Global Patterns of Insolation Receipts

Figure 1: Annual (1987) pattern of solar radiation absorbed at the Earth's surface. (Image created by the CoVis Greenhouse Effect Visualizer).

The combined effect of Earth-sun relationships (angle of incidence and day length variations) and the modification of the solar beam as it passes through the atmosphere produces specific global patterns of annual insolation receipt at the surface as seen on Figure 1. After examining these patterns, the following trends can be identified:

• Highest values of surface insolation received occur in tropical latitudes. Within this zone there are localized maximums over the tropical oceans and deserts where the atmosphere has virtually no cloud development for most of the year. Insolation quantities at the equator over land during the solstices are approximately the same as values found in the middle latitudes during their summer.
• Outside the tropics, annual receipts of solar radiation generally decrease with increasing latitude. Minimum values occur at the poles. This pattern is primarily the result of Earth-sun geometric relationships and its effect on the duration and intensity of solar radiation received.
• In middle and high latitudes, insolation values over the ocean, as compared to those at the same latitude over the land, are generally higher. Greater cloudiness over land surfaces accounts for this variation.

NASA's Surface Radiation Budget Project has used satellite data, computer models, and meteorological data to determine shortwave (solar) surface radiation fluxes for the period July 1983 to June 1991. The following links display these fluxes for January and July globally:

• '''Figure 2: Average Available Solar insolation at the Earth's Surface: January 1984-1991 (K + k)''' -- Highest values of available solar insolation occur at the South Pole due to high solar input and little cloud cover. High values also occur along the subtropical oceans of the Southern Hemisphere. Color range: blue - red - white, Values: 0 - 350 W/m². Global mean = 187 W/m², Minimum = 0 W/m², Maximum = 426 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 3: Average Available Solar Insolation at the Earth's Surface: July 1983-1990 (K + k)''' -- Highest values of available solar insolation over Greenland due to high solar input and low cloud amounts. High values also occur along the subtropics of the Northern Hemisphere. Color range: blue - red - white, Values: 0 - 350 W/m². Global mean = 180 W/m², Minimum = 0 W/m², Maximum = 351 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 4: Average Absorbed Solar Insolation at the Earth's Surface: January 1984-1991 [(K + k)(1 - a)]''' -- Highest values occur along the subtropical oceans of the Southern Hemisphere. Lowest values occur over areas of high surface reflection such as the South Pole, cloudy regions, and areas of low solar input like the high latitudes of the Northern Hemisphere. Color range: blue - red - white, Values: 0 - 350 W/m². Global mean = 162 W/m², Minimum = 0 W/m², Maximum = 315 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 5: Average Absorbed Solar Insolation at the Earth's Surface: July 1983-1990 [(K + k)(1 - a)]''' -- Highest values occur over the subtropical oceans of the Northern Hemisphere due to high solar input and little cloud coverage. Lowest values occur in areas of high surface reflection such as snow/ice covered surfaces like Greenland, cloudy regions such as storm tracks, and areas of low solar input like the high latitudes of the Southern Hemisphere. Color range: blue - red - white, Values: 0 - 350 W/m². Global mean = 158 W/m², Minimum = 0 W/m², Maximum = 323 W/m². (Source: NASA Surface Radiation Budget Project).

In the equations above, the mathematical terms have the following definitions:

• K = Shortwave Direct Radiation
• k = Shortwave Indirect Radiation
• a = Reflectivity of the Surface or Surface Albedo

Net Radiation and Planetary Energy Balance

Figure 6: Global shortwave radiation cascade. (Source: PhysicalGeography.net)

Shortwave radiation from the sun enters the surface-atmosphere system of the Earth and is ultimately returned to space as longwave radiation (because the Earth is cooler than the sun). A basic necessity of this energy interchange is that incoming solar insolation and outgoing radiation be equal in quantity. One way of modeling this balance in energy exchange is described graphically with the use of the cascade diagrams in Figures 6 and 7.

The Global Shortwave Radiation Cascade (Figure 6) describes the relative amounts (based on 100 units available at the top of the atmosphere) of shortwave radiation partitioned to various atmospheric processes as it passes through the atmosphere. The diagram in Figure 6 indicates that 19 units of insolation are absorbed (and therefore transferred into heat energy and longwave radiation) in the atmosphere by the following two processes:

• Stratospheric absorption of the ultraviolet radiation by ozone (2 units); and
• Tropospheric absorption of insolation by clouds and aerosols (17 units).

23 units of solar radiation are scattered in the atmosphere and subsequently absorbed at the surface as diffused insolation. 28 units of the incoming solar radiation are absorbed at the surface as direct insolation. Total amount of solar insolation absorbed at the surface equals 51 units. The total amount of shortwave radiation absorbed at the surface and in the atmosphere is 70 units.

Figure 7: Global longwave radiation cascade. Source: PhysicalGeography.net

Three main losses of solar radiation back to space occur in the Earth's shortwave radiation cascade (Figure 6). 4 units of sunlight are returned to space from surface reflection. Cloud reflection returns another 20 units of solar radiation. Back scattering of sunlight returns 6 units to space. The total loss of shortwave radiation from these processes is 30 units. The term used to describe the combined effect of all of these shortwave losses is Earth albedo.

The Global Longwave Radiation Cascade (Figure 7) indicates that energy leaves the Earth's surface through three different processes. 7 units leave the surface as sensible heat. This heat is transferred into the atmosphere by conduction and convection. The melting and evaporation of water at the Earth's surface incorporates 23 units energy into the atmosphere as latent heat. This latent heat is released into the atmosphere when the water condenses or becomes solid. Both of these processes become part of the emission of longwave radiation by the atmosphere and clouds.

The surface of the Earth emits 117 units of longwave radiation (see Figure 7). Of this emission only 6 units are directly lost to space. The other 111 units are absorbed by greenhouse gases in the atmosphere and converted into heat energy and then into atmospheric emissions of longwave radiation (the greenhouse effect).

The atmosphere emits 160 units of longwave energy (see Figure 7). Contributions to this 160 units are from surface emissions of longwave radiation (111 units), latent heat transfer (23 units), sensible heat transfer (7 units), and the absorption of shortwave radiation by atmospheric gases and clouds (19 units, see Figure 6) . Atmospheric emissions travel in two directions. 64 units of atmospheric emission is lost directly to space. 96 units travel to the Earth's surface where it is absorbed and transferred into heat energy.

The total amount of energy lost to space in the global longwave radiation cascade is 70 units (surface emission 6 units + atmospheric emission 64 units.) This is the same amount of energy that was added to the Earth's atmosphere and surface by the Global Shortwave Radiation Cascade (see Figure 6).

Finally, to balance the surface energy exchanges in this cascade we have to account for 51 units of missing energy [atmosphere and cloud longwave emission (96 units) minus surface longwave emission (117 units) minus latent heat transfer (23 units) minus sensible heat transfer (7 units) = -51 units]. This missing component to the radiation balance is the 51 units of energy absorbed at the Earth's surface as direct and diffused shortwave radiation (see Figure 6).

The following equations can be used to mathematically model net shortwave radiation balance, net longwave radiation balance, and net radiation balance for the Earth's surface at a single location or for the whole globe for any temporal period:

where

• Q* is surface net radiation (global annual values of Q* = 0 , because input equals output, local values can be positive or negative),
• K* is surface net shortwave radiation,
• K is surface direct shortwave radiation,
• k is diffused shortwave radiation (scattered insolation) at the surface,
• a is the albedo of surface,
• L* is net longwave radiation at the surface,
• LD is atmospheric counter-radiation (see Greenhouse Effect) directed to the Earth's surface, and
• LU is longwave radiation lost from the Earth's surface.

NASA's Surface Radiation Budget Project has used satellite data, computer models, and meteorological data to determine surface net shortwave radiation, net longwave radiation, and net radiation balances for the period July 1983 to June 1991. The following links display these balances for January and July globally:

• '''Figure 8: Average Net Shortwave Radiation at the Earth's Surface: January 1984-1991 (K*)''' -- Highest values occur along the subtropical oceans of the Southern Hemisphere. Lowest values occur over areas of high surface reflection such as the South Pole, cloudy regions, and areas of low solar input like the high latitudes of the Northern Hemisphere. Color range: blue - red - white, Values: 0 to 350 W/m². Global mean = 162 W/m², Minimum = 0 W/m², Maximum = 315 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 9: Average Net Shortwave Radiation at the Earth's Surface: July 1983-1990 (K*)''' -- Highest values occur over the subtropical oceans of the Northern Hemisphere due to high solar input and little cloud coverage. Lowest values occur in areas of high surface reflection such as snow/ice covered surfaces like Greenland, cloudy regions such as storm tracks, and areas of low solar input like the high latitudes of the Southern Hemisphere. Color range: blue - red - white, Values: 0 to 350 W/m². Global mean = 158 W/m², Minimum = 0 W/m², Maximum = 323 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 10: Average Net Longwave Radiation at the Earth's Surface: January 1984-1991 (L*)''' -- Net longwave loss is a negative quantity. Highest values of longwave loss occurs where surface temperatures are high and cloud cover is minimal, such as the subtropical deserts of the Northern and Southern Hemisphere. Cold surfaces have low values of loss. Color range: white - red - blue, Values: -100 to 0 W/m². Global mean = -48 W/m², Minimum = -125 W/m², Maximum = -11 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 11: Average Net Longwave Radiation at the Earth's Surface: July 1983-1990 (L*)''' -- Net longwave loss is a negative quantity. Highest values of longwave loss occurs where surface temperatures are high and cloud cover is minimal, such as the subtropical deserts of the Northern and Southern Hemisphere. Cold surfaces have low values of loss. Color range: white - red - blue, Values: -100 to 0 W/m². Global mean = -47 W/m², Minimum = -144 W/m², Maximum = -4 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 12: Average Net Radiation at the Earth's Surface: January 1984-1991 (Q*)''' -- Total net radiation is the sum of shortwave and longwave net radiation. It is dominated by the shortwave portion. Highest values occur along the subtropical oceans of the Southern Hemisphere. Lowest values occur over areas of low solar input such as the North Pole, and areas of high surface reflection such as the South Pole. Color range: blue - red - white, light green = 0 W/m2, Values: -50 to 250 W/m². Global mean = 114 W/m², Minimum = -60 W/m², Maximum = 261 W/m². (Source: NASA Surface Radiation Budget Project).
• '''Figure 13: Average Net Radiation at the Earth's Surface: July 1983-1990 (Q*)''' -- Total net radiation is the sum of shortwave and longwave net radiation. It is dominated by the shortwave portion. Highest values occur along the subtropical oceans of the Northern Hemisphere. Lowest values occur over areas of low solar input such as the South Pole, and areas of high surface reflection such as the North Pole. Color range: blue - red - white, light green = 0 W/m², Values: -50 to 250 W/m². Global mean = 111 W/m², Minimum = -65 W/m², Maximum = 249 W/m². (Source: NASA Surface Radiation Budget Project).

Global Heat Balance: Introduction to Heat Fluxes

Figure 14: Balance between average net shortwave and longwave radiation from 90° North to 90° South. (Source: PhysicalGeography.net)

Figure 14 illustrates the annual values of net shortwave and net longwave radiation from the South Pole to the North Pole. On closer examination of this graph one notes that the lines representing incoming and outgoing radiation do not have the same values. From 0 - 30° latitude North and South incoming solar radiation exceeds outgoing terrestrial radiation and a surplus of energy exists. The reverse holds true from 30 - 90° latitude North and South and these regions have a deficit of energy. Surplus energy at low latitudes and a deficit at high latitudes results in energy transfer from the equator to the poles. It is this meridional transport of energy that causes atmospheric and oceanic circulation. If there were no energy transfer the poles would be 25° Celsius cooler, and the equator 14° Celsius warmer!

The redistribution of energy across the Earth's surface is accomplished primarily through three processes: sensible heat flux, latent heat flux, and surface heat flux into oceans. Sensible heat flux is the process where heat energy is transferred from the Earth's surface to the atmosphere by conduction and convection. This energy is then moved from the tropics to the poles by advection, creating atmospheric circulation. As a result, atmospheric circulation moves warm tropical air to the polar regions and cold air from the poles to the equator. Latent heat flux moves energy globally when solid and liquid water is converted into vapor. This vapor is often moved by atmospheric circulation vertically and horizontally to cooler locations where it is condensed as rain or is deposited as snow releasing the heat energy stored within it. Finally, large quantities of radiation energy are transferred into the Earth's tropical oceans. The energy enters these water bodies at the surface when absorbed radiation is converted into heat energy. The warmed surface water is then transferred downward into the water column by conduction and convection. Horizontal transfer of this heat energy from the equator to the poles is accomplished by ocean currents.

The following equation describes the partitioning of heat energy at the Earth's surface:

The actual amount of net radiation being partitioned into each one of these components is a function of the following factors:

• Presence or absence of water in liquid and solid forms at the surface.
• Specific heat of the surface receiving the net radiation.
• Convective and conductive characteristics of the receiving surface.
• Diffusion characteristics of the surface's overlying atmosphere.

Further Reading

• PhysicalGeography.net