Answers to a weather question!

Matt, I submitted the question of “Why there was so much difference in humidity between San Fransisco and NYC

Some Telltale readers may find this interesting.


Dear Charlie Wilson,

Thank you for your recent question.

First, it’s important to realize that west-coast locations such as San Francisco aren’t always dry, and east-coast locales such as New York City aren’t always humid.  San Francisco has plenty of mornings with fog and high humidity, and New York City is typically fairly dry during the winter.

However, for much of the year, especially during the summer, you’re correct at noticing that San Francisco often has fairly dry conditions and New York City is often humid.  This is mainly due to two factors: evaporation and transpiration.

Evaporation is a change in the state of water from liquid to gas (water vapor).  More evaporation means more humid air since it adds water vapor to the air.  Transpiration is similar to evaporation, but it refers to water vapor introduced to the air by plants.

Along the Atlantic coast, warm waters flow from south to north thanks to an ocean current called the Gulf Stream.  These warm waters naturally warm the air above them and introduce moisture (water vapor) into the air through evaporation.  On the other hand, along the Pacific coast, an ocean current brings cool water down from the north, which cools the air above it.  Some evaporation occurs here, but at a certain point (the dew point), the air reaches saturation and cannot take on any more water vapor.

Along both coasts, as winds blow the moistened air inland, solar radiation (energy from the sun) warms the air.  Along the east coast, since the air is already warm, it doesn’t warm up too much more, and with the moisture content staying the same, the air remains moist and humid.  However, along the west coast, the cooler air may warm quite a bit, and with the moisture content staying the same, the relative humidity decreases.

Transpiration adds additional moisture (water vapor) to the air along the east coast since there are so many plants, especially leafy trees and crops, that are present.  Transpiration is much less of a factor along the west coast since there are generally fewer trees and crops in this part of the country.

Please contact us again if you have any questions or comments.


Corey Davis

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