Weather Blog

Monday was Seattle's most humid Sept. day ever

Monday was Seattle's most humid Sept. day ever

Does it feel like we're in the wrong time zone this week? Some mentioned the muggy weather has had the feel of Hawaii. Others said it was like "I was back home in Atlanta."

Well, you were both right.

A long fetch of tropical moisture that originated near Hawaii rode an "atmospheric rain train" into the Pacific Northwest on Monday. Typically we refer to these systems as a "Pineapple Express" due to their near Hawaiian origin, but the change this time was that the plume of heavy rain instead went far to our north into Central B.C. The rain has since passed, but the jet stream is still sending the tropical plume of moist air our way. Check out this illuminating graphic from the UW forecast models, courtesy of Cliff Mass' excellent weather blog:

But that pattern occurs a few times every fall and winter (it is quite early this year), but it never seems that humid? Part of it is that we are still in the warmer part of the year so temperatures are a bit warmer. But the other facet to the sticky madness was that we had a strong upper ridge of high pressure move in over the top of us. Higher pressure brings sinking air and that effectively trapped the moisture near the surface. As Paul Deanno wrote in the morning weather discussion: "This is similar to turning on a hot shower and closing the bathroom door for hours, as the humid air is trapped and has nowhere to go."

How humid was it? Record levels.

The average dew point temperature at Sea-Tac was 63 degrees on Monday, reaching a peak of 66 degrees. That was the highest dew point average ever recorded in Seattle in September, smashing the old record of 61. The normal dew point for this time of year is right around 50.

A dew point of 66 is more on par with the Midwest and Southeast. To compare, Atlanta on Monday had a nearly identical temperature and dew point set up as Seattle. Yuck! And on Tuesday morning, we were even worse! Our morning dew point was higher than Houston, New Orleans, Atlanta, San Antonio, Dallas, and Tallahassee, Florida.

Also of note, humid air keeps it warm at night as well, as the water vapor keeps in heat, plus temperatures can't fall below the dew point as when it reaches that dew point temperature, it just becomes foggy. (The definition of a "dew point" is the temperature at which the air becomes saturated...i.e., 100% humidity.)

The low temperature Monday morning was 62, making it the warmest fall morning in Seattle history, breaking the old record of 60. Tuesday morning dropped to 60 degrees and thus would have tied the record had it not been for Monday.

Why does it not normally feel muggy here?

So you might be wondering: Why is it that despite the fact that we live near the Pacific Ocean and Puget Sound, and we have generally high humidity, that it doesn't normally feel as humid as it does in the Midwest and East Coast?

While officially, the dew point is the temperature at which the air becomes saturated and 100% humidity, it can informally be used as a comfort meter. Around here, our summer dew points are generally in the 50s, while in the Midwest and back East, they can get into the 60s and even 70s. Warm air can hold more moisture, so air at 55 degrees and 100% humidity doesn't hold as much moisture as air at 70 degrees and 100% humidity.

But air temperature also plays a big part of it. Your body doesn't really feel the humidity until the air temperature gets over 70 degrees or so. So, for example, on a typical summer morning, the dew point in Seattle might be 55 degrees. The humidity will be high in the early morning because the morning low was likely close to 55 degrees -- but since it's cool outside, you don't notice it as much.

But as the day warms up and we get into the 70s and low 80s, the humidity will drop because we'll get further away from the dew point, and it won't feel as humid. (There's a second factor here I'll bring up in a second.) An 84 degree day with a 55 degree dew point is actually quite comfortable.

Now, to contrast, if you have a dew point of 75 and an 84 degree day, that feels much more miserable as the air is holding a lot more water vapor.

Why humid air is so uncomfortable?

The reason humidity makes it feel hotter is it makes it harder for your body to radiate heat.

The body cools by sweating, which then evaporates into the air. The warmer molecules in those sweat droplets evaporate first, leaving the cooler ones behind and, in turn, make you feel cooler. If there is more moisture in the air, it's harder for water to evaporate, so it's harder to cool off.

Northwest is lucky to have muggy days be so rare

We are incredibly lucky with our pattern here that it's either cool and humid or hot and dry.

Most of the time, the air is flowing in off the Pacific Ocean, whose water temperatures are typically in the 50s. So while it is indeed moist air, the dew points are in the 50s and in the comfort range. But those ocean breezes also keep the temperatures cooler, so humidity isn't much of a factor anyway because your body is not warm enough to need to sweat.

To get a hot day here, we need the winds to come from the east to push back the cool, ocean breezes. But those east winds come from the desert-like Eastern Washington. As an added bonus, that air sinks down the western slopes of the Cascade Mountains. As air sinks, it gets warmer and drier, so our hot days here are typically very dry -- just like the desert heat of southern California and Arizona. Dew points on our really hot days can be in the 40s or lower. In the desert, sometimes dew points can get to singled digits. There was a day this summer where Las Vegas had a 102 degree temperature and -2 dew point -- 2% humidity! I think that's too much the other extreme as skin would dry out.