– 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?
In: 4147
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.
It helps when the place is close to the ocean or other bodies of water. I’m in Vancouver, Canada, this area is a Temperate Rainforest. It rains a lot here, we’re also right next to the ocean.
I lived in the mountains of Philippines. Wake up in the morning: blue sky! Then, as the day shifted to the afternoon, clouds would build up until a torrent came down. Rinse and repeat.
Oo! An answer I know the answer to! I’ll do my best to ELI5, but it may be more like ELI12 or 13.
First, my credentials: aviation meteorologist and forecaster.
There are a couple of reasons. I’ll start at a “local scale” and expand out to “global scale.”
Locally, around large bodies of water (oceans, Great Lakes, etc), you have daily heating and cooling during the day. The land heats and cools much quicker than the water. This creates extremely localized low pressure zones over land due to hot air rising (low pressure) and cooler air over the water getting “sucked in” to land. That cool, moist air combines with the warm air and gets pulled upward by the low pressure. The cool, moist air condenses like a glass of ice tea on a hot day and, voila, daily rain and thunderstorms. Those storms usually occur around 10 miles inland, and usually between 3-5pm, and usually in the same places every day. These are called “sea breeze fronts”. It’s fun watching them form on satellite.
%%%
Let’s zoom out a bit to regional scale.
**Regional 1**
There are four main types of air masses that identify the type of air that sits over a particular area. There are
* maritime tropical (moist warm air)
* maritime polar (moist cold air)
* continental tropical (dry warm air)
* continental polar (dry cold air)
Anywhere any of these air masses come into contact, you’re going to have weather. A continental polar air mass out of Canada coming south and combining with a maritime tropical air mass from the Gulf of Mexico, that’s like an unstoppable force hitting an unmovable object. Extreme opposites. You’ll get massive storms really regularly. Hence tornado alley.
That’s an extreme combination and fun to observe. But it’s not always that dramatic. Basically any time you have regular mixing of any two different air masses, you’re going to get weather.
**Regional 2**
Any time you have land formations that push air up – mountain ranges, for example – you’ll find you have a lot of rain on the side the wind blows from (called the windward side) and very little rain on the other side of (called the leeward side).
The higher the mountains, the more lush and green they are on the windward side, and the bigger the desert on the leeward side.
This is because as air rises, it can hold less and less moisture due to it being colder and colder with height. Once it drops its moister, it flows down the leeward side of the mountains as very dry air.
Or, to use the above air masses… A maritime tropical air mass modifies to a continental tropical air mass once it passes over a mountain.
This only really happens for mountain ranges with a north-south orientation.
%%%
Ok. Let’s zoom out to the global scale.
Due to the way air circulates, there is a permanent high pressure over the north and south poles meaning precipitation rarely falls there.
There is a permanent low pressure that stretches all the way around the earth at the equator. Meaning it is almost always raining at the equator. The places that are the wettest places in the world have several global, regional, and local causes for it: they are on the equator, they are next to huge bodies of water, and they have lift from land formations like mountains.
So, we have the permanent low at the equator. If you go north by 30° latitude, there’s another permanent high. The permanent highs will suppress (but not fully stop) other causes of weather. It’s what causes the doldrums in the ocean, where sailors would get stuck for weeks with no wind in sight.
Go north another 30° latitude to 60°, there’s another permanent low. You’ll see enhanced weather events.
Another 30°, and we are at the poles again. Permanent high.
So those are 3-4 reasons why some places are always much more rainy or prone to what we call “weather” than other places. There’s more to it, of course, but this should make it so you can ask yourself a few questions about a specific location and understand why it’s always so wet or dry.
Late to the party, but I live in Lahaina, Maui. I live just down the mountain, at sea level, from the mountain that is considered to be the second wettest place on earth. On the highest point, Puu Kukui, there is an average of 400 inches of rain a year. At sea level, we get about 15 inches of rain a year. Every day, wind blows over the water and carries moisture along. When that moisture hits the mountain, it rises up. At the same time, when the sun heats up the mountain, it pulls the moisture up with hot rising air. Both of these phenomena cause clouds to condense and form at the top of the mountain almost every day, and then the rain comes down. We don’t normally get this rain in Lahaina, as it goes down a river on the other side and fills old lava tubes that drain all the way down into the ocean.
It does NOT rain constantly on Maui. In fact, parts of the island are more or less desert. Source: me. Lived there for years.
In addition to other comments I wanted to add that plants give off a shit ton of moisture.
[Corn sweat](https://www.washingtonpost.com/climate-environment/2022/08/02/corn-sweat-midwest-plains-heatwave/) is a real thing here in the Midwest.
Latest Answers