– how can a place be constantly extremely rainy? Eg Maui is said to be one of the wettest places on earth where it rains constantly. What is the explanation behind this? Why would one place be constantly rainy as opposed to another place?
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The amount of moisture that air can hold depends a lot on its temperature. Most of the moisture in air comes originally from evaporation of the oceans. These two ideas are why tropical islands tend to be really rainy.
Mountainous islands located where the ocean water is warm are right next to a huge zone of air that quickly becomes very humid. That air will migrate across the island with the normal wind patterns. The real problem comes from that air rising up in the atmosphere when it encounters the mountain(s). Rising air cools off, and all that moisture that filled it when warm is simply not able to stay in the air, so it condenses out as clouds and falls, as rain. Tropical rainforests on mountainous islands is what results. They are really quite common. Gilligan’s Island.
Generally speaking, land located downwind from open water gets a lot more precipitation over the course of the year. Moisture enters the air while it is over the water, and drops it back out once it crosses over land (especially when the land is colder than the water; think about Buffalo NY and its massive snowfalls as moisture from unfrozen Lake Erie gets pushed over the cold land to its east).
The presence of mountains is a very important cause of rainfall in many parts of the world, so the upwind side of the mountains gets lots of rain. The downwind side is in a rain shadow (all the moisture dropped out when the air crossed the mountains). The US northwest and west coast of Canada gets lots and lots of rain (and snow), and there are temperate rain forests all along the coast. Inland, though, on the downwind side of those coastal mountains, is generally really dry.
The idea is basically that of an atmospheric conveyor belt, moving moisture from the oceans onto the nearby land where the wind blows off the oceans. Sometimes there is so much moisture and the mass of air is so huge that the weather folks call it an “atmospheric river”. Not really a river but it carries lots and lots of moisture from the ocean to the land. Seasonal Monsoons.
When the wind mostly blows off the land and out onto the oceans, the air is usually very dry and the region does not get a lot of rain. Just the opposite of what happens when the wind blows from the ocean onto the land.
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