Inside the inner planets
The first rule of planetary weather is an obvious one – planets are generally cooler the farther they are from the Sun. But temperature also depends strongly on the thickness of each planet's atmosphere. When that insulating blanket is thinner than ours, temperatures can vary greatly between daylight and darkness. Mercury offers the most vivid example. Due to its small size, Mercury has a relatively puny gravitational field that can't hang on to much of an atmosphere. The best it can manage is a few molecules burned off the surface by the intense sunlight (Mercury is only about 40 percent as far from the Sun as Earth is). Absent an atmosphere, Mercury also lacks clouds, winds, and most everything else that we think of as weather. The exception is some very big swings in temperature, from around 400°C/750°F in the sunshine to –178°C/–290°F at night. The contrast is accentuated by Mercury's extremely long days and nights. Because the planet spins so slowly on its axis but flies so quickly around the Sun, each point on the planet gets sunny and hot for an entire year (which lasts 88 Earth days), then goes dark and cold for the next year.
If Mercury hasn't much of an atmosphere, Venus may have too much of one. The air pressure on the Venusian surface is almost 100 times that on Earth. Surface winds are tepid, typically blowing at no more than about 16kph/10mph, but these gentle breezes pack substantial force due to the atmosphere's heft. Of course, you wouldn't have much chance to experience any of this if you were suddenly transported to Venus, since you'd be burnt to a crisp almost instantly. Temperatures on Venus typically run close to a blistering 460°C/860°F, and they only drop to about 445°C/833°F at their lowest. This is the result of a runaway greenhouse effect, one that famed cosmologist Carl Sagan first explored in the 1960s.
It's believed that Venus may have had vast oceans early in its history. At some point – perhaps as far back as four billion years ago – the Sun's slowly increasing firepower would have begun to evaporate an increasing amount of Venusian water. Since water itself is a greenhouse gas, presumably this led to warmer temperatures, which evaporated even more water, and so forth. Today, the oceans as well as the water vapour are gone, broken into lighter molecules of hydrogen and oxygen that escaped in the scorching heat. What's left is mainly carbon dioxide, which makes up 97 percent of Venus's thick, hazy atmosphere. This greenhouse gas keeps the planet toasty regardless of the time of day or the time of year. (In case you're wondering, global warming isn't expected to produce this stark an outcome on Earth. We don't have enough fossil fuel – and we're too far from the Sun – to fully replicate the Venusian atmosphere.)
Despite the incredible heat on Venus, a thick deck of clouds made of sulphuric acid sits at heights of about 48–64km/30–40 miles, which helps to make the surface surprisingly dim, perhaps only about as bright as twilight on Earth. Within the clouds, there's a zone where the air pressure and temperature aren't too far from earthbound norms. That might make this region the most hospitable part of Venus for humans to explore someday, although winds there can whip along at speeds up to 360kph/225mph.
Compared to Venus, Mars is a cold orb indeed. Its temperatures can drop to a very chilly –140°C/–220°F, and at best they reach around room temperature. As with Venus, the vast majority of the Martian atmosphere (about 95 percent) is made up of carbon dioxide. But Mars is much further from the Sun than Venus, and its atmosphere is very thin (only about 1 percent as dense as ours), so there isn't anything close to a runaway greenhouse effect. Like Earth, Mars has year-round ice caps at both poles; these are primarily made of carbon dioxide, though there's also evidence of frozen water in both ice caps.
Early in its history, when the still-young planet bristled with gas-spewing volcanoes, Mars might have been warm enough to support oceans. But the planet has less than half the gravitational force of Earth, so it's believed that many of the components of that early atmosphere escaped to space as volcanic activity died down and the planet cooled. Unlike Venus, some of Mars' strongest winds are near the surface, where they can kick up gigantic dust storms (the source of the planet's characteristic tan colour). Colossal dust devils can grow as big as Earth's biggest tornadoes – as wide as 2km/1.2 miles and as tall as 10km/6 miles – dwarfing anything seen across our own deserts. Mars also experiences mists over its polar regions during winter, and clouds of water vapour or carbon dioxide occasionally form.