Introduction
What is Precipitation?
- Precipitation is when water falls from clouds to the ground in solid or liquid forms.
- Examples: Snow, hailstones, rainfall.
- It happens because of water vapor in the air changing due to temperature and pressure.
Water on Earth
- 70.8% of Earth’s surface is covered with water.
- Water is not spread evenly-some places have a lot, some have very little.
- Water vapor in the air turns into different forms like dew, fog, snow, or rain depending on the weather.
Forms of Precipitation
Snow
How it Forms:
- When the air temperature drops below 0°C (freezing point), water vapor turns directly into snowflakes (called sublimation).
- This happens without becoming liquid first-gas turns straight to solid.
Where it Happens:
- Common in high-latitude areas (like Kashmir) and high-altitude places above the snowline.
- Tropical areas like Maharashtra don’t get snow at sea level because it’s too warm.
Features:
- Snow is soft, flaky, and opaque (not see-through).
- It piles up in layers and doesn’t flow like water.
Effects:
- Heavy snow can block roads and communication.
- People need protection from frostbite (cold injury).
- Melting snow provides water to the region.
Snow vs. Ice:
- Snow turns into ice when lower layers get pressed by upper layers, becoming hard and transparent.
Hail
How it Forms:
- Hot air rises fast from the Earth’s surface, carrying water vapor upward.
- The air cools at higher altitudes, forming dark clouds.
- Water droplets freeze into hailstones due to cold temperatures and strong winds.
- Hailstones grow bigger as more layers of ice form around them.
Where it Happens:
- Seen in summers in India, Africa, and Southeast Asia.
- Doesn’t happen in equatorial areas (too hot) or cold zones (no strong upward air).
Features:
- Hailstones are heavy and solid, falling fast to the ground.
- They have concentric layers (like onion rings) from repeated freezing.
Effects:
- Can destroy crops and damage property or even hurt people.
Rainfall
How it Forms:
- Warm air with water vapor rises, cools, and condenses into tiny water droplets.
- These droplets join dust particles to form clouds.
- When droplets get too heavy, they fall as rain.
Types of Rainfall:
Convectional Rainfall:
- Happens in hot equatorial areas (e.g., Congo Basin, Amazon Basin).
- Sun heats the ground, warm air rises, cools, and causes rain, often with thunder and lightning.
- Common in afternoons daily in these regions.
Orographic Rainfall:
- Moisture-laden winds from seas or lakes hit mountains and rise.
- Air cools as it rises, condenses, and rains on the windward side (facing the wind).
- The leeward side (opposite) gets less rain, called the rain shadow area.
- Example: Western Ghats in Maharashtra get more rain on the west side.
Cyclonic Rainfall:
- Occurs in low-pressure areas called cyclones.
- Air moves to the center, rises, cools, and rains.
- Common in temperate zones, often with stormy winds and floods.
Other Forms of Condensation (Near the Surface)
Fog
How it Forms:
- Air near the ground cools, and water vapor turns into tiny floating droplets.
- When these droplets get dense, fog forms, reducing visibility.
Where it Happens:
- Seen in winter, e.g., London has thick fog till afternoon.
- Less dense in tropical areas like India due to warmer weather.
Dew
How it Forms:
- Moist air touches cold objects (like grass) at night, turning vapor into small water droplets.
Where it Happens:
- Common on winter mornings in many places.
Frost
How it Forms:
- When temperature drops below 0°C, dew droplets freeze into frost.
Where it Happens:
- Seen in cold winter areas on surfaces like grass or roads.
How to Measure Precipitation
Rain Gauge
What it is:
- A tool to measure rainfall, with a funnel and a bottle to collect water.
How it Works:
- Rain falls into the funnel and collects in the bottle.
- Water is measured in millimeters (mm) using a measuring jar.
- In heavy rain areas, readings are taken every 3 hours.
Precautions:
- Place it in an open area, 30 cm above ground, to avoid obstructions.
Fun Fact:
- 1 mm rain over 1 sq.km = 10 lakh liters of water!
Measuring Snowfall
Melt the snow in the rain gauge container and measure the water.
120 mm of snow equals 10 mm of rainfall.
Special Rainfall: Acid Rain
What it is:
- Rainfall mixed with acids (like nitric or sulfuric acid) due to air pollution.
How it Forms:
- Polluted gases from industries mix with water vapor, forming acidic rain.
Effects:
- Harms plants, animals, buildings, and human health.
Effects of Precipitation
Positive Effects:
- Main source of drinking water.
- Good rainfall at the right time boosts crop growth (important for India’s farmers).
- Melting snow provides water in some regions.
Negative Effects:
- Too Much Rain:
- Floods destroy homes, crops, and lives.
- Too Little Rain:
- Droughts cause food shortages and economic problems.
- Fog:
- Reduces visibility, causing transport delays or accidents.
- Frost:
- Damages crops and makes roads slippery.
- Hail:
- Breaks crops and property.
India’s Rainfall:
- Monsoons are key for agriculture, but rain is uneven and unpredictable.
- Good rain helps; untimely rain harms crops.
Why Rainfall Varies
Factors Affecting Rainfall:
- Amount of water vapor in the air.
- Air pressure (low pressure = more rain).
- Temperature (cooler air holds less moisture).
- Topography (mountains cause orographic rain).
- Latitude (tropical areas get more rain).
Examples:
- High rain in tropical areas due to heat and moisture.
- Less rain in eastern Europe due to distance from oceans.
- Eastern Australia gets more rain due to coastal winds.
Activities and Experiments
Make Rain Experiment:
- Materials: Glass container with metal lid, nail, hammer, hot water, ice cubes, salt.
- Steps:
- Punch small dents in the lid (no holes).
- Fill 1/3 of the container with steamed water.
- Close lid tightly, add ice cubes, salt, and a little water on top.
- Wait 10-15 minutes to see “rain” droplets fall inside.
- Explanation:
- Steam rises, cools on the cold lid, forms droplets, and falls like rain.
Measure Rainfall:
- Use a rain gauge at school for one week.
- Record daily rain in mm and make a bar graph.
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