Explore Insolation — angle of incidence and distribution; heat budget of the earth — heating and cooling of atmosphere (conduction, convection, terrestrial radiation, advection); temperature — factors controlling temperature; distribution of temperature — horizontal and vertical; inversion of temperature
Insolation, heat budget & Temperature Inversion
Insolation — angle of incidence and distribution
Introduction to Earth's Energy System
The earth receives almost all its energy from the sun and radiates it back to space, maintaining a balance over time.
Uneven heat distribution causes atmospheric pressure differences, leading to heat transfer via winds.
This section explains:
The processes of atmospheric heating and cooling.
The resultant temperature distribution over Earth's surface.
Solar Radiation (Insolation)
Energy received by the earth in short wavelengths is termed insolation.
The earth, resembling a geoid, intercepts only a small portion of the sun's energy.
Average Solar Energy: 1.94 calories per sq. cm per minute at the top of the atmosphere.
Earth's Position and Solar Output:
Aphelion (4th July): Earth is farthest from the sun (152 million km).
Perihelion (3rd January): Earth is nearest to the sun (147 million km).
The insolation received on 3rd January is slightly more than on 4th July.
Other factors like land-sea distribution and atmospheric circulation mask these variations.
Variability of Insolation
The amount and intensity of insolation vary:
During a day, a season, and a year.
Factors Influencing Insolation:
Rotation of Earth on its axis.
Angle of inclination of the sun's rays.
Length of the day.
Atmospheric transparency (less significant).
Configuration of land and its aspect (less significant).
Earth's Axis: The tilt of 66½° relative to its orbital plane greatly affects insolation distribution.
Angle of Solar Rays:
Higher latitudes receive slant rays, covering larger areas with lower energy per unit area.
Slant rays pass through more atmosphere, causing greater absorption, scattering, and diffusion.
Passage of Solar Radiation through the Atmosphere
The atmosphere is largely transparent to shortwave solar radiation.
Absorption: Water vapor, ozone, and other gases absorb much of the near-infrared radiation within the troposphere.
Scattering:
Suspended particles scatter visible light, adding color to the sky.
Examples:
Red color of sunrise and sunset.
Blue color of the sky.
Spatial Distribution of Insolation
The insolation received at Earth's surface varies:
Tropics: About 320 W/m².
Poles: About 70 W/m².
Key Patterns:
Maximum insolation occurs over subtropical deserts due to minimal cloud cover.
The equator receives less insolation than the tropics.
At the same latitude, continents receive more insolation than oceans.
Higher latitudes receive less radiation in winter compared to summer.
There are different processes by which the atmosphere is heated and cooled:
Conduction:
Earth transmits heat to the atmospheric layers near its surface in long wave form.
Process:
The air in contact with the land gets heated slowly.
The upper layers in contact with the lower layers also get heated.
Occurs when two bodies of unequal temperature are in contact, transferring energy from warmer to cooler bodies until equilibrium is reached.
Important for heating the lower layers of the atmosphere.
Convection:
Vertical heating of the atmosphere as air rises in currents after being heated.
Confined to the troposphere.
Advection:
Horizontal transfer of heat through air movement.
More significant than vertical movement.
Examples:
Diurnal variations in middle latitudes are often caused by advection.
In tropical regions, winds like the "loo" in northern India during summer are results of advection.
Terrestrial Radiation
The earth absorbs shortwave insolation, heats up, and radiates energy back in long wave form.
The atmosphere is indirectly heated by this longwave terrestrial radiation.
Process:
Longwave radiation is absorbed by atmospheric gases, especially greenhouse gases like carbon dioxide.
The atmosphere radiates and transmits heat back to space, maintaining temperature balance.
Heat Budget of the Planet Earth
The earth neither accumulates nor loses heat, maintaining a constant temperature through a heat budget.
Process:
Insolation: Total energy received at the top of the atmosphere is 100%.
Energy Reflection:
35% is reflected back to space (called the albedo).
Breakdown:
27% from cloud tops.
2% from snow and ice-covered areas.
Energy Absorption:
65% is absorbed:
14% within the atmosphere.
51% by the earth's surface.
The earth radiates back 51% as terrestrial radiation:
17% radiated directly to space.
34% absorbed by the atmosphere:
6% absorbed directly by the atmosphere.
9% through convection and turbulence.
19% through latent heat of condensation.
Total radiation returned to space:
17% directly from the earth.
48% from the atmosphere.
Total: 17 + 48 = 65%, balancing the 65% absorbed from the sun.
This heat budget ensures that the earth does not significantly warm or cool despite the constant transfer of heat.
Variation in the Net Heat Budget at the Earth’s Surface
There are variations in the radiation received at the earth’s surface.
Some regions have a surplus radiation balance, while others have a deficit.
Latitudinal Variation:
Between 40° north and south latitudes, there is a surplus in net radiation balance.
Near the poles, there is a deficit in net radiation balance.
Surplus heat from the tropics is redistributed toward the poles, preventing:
Progressive heating of the tropics due to excess heat.
Permanent freezing of high latitudes due to excess deficit.
Temperature — factors controlling temperature; distribution of temperature — horizontal and vertical
Temperature
The interaction of insolation with the atmosphere and the earth’s surface creates heat, which is measured in terms of temperature.
Heat represents the molecular movement of particles comprising a substance, while temperature is the measurement of how hot (or cold) a place is.
Factors Controlling Temperature Distribution
The temperature of air at any place is influenced by:
(i) The latitude of the place
(ii) The altitude of the place
(iii) Distance from the sea, air-mass circulation
(iv) The presence of warm and cold ocean currents
(v) Local aspects
The Latitude: The temperature of a place depends on the insolation received. Since insolation varies according to latitude, temperature also varies accordingly.
The Altitude: The atmosphere is indirectly heated by terrestrial radiation from below. Therefore, places near sea level record higher temperatures than places situated at higher elevations. Temperature generally decreases with increasing height.
The rate of decrease of temperature with height is termed the normal lapse rate, which is 6.5°C per 1,000 m.
Distance from the Sea: The sea heats up and cools down slowly compared to land. Land heats up and cools down quickly. Therefore, temperature variation over the sea is less than over land. Places near the sea experience moderating influences from sea and land breezes.
Air-mass and Ocean Currents: Air masses and ocean currents also affect temperature.
Places influenced by warm air masses experience higher temperatures, while places influenced by cold air masses experience lower temperatures.
Similarly, coastal areas influenced by warm ocean currents experience higher temperatures compared to those influenced by cold currents.
Distribution of Temperature
The global distribution of temperature can be studied by analyzing temperature distribution in January and July. Isotherms, lines joining places with equal temperatures, are used to represent temperature distribution.
In general, the effect of latitude on temperature is well pronounced on the map, with isotherms generally parallel to the latitude.
The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere.
In the northern hemisphere, the land surface area is much larger than in the southern hemisphere, making the effects of landmass and ocean currents more pronounced.
In January, the isotherms deviate to the north over the ocean and to the south over the continent, such as the North Atlantic Ocean.
The presence of warm ocean currents like the Gulf Stream and North Atlantic drift make the Northern Atlantic Ocean warmer, causing isotherms to bend towards the north.
Over land, the temperature decreases sharply, and isotherms bend towards the south in Europe and the Siberian plain.
The mean January temperature along the 60° E longitude is minus 20°C at both 80° N and 50° N latitudes. Other regional temperatures include:
27°C in equatorial oceans,
24°C in the tropics,
2°C to 0°C in middle latitudes,
-18°C to -48°C in the Eurasian continental interior.
The Effect of the Ocean in the Southern Hemisphere: In the southern hemisphere, isotherms are more or less parallel to latitudes, and temperature variation is more gradual.
The isotherms of 20°C, 10°C, and 0°C run parallel to 35° S, 45° S, and 60° S latitudes, respectively.
In July: Isotherms generally run parallel to the latitude. The equatorial oceans record warmer temperatures (more than 27°C).
Over land, more than 30°C is observed in the subtropical continental region of Asia, along the 30° N latitude.
The isotherm of 10°C runs along the 40° N latitude, while the 10°C temperature is also seen along the 40° S latitude.
Figure 9.5 shows the range of temperature between January and July. The highest range of temperature is more than 60°C over the north-eastern part of the Eurasian continent due to continentality.
The least range of temperature (3°C) is found between 20° S and 15° N.
Temperature Inversion
INVERSION OF TEMPERATURE
Normally, temperature decreases with increase in elevation. This is called the normal lapse rate.
At times, this situation is reversed, and the normal lapse rate is inverted. This is known as inversion of temperature.
Inversion is usually of short duration but quite common nonetheless.
A long winter night with clear skies and still air is an ideal situation for inversion. The heat of the day is radiated off during the night, and by early morning hours, the earth is cooler than the air above.
Over polar areas, temperature inversion is normal throughout the year.
Surface inversion promotes stability in the lower layers of the atmosphere.
Smoke and dust particles get collected beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere.
Dense fogs in the mornings are common, especially during the winter season.
This inversion commonly lasts for a few hours until the sun rises and begins to warm the earth.
Inversion in Hills and Mountains: Inversion also takes place in hills and mountains due to air drainage.
Cold air produced during the night in the hills and mountains flows under the influence of gravity.
The cold air, being heavy and dense, acts almost like water and moves down the slope, piling up deeply in pockets and valley bottoms with warm air above. This is called air drainage.
Air drainage protects plants from frost damage.
Plank’s Law: Plank's law states that the hotter a body is, the more energy it will radiate, and the shorter the wavelength of that radiation.
Specific Heat: Specific heat is the energy needed to raise the temperature of one gram of a substance by one degree Celsius.
