When the tropics become the ‘fall’ guy: Why India doesn’t have an Autumn season

Even as India transitions from one monsoon (South-West) to the other (North-East) in quintessential tropical weather, the landscape in higher latitudes (away from the Equator) is bursting into a riot of colours, as the ‘fall’ season sets in.

It is quite a sight to behold as swathes of green trees fade away only to be replaced by dazzling amber, maroon, gold and bronze hues, with the Sun becoming less bright and intense, and days becoming shorter.

Tropics miss out

Except for the fringes of Kashmir or Himachal Pradesh, most of India misses out on this beautiful makeover, which is determined by the seasonal movement of the Sun. In fact, during this time, South India battles humid and often stormy or rainy weather.

A view of New Hampshire. Fall season has begun, and the New England region in the US, of which New Hampshire is a part, is a riot of colours. Photo courtesy: Prashant Kukreti

It may not quite be the case in India, but in some tropical regions such as Malaysia and Singapore, there is just one season — the hot season — all year round. The days are sunny and are often interspersed with bouts of showers.

In others, there are two seasons — dry and rainy. The dry season is hot with little or no rain, and takes up most of the year. Heavy rainfall comes in the rainy season, which lasts between one and three months (just as during India's monsoons).

This is because the tropics get more exposure to the sun, which aids in the process of convection (cloud-building and rainfall). Most of these regions abound in lush-green evergreen trees and forests that retain their colour all through the year.

It also explains why the tropics — situated close to the Equator, which is an imaginary line passing right through the middle and encircling the globe — may be considered the ‘fall guy’ in the larger scheme of the colourful transition from Summer to Winter.

The transition to Winter

Autumn marks the transition to Winter — in September (the Northern Hemisphere) or March (the Southern Hemisphere), depending on which side of the Equator you are — though meteorologists and astronomers have their own takes on this.

In the temperate zones, the duration of daylight becomes noticeably shorter and the temperature cools considerably. And this is what leads to the spurt of colours of leaves (autumnal change) over the landscape as the Sun moves farther away.

Trees take water from the ground through their roots and carbon dioxide from the air. They use sunlight to turn water and carbon dioxide into oxygen and glucose. Glucose is a type of sugar that trees use as food.

This process is known as photosynthesis, and chlorophyll is a vital facilitator of photosynthesis. Chlorophyll is a green colour pigment that is abundantly found in leaves; therefore, it is chlorophyll that gives leaves their green colour.

Other than chlorophyll, leaves contain two other pigments: carotene (orange colour) and xanthophyll (yellow colour). During the Summer, chlorophyll is used abundantly for photosynthesis. This masks the orange and yellow colour pigment that lies beneath.

The change in colour during Autumn is occasioned by the trees’ hoarding the green chlorophyll, as if in knee-jerk reaction to the shorter, cooler days and chilling nights of Autumn. The red, yellow, and orange pigments remain in the leaves.

These ingredients from the leaves are put into cold storage for the Winter, deep underground where they are kept safe for as long as the cold lasts before being used again when Spring sets in.

Sunlit all year long

In short, during Spring and Summer, when there is plenty of sunlight, plants make a lot of chlorophyll. As chlorophyll goes away, other pigments start to show their colours, making leaves turn yellow or red in fall.

Alongside, the trees also conserve the moisture from the leaves, which causes them to quickly dry out, change colour, and fall to the ground. Trees that drop their leaves annually in this manner are called deciduous trees.

Tropical regions (of which India is one) are close to the Equator, and they receive strong sunlight all year long. And that is precisely why these regions may not have the four seasons (Spring, Summer, Autumn and Winter).

Unlike their counterparts in the temperate zones, the broad-leafed evergreen trees are not subjected to seasonal stresses. Neither do they need to bother about any significant day-length differential. Therefore, they have ample supplies of chlorophyll and an assured green colour.

Most of India falls under the tropical or sub-tropical category, except for the higher reaches of the Himalayas. For the rest of the country, though, trees do not change colour since they hardly need to prepare for Winter by dispensing with their leaves.