Insolation

Solar radiation

Reason — The Sun continuously radiates heat and light energy in all directions, known as solar radiation or insolation. It is the primary source of energy for the Earth's atmosphere and climate.

Both A and R are true and R is the correct explanation of A.

Explanation — The lower layers of the atmosphere are denser and contain more carbon dioxide, water vapour, and dust particles, which absorb and retain more heat. Therefore, the heat absorption capacity is greater in the lower layers than in the upper layers.

Insolation

Reason — The amount of solar energy received by the earth is called insolation. The sun's energy is dissipated and scattered before it reaches the earth and only one part out of two billion parts reaches the earth.

Terrestrial radiation

Reason — Solar radiation (insolation) heats the Earth's surface, which then radiates heat back into the atmosphere as long-wave radiation. This outgoing long-wave radiation is called terrestrial radiation.

Heat balance

Reason — There exists a state of equilibrium on earth between the incoming insolation from the sun and the outgoing terrestrial radiation from the earth. When both incoming radiation and outgoing radiation are balanced, it is called heat balance.

51%

Reason — Out of the total 100% solar energy, 35% is reflected back into space and 14% is absorbed by the atmospheric layers including ozone. Hence, only the remaining 51% reaches the earth's surface.

direct contact; circulatory motion

Reason — The heat radiated by the earth's surface warms the layers of air above it by direct contact and this is called conduction, whereas the transfer of heat by circulatory movement, as in the circulation of air and water, is called convection.

A : 14; B : 34

Reason — The atmosphere absorbs only 14 units of the sun's incoming radiation but absorbs 34 units of the earth's terrestrial radiation. Thus, the atmosphere is heated more by terrestrial radiation than by incoming insolation and acts like a blanket, preventing extremes of temperature on the earth.

Altitude

Reason — Altitude is the height of a place above the mean sea level. As altitude increases, the temperature generally decreases due to the normal lapse rate.

conduction, convection

Reason — Land is heated by conduction, as heat is transferred through direct contact. Water is heated by convection, as heat is distributed by the circulation of water from one level to another.

night time

Reason — During the day, air over the land becomes warmer and rises and cool air from the oceans moves in to take its place, causing the sea breeze. At night the opposite happens as the land surface gets cooled more quickly than the ocean surface, so the breeze blows from land to sea and is known as land breeze.

Inversion of temperature

Reason — On calm, cold winter nights, heat from the earth's surface escapes rapidly back into space and the cool air from the mountain sides sinks into the valley. This causes a reversal in the vertical distribution of temperature, in which temperature decreases down in the valleys and is known as inversion of temperature.

The Sun continuously radiates heat and light energy in all the directions. It is known as solar radiation.

Solar radiation is the only primary source of light and heat on the Earth.

The amount of solar energy received by the Earth is called insolation.

Two main characteristics of insolation are-

1. The Sun's energy reaches the Earth as short wave rays.
2. Out of the total 100% solar energy, only 51% reaches the Earth, 35% is reflected back into space and only 14% is absorbed by the atmospheric layers including the ozone during insolation.

Two advantages of convectional heating of the atmosphere are-

1. Convection currents in the atmosphere cause sea breeze. During the day, the air over land becomes warmer and rises. Cool air from oceans moves in to take its place causing sea breeze.
2. At night, land breeze happens due to the opposite of this. Land surface gets cooled more quickly than the ocean surface. Air over land flows towards the ocean causing land breeze.

Four factors that affect the temperature of a place are-

1. Latitude of a place
2. Altitude of a place
3. Distance from the sea
4. Slope of the land

Due to spherical shape of the Earth, mid latitudes get less insolation resulting in moderate climate with normal temperature ranging from 15°C to 30°C. The duration of sunshine is greater in summer than winter. So the summer is warm and winter is cold.

The temperature on a mountain is lower than that on a sea shore because temperature decreases with increase in altitude. On average, the temperature falls by 1°C for every 166 metres of ascent (normal lapse rate).

India is cooler in December than in July because of the tilt of the Earth's axis.

On 21st June, the Northern Hemisphere is tilted towards the Sun, and the Sun's rays fall vertically over the Tropic of Cancer. India receives more direct sunlight and experiences summer, making July hot.

On 22nd December, the Northern Hemisphere is tilted away from the Sun, and the Sun's rays fall vertically over the Tropic of Capricorn. India receives slanting rays of the Sun and therefore experiences winter, making December cooler.

Heat balance is achieved when the incoming solar radiation (insolation) is equal to the outgoing terrestrial radiation from the Earth. Although the atmosphere absorbs 14 units of incoming solar radiation and 34 units of the Earth's terrestrial radiation, the overall balance between incoming and outgoing radiation maintains the Earth's average temperature.

The temperature of a place depends on the latitude of a place as temperature decreases with increase in latitude on either side of the equator due to the spherical shape of the Earth and its annual revolution around the Sun.

Also, the mid day Sun is almost overhead within the tropics but at oblique angles outside the tropics.

(i) North India lies away from the sea and experiences a continental climate, whereas South India is surrounded by water on three sides and experiences a maritime climate. The moderating influence of the sea keeps temperatures more uniform in South India. Hence, North India has a greater annual range of temperature than South India.

(ii) The temperature of Delhi is lower than that of Chennai in December because Delhi is farther from the Equator and receives more slanting rays of the Sun during winter. Chennai, being closer to the Equator and surrounded by the sea, experiences a maritime climate, which keeps its temperature moderate. Hence, Chennai remains warmer than Delhi in December.

(iii) Desert areas experience high temperatures during the day and much lower temperatures at night because the sandy surface heats up quickly during the day and loses heat rapidly at night. Also, the dry desert air contains very little water vapour, so it cannot retain heat. As a result, deserts have a large daily range of temperature.

Below labelled diagram shows the Heat Budget of the Earth:

The five temperature zones of the Earth are-

1. Torrid Zone
2. North Temperate Zone
3. South Temperate Zone
4. North Frigid Zone
5. South Frigid Zone

The rate of decrease of temperature with height is about 6°C per km above the sea level, so on climbing every 166 metres, there is a fall of 1°C in temperature. It is known as Normal Lapse Rate.

(i) The sea heats up and cools down more slowly than land. Therefore, places near the sea experience a moderate (maritime) climate with a small daily and annual range of temperature, while places far from the sea experience a continental climate with greater temperature extremes.

(ii) Land is heated and cooled faster than the sea because water is mobile and heat is distributed by convection, whereas land is stationary and heat remains confined to the surface. Hence, land heats up and cools down more quickly than water.

(iii) The ports of the western coast of Europe remain ice-free during winter due to the warming effect of the Gulf Stream and the North Atlantic Drift.

Below labelled diagram shows that the vertical rays are hotter than slanting rays:

The sea heats up and cools down more slowly than land. Therefore, places near the sea have a moderate climate with a small daily and annual range of temperature, while places farther from the sea experience a continental climate with a greater range of temperature.

During the day, the sea breeze blows from the sea towards the land, bringing cool air and lowering the temperature of the coastal region. At night, the land breeze blows from the land towards the sea as the land cools more quickly than the sea. This keeps the coastal region from becoming extremely hot during the day or very cold at night.

Thus, the alternate blowing of sea breeze and land breeze helps maintain a moderate climate in coastal areas.

(i) At higher latitudes, the Sun's rays fall obliquely. These rays travel a longer distance through the atmosphere and spread over a larger area, reducing their heating effect. Hence, temperature decreases with increasing latitude.

(ii) The vertical rays of the Sun give more insolation because they travel a shorter distance through the atmosphere and are concentrated over a smaller area. Slanting rays travel a longer distance and spread over a larger area, so they provide less heating.

(iii) Hot deserts have little cloud cover and scanty vegetation, allowing intense heating during the day and rapid loss of heat by terrestrial radiation at night. As a result, deserts have a high daily range of temperature. In contrast, forests moderate temperature through shade and transpiration, resulting in a lower range of temperature.

Below labelled diagram shows the land and sea breeze:

If the entire solar energy reached the Earth instead of only 51%, the Earth would become excessively hot. The consequences would include:

1. Rise in Temperature — The Earth's temperature would increase drastically, making the climate extremely hot.cts on ecosystems, weather patterns, and human habitats.
2. Melting of Ice Caps and Glaciers — Increased temperatures would melt polar ice, causing a rise in sea level.
3. Disturbed Heat Balance — The balance between incoming solar radiation and outgoing terrestrial radiation would be upset.
4. Changes in Climate and Rainfall — Weather patterns would become irregular, leading to more droughts, floods, and extreme weather.
5. Adverse Effects on Plants, Animals, and Humans — Excessive heat would affect agriculture, ecosystems, and human health.

Thus, only about 51% of solar energy reaching the Earth's surface helps maintain the heat balance necessary for life.

Yes, greenhouse gases affect the Earth's heat budget in the following ways:

1. Trap Terrestrial Radiation — Greenhouse gases such as carbon dioxide (CO₂), methane (CH₄), and water vapour absorb and trap the Earth's outgoing terrestrial radiation, reducing heat loss to space.
2. Maintain Earth's Temperature — They act like a blanket around the Earth, keeping it warm and making it suitable for life.
3. Global Warming — An increase in greenhouse gases due to human activities disturbs the Earth's heat balance, causing global warming and climate change.
4. Human Activities — Burning fossil fuels, deforestation, and industrial emissions increase greenhouse gas concentrations, enhancing the greenhouse effect.