– 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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Instead of a remote island, lets take rain forests. These areas are called rain forests for a reason; it rains a lot.
If you look up their location, they have something similar: they are near the equator. Something about the equator is creating a good environment for these forests to develop; lots of rain. What causes rain? The water cycle.
1. Water exists on earth
2. Water evaporates into the air
3. The water in the air rises up, cools down, and clumps together, forming clouds
4. The clouds condense far enough and become cold enough to become rain, until the water is gone.
5. Water exists on earth (repeat)
Warmth helps the water to evaporate and rise up into the air faster (warm air goes up). On earth, most of the warmth comes from the sun. The sun shines the heaviest and warmest on the equator (on average). So on the equator, air rises the most, and water should evaporate the fastest, creating great conditions for rain to form.
It gets even better through the forming of planet-scale air circulation: the air that goes up at the equator has to come from somewhere. The air comes from the areas north and south of the equator. That means there is a general tendency for the air to move towards the equator, and then go up. So most of the moisture from nearby areas will travel towards the equator, where it will gather and form rain.
Of course the air and moisture has to go somewhere once it has travelled up at the equator, and because it cannot go back the way it came (can’t swim against the current) it will go up and over the incoming air, moving north and south of the equator. This is why the rain forests are not limited directly to the equator, but a larger portion north and south as well.
To go beyond your question; this also causes the extreme drought (lack of rain) in for example the Sahara. By the time the air gets here, it no longer carries enough moisture to form clouds, and the air wants to move down to earth again, meaning that what little moisture was left, is now low enough in the atmosphere that it won’t cool down sufficiently to form droplets.
[The diagrams on this wikipedia article should be of help to visualise the air circulation pockets, and see the shift in average rainfall on earth through changing seasons.](https://en.wikipedia.org/wiki/Atmospheric_circulation)
To answer your question for islands (like the British Isles as well): land warms up faster than water. This is why a city on the coast will be cooler in the summer than a city further inland: the air above the sea simply warms slower. However, the air above the sea will carry more moisture. So once above land, this moisture has a nice opportunity to start rising with the warmer air, and form clouds that bring rain. For all the air moving from the Atlantic Ocean towards Europe, the British Isles will be the first landmass that the air will encounter, so most rain will fall there before the air will move on across the Channel and the North Sea towards the rest of Europe, which will have relatively less rain than the British Isles receive.
Another way to force air to go upwards is mountains; air cannot move through the earth or a mountain, so when blown against it, it can only go up. This rapid rising of air causes the air to cool rapidly as well, forcing out clouds and rainfall on the side of the mountain that gets the most wind from the sea. In the above example, you could see the British Isles as a sort of mountainous wall, shielding the rest of Europe from rain.
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