Notes Ch 2 Understanding the Weather Class 7 Social Science

Introduction

Weather: The condition of the atmosphere at a specific time and place (e.g., hot, cold, rainy).
Atmosphere: A layer of gases (air) surrounding the Earth.
Troposphere: The lowest layer of the atmosphere where weather happens and where plants, animals, and humans live. It is 6–18 km thick, thinner at the poles and thicker in the tropics.
Weather affects our daily choices, like what clothes to wear (thick in winter, light in summer).

Weather Elements

Weather is described using five main elements:

Temperature: How hot or cold the air is.
Precipitation: Water falling from the sky (rain, snow, sleet, hail).
Atmospheric Pressure: The weight of the air pressing down on Earth’s surface.
Wind: The movement of air, with speed and direction.
Humidity: The amount of water vapor (gaseous water) in the air.

Why Measure Weather?

To understand and compare weather conditions (e.g., Krishnan in Chennai feeling “chilly” at 20°C might seem warm to Amir in Kashmir).
To plan activities (e.g., farming, travel, or events) by knowing the weather in advance.
To prepare for extreme weather like storms or heat waves.

Traditional Weather Prediction

People have observed nature for centuries to predict weather:
Birds flying low: Indicates rain.
Ants moving eggs to higher ground: Suggests heavy rain.
Frogs croaking loudly: Signals rain.
Pine cones: Close in humid conditions, open in dry conditions.
Squirrels gathering nuts: Predicts a harsh winter.
In India, traditional methods are still used to predict monsoons.

Weather Instruments

Scientists use tools to measure weather accurately. These are often found in weather stations.

1. Temperature

Instrument: Thermometer

Types: Clinical, laboratory, digital, or maximum-minimum thermometers.
Measures: How hot or cold the air is.
Scales: Celsius (°C) and Fahrenheit (°F). Example: 15°C = 59°F.

Uses:

Calculate range (max temp – min temp in 24 hours).
Calculate mean daily temperature (max temp + min temp ÷ 2).

Example: A city’s weekly temperature data (28 Feb–6 Mar 2025):

Max temp: 32°C (3 Mar).
Min temp: 14°C (5 Mar).
Range: 32°C – 14°C = 18°C.

Fact: India Meteorological Department (IMD), set up in 1875, has the motto “From the sun arises rain” (from Manusmriti).

2. Precipitation

Instrument: Rain gauge

How it works: A funnel collects rainwater in a cylinder, and a scale measures the depth in millimeters (mm).
Example: 5 mm of water in the gauge = 5 mm of rainfall.
Types: Rain, snow, sleet (partly frozen rain), hail (small ice balls).
Activity: Make a rain gauge, place it in an open area, and measure daily rainfall for a month. Calculate weekly averages and note variations.

3. Atmospheric Pressure

Instrument: Barometer

Measures: The weight of air pressing down, in millibars (mb).
Normal pressure at sea level: ~1013 mb.
Low pressure (<1000 mb): Indicates a depression or storm.

Effects:

Higher pressure near sea coasts; lower in mountains (less oxygen, causing breathlessness).
Low-pressure systems can lead to storms or cyclones.
Uses: Important for pilots, sailors, and predicting storms.

Example: At high altitudes like Khardung La (5600 m), pressure is ~650 mb, making it hard to breathe.

4. Wind

Instrument:

Wind vane: Shows wind direction (pointer turns toward the wind).
Anemometer: Measures wind speed (cups rotate faster in stronger winds, measured in km/h).
Wind sock: Used at airports or industries to show wind direction.

Importance:

Helps predict rain (farmers), affects soil drying, and is critical for pilots and sailors.
Wind moves from high-pressure to low-pressure areas.

Example: Flying seeds depend on wind to spread.

5. Humidity

Instrument: Hygrometer

Measures: Relative humidity (% of water vapor in the air).
Range: 0% (no water vapor) to 100% (saturated air).
Dry weather: 20–40%; Humid weather: 60–80%.