SQA Higher Geography

Energy from the sun

Almost all the energy that drives the atmosphere and the oceans arrives from the sun as short-wave radiation, called insolation (incoming solar radiation). The Earth sends energy back to space as long-wave (infrared) terrestrial radiation. Over a long period the two are in balance, so the planet does not keep getting hotter or colder, and the balance is called the global heat budget or global energy budget.

The budget has to balance for the planet as a whole, but not at every place on it. Some latitudes receive more energy than they lose and others lose more than they receive, and the movement of air and water between them is what makes weather and climate.

What happens to insolation

Of the radiation that reaches the top of the atmosphere, roughly 30 per cent never warms anything. It is reflected straight back to space by clouds, by dust and gas molecules in the air and by bright surfaces such as ice and snow. Roughly 20 per cent is absorbed by the atmosphere itself, mostly by water vapour, ozone, carbon dioxide and clouds, and roughly half passes through and is absorbed by the land and the oceans. These proportions are approximate and vary from one estimate to another, but the pattern does not: the surface receives about half of the insolation, and the atmosphere is mainly warmed from below.

The warmed surface then does three things with the energy:

• it radiates long-wave energy upwards, and much of this is absorbed by greenhouse gases and radiated back down, which is the natural greenhouse effect and keeps the average surface temperature near 15 °C rather than well below freezing;
• it heats the air directly, by conduction and convection, which is called sensible heat transfer;
• it evaporates water, and the energy used is stored in the water vapour as latent heat, which is released when the vapour condenses higher in the atmosphere.

Albedo

Albedo is the proportion of insolation that a surface reflects, and it is written as a percentage or a decimal. Fresh snow and ice reflect most of the radiation that falls on them (commonly 80 to 90 per cent), thick cloud reflects a great deal, light sand and desert reflect a fair amount, and dark surfaces such as forest and open ocean reflect very little (often below 10 per cent for the sea). Dark surfaces absorb most of what arrives, so they warm faster than bright ones.

Albedo matters for climate because it can feed back on itself. Ice melts, a darker surface is exposed, more energy is absorbed, and so more ice melts.

Why insolation varies across the Earth

Latitude and the angle of the sun

The Earth is a sphere, so the sun's rays strike low latitudes at a steep angle (close to overhead) and high latitudes at a shallow angle. Near the equator a beam of radiation is concentrated on a small area. Near the poles the same beam is spread over a much larger area, so each square metre receives less energy.

The shallow angle also makes the rays travel through a much thicker layer of atmosphere before they reach the ground. More of the radiation is absorbed, scattered and reflected on the way, and so less arrives at the surface.

Season and day length

The Earth's axis is tilted at about 23.5 degrees, so the latitude where the sun is overhead at noon moves between the Tropic of Cancer and the Tropic of Capricorn through the year. A hemisphere tilted towards the sun has high-angle rays and long days, and receives much more energy in summer than in winter. Close to the poles there are months of continuous daylight in summer and months of darkness in winter, and the annual total is low even though summer days are long.

Cloud cover and the atmosphere

Cloud reflects insolation back to space by day, which cools the surface, but it also holds in heat at night. Places with persistent cloud, such as the equatorial rainforests, receive less radiation at the surface than the cloudless subtropical deserts, which have the highest surface insolation on Earth for this reason.

Surface type and altitude

Surface albedo changes how much of the radiation that arrives is absorbed. Altitude matters because high places have a thinner and drier atmosphere above them, so more radiation gets through in the day, but the air holds little heat, so that heat is lost quickly and temperatures are lower.

Surplus and deficit

When the energy absorbed in a latitude band is compared with the energy that band radiates to space, the two groups of latitudes separate:

The surplus occurs because insolation is high at low latitudes, with the sun at a high angle all year, and the loss of long-wave radiation is more even across the globe, so there is a large net gain. The deficit at high latitudes occurs because the angle is low, the days are short for part of the year, and snow and ice reflect much of what arrives.

If energy were not moved, the tropics would get steadily hotter and the poles steadily colder. They do not, because the atmosphere and the oceans carry heat from the surplus areas towards the poles, as described in the notes on atmospheric and ocean circulation. Roughly three quarters of the poleward transfer is done by the atmosphere (by winds, and by the latent heat in moist air) and the remainder by ocean currents.

Worked example: the effect of albedo

A patch of ice with an albedo of 0.8 receives 200 units of insolation. It reflects $200 \times 0.8 = 160$ units and absorbs $200 - 160 = 40$ units.

The same patch after the ice has melted, leaving dark water with an albedo of 0.1, reflects $200 \times 0.1 = 20$ units and absorbs $200 - 20 = 180$ units, which is $180 \div 40 = 4.5$ times as much.