Weather and Climate
Introduction
We talk about the WEATHER every day — 'It is hot today' or 'It might rain.' But what is the DIFFERENCE between WEATHER and CLIMATE? And what are the ELEMENTS that make up weather? This chapter explores the FACTORS that create our daily weather and the INSTRUMENTS used to measure them.
1. Weather vs Climate
| Aspect | Weather | Climate |
|---|---|---|
| Definition | ATMOSPHERIC conditions at a SPECIFIC time and place | AVERAGE weather conditions over a LONG period (30+ years) |
| Duration | SHORT-TERM (hours to days) | LONG-TERM (decades to centuries) |
| Change | CHANGES frequently | RELATIVELY constant |
| Example | 'It is raining today in Mumbai' | 'Mumbai has a TROPICAL monsoon climate' |
| Study | METEOROLOGY | CLIMATOLOGY |
Simple Rule of Thumb
- Weather is what you WEAR today
- Climate is what you KEEP in your WARDROBE
2. Elements of Weather and Climate
| Element | Definition | Measured By |
|---|---|---|
| Temperature | How HOT or COLD the air is | THERMOMETER |
| Air pressure | The WEIGHT of air pressing down | BAROMETER |
| Wind | MOVING air (from high to low pressure) | ANEMOMETER (speed), WIND VANE (direction) |
| Humidity | Amount of WATER VAPOUR in air | HYGROMETER |
| Precipitation | Water falling from sky — RAIN, SNOW, HAIL | RAIN GAUGE |
| Cloud cover | Fraction of sky covered by CLOUDS | Visual observation (oktas) |
| Sunshine | Duration of SUNLIGHT | SUNSHINE RECORDER |
3. Temperature
Factors Affecting Temperature
| Factor | Explanation |
|---|---|
| Latitude | CLOSER to equator = HIGHER temperature; CLOSER to poles = LOWER |
| Altitude | HIGHER altitude = LOWER temperature (6.5°C drop per km) |
| Distance from sea | COASTAL areas = moderate temperatures; INLAND = extreme |
| Ocean currents | WARM currents = warmer coasts; COLD currents = cooler coasts |
| Vegetation | FORESTS = cooler (shade, transpiration); DESERTS = hotter |
| Cloud cover | CLOUDS = cooler days, warmer nights |
Temperature Scales
| Scale | Freezing point of water | Boiling point of water |
|---|---|---|
| Celsius (°C) | 0°C | 100°C |
| Fahrenheit (°F) | 32°F | 212°F |
| Kelvin (K) | 273 K | 373 K |
Isotherms
- Lines on a map JOINING places with the SAME temperature
- CLOSE isotherms = RAPID temperature change
- WIDE isotherms = GRADUAL temperature change
4. Atmospheric Pressure
What is Atmospheric Pressure?
The WEIGHT of the COLUMN of air above a point. It is measured in MILLIBARS (mb) or HECTOPASCALS (hPa).
- HIGH pressure = CLEAR skies, sunny weather
- LOW pressure = CLOUDY, rainy, stormy weather
Factors Affecting Pressure
| Factor | Effect |
|---|---|
| Temperature | WARM air RISES = LOW pressure; COLD air SINKS = HIGH pressure |
| Altitude | HIGHER altitude = LOWER pressure |
| Water vapour | MOIST air = LOWER pressure than DRY air |
Isobars
- Lines on a map JOINING places with the SAME pressure
- CLOSE isobars = STRONG winds
- WIDE isobars = GENTLE winds
5. Winds
What is Wind?
Air MOVING from HIGH pressure areas to LOW pressure areas.
Types of Winds
(a) Permanent Winds (Planetary Winds)
- TRADE WINDS: Blow from SUBTROPICAL high pressure to EQUATORIAL low pressure
- WESTERLIES: Blow from SUBTROPICAL high to SUBPOLAR low
- POLAR EASTERLIES: Blow from POLAR high to SUBPOLAR low
(b) Seasonal Winds (Monsoons)
- Winds that CHANGE direction with the seasons
- INDIA has the MOST FAMOUS monsoon system
- Summer monsoon: WIND from ocean to land (brings RAIN)
- Winter monsoon: WIND from land to ocean (DRY)
(c) Local Winds
- SEA BREEZE: Wind from sea to land (DAYTIME)
- LAND BREEZE: Wind from land to sea (NIGHTTIME)
- LOOSE: Hot, dry wind from the Sahara
- CHINOOK: Warm, dry wind in the Rocky Mountains
6. Humidity and Precipitation
Humidity
HUMIDITY is the amount of WATER VAPOUR in the air.
- Absolute humidity: Total water vapour in the air
- Relative humidity: How MUCH water vapour the air holds COMPARED to how much it CAN hold at that temperature
- When relative humidity REACHES 100% = CONDENSATION and PRECIPITATION
Types of Precipitation
| Type | Description | Example |
|---|---|---|
| RAIN | Liquid water droplets | Most common in India |
| SNOW | Ice crystals (frozen) | Himalayas, polar regions |
| HAIL | Balls of ice | Thunderstorms, damage crops |
| SLEET | Rain that freezes on the way down | Mixed rain and snow |
| DRIZZLE | Very fine rain | Light, continuous |
Types of Rainfall
| Type | How It Forms | Where It Occurs |
|---|---|---|
| CONVECTIONAL | Air HEATS, RISES, cools, condenses, RAINS | Equatorial regions (heavy afternoon rain) |
| RELIEF/OROGRAPHIC | Moist air hits MOUNTAINS, rises, cools, RAINS | Windward side of mountains |
| CYCLONIC/FRONTAL | WARM air meets COLD air, warm air rises, RAINS | Temperate regions (Western Europe) |
7. Weather Instruments
| Instrument | Measures | How It Works |
|---|---|---|
| THERMOMETER | Temperature | Liquid (mercury/alcohol) expands with heat |
| BAROMETER | Air pressure | Mercury rises with high pressure, falls with low |
| ANEMOMETER | Wind speed | Cups spin faster in STRONG winds |
| WIND VANE | Wind direction | Arrow POINTS where wind comes FROM |
| HYGROMETER | Humidity | Measures moisture in the air |
| RAIN GAUGE | Rainfall amount | Collects rain in a CYLINDER; measured in mm |
| STEVENSON SCREEN | Protects instruments | WHITE box with slats; keeps instruments at STANDARD conditions |
ICSE Exam Focus
2-mark questions
- What is the DIFFERENCE between weather and climate?
- Name THREE elements of weather.
- What instrument is used to measure RAINFALL?
4-mark questions
- Explain the FACTORS affecting temperature distribution.
- What are the THREE types of rainfall? Explain each.
- What is the DIFFERENCE between sea breeze and land breeze?
6-mark (essay) questions
- Describe the ELEMENTS of weather and the INSTRUMENTS used to measure them.
- Explain the DIFFERENT types of WINDS with examples.
Self-Test
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What is the DIFFERENCE between WEATHER and CLIMATE? Answer: Weather is the SHORT-TERM condition of the atmosphere at a specific time and place. Climate is the AVERAGE weather condition over a LONG period (usually 30+ years). Weather changes DAILY; climate is RELATIVELY STABLE.
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How is RELIEF rainfall different from CONVECTIONAL rainfall? Answer: Relief rainfall occurs when moist air hits a MOUNTAIN BARRIER and is forced to rise. Convectional rainfall occurs when air HEATS up, RISES, and cools — common in equatorial regions with afternoon thunderstorms.
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What factors AFFECT the temperature of a place? Answer: Latitude (distance from equator), altitude (height above sea level), distance from the sea (continentality), ocean currents, vegetation cover, and cloud cover.
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How does a BAROMETER work? What does it measure? Answer: A barometer measures ATMOSPHERIC PRESSURE. High pressure = fair weather, low pressure = rainy/stormy weather. Mercury barometers show pressure in MILLIBARS.
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Why does a SEA BREEZE blow during the day? Answer: During the day, the LAND heats up faster than the sea. Warm air over land RISES, creating LOW pressure. Cooler air from the SEA flows in to REPLACE it — creating a sea breeze.
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What is RELATIVE HUMIDITY? When does precipitation occur? Answer: Relative humidity is the AMOUNT of water vapour in the air compared to the MAXIMUM it can hold at that temperature. When it reaches 100%, the air is SATURATED — leading to CONDENSATION and PRECIPITATION.
