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Why is the Eastern US more humid than the west?  RSS feed

 
pollinator
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I am visiting my family on the east coast in DC and it struck me that I am baffled by why it is more humid on this side of the continent. I feel like I understand west coast weather patterns alright, and virtually all our moisture comes off the ocean. As I understand it the prevailing winds at our temperate northern latitudes (Northern California is similar to DC) are westerly. If we get anything from the east it’s hot and dry from inland deserts. We get fog fairly often (ie San Francisco has nothing on the heart of the Redwood coast), but if it’s over 65 it’s almost always less than 50%rh. So the coastal nw US could be summed up as summer dry warm (not hot) and the winter wet and cool (not cold). 90% of rainfall is in Oct-June most places I’ve lived in the west coast.

It seems the same latitude on the east coast would be summer hot and humid and winter cold and dry, but much more evenly distributed rainfall than the NW. Is the east coast summer humidity due to the westerly winds coming off the deciduous forests inland? This would explain drier winters. Are winds in the east coast more variable as well? I guess it’s not that simple, but can a get some meteorology up in here? Thanks for any help in my unbefuddling.
 
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It is from the confluence of the jet stream and the currents of the Atlantic.

We not only get any of the storms traveling from west to east across the country, the currents swirling from the warm waters of the Atlantic near the Caribbean, draws upwards towards the cool waters of the North Atlantic, swirling up along the coast as it does so. You can actually see it in the shape of the coastline.

This can sometimes combine with low pressures over Canada that push any storms downward too, called an "Alberta Clipper" that can combine with other storms if the timing is just right. The movie "the Perfect Storm" had this, (3) storms converging off the coast of Maine and Nova Scotia causing incredible seas that took many lives.

...

Not asked, but given...the drawing effect of warmer water to colder (remember the laws of thermal dynamics here) are dramatic because the Gulf of Maine is incredibly deep...for a bit. As you go way out to sea, there is a massive rise, creating a toilet bowl effect. It is below the waterline, but not by a lot proportionally speaking, so the Gulf of Maine water is "trapped" so to speak, and not much water mixes with Caribbean heated water, so the water here is incredibly cold even in the summer. This affects weather. It is why hurricanes die out as soon as they get here...that cold water puts the storm out so to speak. Of course that "cold" water helps keep temps where I live close to the coast, cooler in the summer, but that same mass being warmer (50ish degrees) helps the temps along the coast to be "warmer" in the winter. The temp differential is about 10 degrees.
 
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Forests ?
 
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Yep, exactly what Travis said, above. When I lived in Massachusetts, I remember the weather would variously come up the coast with heavy rain or sometimes hurricanes, or down from Canada with heavy snow or cold snaps, or maybe other directions too. I lived on Cape Cod where the warm Gulf stream just barely reached before heading off to the British Isles, so we'd get a lot of humidity.
 
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The West coast has low humidity because of the cold northern current running just offshore.  The warmish air from the interior of the Pacific Ocean flows eastward over the narrow strip of cold water flowing south along the west coast, cooling the air and precipitating the excess moisture out in the form of fog.  This cool foggy air then  comes onshore, passing over the warm/hot land where its temperature rises and its humidity drops.  This combination of cold, then hot, functions just like a dehumidifier.  The east coast has tropical humid air moving up from the gulf of Mexico during the summer combined with all the moisture transpired from the broadleaved deciduous trees during the summer.  The humidity here in SC really climbs in spring once the deciduous trees have leaved out in the spring.
 
Travis Johnson
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I never realized how "cold" Maine really was until I went elsewhere, like Minnesota and their winters. Yes it gets the same temperature like -20 below zero (f), but there I noticed it felt warm because it was a "dry" cold. In the summer they were humid because their 10,000 lakes were open and pumping out humidity, but when they froze over, a dry cold winter fell over the land.

NOT HERE!

Here, with the Gulf of Maine driving in salt sea air, it cuts right through any clothing, and it feels cold. To understand it, you have to be on the ocean where you have clothing washed in salt water, or fresh water; there is a huge difference in warmth. Naturally, on the water it is only a matter of time before sea-spray gets into your clothing and it is impossible to stay warm. In Janurary, even harder.

I never was a fishermen, but my first wife was. her father had a house out on Criehaven (Ragged Island on the maps), some 28 miles out, and it was interesting life, like finding little lobsters like this...



12-Pound-Lobster.JPG
[Thumbnail for 12-Pound-Lobster.JPG]
 
Ben Zumeta
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Thanks all, this is starting to make a lot more sense. It seems now the better question may have been “why is the west coast so dry in the summer?”, as this is a more novel climate pattern (<5% of the earth is summer dry winter wet). But we got a great explanation of why that is already with the dehumidifier analogy above, thanks all!
 
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Travis Johnson wrote:I never realized how "cold" Maine really was until I went elsewhere, like Minnesota and their winters. Yes it gets the same temperature like -20 below zero (f), but there I noticed it felt warm because it was a "dry" cold. In the summer they were humid because their 10,000 lakes were open and pumping out humidity, but when they froze over, a dry cold winter fell over the land.

NOT HERE!

Here, with the Gulf of Maine driving in salt sea air, it cuts right through any clothing, and it feels cold. To understand it, you have to be on the ocean where you have clothing washed in salt water, or fresh water; there is a huge difference in warmth. Naturally, on the water it is only a matter of time before sea-spray gets into your clothing and it is impossible to stay warm. In Janurary, even harder.

I never was a fishermen, but my first wife was. her father had a house out on Criehaven (Ragged Island on the maps), some 28 miles out, and it was interesting life, like finding little lobsters like this...



its even different from north to south in Maine. I'm as far north as it gets and our winters here are bone dry because we don't have the gulfs effects here. i can stand outside all day in -20f w/ the right gear. not so along the coast even if it doesn't get down to - temps!
 
pollinator
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The east coast get the
1)'hurricane' tropical gulf stream weather systems.
2) Canada cooler weather system
3) Moisture from the tornado alley (think monsoon from gulf of mexico, a separate/parallel system from the gulf stream hurricane)
4) Any moisture coming from monsoons from Arizona
5) Any moisture left in the dry desert air coming from the West Coast (Pacific Ocean)
6) Most importantly a 'close-loop' plant>transpiration>air>condensation>dew/rain>plant, why plants are making half of there own recycled rain independent from the ocean/sea driven weather systems
 
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Respective of nothing to do with the topic of this thread, that is one enormous lobster, Travis.  Impressive doesn't begin to describe it!
 
Travis Johnson
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Marci Sudlow wrote:Respective of nothing to do with the topic of this thread, that is one enormous lobster, Travis.  Impressive doesn't begin to describe it!




Mainers throw ones like that back!

I am actually not joking, but not for the reason most people would think. Big lobsters like this make agrressive breeders so any lobster over about 5 pounds has to be tossed back by law. So are lobsters that are too small. So are lobsters with eggs. Those are marked so they never will be taken again.

All this ensures Maine has a Lobster Industry years from now.
 
Ben Zumeta
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I asked why fisheries don’t set salmon max sizes, and got laughed out of the fisheries biologists’ meeting for NW California. Despite this following the head biologist pointing out how size is exponentially correlated with egg production, I guess they figured it was too much to ask of “sportsmen”.
 
Travis Johnson
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Ben Zumeta wrote:I asked why fisheries don’t set salmon max sizes, and got laughed out of the fisheries biologists’ meeting for NW California. Despite this following the head biologist pointing out how size is exponentially correlated with egg production, I guess they figured it was too much to ask of “sportsmen”.




It does work, but not always well. For instance these laws only apply to Maine and not the other states and providences that have lobster too. This has created a problem because if a trawler is fishing in the Outer Banks, say for Cod, they might take on lobsters as an incidental catch. Because of our oppressive lobster laws, they will take their catch to Mass or Nova Scotia where the size limits are much more lienient for lobsters. This has pretty much made Portland's fish market non-existant now.

There are a few things to glean from this; because lobster is almost Maine's identity, we are protecting our resource vigorously. The other thing is, Maine lobster is truly sustainable.
 
pollinator
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As to the topic of the thread, Travis nailed it. But there's one answer I haven't seen.

Why is the Eastern US more humid than the west? Because there's more water in the air, of course.

-CK
 
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