RBSE Class 11 Geography Notes Chapter 12 Insolation and Heat Budget

Go through these RBSE Class 11 Geography Notes Chapter 12 Insolation and Heat Budget contain important concepts so that students can score better marks in the exam.

Rajasthan Board RBSE Class 11 Geography Notes Chapter 12 Insolation and Heat Budget

→ Atmospheric cover protects the earth from heat during the day and cold during the night.

→ Sun is the main source of heat for Earth, the temperature on the surface of the Sun is 6000°C.

→ The sun’s heat is received by radiation on the earth. The average distance between Earth and Sun is 15 million km.

→ It takes 8 minutes and 20 seconds for sunlight to reach the Earth.


  • Solar energy reaching the earth from the Sun is called insolation.
  • The radiation that is emitted from the surface of the Sun is called solar radiation.
  • The solar radiation refers to the reception of insolation by the Earth.
  • Energy from the Sun reaches the Earth in the form of short waves.
  • This solar energy, which is received on the Earth’s surface, enables all the physical and biological phenomena.
  • Insolation is measured by an instrument called Pyroheliometer.

Distribution of Temperature

  • Distribution of temperature on the earth’s surface is not even everywhere.
  • The earth is divided mainly into three tropics based on the receipt of temperature.
  • The temperature refers to the atmospheric temperature, and the sun is the source of it.

RBSE Class 11 Geography Notes Chapter 12 Insolation and Heat Budget

Horizontal Distribution of Temperature

  • The horizontal distribution of the temperature refers to the latitudinal distribution of temperature.
  • While proceeding towards the poles from the equator, the temperature distribution changes.
  • The distribution of temperature on the map is represented by Isotherms.
  • The line joining the places having the same temperature is called Isotherm.
  • For the analysis of temperature, the months of January and July have been selected because in these months minimum and maximum temperature is found in most parts of the world.

Horizontal Distribution of Temperature in January

  • In the month of January, the rays of the Sun fall vertically on the Tropic of Capricorn, due to this summer prevails in the Southern Hemisphere and winter prevails in the Northern Hemisphere.
  • At this time, the minimum temperature is found in Siberia and Greenland.
  • In this period, the isotherms are folded according to the condition of continents and oceans.
  • In the Northern Hemisphere, due to unequal distribution of water and land, the isotherms are found to be more curved.

Horizontal Distribution of Temperature in July

  • In July, the rays of the sun fall vertically on the Tropic of Cancer, due to which summer occurs in the Northern Hemisphere and winter occurs in the Southern Hemisphere.
  • In summer, the effect of temperature is on a large area. During this, time, the minimum temperature is found in Antarctica.
  • The isotherms in the Southern Hemisphere are often found parallel to the latitudes.

Vertical Distribution of Temperature

  • The vertical distribution of temperature refers to the temperature found at different heights above the surface.
  • On going upwards from the surface, 1°C temperature reduces for every 165 meters, which is called normal lapse rate of temperature.
  • This decline in temperature is not similar at all places in the atmosphere. This temperature lapse rate is only found in the troposphere.

RBSE Class 11 Geography Notes Chapter 12 Insolation and Heat Budget

Inversion of Temperature

  • Increase in temperature with increase in height, contrary to decrease in temperature is known as Inversion of Temperature.
  • For the inversion of temperature, long nights, clear sky, cool air, dry air and glacial conditions etc. are the ideal conditions. In this situation, heat radiation from the lower layers of the surface and air is at a faster pace.
  • Thermal inversion is often found in inter-montain valleys, thus settlements and fruit gardens are grown in the upper part of the hill slopes in mountain valleys.

Factors Affecting the Distribution of Temperature

  • Factors affecting the distribution of temperature include distance from the equator, height from sea level, distance from sea level, sea currents, prevailing winds, slope of land, nature of the surface and clouds and rainfall.
  • Due to the perpendicular and oblique sun rays, the receipt of temperature is respectively more and less.
  • Due to the nature of the water, generally constant and moderate temperature is found near the sea. According to the nature of currents, the winds are hot or cold.
  • The structure and appearance of the soil and the profile of the slope towards the wind or opposite to it, also affect the temperature.

Heat Budget of Earth

  • The heat budget refers to the temperature received by the Earth and the atmosphere and the balance of heat loss by that determines the temperature.
  • The volume of insolation received by sun and the radiation released by the earth is almost indentical.
  • Only 1 part of the two billion parts of solar terrestrial radiation reaches the Earth, the remaining heat is dissipated by the process of atmospheric absorption, reflection and scattering.
  • 35% of the heat coming from the sun gets reflected in space even before reaching the Earth’s inner atmosphere and 14% is absorbed by the atmosphere. In this way the earth receives only 51% of the energy.
  • 51% of the heat obtained by the Earth is returned in the form of 23% by radiation, 9% by convection and 19% by evaporation.

RBSE Class 11 Geography Notes Chapter 12 Insolation and Heat Budget

Insolation and Heat Budget Notes Important Terms

→ Insolation: The solar energy that is received from the sun is called insolation. This energy reaches surface of the earth from the sun in the form of short waves radiation whose speed is 1,86,000 miles per second.

→ Heat Budget: Radiation of heat received from the Sun and its distribution on surface of the earth is called Heat Budget. About 35% of the total heat radiated from the sun is returned in its original form into outer space by reflection and scattering. Similarly, 14% is absorbed by the atmosphere through clouds, dust particles and some permanent gases. The remaining 51% of the energy is received by earth in the form of a heat budget.

→ Solar Radiation: The energy radiated from the surface of the Sun in form of short waves, which is transmitted at a speed of 1,86,000 miles km / second.

→ Latitude: The angular distance measured from the Earth’s center to a point on the north or south part from the equator is called latitude.

→ Lower Latitudes: The area between 30° North and 30° South latitudes from the equator is called the lower latitudes.

→ Median Latitudes: The area between 30° to 60° latitude in both the hemispheres is called median latitudes.

→ Higher Latitudes: The area between 60° to 90° latitude in both the hemispheres is called higher latitudes.

→ Torrid/Tropical Zone: Part of the Earth located between the tropics on either side of the equator. In this zone, except for a few months of the year, the sun rays shine almost vertically.

→ Tropic of Cancer: (23 1/2°) Northern latitude circle. In the northern hemisphere, this is the final boundary of the horizontal position of the Sun, above which, the sun’s rays never fall vertically. Only on one day June 21, the sun rays fall vertically on the tropic of cancer.

→ Tropic of Capricorn: 23° 32′ (approximately 23° 30′) southern latitude. This is the southernmost border, below which, the sun rays never fall vertically. Only on one day in the year (22nd December) the sun shines vertically oyer the Tropic of Capricorn.

→ Arctic Circle: 66° 30′ Northern latitude circle. Due to the axial tilt of the Earth, in the event of maximum leaning towards the north, there is no sunset on the Arctic Circle for one day, and in the event of more inclination towards the south, there is no sunrise for one day.

→ Antarctic Circle: (66 1/2°) Southern latitude circle. Due to the inclination of the axis of the Earth, there is no sunset on the Antarctic circle during the summer solstice of the southern hemisphere, and there is no sunrise during the winter solstice.

→ Isotherm: The line displayed on the map which joins the places with the same temperature at a particular time, or places with the same average temperature over a given time period. In Geography, often those isotherms are used, which are drawn to the places with the same average monthly temperature.

→ Normal lapse rate: There is an average rate of change in temperature of the air, without the inclusion any additional heat in the air as we go high up above the sea level, which is called normal lapse rate or environmental temperature lapse rate.

→ Inversion of temperature: The tendency or process of increase in temperature instead of decrease along with increase in height from the surface.

→ Glaciation: This is an extensive process in which glaciers or ice sheets expand on the ground due to temperature getting excessively less due to climate change. This process occurs in various ice ages.

→ Inter-mountain Valley : The valleys found between two or more mountains.

→ Settlement: Group of man-made houses on the surface of earth. The house of residence can be of any kind, from the grass-mud cottage to a large building made of brick, stone and concrete.

→ Sirrocco: The hot, dry and sandy-wind which evolves from the Sahara Desert and proceeds to the north, which crosses the Mediterranean Sea and enters the coastal countries of Italy, Spain etc. This is a suddenly blowing short-term wind, which sustains for one or two days.

→ Chinook: In North America, the southern-western and hot wind blowing through the eastern slope of the Rocky Mountains is called Chinook. This south-western wind crosses down the Rocky Mountains and heats the Western plains.

→ Loo: In northern India, the hot summer winds blow in the form of a hot wave in the direction of west to east. These winds often blow for two or three times in the month of May-June, which last for 2-3 weeks.

→ Absorption: The process of absorption of insolation by various gases, flora, water vapour etc. present on the atmosphere is called absorption.

→ Albedo: A fraction of all the solar radiation falling on a some cosmic or celestial body or the Earth, which is reflected and reaches the space again. It is expressed in decimal or percentage.

→ Terrestrial Radiation: Radiation of insolation heat received by earth through long waves. As a result of this, the lower part of the atmosphere gets heated.

→ Scattering: The atmosphere is composed of microscopic particles of air molecules, water vapour and dust particles. The short waves of solar energy are dispersed in every direction by these molecules or particles, and this process is called scattering.

→ Convection: The transmission of energy from one point to another by actual movement of molecules themselves in a liquid or gas. When a liquid substance is heated in its lower part, it spreads in it and decreases the density, causing the hot part to rise and the cool part reaches at its place causing a convection current. Thus, through this process, the entire liquid substance gets heated up.

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