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Extraterrestrial Weather

Seasons:  
  • Every planet in the Solar System has seasons. In relation to them, the seasonal variation in Earth's weather isn't very much.
  • Seasons are caused by the tilt of a planet's axis. Earth's axis is tilted 23 degrees, so planets with less of a tilt could have less variation between summer and winter and planets with more could have more.
  • Earth's orbit is almost circular, which means when the northern and southern hemispheres are averaged, there is little change in the climate. Planets with elliptical orbits will have very different seasonal variations.
  • Planets closer to the star will also have greater weather variations than planets further away.
  • On any planet: the North Pole tilted towards the star is the Summer Solstice. The South Pole tilted towards the star is the winter solstice.
  • Weather is affected by the tilt of a planet's axis (seasons), the shape of the orbit around its star, the presence or lack thereof of an atmosphere, the average distance from the star, and the length of its day.
The Coriolis Effect
  • In the Northern Hemisphere, the rotation of the planet deflects a moving mass to the right. In the Southern Hemisphere, it's to the left.
  • This affects winds and currents (atmospheric and ocean movements).
  • Because things go from high to low pressure, in the Northern Hemisphere, winds going towards low pressure deflect to the right and move around the area with low pressure counterclockwise. When going away from a high pressure area, they deflect to the left and move counterclockwise. These directions reverse in the Southern Hemisphere. Ocean currents work similarly.
  • The Coriolis parameter increases with increasing latitude, so long waves in the atmosphere and ocean can form and move slowly for long distances. Weather moves along these waves.
  • These patterns can have a very small effect on Earth's rotation in return. Like in Northern Winter, winds are stronger than they are in summer. This causes a decrease in global atmospheric angular momentum from northern winter to summer. This is compensated for by an increase of angular momentum in the solid Earth (which makes it rotate faster). This also makes the day very, very slightly shorter.
  Planet Specific:
Mercury:
  • Mercury rotates thrice every two of its years. It's the only planet in the Solar System that has that ratio something other than 1:1.
  • This, and Mercury's weirdo orbit mean that at some latitudes, after the sun rises it looks like it gets bigger as it gets higher in the sky. Then it looks like the sun would stop growing. stop, go backward a little, stop again, then continue down towards the horizon and get smaller. This is because of the variation of the movement of the sun against the background stars from the ground.
  • From other latitudes you'd also see weird things, although different.
  • That can make it hard to tell the boundaries between seasons.
  • Mercury has the most extreme temperature variations in the solar system. At some parts of the planet, it goes from about -280F or -300F at night (because there is no atmosphere to trap heat) to 800F during the day. But at the poles, there is almost no change in temperature because it is not tilted on its axis. This also means so seasonal variation.
  • Because of the lack of atmosphere, weather changes show themselves as temperature changes rather than storms.
  • There are 176 Earth days between sunrises.
  • The sun looks 3 times bigger and over 10 times brighter than it does on Earth from the surface.
Venus:
  • Venus is tilted on its axis 3 degrees.
  • Its atmosphere is dense and acidic (made of carbon dioxide and clouds made of sulfuric acid), which makes for a powerful greenhouse effect so the surface is about 865F all year. This makes it the hottest planet in the Solar System.
  • Its smaller orbit (than Earth's) makes its seasons shorter and variations in temperature and conditions slight. The seasons are 55-58 days, in comparison to 90-93 days on Earth.
  • In contrast, sunrises on Venus are separated by 117 Earth days. It also rotates "backwards", meaning that the sun rises in the west and sets in the east.
Mars:
  • The distance between Mars and the sun goes between 1.38 AU and 1.67 AU during Mars's orbit.
  • Its tilt on its axis is a little greater than Earth's.
  • These two things combined make the seasonal changes much more dramatic than we have on Earth.
  • The sun warms Mars's atmosphere and makes the air move quickly, which lifts dust off the ground and makes dust storms common. The Martian atmosphere is only about 1% as dense as Earth's at sea level, making it so only the smallest grains of dust stay in the air, and they're as fine as smoke.
  • The atmospheric pressure is 25% lower in the winter than in the summer. Around the winter solstice, the North Pole is tilted away from the sun and the northern polar ice cap expands because CO2 is freezing and becoming part of the dry ice icecap. That reverses half a year later during the summer solstice. The planet is 10% closer to the sun during the southern summer than during the northern. During the winter solstice, the northern polar cap absorbs less CO2 than the southern polar cap during the summer solstice. It's enough of a difference that the atmosphere is noticeably thicker during the northern winter.
  • The speed of Mars's orbit changes during its year. It's slowest at the aphelion (when it's farthest from the sun) and fastest at the perihelion (when it's closest to the sun). That means that the seasons vary much more than those on Earth. Northern spring and fall last 171 Earth days, northern summer is 199 days, and northern winter is 146 days. This means that the seasons don't start on the same Earth day ever Martian year.
  • During the day, temperatures can get up to 32F. But again because of the thin atmosphere that doesn't hold heat, temperatures drop to around -200F at night.
Jupiter:
  • Jupiter also has a tilt of only 3 degrees, which makes for absolutely no difference between the seasons.
  • The distance from the sun makes the seasons change slowly, they're each about three years.
  • It's the fastest spinning planet in the Solar System, which makes the planet flatten at the poles and bulge at the equator.
  • Jupiter has a dense and turbulent atmosphere that can cause lots of storms, the most famous of which is of course the Great Red Spot, which has been raging for centuries.
  • It has a wide range of temperature, because of the chemical compositions of the atmosphere. Like, the white clouds (the highest up) are made up of crystals of frozen ammonia at a temperature of about -220F. As you go farther beneath the clouds, the temperature increases. It gets to 70F when the atmosphere is about 10 times greater than Earth's. It continues to increase as you go deeper, and the core is hotter than the sun.
Saturn:
  • Saturn's axial tilt is almost 27 degrees.
  • The seasons last over seven years, although because of the kind of planet and the distance from the sun, changes between seasons don't mean the same thing as on Earth.
  • Temperatures get down to about -285F.
  • Titan, Saturn's largest moon, is believed to have seasons, clouds that rain, and a mostly nitrogen atmosphere. But instead of a water cycle, it has a methane cycle, which makes up the rain, lakes, rivers, and seas. It's also very cold.
Uranus:
  • Uranus's orbit is pretty circular.
  • It's tilted on its axis 98 degrees. This means that the seasons are 21 years long and the weather is weird, although it is always cold. For about 1/4 of a year on Uranus (≈84 Earth years) the sun is directly over a pole, and the other side has a dark winter.
  • Its atmosphere is deep and mostly hydrogen and helium. The planet looks blue because methane in the atmosphere absorbs red light.
  • When the decades-long winter ends and the sun starts warming the other side of the planet, huge storms (as big as North America and maybe -300F) form as the atmosphere warms.
  • Temperatures are about -360F.
Neptune:
  • Neptune's axial tilt is 28.5 degrees.
  • Not much seasonal variation, and the seasons last over 40 years.
  • Average temperature is again about -360F.
  • It's a very windy planet, with speeds as much as almost 1,200 mph, the fastest in the Solar System.
Pluto:
  • Since it's so far away, it's hard to know much about Pluto's seasons.
  • The tilt is almost 120 degrees and the funny orbit, the weather probably varies greatly, maybe even with atmospheric variations like on Mars. Maybe the thin atmosphere entirely freezes and falls as snow when Pluto gets farther from the sun.

Sources: nasa.govspaceplace.nasa.govastronomy.com
This is just notes for my worldbuilding, from the websites linked at the bottom. A lot of it is verbatim, because that's how I take notes.

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