PrecipitationClass 9 Geography Chapter 5 Notes – Maharashtra Board

Summary: Precipitation

About 70.8% of the Earth's surface is covered by water. Water vapour in the air condenses into different forms depending on conditions — dew, frost, fog, clouds — and when water falls from clouds to the ground as a liquid or solid it is called precipitation. Rain, snow and hail are the main forms.

Snow forms when vapour turns directly into ice crystals below freezing point. It falls at sea level in high latitudes but only above the snowline in the tropics. Hail forms in tall storm clouds where strong rising air carries drops up again and again, adding ice layers; it falls in summer in places like India.

Rain falls when cloud droplets grow too heavy to float. Convectional rain comes from strong heating and rising air — almost every afternoon in equatorial areas. Orographic (relief) rain falls when moist winds are forced up mountains; the far side is a rain-shadow area. Cyclonic rain falls when air rises in a low-pressure system.

When condensation happens near the ground we get fog (tiny droplets floating in air), dew (droplets on cold surfaces) and frost (dew frozen below 0 °C). Rainfall is measured with a rain gauge in millimetres.

Rain is our main source of fresh water. Too much brings floods, too little brings drought; Indian farming depends heavily on the monsoon. Polluted air can make rain acidic (acid rain), which harms living and non-living things.

Key points to remember

• Precipitation = water falling from clouds as liquid or solid: rain, snow, hail
• Forms of condensation near the ground: dew, frost, fog
• Snow: vapour → ice crystals directly below 0 °C
• Snow is soft and opaque; packed snow becomes clear, solid ice
• Hail: strong updrafts carry drops up repeatedly, building layered ice balls; not found at the equator or in cold zones
• Convectional rain: heating → rising air → cooling → rain; equatorial afternoons, Amazon and Congo basins
• Orographic rain: moist winds rise over mountains; windward side wet, leeward side rain-shadow
• India's monsoon rain is mainly orographic; Konkan wet, area east of Sahyadris rain-shadow
• Cyclonic rain: air converges into a low-pressure centre and rises
• Rain gauge measures rainfall in mm; 1 mm over 1 sq km = 10 lakh litres
• Rainfall depends on vapour, pressure, temperature, relief and latitude
• Acid rain forms when vapour reacts with polluting gases (sulphuric and nitric acids)

Precipitation – Notes

1. Condensation and precipitation

• Water covers about 70.8% of the Earth's surface but is unevenly spread. Evaporation puts water vapour into the air.
• When moist air cools, the vapour condenses. Depending on conditions we see dew on winter mornings, snow on high mountains, rain in the monsoon, thick fog in places like London in winter, and sudden hailstorms.
• Precipitation means water falling from clouds to the ground in liquid or solid form. Rain, snow and hail are the main forms; dew, frost and fog form near the ground and are forms of condensation.
• The 'make rain' activity shows the idea: steam rises in a closed jar, the lid cooled by ice and salt makes it condense, and the drops gather and fall — the same thing happens in the sky on a huge scale.

2. Snow and hail

• Snow: when the air is below freezing point, water vapour changes straight into ice crystals (snowflakes) without becoming liquid. (The book calls this sublimation; scientists call vapour-to-solid change deposition.)
• In high latitudes and temperate areas snow can fall even at sea level; in the tropics it falls only on mountains above the snowline, such as the Himalayas. That is why Kashmir gets snow and Maharashtra does not.
• Heavy snow blocks roads and cuts off communication; people must protect themselves from frostbite. When it melts in spring, rivers get water.
• Snow is loose and opaque. Under the weight of upper layers, lower layers become compact, clear and solid — this is ice. Ice on a frozen lake forms because the water freezes, not because of snowfall.
• Hail: strong heating makes air rise very fast; it cools and forms dark storm clouds. Drops are carried high and freeze. As they fall, rising air pushes them up again, and each trip adds a new layer of ice, so hailstones have concentric layers. Finally they are too heavy and fall fast.
• Hail falls mainly in summer in India, parts of Africa and South-East Asia. It does not occur in equatorial areas (too warm in the lower air) or in cold zones (no strong rising air). It damages crops and property.

3. Types of rainfall

• Rain forms when rising air cools, vapour condenses on dust particles into droplets, clouds form, and droplets join until they are too heavy to stay up.
• Convectional rainfall: the sun heats the ground strongly; the air above expands, becomes light and rises, cools, and its moisture condenses. In equatorial regions (Amazon basin, Congo basin) it rains like this almost every afternoon, with thunder and lightning. It covers a limited part of the world but is very regular.
• Orographic (relief) rainfall: moist winds from the sea meet a mountain range and are forced up its slope. They cool, condense and rain heavily on the windward side. After crossing the crest they descend, warm up and can hold more moisture, so the leeward side gets little rain — a rain-shadow area.
• In Maharashtra, south-west monsoon winds from the Arabian Sea rise over the Sahyadris: Konkan and the ghats get very heavy rain, the plateau just east of the Sahyadris is a rain-shadow area, and rainfall increases again towards eastern Maharashtra (Vidarbha). India's monsoon rain is mostly orographic.
• Cyclonic rainfall: a cyclone is an area of low pressure surrounded by higher pressure. Air flows in towards the centre and rises; it cools and rain falls wherever the cyclone moves. Temperate cyclones cover large areas; tropical cyclones are smaller but stormy, with strong winds and floods.
• Orographic rain occurs in most parts of the world. Convectional rain is the most certain (equatorial belt); orographic and cyclonic rain are less certain, so such areas may face floods or droughts.

4. Fog, dew, frost and the rain gauge

• Fog: when air near the ground cools, vapour condenses into tiny droplets that float; when they become dense, visibility drops.
• Dew: moist air touching very cold surfaces (grass, car tops) condenses into small drops that stick to them.
• Frost: if the temperature is below 0 °C, these drops freeze into a white icy coating. Dew and frost are common on clear winter nights.
• Rain gauge: a funnel of fixed diameter collects rain into a bottle; the water is measured in a jar marked in millimetres. The gauge is placed in open ground on a raised platform about 30 cm high so splashes and runoff do not enter. In heavy-rain areas it is read every three hours.
• 1 mm of rain over 1 sq km gives 10 lakh litres of water (1,000,000 m² × 0.001 m = 1,000 m³).
• Snow is measured by melting it and measuring the water. The book says 120 mm of ice equals 10 mm of rain; this ratio (about 12 : 1) applies to freshly fallen snow, not solid ice.
• Factors affecting rainfall: water vapour in the air, air pressure, temperature, relief (mountains) and latitude.

5. Effects of precipitation and acid rain

• Precipitation is the main source of drinking water on Earth.
• Heavy rain can cause floods and loss of life and property; failure of rain causes drought, food shortage, imports, distress for farmers and damage to the economy.
• India's farming depends largely on the monsoon, which is erratic. Timely, good rain increases crops; untimely rain damages them.
• Dense fog reduces visibility and disrupts road, rail, water and air transport; accidents may occur and flights or trains get cancelled. Frost harms crops and makes roads slippery. Fog and dew help some crops but spread diseases in others.
• Acid rain: in industrial areas, gases such as sulphur dioxide and nitrogen oxides mix with water vapour and form sulphuric and nitric acids, which fall with rain. Acid rain harms plants, fish and soil, and corrodes buildings, statues and metal.

Types of rainfall compared

Key terms

Precipitation – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Precipitation – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Precipitation

⏱ About 40 minutes

1. Forms (12 min)
   Learn rain, snow and hail, and fog, dew and frost, with how each forms.
2. Rainfall types (15 min)
   Draw simple diagrams of convectional, orographic and cyclonic rainfall; mark windward and rain-shadow sides on Maharashtra's profile.
3. Check yourself (13 min)
   Answer the 'Try it yourself' questions, take the quiz and play 🎮 Play.