ENERGY and RADIATION

Forms of Energy

"Energy" is defined (not always very helpfully) as the 'ability to do work' - heat something, move something, change the form of something. As such there are several forms of energy, but only four are of major concern in the natural environment:

Claude: Seaport with the embarkation of the Queen of Sheba

Courtesy: National Gallery, London

radiant energy (radiation) - energy emitted outwards as a consequence of the molecular motions within a body. This energy can be transmitted through the vacuum of space. (Sunlight is the best known form)

Breugel: The Harvesters

Courtesy: Metropolitan Museum of Art, New York

thermal (internal) energy - this is associated with the molecular motions within the body and is retained as a property of the body, which we sense as the heat of the body

Monet: The Terrace at Sainte-Adresse

Courtesy: Metropolitan Museum of Art, New York

kinetic energy - the energy associated with the motion of the body, whether as small as an earthworm burrowing through the ground or the flight of a jet

Constable: Weymouth Bay

Courtesy: National Gallery, London

potential energy - energy associated with position, usually relative to the surface of the earth, with higher bodies having more potential energy.

Energy can be changed from one form to another, and it can be stored, but it can never be destroyed (This is a short, general statement of the Laws of Thermodynamics). We shall consider below the change from radiant to thermal (and vice versa), and later the transformation to kinetic and potential energy.

Radiation

The full title is "electromagnetic radiation" to differentiate it from the nuclear kind of radiation. It is emitted by everything having a temperature above absolute zero - and thus is entirely natural and benign. The amount and character of the radiation depends on the temperature of the emitting body. We can divide this natural radiation into 2 kinds:

An instrument for measuring the amount of solar radiation arriving at the surface - the radiation is reflected by the white part of the instrument, absorbed by the black surface. The temperature difference between the two (measured by sensors under the surface) is related to the radiation amount. (Courtesy of Eppley Laboratories http://www.eppleylab.com/)

solar radiation:

emitted by the sun at high temperature. A tremendous amount is emitted, of which a small portion arrives at the earth. We see it as sunlight.

The instrument at left measures total solar radiation - the (yellowish looking) direct beam and the (bluish) diffuse (or sky) radiation. This shadow band obscures the direct beam and measures only the solar energy arriving after being scattered by air molecules. (Courtesy of Eppley Laboratories http://www.eppleylab.com/)

Another type of solar radiation measuring instrument, clearly showing the glass dome - which only transmits solar radiation, and prevents terrestrial radiation from reaching the sensor surface (Courtesy Yankee Environmental http://www.yesinc.com/)

terrestrial radiation:

emitted by the surface of the earth and everything thereon (including us), and by the atmosphere and everything which it contains. The earth is much colder than the sun and there is much less of this terrestrial than solar radiation emitted. Although we cannot "see' this radiation, it acts in much the same way as sunlight - we absorb it into our bodies, which then heat up. However, out bodies also emit radiation of this type (which is a cooling process). Some of the emitted radiation may be absorbed by other things on or near the surface of the earth, but some will be transmitted through the atmosphere and escape to space.

Although this instrument appears similar in construction to the one at left, a different type of dome ensures that only Infra-red radiation is measured.

(Courtesy Yankee Environmental http://www.yesinc.com/)

Evening light at Rural Hall, Forsyth County. The low sun in the evening (and morning) means that light has to traverse a great depth of the atmosphere to arrive at the surface. This allows interaction between the sunlight and the air molecules, often leading to spectacular optical effects.

The solar radiation arriving from the sun is the driving force for our weather and climate. Much of the sunlight is absorbed at the surface of the earth, and causes the earth to heat. Since the amount arriving depends on your location (the latitude), the time of year and time of day, the heating also varies (Many other factors play roles, but is it obvious to say that you get hotter if you stand in the tropics, not at the poles, that you are in summer rather than winter, and that it is midday rather than dusk). The variability is ultimately responsible for all of our weather - unequal heating from place to place creates wind and influences cloud and rain. Eventually, however, the heat that is absorbed and used to create weather is returned to space by terrestrial radiation.

Shadows suggest a 'cool' spot and indicate the rapid changes possible in the amount of solar radiation which is received.

The Greenhouse Effect

Certain gases in the atmosphere are known to be 'radiatively active' or as the 'greenhouse gasses'. With minor exceptions, this means that:

a) they readily transmit solar radiation, allowing the sunlight to penetrate to earth relatively unimpeded, while

b) they readily absorb terrestrial radiation, thus making it more difficult for this energy to escape to space.

The "trapping" of terrestrial radiation means that there is more energy retained by the earth and its atmosphere, leading to a planet which is warmer than it would be in the absence of the atmosphere.

This greenhouse effect is entirely natural - indeed, by keeping the planet warm, it makes possible life as we know it.

However, there is concern because human actions are increasing the concentration of many of the radiatively active gases in the atmosphere, increasing the amount of energy trapped, ultimately leading to an addition (human-induced) warming.

The major greenhouse gases are:

water vapor (the main one, and entirely natural), carbon dioxide (naturally occurring, but increasing as a result of many industrial processes), methane (also natural in small amounts, but increasing because of increased agricultural activity), CFCs (entirely the result of human production), various oxides of nitrogen (again, mainly from human activity), and several more

Non-radiative energy flows

So far we have indicated that when solar radiation is absorbed at the surface (or is absorbed by air molecules) it causes heating. We have also inferred that as the surface or molecule heats, it emits increasing amounts of terrestrial radiation. Indeed, if radiation was the only process acting, we should have a very simple situation:

a 'body' (i.e. anything that gets in the way of radiation - including things we cannot see, such as air) absorbs (solar) radiation and heats up. As it heats it emits (terrestrial) radiation and cools down. Since the hotter the body, the more it emits, provided the incoming radiation is constant, we would eventually find that our body would come to a constant temperature - it would be in equilibrium with the radiation - the body would achieve just the right temperature so that the absorbed energy is just - and exactly - balanced by the emitted energy.

Of course, because the sun moves across the sky, and clouds and pollution come and go, the incoming radiation is never constant for more than a few minutes, and body temperatures vary with time.

The other complicating factor is the presence of the non-radiative energy flows. There are three of these:

1 - sensible heat; this is heat which you can sense as temperature. It flows from hot areas to cool ones. Most of the time the surface of the earth is hot (because that is where most solar radiation is absorbed) and the air above is cool. Consequently most of the time the sensible heat flow moves energy from the ground into the air.

2 - ground heat: refers to the sensible heat flow between the surface and the underlying material, whether that is bare rock, soil and vegetation, or water. (Ground heat flow is just a convenient term to cover them all. We shall look at this flow specifically for the oceans later. AT the moment, thinking of land, it is most convenient to say that during from the middle of spring to the middle of autumn the ground is warmer than the soil, so the flow is downwards - heating the soil and allowing seeds to germinate and plants to grow. In the opposite seasons the surface is colder than the interior, and the flow is back towards the surface. For the year as a whole, therefoe, there is almost zero ground heat flow, but there is plenty on individual days.

3 - latent heat: this is the energy associated with evaporation. Energy is needed to evaporate water - weather in the soil a puddle, or the ocean. That energy is carried aloft as water vapor, eventually being released when condensation occurs and clouds are formed.

Energy budgets

In the previous section we considered the simple relationship between temperature and energy when only radiation was involved. In the actual case the same principles apply - absorption causes heating, heating causes emission of radiation and stimulates the other flows, and the body tries to adjust its temperature so that all of the flows are in balance. It is just more complicated to consider (and a lot more complicated to calculate)

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