Weather Blog

Seattle really one of the most humid cities in the U.S.? You bet!

Seattle really one of the most humid cities in the U.S.? You bet!
Photo: Kathleen Higgins

It began as an innocuous quote inside a story in the New York Times Wednesday about new technologically-advanced homes in Seattle that do wonders for efficient climate control, and really didn't have much to do about our city's climate in general (emphasis is mine):

"When you visit Sloan and Jennifer Ritchie’s new passive house in the Madison Park neighborhood here, it takes a while to notice all the things you’re not hearing... There is no furnace or air-conditioner clicking on or off, no whir of forced air, and yet the climate is a perfect 72 degrees, despite the chilly air outside. Then there are the things you’re not feeling. In one of the most humid cities in the country, you aren’t sticky or irritable, and the joints that sometimes bother you are mysteriously pain-free."

The claim that Seattle is one of the most humid cities touched off a mini fire storm (OK, so it was a few Tweets and a few comments on the article itself, but they're a vocal few) from Seattleites defending our turf that there is no way we are more humid than the East Coast and the article's author clearly doesn't live here.

But what if I told you that she was right? Seattle is in fact one of the most humid cities in the United States. According to currentresults.com, Seattle ranks 7th nationally among major cities, even a tick ahead of Miami and just a few tenths of a percent behind other big Florida cities. (I dare you to go tell your friends Seattle is officially more humid than Miami -- might even win a bet or two.)

Here's the Catch:

The catch is there is a big difference between relative humidity and another measure of moisture content: the dew point.

A lot of people think humidity is a percentage of water vapor in the air, but it's actually more complex: It's the ratio between water evaporating from liquid water to water vapor and condensating from water vapor back to liquid form at any given time. The lower the humidity, the easier time a puddle has evaporating into vapor. At 100% humidity, the air is saturated and a puddle won't evaporate.

But warmer air can hold more moisture than colder air. In other words, if you have a bottle of saturated air at 55 degrees, it will have less moisture inside the bottle than one filled with 75 degree air at saturation. But both bottles would have 100 percent humidity.

Dew Point Guide for Seattleites:

<46 degrees: Hot and dusty (For Seattle, likely a thermal trough situation with that dry, easterly wind)
46-53 degrees: Typical and comfortable
54-56: A hint of mugginess, but still not bad.
57-58: Starting to feel pretty muggy
59-63: Yuck
>63: Break out the plastic flamingos

This is where the dew point comes in -- the temperature at which air becomes saturated. A higher dew point indicates the air has more moisture in it and will feel more sticky to a person, even with identical humidity to a cooler temperature.

Seattle's No. 7 ranked-humidity yet lack of "stickyness" comes from our cooler combination of lower temperatures and dew points.

For example, on Wednesday in Miami, there was an afternoon temperature of 90 degrees with a 73 dew point -- a relative humidity of 58 percent.

Meanwhile in Seattle, it was 72 degrees with a 55 dew point that afternoon -- a relative humidity of 55 percent. Nearly the same humidity, but way more sticky and uncomfortable in Miami than in Seattle because there is a whole lot more moisture content to the air.

The higher humidity makes it more difficult for your body to cool itself by sweating, which is a problem when it's 90 degrees out in Florida, but not so much when it's generally in the 50s, 60s and 70s around here.

Our predominant pattern is to have air come off the Pacific Ocean that has water temperatures in the 50s. So our high humidity readings come on our frequent days when both temperatures and dew points are in the 50s and not really an issue for your body trying to cool itself off. (And on our hot summer days when we do get into the 80s and 90s, it's usually due to a dry, desert-like east wind that drops the humidity well into the 20-30 percent range.)

And Old Spice agrees, consistently ranking Seattle near the bottom of the list of America's sweatiest cities despite our high average humidity, while at least in 2006, Miami came in at No. 13.

So the author is correct in that Seattle is among the most humid cities in America, but the Seattleites' angst is also correct that it rarely makes us "sticky or irritable" (it's the 3 weeks straight of rain that does that.)

P.S. Pretend Thursday's weather isn't happening

There was some irony that the day this blog was written, Seattle was in the midst of a rare muggy day with both high humidity and dew point. In the late morning hours amid a steady drizzle, the temperature was 64 with a 63 dew point -- just 3 degrees off Seattle's highest dew point reading of the last 15 years (66) -- with a humidity of 94 percent. It's not Miami muggy, but certainly to the point that those not living in the homes featured in the NYTimes article could well indeed be feeling sticky and irritable.