I grew up in Boston which has high humidity. I now live in San Francisco which has low humidlity. Why?

Question

Both Boston and San Francisco are surrounded by water. Why does the humidity vary so widely?

Answer 1

Boston? Humid? Come visit Savannah, GA and everything else will feel like a desert

Relatively though, it probably has to do with the wind. If the wind is blowing from the ocean to the city, it will bring more water vapor with it. Likewise, when the wind goes over land before the city, it doesn't have a chance to pick up as much humidity. On Wikipedia it says that the west coast on average is less humid than the east coast.

But, seriously, if you want to feel humid, visit the Southeast.

Answer 2

The adjacent ocean waters there at San Fran are much cooler, as is the air in contact with the water.  That cooler air cannot hold as much moisture as the warmer air that you saw in Boston.  Thus with less moisture in the air, the humidity is likewise lower.

Answer 3

I grew up in San Francisco all my life and yes the humidity is relatively low through much of the summer. That is because the California Current that runs down our coast brings cooler water from the Gulf of Alaska. In addition to cooler water when we have a heat wave winds blow offshore, further drying out the air. Dew points get as low in the 20 range and humidity can be in the single digits. However, during mid July it does get humid because the southwest monsoon will occasionally have Gulf Surges which brings higher dew points at the surface. Dew points in San Francisco can reach the low to mid 60's and it can be quite muggy. Thunderstorms will occasionally break out across our area and they usually occur late overnight but they can happen anytime of the day.

Answer 4

Cswx may be a top contributor and a meteorologist but his/her answer is not enlightening. A few clarifications:

"Humidity" in this case is not a measure of the amount of water in the air; it is a RELATIVE measure of the amount water vapor the air could hold AT A GIVEN TEMPERATURE.

It sounds as if cwxz... is saying that SF is surrounded by cold water and Boston is surrounded by warm water. This does not seem likely. Boston is much colder in winter than SF. I've been there and frozen my tush off; although I did not measure the ocean temperature, I doubt that it's ever very warm.

Your statement that SF has low humidity is not right, either. SF has a lot of fog and is often at 99 percent condensing. SF's relative humidity (and the reported humidity is always relative) is rarely below 50 percent, virtually never dry.

So I question the premise of your question. If the question were "Why do Boston's summers FEEL so much more humid than SF's?" (a valid premise) then the answer would probably be something to the effect that Boston experiences a lot of hot humid air that moves off the land toward the east, cooling as it approaches the Atlantic, which increases the RELATIVE humidity. Conversely, in winter, Boston is much colder and gets a lot of cold dry air from the land mass to the north and west, whereas SF is more regulated by its marine environment; thus Boston would be expected to have greater extremes in its relative humidity.