The question: What realistic environmental signs would telegraph to the occupants of a floating landmass that their home was slowly losing altitude?
The planet: I'm working on a story that takes place on a gas planet, but with breathable air (I'm sure it has a solid core below its dense cloud layer somewhere, but it's not relevant to the story). It's close enough to its star for the planet to have a well-lit atmosphere during the day, and probably doesn't get totally dark at night, due to its gaseous nature. The length of a day, and the gravitational pull, are the same as ours on Earth. There are no land masses as such, but there are incredibly slow, floating sky whales of epic proportions (like 10+ miles long) that people live pastoral lives upon as if they were land masses (neither the humans nor sky whales are native to this world, so don't worry about how they evolved to be this way).
The event: At one point, the whale the protagonists are living on slowly begins to lose altitude. Not in a way that's immediately noticeable, but gradually over years and years. My question is how would the occupants know this was happening? They know they live on a giant flying creature, and have seen others in the distance at different altitudes. The whales live thousands of years, and this one's behavior has remained unchanged for all of recorded history. The people revere them as gods and don't suspect they ever change course (and would probably believe it was the apocalypse). How would they realize something was wrong?
The signs: I know that the air pressure would increase, raising the boiling temperature of water, but what else would happen? Would the sun's path across the sky change? Are different weather patterns more likely at lower altitudes? Basically, what believable signs would alert the occupants to the problem before a catastrophic pressure increase killed everyone on the island/whale?
UPDATE: Thanks for all the great suggestions! I don't want to select a "correct" answer, because everyone pointed out different things that are just as relevant. To summarize what I've learned, and plan to use:
- Scientific observations (such as barometric pressure) collected by the protagonist.
- Anecdotes from older residents or visiting traders.
- More intense thunderstorms.
- Strange animal migrations (to higher ground or never returning).
- Vertical migration of natural flora.
- Shorter days, and sunset shadows lower on the wall, as the horizon rises.
- Failure of delicate crops due to less light and heat.
- Weight scales off a little due to increased gravity (very subtle).
- Rise in respiratory health problems due to change in atmospheric gases.