:

• Higher near the coast/sea level (~1013 mb) and lower at high altitudes (e.g., ~650 mb at Khardung La, Ladakh, at 5600 m).
• Low pressure (below 1000 mb) can indicate a depression, which may develop into a storm or cyclone.
• At high altitudes, lower pressure means less oxygen, causing breathlessness and dizziness — which is why people need to acclimatise gradually.
• Measured using a barometer in units called millibars (mb).

Wind is the movement of air from high pressure to low pressure areas. It is described by two factors:

• Direction – measured using a wind vane (the pointer shows where wind comes from).
• Speed – measured using an anemometer (cups spin faster in stronger wind; speed calculated in km/h).

A special wind vane at airports is called a wind sock — it helps pilots judge wind direction during take-off and landing. Wind data is vital for pilots, sailors, farmers, and firefighters.

Humidity is the amount of water vapour in the air. It is measured as relative humidity on a scale of 0% to 100%:

• 0% = completely dry air (impossible in nature).
• 100% = air fully saturated with water vapour.
• Dry weather: 20–40% relative humidity.
• Humid weather: 60–80% relative humidity.

Humidity affects how fast water evaporates — high humidity means slow evaporation, so wet clothes dry slowly and sweat evaporates less (making us feel hotter and stickier). Measured using a hygrometer.

A weather station brings all weather instruments together in one place to measure and track weather regularly. An Automated Weather Station (AWS) uses sensors to record temperature, humidity, wind speed, precipitation, and pressure automatically — without needing a person present.

Meteorologists analyse data collected over long periods to predict future weather. Accurate predictions help:

• Warn fishermen about storms at sea.
• Alert governments to prepare for floods or cyclones.
• Help farmers plan sowing and harvesting.
• Guide aviation and navigation.

The IMD issues colour-coded weather warnings: No Warning → Watch → Alert → Warning (Take Action).