Introduction

During the early life of the earth, there was an abundance of hydrogen and helium. Extensive volcanism increased nitrogen, sulfur, water vapor, argon, and carbon dioxide. Then heavy rains pushed the bulk of CO2 into the oceans. Currently, the proportion of gases in the atmosphere is

Nitrogen – 78%

Oxygen – 21%

Argon – 0.93%

Carbon dioxide – 0.03%

Layers of the Atmosphere:

The atmosphere extends up to 10000 km above the earth’s surface. The parts of the atmosphere are:

           Troposphere:

• 90% of the atmosphere is within 32 km. The tropopause is at 8 km at the poles and 18km at the equator. At the equator, cumulonimbus clouds are formed.

Climatic elements like temperature, pressure, humidity, precipitation, and clouds lead to variations in local climate and weather phenomena. The variable water content in the atmosphere leads to great contrast in weather and climate. Without water in the atmosphere, there would be no weather or climate

• Temperature decreases with height. It is transparent to shortwave radiation but G.H.G’s absorb long-wave terrestrial radiation.
• The weather phenomenon is formed in the troposphere.

 Stratosphere

• Temperature increases with height. The ozone layer is present here and UV radiation is absorbed.

The stratosphere lies above the troposphere and has no dust, smoke, or water vapor however seasonal temperature variations are seen.

• Polar stratospheric clouds are present. Spy planes, commercial planes, and weather balloons fly here.

Mesosphere

• No GHGs and temperature decrease with height. Noctilucent clouds are seen.

Meteors burn in this layer.

Thermosphere – Ionosphere

• Gases in ionic state trap insolation. Temperature increases with height.

The ionosphere layer makes short wave radio transmission possible over long distances.

• Largest layer of atmosphere. Useful in radio communication.
• Reflects low frequency and high-frequency waves but not very high or ultra-high.

Heat Budget:

Sun’s energy reaches as solar or radiant energy by a process called insulation. Out of this 35% is reflected back by the dust and the atmosphere, 14% is absorbed by the water vapor in the atmosphere and is scattered by dust particles in the air this causes the “Blue Sky phenomenon”.

The remaining 51% is absorbed by the earth’s surface and heats it. The surface through conduction and convection heats the air above it. Radiation of heat at night by the surface continues during the night and since insolation is absent the surface cools.

The land is heated more rapidly than water as the heat in the water is spread out evenly due to the motion of the water. The opaque nature of land means that more heat is absorbed and so land is heated and cooled faster. The transparent nature of water also leads to less heat being absorbed. Specific heat of water is higher so it takes longer to heat. Interiors of continents also face higher temperature variations than coastal areas as the moderating influence of the sea is not seen there.

The differential heating gives rise to land and sea breezes. During the day as land gets heated faster than water the air above it is warmer and less dense. The sea remains cool and so a high-pressure belt is formed and we experience sea breeze. This is usually stronger in the tropics than in temperate regions. During the night, the land is cooled faster than the sea and so a low-pressure belt is seen oversea. Winds now move from land to sea and we get a land breeze.

Slopes are affected by temperature as a steep slope is heated faster than a gentle slope. Mountains having east-west alignment also see their south-facing ‘sunny slope’ have a higher temperature than their north facing ‘sheltered slope’. The southern slopes thus see more cultivations and settlements.

Dry soils heat and cool faster than wet soils. The darker soils are also better absorbers of heat than lighter soils. The leaves of trees in jungles lose water during the day due to evapotranspiration and this makes the air cooler. The relative humidity increases and mist and fog may form.

Albedo is the percentage of solar radiation reflected by the surface of the solar radiation that is incident. Lower albedo is for dark soil and higher albedo is snowfall.

Movement of Air:

Air moves from a high-pressure area to a low-pressure area [advection]. When the air gets warm it expands as it becomes lighter [convection].

When air is heated it goes up. If it has the moisture it condenses and brings rain. However, this causes instability. Similarly, when the air cools it can’t rise it sinks causing high-pressure conditions or anticyclonic conditions on the ground.

Temperature inversion:

In hill areas, a hot day followed by a cloudless night sees the air at a higher altitude to cool much faster. This cold, dense air sinks downwards and pushes the warmer air to rise. The temperature at the bottom of the valley is lower than at the top. This is called “temperature inversion”. Normally temperature decreases with height but when it increases.

Case 1: On a cool night the air above cools more rapidly than the air below. The warm air rises upwards than the colder air. This causes temperature inversion.

Case 2: In a valley, the cool air descends downwards and the warm air rises. This cool air causes frost and damages crops.

Effects of temperature inversion:

1. Fog – The water droplets condensed around a dust particle. It reduces visibility and damages crops.
2. Frost
3. Atmospheric Stability – discourages rainfall.

Smog is when a water droplet condenses around a pollutant like SO2. It too reduces visibility and is a health hazard. It occurs in a cool humid climate.

Photochemical smog: occurs in a warm, dry, and sunny climate. It is due to unsaturated hydrocarbons and ozone in the presence of sunlight. It is oxidizing smog.