Chapter Notes

Solar Radiation, Heat Balance and Temperature

Our planet Earth is wrapped in a blanket of air called the atmosphere. This mixture of gases is essential for life and is constantly in motion. The movement of air is what we feel as wind. The primary engine driving this motion and all weather patterns is energy from the sun.

The Earth receives energy from the sun and radiates that same amount of energy back into space. This maintains a stable temperature over time, preventing the planet from continuously heating up or cooling down. However, this energy is not distributed evenly across the globe. This uneven heating creates pressure differences in the atmosphere, causing winds that transfer heat from one region to another. This chapter explores how the atmosphere is heated and cooled and how this process determines the distribution of temperature on Earth.

Solar Radiation

Nearly all energy the Earth receives comes from the sun in the form of short wavelengths. This incoming solar energy is called insolation.

Because the Earth is a sphere-like geoid, the sun's rays strike the top of the atmosphere at an angle. On average, the Earth receives about 1.94 calories per square centimeter per minute at the top of its atmosphere.

The amount of solar energy reaching us varies slightly throughout the year because the distance between the Earth and the sun changes.

• Aphelion: On 4th July, the Earth is farthest from the sun (152 million km).
• Perihelion: On 3rd January, the Earth is nearest to the sun (147 million km).

Consequently, the Earth receives slightly more insolation on January 3rd than on July 4th. However, this small variation doesn't have a major impact on our daily weather. Other factors, like the distribution of land and sea, have a much greater effect.

Variability of Insolation at the Surface of the Earth

The amount of insolation reaching the Earth's surface changes daily, seasonally, and yearly. The main factors causing these variations are:

• The rotation of the Earth on its axis.
• The angle of inclination of the sun’s rays.
• The length of the day.
• The transparency of the atmosphere.
• The configuration of land (its aspect or direction it faces).

The two most significant factors are the Earth's tilt and the angle of the sun's rays.

The Earth's axis is tilted at an angle of 66.5° to the plane of its orbit around the sun. This tilt has a major influence on how much insolation different latitudes receive throughout the year, causing our seasons.

The angle at which the sun's rays strike the Earth's surface is also critical. This angle depends on a place's latitude.

• At lower latitudes (near the equator): The sun's rays are nearly vertical. They are concentrated over a smaller area, delivering more energy per unit area.
• At higher latitudes (near the poles): The sun's rays are slanted. They spread out over a larger area, so the energy is less concentrated. These slant rays also have to travel through a thicker layer of the atmosphere, which leads to more energy being absorbed, scattered, or diffused before it reaches the ground.

The Passage of Solar Radiation through the Atmosphere

The atmosphere is mostly transparent to the incoming short-wave solar radiation, allowing much of it to pass through to the surface. However, some interactions do occur:

• Absorption: Gases in the troposphere, such as water vapour and ozone, absorb some of the near-infrared radiation.
• Scattering: Tiny suspended particles in the atmosphere scatter the visible light in all directions. This scattering is what gives the sky its colour.

Spatial Distribution of Insolation at the Earth's Surface

The amount of insolation received varies significantly across the globe.

• In the tropics, the insolation is high, around 320 Watt/m².
• In the polar regions, it is much lower, around 70 Watt/m².

Interestingly, the maximum insolation is not at the equator but in the subtropical deserts, where cloud cover is minimal. The equator receives slightly less insolation than the tropics due to persistent cloudiness. Generally, at the same latitude, continents receive more insolation than oceans.

Heating and Cooling of Atmosphere

The atmosphere gets heated through several different processes after the Earth's surface is warmed by insolation.

Conduction

The Earth's surface, after being heated by insolation, transfers this heat to the layer of air directly in contact with it. This process is called conduction. Conduction is the transfer of heat between two bodies in contact that have different temperatures. It is most important for heating the lowest layers of the atmosphere.

Convection

Once the lower layer of air is heated, it expands, becomes lighter, and rises. This vertical movement of air currents transfers heat upwards. This process of vertical heating of the atmosphere is known as convection. Convective transfer of energy is primarily confined to the troposphere.

Advection

The transfer of heat through the horizontal movement of air (wind) is called advection. This process is more significant in creating weather variations than vertical movement.

Terrestrial Radiation

The Earth's surface is heated by insolation, which arrives as short-wave radiation. Once heated, the Earth itself becomes a radiating body, but it emits energy back into the atmosphere in the form of long-wave radiation. This process is known as terrestrial radiation.

This long-wave radiation is what primarily heats the atmosphere from below. Atmospheric gases, especially carbon dioxide and other greenhouse gases, are very effective at absorbing this long-wave radiation. This is why the atmosphere is heated indirectly by the Earth's radiation, not directly by the sun.

Heat Budget of the Planet Earth

The Earth maintains a constant temperature because it is in a state of balance. The amount of heat it receives from the sun is equal to the amount it loses back to space. This balance is called the heat budget or heat balance.

Let’s imagine the total incoming insolation at the top of the atmosphere is 100 units.

1. Reflection: Before even reaching the surface, 35 units are reflected back to space.

   • 27 units are reflected from the tops of clouds.
   • 6 units are scattered to space by the atmosphere.
   • 2 units are reflected by snow and ice-covered areas on Earth.
   • This total reflected amount of radiation is called the albedo of the Earth.

2. Absorption: The remaining 65 units are absorbed.

   • 51 units are absorbed by the Earth's surface.
   • 14 units are absorbed by the atmosphere (gases, water vapour).

3. Radiation from Earth and Atmosphere: To maintain balance, these 65 absorbed units must be radiated back to space.

   • The Earth radiates back its 51 units as terrestrial radiation. Of this, 17 units go directly to space.
   • The other 34 units of terrestrial radiation are absorbed by the atmosphere.
   • The atmosphere now holds a total of 48 units of energy (14 units from insolation + 34 units from terrestrial radiation).
   • The atmosphere then radiates these 48 units back to space.

Variation in the Net Heat Budget at the Earth's Surface

While the planet as a whole is in balance, different parts of the Earth are not.

• There is a surplus of radiation energy in the regions between 40° North and 40° South latitudes.
• There is a deficit of radiation energy in the polar regions.

This imbalance is crucial. The excess heat from the tropics is transported towards the poles by winds and ocean currents. This redistribution of energy prevents the tropics from getting progressively hotter and the poles from getting permanently frozen.

Temperature

Heat is the energy produced by the movement of molecules in a substance. Temperature is the measurement of how hot or cold that substance is, measured in degrees. The interaction of insolation with the Earth's surface and atmosphere creates the heat that we measure as temperature.

Factors Controlling Temperature Distribution

The temperature at any given place is influenced by several factors:

• Latitude: The amount of insolation a place receives depends on its latitude. As explained earlier, lower latitudes are generally warmer than higher latitudes.
• Altitude: The atmosphere is heated from below by terrestrial radiation. Therefore, places at higher elevations are cooler than places near sea level. Temperature generally decreases with increasing height at a rate of 6.5°C per 1,000 meters. This is known as the normal lapse rate.
• Distance from the sea: Land heats up and cools down much faster than water. Places near the coast experience a moderating effect from the sea (known as continentality), with less extreme temperature variations between summer and winter. Inland areas have a much larger temperature range.
• Air-mass and Ocean currents: The movement of air masses affects temperature. A place will experience higher temperatures when under the influence of a warm air mass and lower temperatures with a cold air mass. Similarly, coasts with warm ocean currents are warmer than coasts with cold ocean currents.
• Local aspects: The local landscape, such as the slope and orientation of the land, can also influence local temperatures.

Distribution of Temperature

The global distribution of temperature is often shown on maps using isotherms, which are lines connecting places that have the same temperature.

Temperature in January (Northern Hemisphere Winter)

• Isotherms are generally parallel to the latitude, but they bend significantly over land and sea.
• In the Northern Hemisphere, isotherms bend southward over continents (showing colder temperatures) and northward over oceans (showing warmer temperatures). This is because continents are much colder than oceans in winter.
• For example, the warm ocean currents of the Gulf Stream and North Atlantic Drift make the North Atlantic Ocean much warmer than the adjacent landmasses in Europe and North America.
• In the Southern Hemisphere, which has much less landmass, the isotherms are more regular and parallel to the latitudes.

Temperature in July (Northern Hemisphere Summer)

• In July, the isotherms again generally run parallel to the latitudes.
• The highest temperatures (over 30°C) are found over the subtropical continental regions of the Northern Hemisphere, like in Asia.
• The temperature variation in the Southern Hemisphere is more gradual.

The annual range of temperature (the difference between the warmest and coldest months) is greatest over the continents. The highest range, over 60°C, is found in the north-eastern part of the Eurasian continent due to its extreme continentality. The smallest range (less than 3°C) is found over the oceans near the equator.

Inversion of Temperature

Normally, temperature decreases as you go higher in altitude (the normal lapse rate). However, sometimes this situation is reversed, and temperature actually increases with height. This is called an inversion of temperature.

An inversion is usually short-lived but common. The ideal conditions for it are:

• A long winter night
• Clear skies
• Still air

During the night, the ground radiates heat and cools down rapidly. By early morning, the ground is cooler than the air above it, creating a layer of cold, dense air near the surface with warmer, lighter air on top.

In mountainous regions, a phenomenon called air drainage can cause temperature inversions. At night, the cold air on the hills and slopes becomes dense and flows down into the valley bottoms, displacing the warmer air upwards. This can protect plants in the valleys from frost damage.