Chile is a very long narrow country, bordered on one side by one of the highest mountain ranges in the world. This dramatically affects its climate. In the north, the prevailing winds are the SE Trade Winds, which means they come across the Atlantic, bringing rain to the Amazon Basin and northern Argentina. Here, Chile lies in the rain shadow of the Andes, and is very dry - the Atacama Desert is the driest on earth. However further south the prevailing winds switch to the westerlies, bringing rain from across the Pacific. As a result the centre and south of Chile has very high rainfall, while it's dry on the other side of the Andes in southern Argentina.Originally Posted by Azelor
The prevailing winds for NE USA are the westerlies, as you say, and all else being equal, North America should be relatively dry east of the Rockies. However there's a local anomaly which dramatically changes this; the Gulf Stream. This is a water current in the Atlantic. The northern equatorial current comes across the Atlantic, where the gently curved coast of the USA acts like a big scoop, sweeping the current north up the coast. The outward flaring shape in the north then directs the current back out across the Atlantic. This has a profound effect on the climate of both North America and Europe.
All regions experience winds that don't adhere to the prevailing wind direction. This is because air circulates around high and low pressure areas, thus the local wind direction is dependent on the position of the low or high. As the weather system moves through the area (in the direction of the prevailing winds), the local wind direction changes, in exactly the same way that wind direction reverses as a hurricane passes overhead.
If you imagine a weather system like a giant spinning disc, it is travelling around the world based on the prevailing winds, but the disc itself is also spinning. If it's spinning clockwise, the wind direction at 12 o'clock is westerly, the wind direction at 3 o'clock is northerly, the wind direction at 6 o'clock is easterly, and the wind direction at 9 o'clock is southerly. As you can see, depending on where exactly the weather system passes through, the wind could be coming in any direction.
As weather systems sweep west-to-east over the coast of the NE United States, the "backswing" of the system is going in the opposite direction, east-to-west, thus it creates onshore winds as it passes over. Because the Gulf Stream dramatically increases the water temperature off the eastern coast of North America, that backswing collects a huge amount of water off the ocean and drops it on the coast as it comes past.
Storms are driven by the same consideration. Tornadoes are a product of severe thunderstorms, and thunderstorms are created where warm and cold air mix. So-called "Tornado Alley" has such devastating tornadoes and thunderstorms because of geography. It's unusual on earth in that you have a huge relatively flat expanse of land that extends from polar latitudes in Canada right down to the tropics. Cold air sweeps off the arctic and heads south, while water-laden air off the Gulf of Mexico sweeps north.
Unusually, there are no mountain ranges to interrupt these two air flows. As a result the water remains in the warm air as it travels hundreds of kilometers, and the cold air continues to surge south uninterrupted. When these two hugely powerful wind forces collide the result is, literally, electric.
The only other places on earth that have such a potent mix of warm and cold air are over oceans, such as the Indian Ocean, so we don't notice the violent storms and waterspouts there as much.
Hurricanes are different again. Hurricanes are actually the local name for a cyclonic storm, which are known by different names in different parts of the world; hurricane, tropical cyclone, and typhoon. The cause of these incredibly powerful storms isn't completely understood, but primary factors are the earth's coriolis force (which gives weather its spinning motion), atmospheric humidity, and high ocean temperature. Thus they only form in a narrow band through the tropics on either side of the equator.
Cyclones form right through this band both in the northern hemisphere and southern hemisphere, but most of the powerful cyclones are limited to two basins; the NW Pacific and the Atlantic. A theorised reason for this is a phenomenon called the Tropical Wave. This is caused by atmospheric disturbances in Northern Africa. The prevailing winds carry the hot, extremely dry air west across the Atlantic. This region, being subtropical and tropical, is normally very humid, and as the wave passes tropic air sweeps back in. This sudden change from dry to humid air creates very unstable atmospheric conditions, spawning violent downpours, thunderstorms, and the perfect conditions for creating cyclonic storms.
Where you find these factors on your own world, that's where you'll get this phenomena.
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