• The Earth basks in the Sun's radiant embrace, deriving both heat and light from this celestial powerhouse. The Sun, akin to other stars, is a fiery orb of hot plasma ceaselessly emitting energy into the vast expanse of space—a phenomenon we call solar radiation.

Solar radiation 

• This solar radiation serves as the primary energy source for our planet, vital for sustaining the Earth's biosphere. 
• The delicate balance of energy exchange between the Sun and Earth, once a harmonious dance, is now disrupted by human activities. 
• The relentless exploitation of the environment and the release of greenhouse gases have triggered global warming and climate change.
• Deep within the Sun, the relentless force of gravity and intense temperatures create an environment conducive to nuclear fusion, giving birth to the heat that sustains our solar system. Meanwhile, the Earth, with its geoid shape, receives only a fraction of the Sun's energy, as the rays fall obliquely on the upper atmosphere.
• In essence, the Sun's radiance, once a symbol of harmony, now illuminates the consequences of our actions on Earth.
• In the Earth's atmosphere, two distinct types of thermal radiation are observed. The first originates from outer space, characterized by lower-wavelength radiations predominantly from the Sun, referred to as insolation.
• The second type is the thermal radiation emitted from the Earth's surface, manifesting as high-wavelength radiations known as terrestrial radiations.

Insolation

• The term "insolation" describes the amount of incoming solar radiation in the form of short waves. The insolation received on the Earth's surface amounts to 1.92 calories per square centimeter per minute.
• However, insolation is not distributed uniformly across the Earth; it varies both by location and time. The summer season witnesses higher insolation, while the winter season experiences comparatively lower levels.
• The Earth receives only a minute fraction, 1/2000th million part, of the total energy radiated from the Sun's surface.
• Reflection of insolation occurs through various mediums such as clouds, snowfields, oceans, and other water bodies, resulting in a 35% loss of insolation.
• The insolation that manages to penetrate the atmosphere does not directly heat the air but rather heats the Earth's surface.
•  As the Earth's surface gets heated, the adjacent air is also warmed, leading to the heating of the Earth's atmosphere from below. 
• This phenomenon contributes to the lower atmosphere having a lower temperature than the higher atmosphere.
• The Equator receives the maximum insolation, with a gradual reduction toward the poles. At the poles, the received insolation is at its minimum, merely 1/40th of that received at the Equator.
• The solar radiation entering the Earth's atmosphere passes through various layers before reaching the surface. 
• Components such as water vapor, ozone, and other gases in the troposphere absorb a significant portion of the near-infrared radiation.
• Factors influencing insolation include:
  • Angle of the Sun Rays: Sun rays at the Equator are more or less vertical, resulting in a higher angle of incidence and, consequently, a higher amount of insolation received.
  • Towards the poles, the Sun rays become more oblique, leading to a lower angle of incidence and, subsequently, a lower amount of insolation.
  • Rotation of the Earth on its Axis: The Earth's axis, inclined at an angle of 66.1 degrees to the plane of its orbit around the Sun, significantly influences the distribution of insolation at different latitudes.
  • Distance between the Earth and the Sun: The Earth receives the minimum insolation during aphelion (4th July) and the maximum insolation during perihelion (3rd January).

• Effect of Atmosphere The atmosphere absorbs and scatters short waves. Absorption reduces direct beam radiation and scattering reduces direct beam, adding diffuse beam.
• Length of the Day The longer length of the day ensures a larger amount of insolation received. Length of the day varies with the latitudes and the position of Earth on its orbit during the topography revolution around the Sun.
• Transparency of the Atmosphere The atmosphere is largely transparent to shortwave solar radiation. Within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation. 
• Very small-suspended particles in the troposphere scatter the visible spectrum both to space and towards the Earth surface. This process adds colour to the sky. The red colour of the rising or setting Sun and the blue colour of the sky are the results of the scattering of light within the atmosphere.

Sunspot Count

• A sunspot, a darker region on the Sun's surface, possesses a cooler temperature compared to its surroundings.
• A higher number of sunspots contributes to an increased amount of insolation and is also responsible for phenomena such as Aurora Borealis, Aurora Australis, Polar Auroras, and Magnetic Storms.

Equilibrium in Atmospheric Heating and Cooling

• Equilibrium in the context of atmospheric heating and cooling refers to the balance between incoming solar radiation received by the atmosphere and Earth and the outgoing heat either reradiated or reflected. 
• This heat balance fluctuates both seasonally and with altitude. Generally, regions between 40° N and 40° S of the Equator exhibit a positive regime, receiving more radiation than they lose. Conversely, areas poleward of 40° N and 40° S experience a deficit, receiving less radiation than they lose.
• This imbalance prompts the transfer of heat from low to high latitudes through air masses and ocean currents. Consequently, the tropics do not continuously heat up, and high latitudes do not remain perpetually frozen due to this exchange.
• The processes of atmospheric heating and cooling involve four key mechanisms:
  • Conduction
  • Radiation
  • Advection
  • Convection

Conduction

• The upper layers in contact with the lower layers also get heated. This process of heating of the atmosphere is called Conduction
• Conduction holds significance in warming the lower layers of the atmosphere. It involves the flow of energy from a warmer body to a cooler one when two bodies of unequal temperature come into contact. 
• This transfer of heat persists until both bodies reach the same temperature or the contact is severed.
• The Earth experiences heating from insolation, with the air in contact with the land gradually warming.

Convection

• As the air in contact with the Earth warms, it ascends vertically in the form of currents. This convection process aids in further distributing the heat within the atmosphere and is known as vertical heating.
• The convective transfer of energy is confined to the troposphere.

Radiation

• Radiation is a mode of heat transfer where heat is radiated or transmitted from one place to another without the need for a material medium.
• The solar radiation received by the Earth is in short-wave forms, heating up its surface. The Earth, in turn, acts as a radiating body, emitting energy in the form of long waves to the atmosphere. Heat transfer occurs through electromagnetic waves in this process, referred to as terrestrial radiation.

Advection

• Advection involves the horizontal movement of air, facilitating the transfer of heat. Horizontal air movement is notably more significant than vertical movement.
• In middle latitudes, diurnal variations in daily weather are primarily caused by advection alone. In tropical regions, particularly in Northern India during the summer season, local winds known as "lo" result from the advection process.

Heat Budget

• The Earth, as a whole, neither gains nor loses net heat; instead, it maintains its temperature by balancing the insolation or heat received with terrestrial radiation or heat loss. 
• This equilibrium is known as the Heat Budget, ensuring that the Earth remains neither excessively heated or cooled despite the substantial heat exchanges occurring within the system.
• The amount of heat received in the form of short-wave radiation insolation equals the amount lost by the Earth through short-wave terrestrial radiation. 
• Assuming that the insolation received at the top of the atmosphere is 100%, approximately 35 units are reflected back into space even before reaching the Earth's surface.
• Out of these 35 units, 27 units are reflected from the top of the clouds, 6 units from the top of the atmosphere, and 2 units from the snow and ice-covered areas of the Earth. The reflected radiation collectively forms the Albedo of the Earth.

Albedo

• Albedo denotes the percentage or decimal value expressing the amount of sunlight (solar radiation) reflected by a surface. A perfect reflector has an albedo of one, while an absorber has an albedo of zero. Fresh snow boasts the highest albedo, while asphalt possesses the lowest.
• The atmosphere, in turn, radiates and transmits heat to space, maintaining a constant temperature at the Earth's surface, as the Sun's received heat is effectively transmitted back to space.
• Of the total 100 units, excluding the initial 35 units, the remaining 65 units are absorbed. The atmosphere absorbs 14 units, and the Earth's surface receives the remaining 51 units.
• Out of the 51 units absorbed by the Earth, 34 units come directly from the Sun, while the remaining 17 units are acquired from diffused light.
•  The 51 units absorbed by the Earth are subsequently radiated back in the form of terrestrial radiation.
• Out of this, 17 units go directly into space and 34 units are absorbed by the atmosphere.

• The units of radiation returning to space from the Earth's surface and atmosphere are 17 and 48 respectively, which add up to 65.
• These returning units balance the 65 units that are received from the Sun. This is called heat balance.
• In some parts of the Earth there is a surplus in heat balance but in some parts the heat balance is negative. It is higher in Southern latitudes but low near the poles. Redistribution of heat energy takes place from tropics towards poles. As a result, the tropics do not overheat due to heat accumulation nor are the higher latitudes completely frozen.

Temperature

• Insolation interacts with the atmosphere and Earth's surface, generating heat measured in terms of temperature.
• Temperature is typically calculated in Celsius, Fahrenheit, or Kelvin scales.
• The instrument used for temperature measurement is called a thermometer. Lines connecting regions of equal temperature are known as isotherms.

Isotherms

• Isotherms are lines connecting places with equal temperatures. The horizontal distribution of temperature is depicted through isothermal maps, with the temperature reduced to sea level to eliminate altitude effects.
•  Three general characteristics of isotherms are:
  • Isotherms trend in an East-West direction.
  • Isotherms exhibit a sudden bend at the land-water edge due to land-water contrasts.
  • The spacing of isotherms indicates the latitudinal thermal gradient.
  • Maximum and minimum temperatures are measured using maximum and minimum thermometers, which are U-shaped and filled with alcohol.
  •  Mean daily temperature is the average of the maximum and minimum temperatures.
  • Al-Azizia in Libya holds the record for the warmest place, while Vostok in Antarctica claims the title of the coldest place on Earth. Dalof in Ethiopia boasts the highest annual average temperature, while the pole of cold Antarctica experiences the lowest mean annual temperature.

Temperature Distributions

Temperature distributions can be described in the following ways.

Horizontal Distribution of Temperature

• The horizontal distribution of temperature on the Earth's surface is represented using isotherms, generally parallel to latitudes.
• The gap between isotherms signifies the temperature gradient, with closely spaced isotherms indicating a steep increase and meteorological turbulence, while widely spaced isotherms indicate a gentle thermal gradient and fair weather.

Vertical Distribution of Temperature

• The average rate of temperature decrease upward in the atmosphere is 6.5°C per kilometer. This vertical gradient of temperature is commonly referred to as the normal lapse rate.
• The vertical distribution of temperature is influenced by the nature of the underlying surface; for example, temperature decreases most rapidly with altitude over continental areas in summer.

Spatial or Zonal Distribution of Temperature

• The distribution of temperature over the Earth's surface is closely linked to the amount of insolation received. Generally, temperature decreases from the Equator to the poles.
• The globe can be divided into three broad zones: Torrid zone (Tropical region), Temperate zone (Mid-latitude region), and Frigid zone (Polar region).
• Torrid Zone The zone between 23° 30' North and 23° 30' South latitudes is known as the torrid zone or tropical zone. 
• As the Earth's axis is inclined at 23° and 30' from the vertical plane, all places between the Tropic of Cancer and the Tropic of Capricorn receive vertical Sun's rays. 
• The Sun's rays are almost vertical in this zone throughout the year, making it very hot and receiving maximum insolation, with the angle of incidence of the Sun's rays between 43° and 90° at the Tropic of Cancer and Tropic of Capricorn.
• Temperate Zone The region between the Tropic of Cancer (23° 30' N) and the Arctic Circle (66° 30' N), and the Tropic of Capricorn (23° 30' S) and the Antarctic Circle (66° 30' S), is called the temperate zone. 
• These zones experience neither extreme heat nor extreme cold. The Sun's rays are never vertical during the year in these regions.
• Frigid Zone The zones between 66° 30' North latitude and the North pole, and 66° 30' South latitude and the South pole, are termed as frigid zones. 
• These areas receive the minimum insolation, with the South pole and South latitude experiencing the least amount of sunlight.
• Therefore, they are very cold. In these zones, the duration of sunlight may be more than 20 hours during summer, but the temperature is quite low even in summer.

Regional Distribution of Geographic Zones

•  The globe can be divided into seven geographic zones which are as follows.

Equatorial Linear Region

• This region extends between 5° N and 5° S latitudes on either side of the Equator. Here, the Sun's rays consistently shine vertically year-round, maintaining high temperatures and minimal annual temperature variations. The elevated temperature leads to increased transpiration, ensuring a constant presence of moisture in the atmosphere.

Inter-tropical Region

• This region spans between 5° and 12° latitudes in both hemispheres. While temperatures remain consistently high, there is a noticeable distinction between the summer and winter seasons.

Tropical Region

• Occupying the space between 12 and 25 latitudes in both hemispheres, the tropical zone experiences a temperature decrease from low latitudes to high altitudes. Summers witness a 14-hour day duration, while winters see a 14-hour night duration.

Sub-tropical Region

• Expanding between 25° to 45° latitudes in both hemispheres, this region encounters relatively low temperatures in winter due to the slant of the Sun's rays. At times, temperatures may drop below freezing.

Temperate Region

• Covering the area between 45° and 66.5 latitudes in both hemispheres, the temperate zone experiences long daylight hours, but the slant of the Sun's rays causes a temperature decrease. This region exhibits significant differences between winter and summer seasons, with winter temperatures occasionally falling below freezing.

Tundra Region

• The Tundra region lacks trees and remains snow-covered for a major part of the year. Typically found in cold climates with limited rainfall, the Sun's rays become extremely oblique, almost parallel to the Earth's surface, resulting in lower insolation and very low temperatures.

Polar Region

• Encircling the North and South Poles, this region experiences limited sunlight, leading to continuous snowfall throughout the year. The absence of insolation results in negligible daily temperature variations.

Factors Affecting Temperature Distribution

• Drainage Inversion Drainage inversions occur when cooler air descends down a slope into a valley, displacing slightly warmer air. This phenomenon, known as a cold-air drainage inversion, is a common winter occurrence in certain mid-latitude regions.
• Upper-air Inversion Temperature inversions in the upper air result from descending air, earning them the designation of subsidence inversions.

Subsidence Inversions

• These inversions are typically linked to high-pressure conditions, a characteristic feature of sub-tropical latitudes throughout the year and Northern hemisphere continents in winter. 
• Subsidence inversions can be quite deep, reaching several thousand meters, with their base positioned a few hundred meters above the ground due to low-level turbulence preventing warmer air from sinking lower.

Ranges of Temperature

• The temperature of a location fluctuates within a day and varies across different seasons. 
• The range of temperature is the difference between the maximum and minimum temperatures.

 Two terms are commonly used to describe temperature ranges:

Annual Temperature Ranges

• The annual temperature range is characterized by the difference between the average temperatures of the warmest and coldest months. Larger annual ranges are observed in the Northern hemisphere, particularly on continents in middle and higher latitudes. 
• Conversely, smaller ranges occur in the Southern hemisphere, near the Equator, and over extensive water bodies.

Diurnal and Seasonal Cycle

• The daily temperature cycle exhibits a gradual increase from sunrise to approximately 3:00 pm, reaching the maximum temperature. Temperature then decreases in the evening and night, reaching a minimum before sunrise.
• The difference between the maximum daytime temperature and the nighttime minimum is referred to as the diurnal range of temperature.
•  Places in the interior of continents typically experience a greater diurnal range compared to coastal areas.
• The seasonal temperature difference results from variations in the angle of the Sun's rays and sunlight duration.
• Generally, in the Northern hemisphere, the maximum temperature is recorded in July, despite the peak insolation occurring around June 21st.
• Similarly, the minimum temperature is typically recorded in January, approximately a month after the period of minimum insolation on December 22nd.
• The equatorial region exhibits the lowest annual temperature range due to minimal variation in insolation between summer and winter. In contrast, the interior of continents in middle latitudes experiences a greater range.

Heat Island

• An urban heat island occurs when certain areas within a city experience higher temperatures than surrounding or neighboring areas on the same day. The temperature difference can range from 3° to 5° Celsius.

Temperature Changes

Temperature changes are classified into two types:

• Diabatic Change: Temperature change caused by heat transfer, involving evaporation or absorption. This process resembles an open system, with heat transfer occurring until temperature equilibrium is reached.
• Adiabatic Change: Temperature change without heat exchange with the surrounding air. It results from the expansion (cooling) or compression (warming) of air as it rises or descends in the atmosphere.

Temperature Anomaly

• The temperature anomaly is the difference between the mean temperature of a location and the mean temperature of the latitude passing through that place. It indicates deviation from the normal temperature.
• Isothermal lines exhibit greater irregularity in the Northern hemisphere due to extensive continents, while they are more regular in the Southern hemisphere dominated by oceans.
•  Isotherms are generally closely spaced in the Northern hemisphere but widely spaced in the Southern hemisphere.
• A positive thermal anomaly indicates a warmer observed temperature than the reference value, while a negative thermal anomaly indicates a cooler observed temperature.

Prelims facts

• In which atmospheric layer the air temperature rises with height?
  • - Stratosphere (UPPSC (Pre) 2019)
• By which process the Earth's atmosphere is mainly heated?
  • - Longwave terrestrial radiation (UPPSC (Pre) 2022
• Cloudy nights are better than the clear nights due to
  • -Terrestrial radiation [IAS (Pre) 2001
• Which imaginary line experiences the least annual range of temperature?
  • -Equator [IAS (Pre) 2010)
• High temperature and high humidity cause conceptional rain to fall mostly in the afternoons near the
  • Equator (IAS (Pre) 2003]
• The energy in atmosphere indirectly get from Sun while directly from
  • -Earth's Surface [UPPSC (Pre) 1997]
• The intensity of volume depends on
  • -Latitude (UPPSC (Pre) 2005

Self Check

1. What is contained in the word 'Albedo"?

(a) Ability to absorb Sun radiation

(b) Ability to change the path of Sun radiation

(c) Ratio of shortwave solar radiation reflected by a surface

(d) Quantity of solar radiation reflected by a surface into the air

2. Consider the following statements.

IAS (Pre) 2007

1. The annual range of temperature is greater in the Pacific ocean than that in the Atlantic ocean.
2. The annual range of temperature is greater in the Northern hemisphere than that in the Southern hemisphere.

Which of the statement(s) given above is/are correct?

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

3. Normally, the temperature decreases with the increase in height from the Earth's surface because IAS (Pre) 2012

1. The atmosphere can be heated upwards only from the Earth's surface.
2. There is more moisture in the upper atmosphere.
3. The air is less dense in the upper atmosphere.

Select the correct answer by using the codes given below.

(a) Only 1

(b) 2 and 3

(c) 1 and 3

(d) All of these

4. Which of the following statement(s) is/are true?

1. The angle of the axis in relation to the plane in which the Earth revolves around the Sun is not constant.
2. The amount of energy given off by the Sun changes with the transparency of the atmosphere.

Codes

(a) Only 1

(b) Only 2

(c) Both 1 and 2

(d) Neither 1 nor 2

5. The annual range of temperature in the interior of the continents is high as compared to coastal areas. IAS (Pre) 2013

What is/are the reason/reasons?

1. Thermal difference between land and water.
2. Variation in altitude between continents and oceans,
3. Presence of strong winds in the interior.
4. Heavy rains in the interior as compared to coasts. 

Select the correct answer by using the codes given below.

(a) Only 1

(b) 1 and 2

(c) 2 and 3

(d) All of these

6. Radiant energy from the Sun that strike the Earth is called MPPSC (Pre) 2011

(a) solar constant

(b) insolation

(c) heat budget

(d) terrestrial radiation

7. Assertion (A) The thickness of the atmosphere is maximum above the Equator. Reason (R) High insolation and intense convec- tion currents flow over the Equator. UPPSC (Pre) 2013

Codes

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and B are true, but R is not the correct explanation of A

(c) A is true, but R is false.

(d) A is false, but R is true.

8. Which of the following is a method of heat transfer?

(a) Convection

(b) Radiation

(c) Conduction

(d) All of these

9. What is isotherm?

(a) The line joining the places of equal temperature

(b) The incoming shortwave radiation

(c) The line joining the places of equal pressure

(d) None of the above

10. The process of vertical heating of the atmosphere is known as

(a) Radiation

(b) Convection

(c) Conduction

(d) Advection

11. The sunspots cause

(a) aurora borealis and aurora Australis

(b) magnetic storms 

(c) polar auroras

(d) All of the above

12. Cloudy nights are hotter than clear nights due to

(a) greenhouse effect

(b) ozone layer weathering

(c) incidence

(d) terrestrial radiation

13. Which of the following statement(s) is/are not true?

UPPSC (Pre) 2002

(a) Presence of water vapour is highly variable in the lower atmosphere.

(b) The zone of maximum temperature is located along the Equator.

(c) Frigid zones are located in both the hemispher between the polar circles and the poles.

(d) Jet streams are high altitude winds affecting the surface weather conditions.

14. The maximum annual range of temperature occurs over

(a) poles

(b) near the Equator

(c) coastal regions

( d) Asia and North America at latitude 60° N

15. The thermal Equator is found

(a) at the Equator

(b) South of the geographical Equator

(c) North of the geographical Equator

(d) at the Tropic of Cancer

16. The mountain is 3000 m high. If the temperature at the bottom is 30°C, what will be the temperature at the top?

(a) 15°C

(b) -10.5° C

(c) 10.5° C

(d)- (d) -15°C

17. The average surface temperature of the Earth's surface is

(a) 10°C

(b) 15°C

(c) 8°C

(d) 5° C

18. In absorption of insolation, the most significant part is played by

(a) carbon dioxide

(b) ozone

(c) oxygen

(d) haze

Know Right answer

1 (c)

2 (b)

3 (c)

4 (b)

5 (a)

6 (b)

7 (a)

8 (d)

9 (a)

10 (b)

11 (d)

12 (d)

13 (b)

14(d)

15 (c)

16 (c)

17 (b)

18 (a)