Humidity, Rainfall and Climatic Regions
NCERT Class 11 - Fundamentals of Physical Geography, Chapters 11 and 12
Humidity, Rainfall and Climatic Regions
Water vapour in the air is humidity. When air cools to its dew point the vapour condenses, and if the droplets grow heavy enough they fall as precipitation. The pattern of temperature and rainfall over the year defines a climatic region.
Humidity and condensation
- Relative humidity is the ratio of the vapour present to the maximum the air could hold at that temperature, expressed as a percentage; it falls when air is heated and rises when air is cooled, even if no vapour is added.
- Dew point is the temperature at which air becomes saturated and condensation begins.
- Condensation near the ground produces dew when the dew point is above freezing, frost when it is below freezing, and fog or mist when condensation takes place around dust particles in the air.
Clouds and precipitation
| Cloud group | Examples | Character |
|---|---|---|
| High clouds | Cirrus, cirrostratus, cirrocumulus | Thin, wispy and made of ice crystals |
| Middle clouds | Altostratus, altocumulus | Grey sheets or rolls that often bring steady rain |
| Low clouds | Stratus, stratocumulus, nimbostratus | Dull grey; nimbostratus gives continuous rain |
| Vertical clouds | Cumulus, cumulonimbus | Cumulonimbus is the thunderstorm cloud, bringing heavy rain, hail and lightning |
- Convectional rainfall - air heated at the surface rises, cools and condenses. It gives heavy afternoon downpours and is typical of the equatorial belt.
- Orographic or relief rainfall - moist air is forced to rise over a mountain, cooling and raining on the windward side; the sheltered leeward side stays dry and is called the rain shadow area. The Western Ghats and the Shillong plateau are Indian examples.
- Cyclonic or frontal rainfall - warm air is forced to rise over cold air along a front, giving prolonged moderate rain over a wide area, typical of the temperate regions.
Cyclones
| Point | Tropical cyclone | Temperate cyclone |
|---|---|---|
| Location | Between about 5 and 30 degrees latitude | Between about 35 and 65 degrees latitude |
| Season | Mostly late summer and autumn | Mostly winter |
| Origin | Over warm oceans, needs sea temperature above about 27 degrees | Along the polar front where air masses meet |
| Fronts | No fronts | Formed by warm and cold fronts |
| Size and life | Smaller but far more violent | Larger, milder and longer lasting |
| Regional names | Cyclone in India, hurricane in America, typhoon in East Asia | Depression or extratropical cyclone |
- A tropical cyclone has a calm centre called the eye, surrounded by the eye wall where winds and rain are most violent.
- Tropical cyclones do not form within about five degrees of the equator because the Coriolis force is too weak there to start the rotation.
Climatic regions
- The most widely used classification is Koeppen's, based on temperature and rainfall and on the vegetation each combination supports.
- Group A, tropical - hot all year; includes the equatorial type with rain throughout the year, the monsoon type with a marked wet and dry season, and the savanna type with a longer dry season.
- Group B, dry - evaporation exceeds rainfall; includes hot deserts and semi-arid steppes.
- Group C, warm temperate - includes the Mediterranean type with dry summers and wet winters, the China type with rain in summer, and the marine west coast type.
- Group D, cold snow forest - severe winters, found only in the northern hemisphere, supporting the coniferous taiga.
- Group E, polar - tundra and ice cap; Group H, highland - climate changes rapidly with altitude.
हिंदी संकेत: वृष्टि छाया क्षेत्र पर्वत के विमुख ढाल पर होता है, जहां उतरती वायु गर्म और शुष्क हो जाती है। पश्चिमी घाट का पूर्वी ढाल इसका भारतीय उदाहरण है।
Exam me kaise aata hai
Match-the-following on rainfall type and example, and on cyclone type and its features, are both common. Factual questions ask what relative humidity means and which cloud brings thunderstorms. Application questions give a climate description and ask which Koeppen group it fits.
UPSC / State PSC ke liye
Be precise on relative humidity - it changes with temperature even when the actual vapour content is constant, which is why the same air feels dry at noon and damp at dawn. On climatic regions, learn the Mediterranean type carefully, since dry summer and wet winter is the reverse of the Indian pattern and is therefore asked often.
Yahan confuse hote hain
✗ Orographic rain falls mainly on the leeward side | ✓ It falls on the windward side; the leeward side is the rain shadow
✗ Tropical cyclones have warm and cold fronts | ✓ They have no fronts; temperate cyclones form along fronts
✗ The Mediterranean climate has wet summers | ✓ It has dry summers and wet winters
Ek nazar me
- Relative humidity falls when air is heated and rises when air is cooled.
- Dew point is the temperature at which condensation begins.
- Cloud groups - high, middle, low and vertical; cumulonimbus brings thunderstorms.
- Rainfall types - convectional, orographic with a rain shadow, and cyclonic or frontal.
- Tropical cyclones need a sea temperature above about 27 degrees and have an eye; they have no fronts.
- Temperate cyclones form along fronts in mid latitudes and are larger but milder.
- Koeppen groups - A tropical, B dry, C warm temperate, D cold snow forest, E polar and H highland.
Ab isi chapter ke questions ExamAtlas mock test me practice karo.