WEATHER CORNER

The weather outside this world is frightful

BY JAN NULL
Special to the Mercury News

Q. For some years, space probes have gathered information about other planets with very diverse weather systems, particularly Venus, Mars and Jupiter. As a result, what profound knowledge have we gained that might help us better understand weather in general and our own (here on Earth) in particular? Fred Friedlander - Saratoga

A.  Actually, I think that our continued better understanding of weather here on Earth is allowing us to study and understand the weather on the other planets. Before we look at the weather around the solar system, let's do a quick recap of the weather here on Earth.

Earth's average temperature is about 47 degrees Fahrenheit, and its extremes range from minus 129 degrees in Antarctica to 136 degrees in Libya. The strongest wind ever recorded was a 231 mph gust on Mount Washington in New Hampshire. Winds in tornadoes can exceed 250 mph, and those in a hurricane can top 175 mph. And the composition of our atmosphere is about 78 percent nitrogen, 21 percent oxygen and 1 percent argon. As we will see, these are all relatively benign conditions in relation to our neighbors and a significant reason that life as we know it exists here and not there.

Venus is Earth's closest neighbor and also is most similar in size and density. But its orbit is between Earth and the sun. This and an atmosphere that is 96 percent carbon dioxide and 3 percent nitrogen give an all-new meaning to greenhouse effect and global warming.

Venus has a pretty inhospitable climate, with a toasty average surface temperature of about 850 degrees. The atmospheric pressure is about 90 times greater than on Earth, or the equivalent of being about 3,000 feet deep in the ocean. And if this is not unpleasant enough, clouds of sulfuric acid swirl more than 200 mph above Venus at an altitude of about 150,000 feet.

On the far side of Earth from the sun, our nearest neighbor is Mars. Because of its distance from the sun, less solar energy reaches the surface of Mars than that of Earth. The average daily temperature ranges from a frigid minus 207 at night to an almost balmy 80 in the afternoon. These strong differences in heating cause extremely strong and persistent winds of 100 mph or more, which create huge storms of reddish dust. Because of the cold temperatures, water on Mars exists only in the form of ice or vapor, but not liquid. The same is true of the very thin Martian atmosphere, which has a surface pressure only 1/200th of Earth's. It is 95 percent carbon dioxide, resulting in a coating of carbon dioxide frost. Clouds of carbon dioxide and water vapor form in the atmosphere, but you would never have to worry about a rainy day because liquid water does not exist. Mars has ice caps on both poles, but unlike Earth's icy poles, the caps on the Red Planet are made up of solid carbon dioxide, or what we commonly call
``dry ice.''

Nearest to the sun, the tiny planet Mercury has a barren and harsh environment. It experiences the largest daily temperature changes in the solar system because it's so close to the sun and has no atmosphere to retain heat. Over the very long days on Mercury, the temperature can rise to about
800, yet fall to about minus 200 during the long nights.

Even though its atmosphere is not very hospitable to humans, Jupiter certainly has interesting weather. As the biggest planet in the solar system, Jupiter has the largest storms. The dense atmosphere is made up of 90 percent hydrogen and 10 percent helium, which create a continuous layer of clouds. Jupiter is essentially a huge conglomeration of gases, as are Saturn, Uranus and Neptune. Because the planet is so far from the sun, the tops of its clouds warm only to about minus 250. One of Jupiter's most distinctive features is the Great Red Spot -- a humongous cyclone about three times the size of Earth that has been raging for at least 300 years. Although winds are light near its center, the winds around its edges blow about 225 miles per hour. Smaller storms also migrate around the planet, driven along six bands of strong winds that encircle each hemisphere. Rising gases from the planet's surface somewhere thousands of miles deep inside the clouds give these bands different colors. Large-scale lightning storms
and auroral activity have also been observed on Jupiter.

The huge ringed planet Saturn is even farther from the sun and receives little solar radiation. It has many similarities to Jupiter, including an atmosphere of 97 percent hydrogen. The average temperature in its clouds is about minus 300.  Actually the interior of Saturn is extremely hot (21,000 degrees at the center), and the planet produces more energy than it receives from the sun. Large cyclones migrate around its girth. These cyclones are driven by very strong winds that have been measured at 1,100 mph. The rings are pretty to look at, but this is not the place for an outdoor event.

And as would be expected, outer planets Uranus, Neptune and Pluto get progressively more inhospitable, largely because of their enormous distance from the sun. The average temperature on Uranus is minus 350, almost balmy compared with Neptune's minus 375 and rocky Pluto's minus 400. Uranus and Neptune are basically frozen wastelands with atmospheres of mostly hydrogen and helium gases.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner@ggweather.com.