Factors that shape climate

Many factors, one climate

Why are hill stations pleasant in May, when the plains below bake, and why does Chennai stay warm even in January? Some of the factors that create a climate work on the scale of the whole planet, others on the scale of a region or even a single valley. The chapter looks at five of them.

Latitude

Latitude tells how far a place is from the Equator; it grows as one goes north or south. Near the Equator, at low latitudes, the Sun’s rays strike the ground almost at a right angle, so all their energy falls on a small area. Near the poles, at high latitudes, the rays come in at a slant: the same energy is spread over a larger area, and it also passes through more of the atmosphere, which weakens it further. So the polar regions get less heat than the equatorial zone, and climates change from hot to temperate to cold as latitude increases. In India, Kanniyakumari and the Nicobar Islands, near the Equator, stay warm or hot for nearly the whole year, while Srinagar in the north is much cooler.

Altitude

India’s hill stations, such as Shimla, Darjeeling, Nainital, Tawang, Shillong, Mount Abu, Mahabaleshwar, Madikeri, Munnar and Udhagamandalam (Ooty), are popular because they are cool. They are higher than the plains below, and temperature falls as altitude rises. One reason is that air pressure, and so the density of the air, decreases with height, and thinner air is cooler. Another is that the Sun heats the ground, which warms the air above it, so air far above the surface is less warm. Ooty and Coimbatore lie at almost the same latitude, yet Ooty’s summer temperatures range from about 10 to 25°C and Coimbatore’s from 25 to 38°C.

Nearness to the sea

Land heats up and cools down quickly, while the sea does both slowly. So in summer the sea keeps coastal air cooler, and in winter it keeps it warmer: the sea acts as a moderator. Moving inland, temperatures become more extreme. Mumbai and Nagpur are at a similar latitude, but Mumbai, on the coast, has summers of around 32°C and winters of around 18°C, a range of about 14°C. Nagpur, far from the sea, reaches about 44°C in summer and falls to about 10°C in winter, a range of about 34°C.

Winds

Winds move masses of warm or cool air. In summer, winds from the west that have crossed the deserts from Arabia to Afghanistan bring dry, hot air to Punjab, Haryana, Rajasthan and Madhya Pradesh and cause severe heat waves. During winter, icy winds from beyond the Himalayas slip into the foothills and bring cold waves. Winds also change humidity and rainfall: dry desert winds bring no rain, but winds from the sea carry moisture over the land and can bring rain, as the monsoon winds do.

Topography

Topography is the shape of the land: its mountains, hills, valleys, slopes and coasts. The Himalayas and the Karakoram ranges protect the subcontinent, to some extent, from the cold winds of the Central Asian deserts. The Thar Desert, being nearly flat, has nothing to shield it from hot, dry winds. Mountains also force moist winds to rise and drop their rain, as the Western Ghats do in the monsoon.

Putting it together: climates and microclimates

All these factors together decide a region’s climate, which is described by its usual temperature, precipitation (rain, snow, fog or mist) and winds over three decades or more. Within a region, a small area can have a microclimate of its own. Enclosed valleys and some forests have one, and so do urban heat islands: cities with many buildings and concrete and very few plants, which trap heat and are often much warmer than the land around them. Microclimates affect local plants, animals, crops and people’s health.

Why it matters

These factors explain why India has climates from alpine to tropical. Next, see how land, sea and wind work together in the monsoons, and how extreme weather turns into climate disasters. Answers to every question are in the NCERT solutions for Chapter 3; see all of the Chapter 3 notes.