What would a world where gravity is a weather condition look like?

Instead of wind mills, you could have gravity mills. Pump water into a higher-altitude reservoir on low-gravity days, and let it flow down -- turning a turbine -- on high gravity days. At least electricity would be cheap.

Or if it varies by region, pump water horizontally (or let it flow slightly downward) from a high gravity region to a low one. Then pump the water upwards there, then horizontally again to the high gravity region. Then let it fall down to turn a turbine that runs all the pumps -- perpetual motion (ish)!

Predicting tides becomes hard. Everything is going to be really windy all the time, as the atmosphere expands in low-gravity regions and contracts in high gravity ones. This makes tall buildings impractical, as they would also have to be built for some maximum gravity rating on top of the constant gravity storms.

The oceans would be weird, and violent. Hurricanes might get far more powerful than what we deal with, if the right gravity conditions occur.

For any sort of civilization to emerge, gravity would have to change/vary really slowly. I don't even want to think of orbits. Kerbal Space Program would be like, really hard in that universe.

Sorry if someone else pointed to this already, but this could be relevant for your story:

"Unlike a body circling a single star, a planet orbiting a pair of stars would have to contend with two gravitational fields. And because the stars themselves orbit each other, the strength of the gravitational forces would constantly change."

Source

I think I read your title differently - as in, gravity would ebb and flow like wind or rain or barometric pressure or temperature. In normal days the gravity might be mostly constant, or may fluctuate a few percent as the day goes on, rising and falling over the diurnal cycle. But at times a gravity storm could blow through, causing wild fluctuations from just a few percent (or even reversing!) to a couple hundred percent, causing travelers to lose their un-secured cargo or to be pinned in place until the storm subsides. Locals would know the dangers and have things easily tied down, or beds for riding a gravity storm in relative comfort, but any huge storms people would evacuate, praying that the fluctuations wouldn’t destroy their homes or farms. (And now I’m imagining the end of O Brother Where Art Thou with the cow on the roof)

We would have different bones/muscles/joints.

We already experience this on earth to a degree. The moon causes the tides and shore dwelling ocean creatures have to contend with surviving during the high and low tidal periods. Not really weather per se, but it's something.

This actually a real phenomenon.

However, gravity isn't the same everywhere on Earth. Gravity is slightly stronger over places with more mass underground than over places with less mass. NASA uses two spacecraft to measure these variations in Earth's gravity. These spacecraft are part of the Gravity Recovery and Climate Experiment (GRACE) mission.

But it's practically unnoticeable on earth in real terms.

So there could be some sort of super dense mineral in some places. Or basically hollow earth in others.

This isn't realistic really but you said it doesn't need to be 100%

In real world terms it doesn't really make sense, so you'll need to dial up the "fantastic" when explaining/handwaving it. Ever read Sanderson? Because he does both "reasonably realistic except X" and "this wind is basically a God, whatever, fuck you" and makes both work pretty well.

Lumpy.

Of course, if this is a fantasy world you're under no obligation to go with a world held together by self-gravitation, and you can even ignore the weight of air. Believable water is going to want to follow gravity as usual, though, so you need to figure out some sort of crazy hydraulic system to move it around. It could be a backstory for some cool canyons and things depending on what you decide.

More interesting questions might be related to how the residents adapt. I imagine lower gravity areas would be favoured, with groups living in the high gravity areas being specialised. Maybe unpredictable gravity could serve as an energy source for whatever civilisations are in your setting - you balance a very large weight somehow (against a non-gravitational force or a weight somewhere else), and have it work machinery as it adjusts to a new equilibrium. If it changes rapidly enough it might even be useful at small scales, like on vehicles.

Probably pretty dead since you couldn't have planets with stable orbits.

Completely scraping off any astronomical/space problems with this and just rolling along with the idea, which I like, I think your world inhabitants could be either:

• as you said, adapted locally

Or

• with a series of physical adaptations that would allow them to move between areas of different gravity. I think this would make things interesting because you said gravity as weather, and so you could play with the idea of gravitational seasons and gravitational storms or draughts.

Low gravity + strong winds sounds like it would wreck everything and everyone. Hurricanes would turn into a disaster situation rather than a mild annoyance. Imagine cars flying around and ramming buildings.

You’d need the planet’s core to contain some kind of non-dense material that will not mix with the dense material … like molten rock but also molten something else that’s much lighter than rock. Basically you need “oil and water” droplets that don’t mix, and are of different densities. Then you need some mechanism for them to churn in a turbulent way. The turbulence makes their movements chaotic and unpredictable.

Only thing I can think of to account for the churning is electromagnetic forces being generated by naturally-occurring nuclear reactions.

So to summarize:

• Mantle composed of two substances. One much heavier than the other, and they don’t mix
• Electromagnetic forces occurring at random places and times causes these substances to churn in a turbulent way
• Turbulent churning of these two materials affects the total amount of mass under characters’ feet at different times, causing unpredictable “gravity weather”
• Those electromagnetic forces somehow result from nuclear reactions happening naturally underground (otherwise where do you get the electromagnetism from?)

I think the people who evolved in the highest gravity regions would become like supermen and end up ruling the whole world due to their superior strength. If not, it would at least give them a significant advantage while technology is still primitive. How would they defend themselves from attack?

Maybe my brain is fried, but I think you aren't think big enough if "long term damage" is the hazard in the highest gravity area vs a person just being flattened like they're in a press (which means people wouldn't go there, but it could have industrial uses).

An interesting part is how states of matter would vary due to gravity causing pressure, though with the relation between pressure and temperature I don't know exactly how it would work.