Laying Some Groundwork: Balancing Radiation

viewed over long time scales, but on our home planet, short-term, fluctuating radiant energy emissions are the norm. We see this every day. The sun -- a huge source of radiant energy -- rises and as it moves across the sky toward setting, our little patch of planet receives varying amounts of insolation. If we consider daily and hourly differences in radiation acting over the whole Earth, and also account for seasonal cycles and cloud cover variations, it becomes obvious that the insolation energy received can be quite variable from one location to the next.

The unequal amounts of insolation received across the globe produces well-known temperature variations. It is thus typically hotter during the day and in equatorial regions, and colder at night and at the poles. The Earth system tries, albeit unsuccessfully, to redistribute that heat energy from warm to cold regions so that the planet would have a uniform temperature. Two fluids are called upon to transport that heat: water in the oceans and gases in the atmosphere. In the atmosphere, the energy redistribution process (generally expressed as heat transport) is known as the weather.

Now, let's concentrate on the radiation balance of a small parcel of the Earth system surrounding us. We'll ignore in this discussions the complications introduced by water, fascinating though it is.

Diagram courtesy Spectrum Educational Enterprises

During daylight hours, the insolation entering our parcel of air, minus the reflected part, is absorbed by atmospheric gases and particles and the surface materials (rock, water, soil, vegetation). Thermal (heat) radiation (emitted by the planet and its elements as long-wave radiation) is also exchanged among the atmospheric constituents and surface elements. Since the rate of energy gained and lost varies among the elements, portions of the system may store some energy as internal heat for a time. With the sun in the sky providing a strong energy input, there is usually a net gain of energy during the day in the atmosphere, mostly at the surface where air molecules are warmed through heat conduction.

Diagram courtesy Spectrum Educational Enterprises

At night, however, it is a very different picture: the sun's direct energy input is cut off, and the strongest energy input to the surface ceases. The earth's surface, however, continues to radiate heat away, and thus cools. How much energy is lost depends on the length of the night and the nature (including the temperature) of the atmospheric and surface elements. Rock and snow, for example, radiate heat differently, with snow losing its heat much faster. Water in the atmosphere (clouds and vapour) absorbs thermal radiation

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