FALL STORMS. Sequential images show the evolution of an early fall storm over the Martian north polar region. Recorded on June 30, 1999, each picture was taken approximately two hours later than the previous. Clouds that appear white consist mainly of water ice; clouds that appear brown consist of dust.

SPRING THAW. Winter in the south is coming to a close and the seasonal frosts of the polar cap are retreating. Small, local dust storms can be observed along the margins of the ice cap, as the colder air blowing off the cap moves northward into warmer regions.

The Weather Channel isn't offering daily reports of the conditions on Mars yet, but it very well could, thanks to NASA's Mars Global Surveyor. The spacecraft has been spinning around the Red Planet for just over one Martian year of 687 days. And the daily snapshots it sends back are every bit as interesting as those recorded by the weather satellites orbiting Earth. Even though Mars is a cold, dry desert, the images reveal that it is far from a dead world.

August 2 marked the equinox on Mars. The Mars Orbiter Camera tracked the changing seasons as the first chill of winter gripped the Northern hemisphere and warm spring winds began shrinking the Southern ice cap. Although there aren't yet anemometers on the Martian surface to measure wind speed, the camera captured huge dust devils--much like our tornadoes--and provided evidence that the sand dunes on Mars are still being shaped by winds. "We have captured a unique record of seasonal and meteorological events, which demonstrates that Mars is active and dynamic today," says Michael Malin, principal investigator for Surveyor's camera at Malin Space Science Systems in San Diego, Calif.

Pictures of the Northern polar regions reveal churning storm clouds that mark the arrival of fall. As the seasonal change progresses and winter darkness comes to the North pole, the clouds will expand to cover much of the northern plains. "It might begin to snow as the polar cap expands," says Malin. Meanwhile, in the southern polar regions, the ice caps began to evaporate.

SUBLIME DUNES. This picture, taken on July 21, 1999, shows the early stages of the defrosting process in the southern ice cap. The sand beneath the frost is dark; once it is exposed to sunlight, it absorbs sunlight and helps speed the defrosting process.

Close-up images reveal dark spots of exposed sand poking through the retreating carbon dioxide ice and streaks of sand blown by winds onto the surface of the frost. Because the martian air pressure is very low--100 times lower than at sea level on Earth--ice on Mars does not melt and become liquid when it warms up. Instead, ice sublimes, changing directly from solid to gas. As polar dunes emerge from the months-long winter night, and first become exposed to sunlight, the bright winter frost and snow begins to sublime. This process begins in small spots and then spreads over several months until the entire dune is spotted like a leopard. The dark spots continue to grow and spread as spring approaches until, eventually, the entire area is frost-free.

DUST DEVILS. Arrows indicate a group of dust devils observed in mid-May 1999 in northern Amazonis Planitia. Dust devils are common in this region and were first seen there during the Viking orbiter missions from 1976 to 1980

In the mid-latitudes, the prevailing weather patterns are huge dust devils that are formed by spinning vortices of air that arise when the ground is heated and general wind flow is light. On Earth, dust devils are relatively small features that form on open fields during the heat of the afternoon. But on Mars they can tower nearly 8 kilometers (5 miles) above the martian surface. The average dust devil, which lasts for a few hours at most during the hottest part of the Martian day, is slow- moving but may carry several tons of dust within its height of 1.2 miles (two kilometers). Martian dust devils may be a major transporter of the fine, pinkish dust that gives the sky its unearthly brownish color, which was observed by the Mars Pathfinder and Viking landers.

Global Surveyor also returned tantalizing evidence of recent shifting sands in dune fields first seen in Mariner 9 pictures of Mars from the early 1970s. Scientists are interested in dune fields isolated within large impact craters because their dark color suggests that the dust covering much of the rest of the planet does not accumulate there.

The recent Surveyor images show dark streaks on the frost-covered slopes of the dunes. These streaks result from avalanching of sand on the steep downwind side of the dune, otherwise known as the slip face. Because the dark sand streaks are superposed upon the bright frost, these streaks can only be as old as the frost--hence, not more than 11 months old. "This indicates that the dunes must be moving and that over time we may be able to see changes that will allow us to measure the rates of wind erosion on Mars," says Malin.

So Mars is not merely a planetary husk orbiting the Sun. It is a living, dynamic planet. And if human explorers ever establish a base on Mars, they will have something very earthly to contend with--the weather.