On average, about 59% of the Earth's surface is covered by clouds at any given time, rising to nearly 100% on the day you have an outside activity scheduled. Clouds have long been thought to have a mystical nature. The English poet Shelley said of clouds that they are "like a child from the womb, like a ghost from the tomb."

Here in North America, as we move from winter toward spring, the temperatures and the amount of daylight increase, but what about the amount of cloudiness? Although there may be fewer completely cloudy days in summer than winter, there are also fewer completely clear days. The short answer to the first question is that, at least for the mid-lattitude land masses, there is usually a greater percentage of sunshine in summer than in winter, but paradoxically, there are also more clouds in the summer. Those puffy, popcorn-like cumulus clouds that are a regular feature of summer afternoons usually don't completely block-out the Sun, and they only make guest appearances during the wintertime.

Global cloudiness is organized into roughly three bands: one about the equator, and one at higher latitudes (approximately 45-60 degrees) in each hemisphere. Inbetween these major cloud bands are two relatively cloud-free areas, one in each hemisphere, where the world's great deserts are found. The polar areas are very different in terms of cloudiness. Whereas the North Pole is surrounded by the Arctic Ocean, the South Pole is near the middle of Antarctica and is less cloudy, primarily because the elevated land mass prevents the passage of ocean storms. Antarctica is the driest as well as the coldest continent. The oceans are almost always cloudier than the land masses.

Clouds regulate the Earth's thermostat. In general, clouds keep underlying areas cooler during the day by reflecting sunlight (shortwave radiation) back to space. At night, however, clouds keep underlying areas warmer since they are able to intercept and re-emit longwave radiation (heat) from the Earth's surface, which would normally be lost to space. This is known as the "greenhouse effect." Areas with persistent cloudiness, the northwest U.S. for example, tend to have small temperature variations from day to night.

Because the entire Earth, over the course of a day, a month or a year, experiences as many hours of daylight as darkness, is the Earth then warmer or cooler because of the presence of clouds? Records from satellite observations indicate that global temperatures are slightly cooler because of clouds. The reason for this is the large amount of cooling that results from the cloud masses found over ocean areas. These clouds are particularly efficient at deflecting incoming sunlight back to space.

Are the continents more cloudy in the winter or the summer, and do clouds make the Earth cooler or warmer?

3/18/96