Weather and wind on a planet with a dense atmosphere

Question

I've been building a world that has a thicker atmosphere than that of Earth, about half the gravity, and twice the amount of oxygen. My reasoning behind these features is mainly to support the growth of large trees and megafauna, and to make it easier for my races of intelligent bats to fly.

Now, one thing I haven't been able to figure out about this world it how these changes, particularly the dense atmosphere, would affect the planet's weather.

For example, I've been thinking that because the atmosphere is thicker, the wind on this planet would put more force on anything it's hitting. The thick atmosphere could trap more heat and make more violent storms, if it doesn't just homogenize the planet's temperature.

Basically, I think this planet would either have extreme weather, or very little weather. But I don't know which.

Therefor, my question: Given the above parameters, what could air circulation and wind speed be like compared to Earth?

Answer 1

In general, a thick atmosphere relates to a normal atmosphere, the same way that syrup relates to water. Keep that in mind, as it's generally applicable for all basic cases I can think of.

For example, I've been thinking that because the atmosphere is thicker, the wind on this planet would put more force on anything it's hitting.

Purely considering the kinetic forces, you'd expect the opposite to happen: slower winds.

A thicker atmosphere means a heavier gas, which means more mass. Assuming the same force is applied, an object with twice the mass will move at half the speed.

Do note that if for some different reason the amount of exerted force dramatically increases, then you will indeed be hit with a harder force. But that has nothing to do with the mass of the gas, but rather the force that propels it.

A falling jar with a liter of syrup will hit you harder than that same falling jar containing a liter of water, but when stirring either liquid with a spoon using the same amount of force, the spoon will move slower in the syrup than in the water.

Do also consider that this works both ways. While the wind (at the same speed) hits you harder, you will also receive more support (to help you stay upright) by the wind on the other side of you.
If we're standing in a pool (feet on the ground, not floating), and I push you, the water will dampen your movement and you won't get pushed as far as you would if the pool were empty. The same is true of the thicker atmosphere, to a lesser extent than the water.

A spoon stays upright easier in a jar of syrup, than in a jar of water. However, a spoon that is forcefully pulled out of syrup will endure more force than when it gets pulled out of water.

In other words, your body receives more stress/pressure (stronger push, stronger push back), but any difference in end result (your actual movement) will likely cancel each other out (or at least scale accordingly).

The thick atmosphere could trap more heat and make more violent storms, if it doesn't just homogenize the planet's temperature.

This sounds realistic. A thicker gas is likely going to be a beter insulator, thus will indeed be slower at homogenizing the temperature. As temperature and pressure are related (volume is constant for the planet, so irrelevant), a higher temperature leads to more pressure, which leads to stronger winds, as wind is primarily created by pressure differences.

Given the above parameters, what could air circulation and wind speed be like compared to Earth?

Any wind created from heat-based sources will likely blow more forcefully (assuming the thicker gas is a better insulator).

However, the increased mass inversely correlates to the speed of the wind. Slower and thicker wind, while exerting the same force, has more time to disperse around you and will therefore not hit you as hard, comparatively.
This may or may not completely negate any added wind force due to heat-based sources.

Since the thicker atmosphere dampens the movement of the wind, it also dampens your body's movement (when hit by the wind or any other force), so you won't be blown away easily, but your body will endure more pressure when a strong wind hits you.

Last but not least: a thicker atmosphere makes you more buoyant. Your mass doesn't change, but your perceived weight does, just like how you feel lighter in water (and would feel even lighter when submerged in syrup). You'll feel lighter on your legs, and will e.g. be able to jump higher (again, assuming a jump of equal force).
If the atmosphere is sufficiently thick, you could even "swim" in it (even though you'd slowly "sink" to the planet's surface).

Answer 2

Regardless of chemical composition, an atmosphere twice as dense that is still subject to the heat ranges of those on Earth will create winds with much stronger forces.

That said, to answer your direct question; such a planet would have extreme weather, because the only way it could exist in that format is to be suffering extreme greenhouse activity, not on the scale of Venus but certainly more than Earth. Such a planet would be in the inner fringes of the goldilocks zone of its home star and between the heat and the fire risk, really wouldn't last in this state of life bearing for long would be my guess.

I haven't come across a lot of sci-fi stories about planets without any life on them, so the rest of this answer assumes that the spirit of the question revolves around how such an atmospheric makeup would impact life.

This planet would be on the fringe of those where complex life may be possible. Complex life is far more fragile than we generally give it credit for and this planet presents it with some significant challenges, but first a word on how to consider atmosphere in terms of biology.

When discussing atmosphere for life, it's best to focus on the concept of partial pressure. In other words, organisms can generally be quite tolerant of changing pressure overall (we wouldn't be able to scuba dive if that wasn't the case) but we still use a set volume of a specific gas, like oxygen and are less tolerant of changes to those levels in gases we need or need to avoid.

Generally speaking, the average human can survive if he or she is breathing between 0.16 and 0.30 ATM of Oxygen, less than 0.02 ATM of Carbon Dioxide and other poisonous gases, and whatever mix of inert (or low reaction gases like Nitrogen) is needed to make the balance. An example of this is the Apollo missions, where the astronauts could survive for an extended period in a pure O₂ environment because the Apollo spaceships were only pressurised to around 0.3 ATM.

As for evolution being able to cater to the conditions described above; evolution is not a magic wand, and complex life still has limits in terms of the environments in which it can form. Most extremophile forms of life of which I'm aware are very simple, often single celled organisms. Increasing O₂ and CO₂ and saying 'let evolution take care of it' may work (may), but would put limits on the type of life possible and how long its life expectancy can be. But to the question at hand...

Firstly a higher oxygen count would actually be detrimental to plant life, for which O₂ is essentially a waste product; not because they don't use it in their own organic chemical reactions like all life, but because they (like us) find it toxic in high quantities. This is why they release their excess in the first place.

Also your chances of fires would be MUCH higher and catastrophic when they happened. Megafauna would not occur as a result of higher O₂ levels, except for maybe insect life because the higher concentrations would allow their bodies to absorb O₂ deeper in (they have no lungs).

All other animals would have shorter life spans because the higher oxygen count would actually age their bodies through free radicals (especially in the case of lungs). Humans in particular would have long term problems if constantly exposed to O₂ levels above 0.3 ATM. With a thicker atmosphere, the O₂ levels you describe would probably be toxic.

Also there is an issue with a thicker atmosphere on a lighter planet; the only way that occurs is if a massive amount of energy is being dumped into the system, think runaway greenhouse effects. In such instances, the O₂ levels might be possible through plant photosynthesis, but only because the CO₂ levels would be off the chart for animal survive-ability. Think Venus for the best analog. It's that increase in CO₂ that presents the single biggest challenge to life on such a planet because of both the toxicity of the gas itself and the increase in heat and pressure involved in a high greenhouse effect.

Combining all these things, it becomes clear that your planet will be hot, have high pressure winds and generally be inhospitable to life, but could be livable if the greenhouse effect is nowhere near as pronounced as that on Venus.

Answer 3

You would have really strong winds even with low wind speeds, and there is a real-life solid example to support this: Venus.

Although the wind speed on the surface are really slow, Venus's atmosphere is so dense, that you can simply push a human with a gentle breeze.

At extremely high pressures, it doesn't really matter whether the atmosphere travels slowly or quickly. They exert a lot of force.