Atmosphere and Climate
Class 09 Social ScienceWhen you look up in the sky, you might see the floating clouds, the sunshine, and feel a breeze as well. All of these are a result of a blanket of air surrounding the Earth, called the atmosphere. It is pulled down around the Earth by gravity, and is a mixture of gases in various proportions, vital for the survival of all living beings on the Earth.
The atmosphere shields us from the harmful radiation coming from the Sun, including ultraviolet radiation. It regulates the Earth’s temperature by trapping some of the Sun’s energy and prevents it from escaping back into space. The atmosphere is a key component of the Earth’s weather and climate systems, influencing factors such as temperature, humidity, and air pressure.
Composition of the Atmosphere
The Earth’s atmosphere is composed of a mixture of various gases. Nitrogen and oxygen are the two primary and most abundant gases that are essential to life on the Earth. Carbon dioxide, argon, helium, neon, krypton, xenon, ozone, and hydrogen are some other gases present in the atmosphere, but are found in lesser quantities.
Besides these, the atmosphere also consists of water vapour and tiny dust particles. The amount of water vapour in the atmosphere varies, but it generally ranges from 0.1 per cent to 0.4 per cent. It plays a significant role in cloud formation and precipitation. The composition of the atmosphere also varies with altitude.
Structure of the Atmosphere
The atmosphere has a layered structure. These layers are defined on the basis of changes in temperature and density with increasing altitude. The density of air is highest near the Earth’s surface and decreases with altitude.
1. Troposphere
The troposphere is the most important layer of the atmosphere, with an average height of about 12 kilometres. In this layer, temperature decreases with increasing altitude. The air we breathe exists here, along with most of the water vapour and clouds. Nearly all the weather phenomena, such as rainfall, fog, and hail, occur in this layer. The troposphere is separated from the stratosphere by a transition zone known as the tropopause.
2. Stratosphere
Above the troposphere lies the stratosphere, extending up to 50 kilometres. This layer is ideal for flying aeroplanes because it is free of clouds and other weather disturbances. It contains a layer of ozone gas, which shields us by filtering the Sun’s harmful radiation, including ultraviolet radiation. The stratopause marks the boundary between the stratosphere and the mesosphere.
3. Mesosphere
Above the stratosphere lies the third layer of the atmosphere, the mesosphere. It extends up to a height of 80 kilometres. In this layer, temperature decreases with increasing altitude. Most meteorites entering from space burn up in the mesosphere.
4. Thermosphere
In the thermosphere, temperature rises very rapidly with increasing altitude, as the gas molecules in this layer absorb the X-rays and short-wave ultraviolet radiation of the Sun. The thermosphere extends from 80 to 700 km. It also helps with radio transmission by reflecting radio waves transmitted from the Earth back towards it.
Ionosphere is a part of the thermosphere. Additionally, the northern and southern lights (auroras) also occur in the thermosphere.
5. Exosphere
The uppermost layer of the atmosphere is known as the exosphere, characterised by very thin air. Light gases like helium and hydrogen float into space from this layer due to weak gravity. All of these layers play a vital role in the Earth’s atmospheric processes and also affect its weather and climate.
Weather and Climate
The term weather refers to the hour-to-hour and day-to-day conditions of the atmosphere. Weather can vary significantly from day-to-day.
The average weather conditions of a place over a longer period of time refer to the climate of the place. Climate refers to the sum total of weather conditions and variations over a large area for an extended period of time, usually thirty years or more.
Elements of Weather and Climate
Various elements influence weather and climate. The major elements of atmosphere that impact human life on the Earth are temperature, precipitation, humidity, wind, and atmospheric pressure. These elements can change under varying conditions.
1. Temperature
The temperature of the atmosphere varies not only between day and night but also across seasons, just as summers tend to be hotter than winters. An important factor affecting temperature distribution is insolation. The amount of insolation decreases from the equator towards the poles. Hence, the temperature decreases from the equator towards the poles.
2. Humidity
When water evaporates from land and different water bodies, it becomes water vapour. When the levels of water vapour in the air are high, it leads to humidity. Therefore, humidity refers to the presence of water vapour in the air, creating moisture. As the air gets warmer, its capacity to hold water vapour increases, leading to higher humidity levels. On such a day, clothes take longer to dry, and sweat evaporates more slowly, leaving us feeling very uncomfortable.
3. Precipitation
It occurs when a part of the atmosphere becomes saturated with water vapour, which condenses and ‘precipitates’, or falls on the Earth due to gravity. This includes drizzle, rain, snow, sleet, and hail. The main factors that affect precipitation include prevailing winds, mountains, and seasons. When precipitation falls on the the Earth in liquid form, it is called rain. Rain is the most common form of precipitation and lowers the temperature of a place. A long absence of rainfall results in a dry climate. Most of the groundwater is collected from rainwater.
4. Atmospheric pressure
The pressure exerted by the weight of air on the the Earth’s surface is called air pressure. As we go higher in the atmosphere, pressure falls rapidly. It is highest at sea level and decreases with altitude. Horizontally, the distribution of air pressure is influenced by the air’s temperature at a given place. In areas with high temperatures, air heats up and rises. This creates a low-pressure area. Low pressure is associated with cloudy skies and wet weather. In areas with lower temperatures, air is cold. It is therefore heavy and sinks, creating a high-pressure area. High pressure is associated with clear and sunny skies. Air always moves from high-pressure areas to low-pressure areas.
5. Wind
The movement of air from a high-pressure area to a low-pressure area is called wind. It can be gentle or strong. You can observe the flow of wind as it blows dry leaves down the pavement, gently blows away dust particles, or uproots trees during a storm.
Seasons in India
The climate of India can be broadly classified as tropical monsoon. The Indian Meteorological Department (IMD) has recognised four distinct seasons in India.
1. Winter
The winter season generally lasts from December to early April. The coldest months of the year are December and January, when the average temperature in the north-west is around 10-15°C. Temperatures increase towards the equator, reaching around 20-25°C in mainland India’s south-east.
2. Summer or pre-monsoon
The summer or pre-monsoon season spans from April to June, or up to July in north-western India. In western and southern regions, the hottest month is April; in northern regions, it is May. The average temperature across most of inland India ranges from 32-40°C.
3. Monsoon or rainy (Advancing monsoon)
The monsoon season generally spans from June to September. This season is dominated by the humid south-west summer monsoon, which slowly sweeps across the country in late May or early June. Monsoon rain begins to recede from north India at the beginning of October, and South India typically receives more rainfall during this time.
4. Post-monsoon (Retreating monsoon)
The post-monsoon season lasts from October to December. In north-western India, October and November are usually cloudless.
Monsoon
The climate of India is strongly influenced by monsoon winds. Monsoon winds are seasonal winds. They can be categorised into the south-west and the north-east monsoon.
The south-west monsoon, also known as the summer monsoon, is characterised by winds blowing from sea to land across the Indian Ocean, the Arabian Sea, and the Bay of Bengal between June and September. It is mainly caused by the unequal heating of land and the sea. During summer, the landmass of India heats up faster than the surrounding oceans. This creates a low-pressure area over the Indian subcontinent, while the Indian Ocean remains relatively cooler and has high pressure. Winds move from high-pressure areas to low-pressure areas, so moist winds blow from the ocean towards the land, bringing rainfall. It accounts for most of the rainfall in the country throughout the year.
The winter monsoon, also known as the north-east monsoon, occurs in India from October to February. During this season, the Indian landmass cools faster than the surrounding oceans, creating a high-pressure area over the land and low pressure over the seas. As a result, cold and dry winds blow from land to sea. These winds generally do not bring rainfall to most parts of India. However, when the north-east monsoon winds pass over the Bay of Bengal, they pick up moisture and cause rainfall in the eastern coast of India, especially in Tamil Nadu, Andhra Pradesh, and parts of Karnataka. Thus, the winter monsoon is important for the rainfall of the south-eastern regions of India.