Introduction to Solar Energy and Atmosphere

• The Earth is surrounded by a layer of air called the atmosphere, which contains gases that support life, like oxygen for animals and carbon dioxide for plants.
• Almost all of Earth’s energy comes from the sun. This energy is essential for heating the Earth and driving the weather systems.
• The Earth doesn’t continuously get warmer or colder because it maintains a balance by radiating back the energy it receives from the sun into space. This balance affects weather and climate.

Solar Radiation (Insolation)

• Solar radiation is the energy the Earth receives from the sun. The energy that reaches the Earth’s surface is called insolation. It comes to Earth in short wavelengths.
• Since the Earth is round, the sun’s rays hit different parts of the Earth at different angles. Areas near the equator receive sunlight more directly, while areas near the poles receive it at a slant, which means they get less energy.

Earth’s Distance from the Sun

• The Earth’s orbit around the sun is not perfectly circular; it is slightly elliptical. This means that the Earth’s distance from the sun changes slightly during the year.
• The Earth is:
  • Farthest from the sun (about 152 million km) on 4th July. This position is called aphelion.
  • Closest to the sun (about 147 million km) on 3rd January. This position is called perihelion.
• Although Earth is closer to the sun in January, the impact on weather is minor because other factors like cloud cover, land-sea distribution, and atmospheric circulation play a bigger role in determining the temperature.

Factors Influencing Insolation

The amount of solar radiation reaching the Earth’s surface varies depending on:

• Rotation of the Earth: As the Earth spins on its axis, different parts receive sunlight at different times.
• Angle of the Sun’s Rays: This depends on the latitude of a place. Near the equator, the sun’s rays hit the Earth more directly, while at higher latitudes, they are slanted, meaning less energy is received.
• Length of the Day: Areas with longer daylight hours (like during summer) receive more sunlight.
• Transparency of the Atmosphere: Dust, clouds, and pollution can block sunlight and reduce the amount of energy reaching the surface.
• Aspect of Land: The direction a slope faces affects how much sunlight it receives. Slopes facing the sun receive more direct sunlight.

How Solar Radiation Passes Through the Atmosphere

• The atmosphere allows most solar radiation to pass through and reach the Earth’s surface, especially in short wavelengths.
• Water vapor, ozone, and other gases in the atmosphere absorb some of the radiation, especially in the infrared part of the spectrum.
• Scattering of light by small particles in the atmosphere explains why the sky looks blue and why the sun appears red at sunrise and sunset.

Heating and Cooling of the Atmosphere

After the Earth’s surface absorbs solar radiation, it heats the atmosphere in various ways. The process by which the atmosphere gets heated is different from how the Earth absorbs heat.

Conduction

• Conduction occurs when two objects of different temperatures come into contact, and heat flows from the warmer object to the cooler one.
• The Earth’s surface heats the air that is in direct contact with it through conduction.
• This process mostly affects the lower layers of the atmosphere close to the Earth’s surface.

Convection

• Convection is the process by which warm air rises and cool air sinks, creating a circulation of heat.
• Warm air near the Earth’s surface becomes lighter and rises, while cooler air sinks to take its place. This process transfers heat from the surface to higher levels in the atmosphere.
• Convection is mostly confined to the troposphere, the lowest layer of the atmosphere where weather occurs.

Advection

• Advection refers to the horizontal movement of air, transferring heat from one place to another.
• This process is more important in middle latitudes where it plays a big role in causing daily changes in weather.
• For example, in northern India, the hot wind known as ‘loo’ during summer is a result of advection.

Terrestrial Radiation

• After absorbing solar energy, the Earth’s surface radiates energy back into the atmosphere in the form of long-wave infrared radiation.
• This long-wave radiation is absorbed by atmospheric gases, especially carbon dioxide, water vapor, and other greenhouse gases. These gases trap heat, warming the atmosphere in what’s known as the greenhouse effect.
• This process of terrestrial radiation ensures that the lower atmosphere stays warm, which is essential for maintaining temperatures suitable for life on Earth.

Heat Budget of the Earth

• The Earth’s heat budget refers to the balance between the amount of energy the Earth receives from the sun (solar radiation) and the amount it radiates back into space (terrestrial radiation).
• The Earth’s temperature remains stable over time because the energy received from the sun equals the energy radiated back into space.
• Out of 100 units of solar energy received at the top of the atmosphere:
  • 35 units are reflected back into space (27 units by clouds and 2 units by ice and snow).
  • 65 units are absorbed (51 units by the Earth’s surface and 14 units by the atmosphere).
  • The Earth radiates 51 units of heat, with 17 units going directly into space and 34 units being absorbed by the atmosphere.
  • The atmosphere radiates 48 units of heat back into space, balancing the total energy received and radiated.

Variation in the Net Heat Budget

• The amount of energy absorbed and radiated varies across different regions of the Earth.
• Tropical regions receive more energy (heat surplus), while polar regions experience less energy (heat deficit).
• Winds and ocean currents transfer surplus heat from tropical regions to polar regions, preventing extreme temperature differences. This redistribution of heat keeps the tropics from becoming too hot and the polar regions from becoming completely frozen.

Temperature

• Temperature is a measure of how hot or cold something is. It reflects the movement of molecules in a substance.
• The temperature of air at a place depends on several factors:

Factors Controlling Temperature Distribution

1. Latitude: The closer a place is to the equator, the more direct sunlight it receives, resulting in higher temperatures. As you move towards the poles, the sunlight becomes slanted, and temperatures decrease.
2. Altitude: As height increases, temperature decreases. This is called the lapse rate, which averages 6.5°C per 1,000 meters. Higher altitudes are cooler because the atmosphere is heated from below, not directly by the sun.
3. Distance from the Sea: Land heats and cools faster than water. Coastal areas, influenced by the sea, have more moderate temperatures, while inland areas experience more extreme temperature changes.
4. Air Masses and Ocean Currents: Warm and cold air masses affect temperature. Areas affected by warm ocean currents (like the Gulf Stream) have higher temperatures, while areas near cold ocean currents are cooler.

Global Distribution of Temperature

• Temperature across the globe is represented using isotherms, which are lines that connect places with the same temperature.
• In January (winter in the Northern Hemisphere), temperatures drop sharply over land, especially in places like Siberia, while oceans remain warmer because they lose heat more slowly.
• In July (summer in the Northern Hemisphere), land areas, particularly in Asia, experience high temperatures, while oceans stay relatively cooler.

Inversion of Temperature

• Normally, temperature decreases as you move higher in the atmosphere, but sometimes this pattern is reversed in a phenomenon called temperature inversion.
• Inversion of temperature occurs when cold air gets trapped near the ground under a layer of warm air, often during clear winter nights. The ground cools rapidly, causing the air near the surface to become colder than the air above it.
• Air drainage in valleys and mountains also causes temperature inversion, where cold air flows down and settles in low-lying areas, trapping warmer air above.
• This can result in fog or frost in the lower layers of the atmosphere.

Conclusion

• Solar Radiation (Insolation): The Earth receives energy from the sun, which is essential for heating the surface and driving weather patterns.
• Heat Transfer: The Earth transfers heat to the atmosphere through conduction, convection, and advection.
• Terrestrial Radiation: The Earth radiates heat back to the atmosphere, which helps maintain the planet’s heat balance.
• Heat Budget: The Earth keeps a balance between the heat it receives and the heat it radiates back, maintaining a stable climate.
• Temperature Distribution: Factors like latitude, altitude, distance from the sea, and air masses affect how temperatures vary across the globe.
• Inversion of Temperature: Sometimes the normal pattern of temperature decreasing with height is reversed, leading to cold air being trapped near the ground.

This explanation covers all the critical details in the chapter, helping you understand how solar energy drives the Earth’s climate and temperature systems. Let me know if you need more clarification on any point!