I know that Mars is rarely over freezing, and often nearly 200 below zero F. But how cold would it feel to an astronaut on Mars? The air is so thin, that it has almost no ability to suck the heat out of you. Water that is 33 degrees F feels much colder than air that is 33 degrees F because it is more dense. The air on Mars is almost non-existant. Mark Twain said the coldest winter he ever spent was a summer in San Francisco (where I live). But in San Francisco, it never gets lower than 55F in the summer. Why does San Francisco at 55degrees feel colder than Lake Tahoe when it's 10 degrees? Because San Francisco has a lot of moisture in the air, and a lot of wind. Its air just sucks the heat out of you quickly. But because Mars' atmosphere is so thin, it has no ability to suck the heat out of you. You will only lose the heat that your body radiates. I'm guessing that an astronaut would feel comfortable in just a heavy sweater under his spacesuit. Anyone know the answer?