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The forecast on Mars is usually dramatic. Mars weather varies quite a bit day to day and, sometimes, hour to hour. That seems a bit unusual for a planet that has an atmosphere that is only 1% as dense as the Earth’s.
As a rule, Mars is dry, cold, and clear. In the summer daytime temperatures at the equator can be as warm as 20 C. Nice short sleeve weather and comfortable for most activity. That same night, temperatures can drop to -90 C. The 110 degree difference in one day can create warm and cold temperature fronts that can lead to dust devils and dust storms that can engulf the entire planet for weeks. Winter temperatures can stay as low as -140 C. The carbon dioxide in the atmosphere freezes and becomes dry ice. The Martian North Pole has a one meter layer of dry ice in the winter, while the South Pole is covered by a permanent eight meter deep layer.
It never rains on Mars because of the thin atmosphere and the lack of a magnetosphere. A magnetosphere is not important for rain to be present, but it is necessary to shield liquid water from solar radiation. Since the Sun’s radiation and the solar wind are constantly bombarding the planet, liquid water can not exist; therefore, rain can not form nor fall. Occasionally, however, clouds do form and snow does fall. Clouds on Mars are very small and wispy and the majority of them are formed by carbon dioxide ice. Scientists believe that a few are compromised of small water particles. Since Mars is so cold the water in these clouds could never fall as rain, but can fall as snow in the upper atmosphere of the planet. Scientists have only seen this a few times and have no evidence that the snow ever reaches the ground.
The most dramatic version of Mars weather is the dust storm. They occur frequently because of the temperature gradient on the planet and because the surface is covered in light dust that is easily picked up by wind. Scientist feared that these storms would cover a rover’s solar panels, making it impossible for long term study of the planet. Fortunately, the storms are interspersed by wind that simply blows accumulated dust off of the panels. It seems that Mars weather is as helpful as it is a hindrance.
Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.