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Laying Some Groundwork: Balancing Radiation

© Keith C. Heidorn

When I began working on my planned articles on fog and frost, I realized two important atmospheric concepts were the basis for both. So rather than make those essays too long, I decided to lay a little groundwork for the "Science of the Sky." Fog and frost may both form as a consequence of nocturnal thermal radiation exchanges near the surface that lower temperatures until atmospheric water vapour changes to a liquid or solid state.

Next week, I'll discuss some relevant aspects of humidity, but today I'll tackle the radiation balance. It is a basic concept for many weather and climate topics including the high profile concern over the so-called greenhouse gases and their role in global warming. Close to the planet's surface, the radiation balance plays a major role in determining the weather conditions we feel. (Note that here I consider radiation as a form of energy delivered over time onto an area.)

To start, let's step back from Earth, way back, about 150 million kilometres (93 million miles) to our personal star, the Sun. The Sun produces a tremendous amount of energy in its solar furnaces that it emits as short-wave radiation into the universe. An extemely small fraction falls on the sunlit side of the Earth, some of which is reflected back. The amount reflected, known as the albedo, is about 36 percent for the whole Earth, but can be above 90 percent for "bright" surface areas such as snow, ice and clouds, and as low as 10 percent or less for dark rocks and forests. The remaining fraction of the incoming solar radiation, called insolation by meteorologists, heats the Earth system through absorption by atmospheric gases, water, rock, soil, vegetation and building materials.

Diagram courtesy Spectrum Educational Enterprises

If the Earth were not able to lose that energy, the planet would soon become a molten ball, which over geological time would evaporate and eventually disperse across the solar system. Fortunately, the physical nature of all matter provides a remedy. Every body, from gas giants to molecules and atoms, radiates energy at a rate dependent on its physical nature (density, molecular and atomic structure, etc.) and its temperature.

Thus, every body in the universe sends its heat energy out to the surroundings and receives energy back from every object in its view. When more energy is gained than lost, the body warms. When more energy is lost than gained, the body cools. When there is a balance between gain and loss, the body can maintain a constant temperature.

Now this basic concept applies best on a system more or less in radiative equilibrium, or when

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