Review from Previous Units

Recall from the unit on atmospheric structure and circulation that the temperature of the atmosphere decreases with height. Clouds will have temperatures approximately equivalent to the surrounding air, so high clouds will be expected to have lower temperatures than low clouds. From what we have just discussed, therefore, we would expect high clouds to emit much less infrared radiation than low clouds, and low clouds will likely emit less infrared radiation than the underlying surface of the earth. Therefore, even though all clouds are somewhat uniform in reflecting solar (visible) radiation from their top sides, they differ significantly in the amount of energy they emit upward by infrared radiation.

Our discussion from the last unit included the topic of reflection of solar radiation from particles (dust, soot, volcanic materials, etc.) in the atmosphere. It was noted that volcanoes can cause temporary global cooling due to this effect. Three major volcanoes have erupted in the past 40 years that have allowed us to observe the impact of such events on global temperatures: Agung in the 1960s, El Chichon in Mexico in the mid 1980s, and Mt. Pinatubo in the Philippines in 1991. In each case, the global temperature dropped immediately and gradually recovered over a period of about three years. Global climate models have been used to estimate the effects of such volcanoes from estimated volumes of particulate material put into the atmosphere. These calculations have been quite accurate in estimating the effects on global climate. The University of North Dakota gives a listing of currently active volcanoes.

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