6
$\begingroup$

So there are loads of things that are taken into account with weather from geology to water currents to axial tilt etc. But i am wondering what the effects would be on weather on planets with varying strengths of gravity in comparison to Earth.

If we call Earth standard for any approximates, averages and estimations, how would a planet that has a greater or lesser gravity come out in the weather game? How would it affect rain, snow, hail, and thunderstorms.

Having an atmosphere in which to contain said weather is implied across the board. Lets also say that all planets are not recently terraformed and that they had atmospheres from beginning.

Edit : Thanks to all for the answers so far. I apologize for it being too broad. I would also like to state i do not mind assumptions made from extrapolated information off of real world data. I understand we do not have all the answers currently and really just want best guess in the face of such.

So to help narrow this Let us assume that we are starting with an earth sized planet, with distance from star being the same as earth and a day length period the same as earth. We will use Mass as the variable within density that is being manipulated.

Let us also call the current thought of axial tilt and habitable climate as true and say that any and all tilts taken into account fall mostly earth like tilts with no more than a 3 degree change and the mirror of that range.

Lets also say the Star is either equal to our sun, or is no more than 1.1x its mass. No matter the size of the Star, the Planet will be within its habitable zone in a orbit as earthlike as possible.

Overall, we are taking the planet to be earth in just about every facet, except gravity via density via mass. Let us say the current air patterns are the same, and the current Water currents are the same. Say even Geography is the same so that we can watch merely how Gravity plays upon the weather.

I am interested mostly in how the changing of the density alters the gravity and how that alters the weather. I am kind of confused on it, because the thicker air could in theory hold a hailstone aloft longer because it has to move through a greater air resistance, but by the same token it would be heavier. At what point would rain become as dangerous as a hailstorm currently if there is a point? At what point would hail float down like heavy snowflakes, or would the atmosphere dissipate before such could occur? At what point would water based precipitation begin occurring as what we think of as sea level with lighter gases forming clouds above them, Or would the differences in the weight of everything compensate for it all so that it all kept at the same levels?

$\endgroup$
  • $\begingroup$ Welcome to worldbuilding.SE. When you have a moment, please take our tour and review our help center pages. Thanks! $\endgroup$ – JBH Apr 16 '18 at 22:57
  • $\begingroup$ This is far too broad. The difference between Venus and Earth is just 10% surface gravity. The difference between climate is like between Hell and Earth. You need a lot of specifics to make even educated guesses about what kind of climate a world has. $\endgroup$ – StephenG Apr 16 '18 at 23:27
  • $\begingroup$ @StephenG, I disagree that the question is too broad. That VTC reason means that to answer the question well, as the question is stated, would require a very, very long answer. That isn't the case here. Just because correcting the question would be very broad doesn't make the question itself too broad. Frankly, I could have succinctly answered the question with the phrase, "Any answer would be meaningless because climate is much more complex than that," and the answer would be 100% correct in just one sentence. $\endgroup$ – JBH Apr 16 '18 at 23:42
  • $\begingroup$ @JBH What you're in effect saying is that you can write an answer that actually says "it's too broad" in a more elaborate way, which I see as pointless. Closing as "too broad" lets the OP consider ways to make a more specific question. And there are already a lot of questions relating to planet climates on this site alone for the OP to see how vague the question is. Even if the OP said "gravity on this planet is 1.0 g" the climate variations are practically infinite. $\endgroup$ – StephenG Apr 16 '18 at 23:52
  • $\begingroup$ @StephenG I am sorry for making the question too broad, and my poor wording in the regards. I am more concerned with this question about how specifically gravity alters weather. I understand how ridiculously expansive the list of variables are for weather. i am not trying to simplify what is books even when simplified. I am looking for specifically gravities effects within the formula. Keeping everything else the same as much as possible, how does gravity work upon weather? $\endgroup$ – AYoungRabbit Apr 24 '18 at 4:15
6
$\begingroup$

The Simple Answers

Less than Earth: You would have a thinner atmosphere and it would extend further away from the surface. You would have less mass, therefore slower wind, fewer storms, and without the mass to carry water vapor, less virulent storms.

Greater than Earth: You would have a thicker atmosphere and it would not extend as far from the surface. You would have more mass, therefore faster wind, more storms, and with that greater mass, more water vapor capacity and thus more virulent storms.

But...

As you mention, so very much goes into atmosphereic conditions that it's very hard to explain what could happen without you providing very specific and complete planetary conditions that balance all the equations (which is a fancy way of saying, you can't say "Earth, but with less gravity" because what gives it less gravity affects the atmosphere too, in very complex ways).

For example, a low G world may have lower wind velcocities... unless its orbital speed is very fast (adds to wind) or is closer to the sun (heat = energy = wind).

Likewise, a high G world would expect greater disturbances and higher wind velocities due to the thicker air, but if you reduce the rotational speed and pull it futher from the sun, those speeds slow down.

Therefore...

So many things go into planetary climate that this incredibly simple answer is almost meaningless. Let's consider Venus...

Venus has a rotation period (day) of 243 earth days. That's very slow! It has an incredibly dense atmosphere despite being 0.9G (90% of Earth). You'd think that some of this is due to heat, it being closer to the sun, but in reality it's 100% due to the chemical composition of the atmosphere, which actually reflects most of the sunlight so that little gets to the ground. Thus, the slow rotation and slightly lower gravity produce the very slow surface winds we would expect from my very simple answer... (only a few kph per hour), but the heat and atmopsheric stiration that doesn't exist on Earth lead to whomping fast winds at the top of the stack (winds circumnavigate the planet every 5 earth days) and so much cloud cover that you can't actually see space from the surface.

Conclusion

Therefore, you can take my answer for what it's worth, but the only way it's useful is if you compare two planets, identical in every way save one: gravity. The problem with that is that what makes the gravity diferent (same diameter, therefore one is less dense (has less mass) than the other), would also affect the atmosphere if only by changing the chemicals that make up the atmosphere (but it would also change the surface conditions, hydrology, magnetosphere, etc.).

Simple, but without the other 1099 variables that affect planetary climate, pretty much meaningless.

$\endgroup$
  • $\begingroup$ "Greater than Earth: ... it would extend further away from the surface." That is seems very counterintuitive. $\endgroup$ – RonJohn Apr 16 '18 at 23:27
  • $\begingroup$ "You would have more mass" That also is counterintuitive. The atmosphere would, though, be denser. $\endgroup$ – RonJohn Apr 16 '18 at 23:27
  • $\begingroup$ "a low G world may have lower wind velcocities[sic]" Mars is low-G and has very high winds. $\endgroup$ – RonJohn Apr 16 '18 at 23:28
  • $\begingroup$ "[If gravity is] less than Earth [the planet] would have a thinner atmosphere": atmospheric pressure depends on the gravity of the planet and on the amount of gas in the atmosphere and its temperature. There is no simple relationship between the three. $\endgroup$ – AlexP Apr 16 '18 at 23:29
  • $\begingroup$ @RonJohn, you're right about the extension of the atmosphere from the surface. However, wouldn't a higher G planet have the capacity to retain more atmosphere? Wouldn't that result in greater mass (as well as density)? I'll correct the answer. However, Mars and Venus both underscore the fact that gravity alone doesn't tell us much about how climate changes, which is the primary point of my answer. $\endgroup$ – JBH Apr 16 '18 at 23:35
5
$\begingroup$

This is a deceptively difficult question, since most of the effects of lower or higher gravity are second-order effects which tend to be more complex than the simple first-order effects.

I need to state my assumptions. First, I assume you're talking surface gravity. Second, I'll assume that the planet is a standard rocky planet like Earth, so a smaller surface gravity happens because the world is smaller and less massive and a larger surface gravity is because the world is larger and more massive. Thirdly, I'm assuming its atmosphere followed an evolution like Earth's and has a composition like Earth's and a surface pressure like Earth's. (This is relevant since I want to ignore the possibility of things like Venus's runaway greenhouse effect.) Finally, I'll assume a similar orbit and rotation period.

(Note: all of these assumptions are arbitrary and we're pretty certain that planets exist which violate any and all of them. But if we start varying multiple parameters at once, it's pretty hopeless. There are other reasonable starting points, such as assuming that the amount of air is the same rather than that the surface pressure is the same.)

The most obvious place to start is that the low gravity planet would have an atmosphere where air pressure drops more slowly with height and an atmosphere which extends much further into space. Likewise, the high-gravity planet would have an atmosphere squashed compared with Earth's. This would have a significant effect on convective weather, but estimating what that effect would be is difficult -- it's not something you can do without sophisticated modelling.

A second big effect on convection is the direct effect gravity on buoyancy. Convection is due to density differences causing air to move under the influence of gravity, so you'd almost certainly have less convention under low gravity and more convection under high gravity. So vertical air movement would get more violent as the surface gravity increases. This probably would mean more convective storms (i.e., thunderstorms.) Could be more hurricanes, too.

OTOH, you should get bigger hail in lower gravity, since hail grows larger the longer it's kept aloft by the winds. The wind force lifting it up is proportional to the hail's cross-section, while the weight pulling it down is proportional to its volume. Even if the windspeed increases with higher gravity, I suspect that on balance, hail will grow larger under lower gravity -- as long as thunderheads form. (OTOH, I'm really hate to get hit by hail under 2Gs!)

It seems likely that the higher the gravity the faster the horizontal winds, also, since buoyancy is also ultimately responsible for them.

Beyond this we quickly get into pure guessing. (Assuming we didn't get their five paragraphs ago...)

$\endgroup$
  • $\begingroup$ I really do not mind the speculation, as long as it is based off what we currently know. Would hail in higher gravity come down bigger or smaller? the air would be thicker, and the wind would be more dense providing greater force, but the water would fall relatively faster than in standard gravity. At 2Gs would clouds sit at the same aproximate levels, or would they alter in level, if so aproximately how much? Would snow fall in smaller or thicker crystals? Thanks for what you have given so far :) $\endgroup$ – AYoungRabbit Apr 25 '18 at 21:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.