Being in the northeast today with the unbelievably humid weather got me wondering… why is the east coast humid and the west coast not? I know nothing about weather, but have deduced this much - both are next to oceans, both have mountain ranges to hold moisture… the airstream moves from west to east, but would that, could that be a reason that it’s not humid there?
It can get humid on the West Coast, but that requires a large mass of tropical air coming up from Mexico. That happens a few times a year.
But I believe it’s those ocean breezes that keep the West Coast relatively dry.
Living in either the San Fernando Valley or San Gabriel Valley I can only recall a few days in my entire life where it’s felt like a typical summer day in St. Louis or even Boston.
Then again, we don’t get much thunderstorm activity out here, except in the winter.
Ahh, the benefits of having a mediterranean climate.
Well, specifically it’s the cold water current ocean breezes that keep the West Coast less humid. When a high pressure system over the Four Corners area, or a low off the Baja coast sends air that’s picked up moisture from warm ocean, it can get very humid in Southern California.
We also have deserts out here, and if the wind comes from the east or northeast, it gets very hot and dry. These are the Santa Ana winds, of Babylon Sisters fame.
The East Coast gets a lot of wind from the Gulf of Mexico and the Atlantic Gulf Stream, both of which are warm water currents.
And then there’s the Midwest. Ugh - I spent a summer in Terre Haute, IN and the heat and humidity there were worse than any summer I spent in GA, SC, or FL.
I spent a summer in San Francisco - I remember that being slightly humid at times, but not nearly what the east coast has. I also lived just outside of L.A. for two years, & though it certainly got damn hot, I remember it being a dry heat. It has been ten years, of course…maybe the weather has changed. But I also remember the fog/smog that would sit over L.A. - there certainly didn’t seem to be a breeze on those heavy smoggy days - & I don’t remember humidity. New York has an ocean, but there is no breeze that can help the humidity in NYC. Is the Pacific Ocean that much colder than the Atlantic that it would effect humidity to that degree?
I do often miss the California weather, though I think I’d miss the change of seasons here in the northeast more.
No, the whole Pacific Ocean isn’t colder, just the specific currents that approach the coastline at that specific region. Ocean currents have a lot to do with the climate of land terrain near them.
Refer to this map of major ocean currents.
As you can see, the East Coast of the US (and the West Coast of Europe) is warmed by tropical waters brought up by the Gulf Stream. These waters warm the air above them and evaporate more easily into the air, creating more humidity.
(This can’t be the only source of east coast humidity though, since places like Indiana and Michigan get pretty damn muggy in the summers, too. But that part of North America is naturally well irrigated.)
The US West Coast gets cold water from the Gulf of Alaska brought down by the California current. This water cools the air above it and cold air can’t hold as much moisture. When it comes ashore and warms up, it feels less humid.
Now, when air circulation patterns bring air over California and Oregon from some other place, like the Gulf of Baja, then we get conditions that are as humid as anyplace back east. In fact, we’re expecting these conditions to start up again tomorrow. The real irony is that the deserts get the worst of that humidity!
Do you mean muggy? Humidity is just a measure of the concentration of moisture in the air. San Francisco routinely has a humidity of more than 80%. The only way the humidity can be higher is when it rains, in which case the it is 100%.
Are you sure about that 80% figure being the maximum without rain? I’ve heard people talking about the humidity here in Flint (MI, that is) being 100%. Holy Hell! The past week and a half has been torture in these parts. 92f isn’t bad in new Mexico, but next to the Lakes, its hellish.
At least, I assume its the Lakes that make it humid. They sure are big and wet.
askol
Yeesh, these people actually expect work out of me! The nerve! 
Hmmm… I don’t see where I posted 80% as being a maximum before one can say “It is raining.” I do know that whenever I look at the weather station at school on rainy days it lists the humidity as being 100%. Perhaps that last sentence was misleading. (Cursed deadlines!) I was refering to the weather patterns in S.F. only.
To clarify: San Francisco has lots and lots of foggy days, with varying degrees of fogginess ranging from cool and slightly damp to large-drops-of-water-dripping-off-of-everything-good-old-fashioned-pea-soup-fog. Thus, the humidity on foggy days ranges from the low end (I think it’s somewhere around 80%) to almost raining (over 90%.) For example, it is currently somewhat clear and cool in S.F. with a humidity of 83%. Since humidity measures only the moisture content of the air (not temperature + moisture) the foggy days can be quite chilly.
Of course, if one is being dripped on, it makes little difference if one calls it fog or rain.
Though I find the forecasts laughable in their inaccuracy, I often do watch the weather channel for the forecast, such as it is. Last summer, which was rainy even by New England standards, it did get to 100% without raining, but, even more odd, it got up to 105% on rainy days on at least 2 occasions…so the air can be supersaturated, then?
I don’t know if there’s any truth to it, but I’ve always thought that the difference in humidity was at least partly controlled by the types and amount of foliage. Where I live, as opposed to say LA, there are trees every where. People just clear out enough trees on their lot to put a house, and leave the rest, so while not nearly as canopied as say a rain forest, it’s a lot more than a city would be…