Chapter-Chapter 2: Understanding the Weather Social Science Part-1 class 7 in english Medium CBSE Notes

Chapter 2: Understanding the Weather

Introduction

Weather affects our daily lives in many ways. It influences the clothes we wear, the food we eat, the activities we perform, and even our travel plans. Understanding weather helps us prepare for changing conditions and natural events such as storms, floods, droughts, and heat waves.

What is Weather?

Weather is the condition of the Earth's atmosphere at a particular place and time. It changes from hour to hour and day to day.

The atmosphere is the layer of gases surrounding the Earth. Most weather phenomena occur in the Troposphere, the lowest layer of the atmosphere.

The Atmosphere

• The Earth is surrounded by a layer of gases called the atmosphere.
• The lowest layer is the Troposphere.
• Almost all weather activities occur in this layer.
• The troposphere extends about 6–18 km above the Earth's surface.
• It is thinner at the poles and thicker near the equator.

Elements of Weather

Weather is determined by five important elements.

1. Temperature

Temperature indicates how hot or cold the atmosphere is.

2. Precipitation

Precipitation includes all forms of water falling from the atmosphere such as rain, snow, sleet, and hail.

3. Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of air on the Earth's surface.

4. Wind

Wind is the movement of air from one place to another.

5. Humidity

Humidity is the amount of water vapour present in the air.

Traditional Weather Forecasting

Before scientific instruments were invented, people observed nature to predict weather conditions.

Examples

• Ants carrying eggs to higher places often indicate rain.
• Frogs croaking loudly may suggest approaching rainfall.
• Pine cones close during humid weather and open in dry weather.
• Bird behaviour is often used to predict storms.

Such traditional knowledge has been passed from generation to generation.

Weather Instruments

Scientists use special instruments to measure weather accurately.

Measuring Temperature

Thermometer

A thermometer is used to measure temperature.

Temperature Scales

• Celsius Scale (°C)
• Fahrenheit Scale (°F)

Example: 15°C is equal to 59°F.

Types of Thermometers

• Clinical Thermometer
• Laboratory Thermometer
• Digital Thermometer

Important Temperature Terms

Maximum Temperature: Highest temperature recorded during a day.

Minimum Temperature: Lowest temperature recorded during a day.

Temperature Range:

Temperature Range = Maximum Temperature − Minimum Temperature

Mean Daily Temperature:

(Maximum Temperature + Minimum Temperature) ÷ 2

India Meteorological Department (IMD)

• Established in 1875.
• Responsible for weather observation and forecasting.
• Provides weather warnings and alerts.
• Helps during cyclones, floods, droughts, and heat waves.

Measuring Precipitation

What is Precipitation?

Precipitation includes rain, snow, sleet, and hail falling from the atmosphere.

Rain Gauge

A rain gauge measures the amount of rainfall received in an area.

How It Works

• Rainwater falls into a funnel.
• The water is collected in a measuring cylinder.
• The collected water is measured in millimetres (mm).

Example: If water collected reaches 10 mm, the rainfall is recorded as 10 mm.

Measuring Atmospheric Pressure

What is Atmospheric Pressure?

Atmospheric pressure is the pressure caused by the weight of air above us.

Important Facts

• Pressure decreases with increasing altitude.
• Mountain regions have lower pressure than plains.
• Lower oxygen levels are found at high altitudes.

Barometer

A barometer is used to measure atmospheric pressure.

Unit of Measurement

Millibar (mb)

• Normal sea-level pressure: about 1013 mb.
• Pressure below 1000 mb often indicates a depression or storm.

Importance

• Helps predict storms and cyclones.
• Important for weather forecasting.
• Useful for aviation and navigation.

Measuring Wind

What is Wind?

Wind is the movement of air from areas of high pressure to areas of low pressure.

Characteristics of Wind

• Direction
• Speed

Wind Vane

A wind vane measures the direction of the wind.

Anemometer

An anemometer measures wind speed.

Importance of Wind Data

• Helps pilots during take-off and landing.
• Assists sailors and fishermen.
• Helps farmers predict rainfall.
• Influences weather forecasting.

Humidity

What is Humidity?

Humidity is the amount of water vapour present in the atmosphere.

Water Vapour

Water vapour is water in gaseous form.

Relative Humidity

Humidity is expressed as a percentage.

• 0% = No water vapour (practically impossible).
• 100% = Air completely saturated with water vapour.

Humidity Levels

• Dry Weather: 20%–40%
• Humid Weather: 60%–80%

Hygrometer

A hygrometer measures humidity.

Effects of Humidity

• High humidity slows evaporation.
• Clothes dry slowly.
• People sweat more.
• Weather feels sticky and uncomfortable.

Weather Stations

A weather station is a place where weather instruments are installed to record weather conditions.

Functions

• Measure temperature.
• Record rainfall.
• Monitor wind.
• Measure humidity.
• Record atmospheric pressure.

Automated Weather Station (AWS)

An Automated Weather Station works automatically without human assistance.

Features

• Uses sensors to collect weather data.
• Provides accurate observations.
• Operates continuously.
• Useful in remote areas.

Uses

• Agriculture
• Aviation
• Navigation
• Environmental monitoring
• Disaster management

Meteorology

Meteorology is the scientific study of weather and atmospheric conditions.

Scientists who study weather are called Meteorologists.

Weather Forecasting

Weather forecasting is the process of predicting future weather conditions using scientific observations and data.

How Forecasting is Done

• Data collected from weather stations.
• Satellite observations.
• Computer models.
• Historical weather records.

Importance of Weather Forecasting

• Provides early warning of cyclones.
• Helps prepare for floods and droughts.
• Protects life and property.
• Supports agriculture.
• Helps fishermen avoid dangerous seas.
• Assists governments in disaster management.

Extreme Weather Events

Climate change has increased the frequency of extreme weather events.

Examples

• Floods
• Droughts
• Cyclones
• Heat Waves
• Cold Waves
• Heavy Rainfall
• Hailstorms

Weather Instruments and Their Uses

Instrument	Measures
Thermometer	Temperature
Rain Gauge	Rainfall / Precipitation
Barometer	Atmospheric Pressure
Wind Vane	Wind Direction
Anemometer	Wind Speed
Hygrometer	Humidity

Key Terms

• Weather: Condition of the atmosphere at a particular place and time.
• Atmosphere: Layer of gases surrounding Earth.
• Troposphere: Lowest atmospheric layer where weather occurs.
• Meteorology: Study of weather.
• Forecast: Prediction of future weather.
• Humidity: Water vapour in the air.
• Precipitation: Rain, snow, sleet, or hail.
• Atmospheric Pressure: Weight of air acting on Earth.
• Wind: Movement of air from high pressure to low pressure.

Chapter Summary

Weather is determined by temperature, precipitation, atmospheric pressure, wind, and humidity. Scientists use instruments such as thermometers, rain gauges, barometers, anemometers, wind vanes, and hygrometers to measure these elements. Weather stations and automated weather stations collect data that meteorologists use to forecast weather. Accurate weather forecasts help people prepare for extreme weather events and protect lives and property.