# What is the ideal day length for extreme weather?

This question is in reference to a previous question by TrEs-2b and SRM: I want to create a planet with a significantly longer day than Earth's. However, I was only able to find a question about a planet with 9-year long days. This is way too long of an rotational period. As evidenced by the linked question, longer days equal more extreme weather (high winds, rainfall, different hemispheres). I am looking for weather like this, but do not want the distinct hemispheres (one super hot and one super cold).

So, what would be the range of day lengths needed to achieve extreme weather without distinct hemispheres?

Clarifications:

• The planet in question is just slightly larger than Earth
• There is one super-continent, similar to the size and shape of Pangaea, in the middle of the planet surrounded by oceans
• Everything else is identical to Earth (atmosphere, pressure, etc.)
• I am looking for the minimum day length to achieve extreme weather and the maximum before the distinct hemispheres form
• What I mean by distinct hemispheres is that the only habitable space is in the middle. Polar regions and deserts are ok.

Bonus points if the orbital period around the pseudo-Sun is one "day," like if the day length is equivalent to 400 Earth days, then it takes 400 Earth days to complete a full orbit.

• To have a day length equal to the orbital period all you have to do is have your planet have a retrograde rotation. Dec 2, 2020 at 1:21
• Difficult: the " distinct hemispheres (one super hot and one super cold)." is what causes the extreme weather. Even on Earth, most of the weather derives its energy from the day/night cycle of heating and cooling, plus the polar areas receiving no day worth mentioning. Dec 2, 2020 at 6:46

We know of quite a few planets with long days (Mercury has a longer day than its orbital period), but I think that Venus is most similar to what you're looking for.

As you probably know, Venus is known for extremely harsh conditions on the surface with respect to pressure and temperature. But it also has wind speeds that can be up to 85 m/s and circle the planet once every four to five Earth days.

Venus days are 116 Earth days while a Venus year is 225 earth days. That puts the day at 62% of the year's length.

Venus is about the same size as Earth, but it definitely doesn't have the same atmospheric conditions. But if you go up in the atmosphere about 50km, you'll have the same pressure as earth, a similar temperature, and the winds are still 85 m/s.

And here's something that addresses your concern about one hemisphere being super hot and the other super cold: with such high winds, the whole planet is nearly equally hot because the heat is distributed by the strong winds. You could have a tidally locked planet with fast winds to distribute the heat around the planet. Basically, any period of rotation long enough to create extremely fast winds can do away with its distinct hemispheres.

In respect to having the orbital period be the same as the day, you're either going to need them to be tidally locked, meaning one side is always facing the sun (which you said you don't want, but you could do it with the winds as I said above) or have it spin in the opposite direction of the orbit.

I hope that helps. I tried to answer the question by giving a real-world example of a planet similar to what you're describing, which I personally find helpful when I'm designing planets, but it might not be helpful in your case. If not, let me know and I'd be happy to do more theorizing.

• I'm rather skeptical that even very high winds could "even out" the temperatures between hemispheres of a tidally locked or slowly-rotating world. An atmosphere will generally have far less thermal mass than the rock it sits above - it's a poor way of ferrying heat from one side of a planet to another. I expect it will be like filling a pool with a hose, and trying to simultaneously empty it with a thimble. Jul 7, 2022 at 19:05

Venusian Days and the Runaway Greenhouse Effect: I think the problem with the question is that there are many factors that can create extreme weather. I'm going off my memory of astronomy class so forgive me if this isn't 100% accurate. Venus's day contributes to its bad weather because the long day causes the sun to cook one side for way too long, then all the water vapor goes to the other side, and since water vapor is a greenhouse gas, the other side gets cooked. Then the whole planet gets so roasted that the interior outgases more water in addition to all the nasty chemicals it has today, forming a feedback loop. I don't think you want to build a planet with a runaway greenhouse effect.

Day Lengths, Wind Cells, and Oceanic Currents: The thing about day lengths longer than 24 hours is that in the absence of being roasted, you can actually get lower wind speeds, and since you want an eccentric but still earth-like planet, that's what you would likely get with a longer day. Instead, I would speed the day up. I just read that without the moon, we might have an eight hour day, and as a result 100 mph winds. Speed that up to 6 hours, and you go from Earth's 3 wind cells to 7 wind cells, which in turn affect how many oceanic currents you have. Oceanic currents and wind cells are where you get hot and cold chasing each other, and as a result very bad weather! So, if you want Earth with really bad weather, that's the direction I would suggest you go.

Tidal Locking: You could also make your day the same length as your year, but this will create a very particular type of planet. Assuming other factors don't cause a runaway greenhouse effect, you will have a frozen night side, a tiny habitable strip along a band of permanent dawn/dusk, and a neverending mega-hypercane on the side facing the sun. My personal spin on this would be to make the orbit just a little eccentric to make the sun move up and down in the sky and give it a little bit of a day-night cycle, but that is just my crazy idea, maybe it would break the tidal locking if you did the math. Additionally, if you tilted the axis of a non-tidally locked planet ninety degrees, I would think you'd get the same effect, except the spin would affect air currents, which would no longer be straight, but would spiral. This option might be more plausible for an earth-like planet around a sun-like star, because the distance and/or presence of a large moon might prevent tidal-locking. EDIT: I was reflecting on this and realized tilting a planet ninety degrees wouldn't be anywhere close to tidal locking, because it would still result in a day-night cycle, just one that is as long as the year, whereas tidal locking has no day-night cycle, and rotates once every year.

Other Factors that Affect Weather: As I said before, other factors affect extreme weather. Tall mountains and large bodies of water have a major effect on weather, and can be placed strategically for maximal weather mayhem. Check out Artifexian's channel. He has a great deal of information on several of the things I just covered. In this video, he explains rotation periods and wind cells. At the end, he also explains that Middle Earth is already full of mountains, so if you just make the body of water in the center bigger and connect it to an ocean, you get never-ending thunderstorms.

Conclusion: I wouldn't just use the length of the day to make a bad weather version of Earth. I would look at other properties of planetary motion and/or unfortunate geographic coincidences as a better vehicle to accomplish this.

• Hi! Good answer to a tough question. Venus doesn't quite work like that. All of the water on Venus would hardly fill an olympic sized swimming pool. It was done outgassing a long time ago. The entire atmosphere blows around the planet once every four Earth days, but the surface itself takes 243 days to go the distance. Mar 6 at 1:25
• You're right. I was talking about the past though. I was told it started out much closer to Earth. I realize at some point it had to lose most of the water, but what I meant is that water itself was the originator of the runaway greenhouse effect on Venus. Mar 19 at 13:14
• To my knowledge, water is a dependent variable in green house gas (percentage based on temperature temperature), not the other way around. This is true until either the planet hits boiling (water stops being a vapor and becomes a gas), or until it runs out of oceans. Thus, it can't start runaway greenhouse on its own. The articles I read suggest CO2 outgassing from vulcanism, if Venus used to be Earth-like. Mar 19 at 19:40