The thin, chill blanket of the Martian atmosphere has an average density less than one-hundredth of the Earth's. At 142 million miles from the Sun, Mars receives less than half the solar energy that reaches the Earth. Yet coupled with the tenuous air and Mars' daily rotation, that energy produces some remarkable weather. Clouds of water ice form high in the Martian air, driven by winds that can gust up to 50 mph on the surface—and more than 125 mph in the vast dust storms that astronomers have seen sweeping the planet.

The Martian weather is driven in the same way as the Earth's weather: The Sun's heat warms the spinning, tilted planet unevenly and is distributed from the relatively warm equator to the chilled poles by complex atmospheric movements. In some ways, the planets are near-twins. A Martian day is only 40 minutes longer than Earth's 24-hour day and its axial tilt is within a degree or so of the angle that gives the Earth its seasons.

But there the resemblance ends. The atmosphere on Earth is dense, and almost three-quarters of our planet is covered in water. Mars, on the other hand, is a desert. Even in its deepest valleys, the atmosphere is no denser than the Earth's stratosphere, and 95% of it is carbon dioxide. As on Earth, the gas has a greenhouse effect. But the Martian atmosphere is too thin to achieve much in the way of global warming.

Apart from the thin, unbreathable air, Mars is cold. Even in midsummer at the equator, surface temperatures rarely exceed 60°F. And on Mars, surface means surface:
Just a few inches above the ground, the temperature falls sharply. At night, it will drop to -100°F. In midwinter at the poles, the temperature plummets below -200°F.

As on Earth, relatively "warm" air at Mars' equator rises and moves poleward, where it cools, descends and returns to the equator. On both planets, cold and warm fronts of low and high pressure air move eastward in a regular pattern that is linked to the rotation of the planet.

Like storms on Earth, Martian storms usually occur at particular latitudes. Most are short-lived, although they can cover vast areas. In 1999, the Hubble Space Telescope photographed a colossal cyclone near Mars' north pole. It was close to the Martian midsummer in the northern hemisphere, when the planet's weather is agitated by increasing temperatures. The storm clouds had the same spiral structure as cyclones on Earth—but this Martian example was more than 1,000 miles across.

The clouds contain mainly water ice crystals that have evaporated from the north polar ice cap and refrozen high in the atmosphere. A similar phenomenon occurs in Antarctica, but without the cyclone. There, the thin, high ice crystals that form the clouds sometimes drift to the ground as so-called "diamond dust." During summer in the north, such clouds are seen throughout most of the northern hemisphere of Mars, but there is no evidence that any of their water crystals ever fall to the surface.

Even in midsummer, most of Mars7 water remains locked in the polar ice caps. These are a mix of water ice and frozen carbon dioxide (CO^). As summer advances, much of the solid CO^ sublimates directly into gas. Summer is also the season for the great Martian dust storms. These dust storms often take place regionally, but in some years they seem to combine to form a dust-laden tempest that can cover the whole planet.

These storms have contributed much to the shaping of the planet's surface—by eroding its rocks, heaping the soil into large sand dunes in some areas and scouring out smooth, flat patches in others. Once, though, the Martian climate contributed another powerful agent of erosion:
rain. Ancient river channels testify to the presence of running water billions of years in the past. But we will have to learn much more about Martian weather before we can explain just when—and why—the rain stopped falling.