August 02, 2024

Why has it been so humid?

Yesterday it felt quite "sticky".   Humid and a bit uncomfortable.  Several folks emailed me about it, and social media had plenty of complaints (see samples below).



The best measure of unpleasant moisture in the air is the dew point temperature, often simply called the dew point.  

The amount of water vapor air can hold increases rapidly with temperature.  If you start with a sample of air with a certain amount of water vapor and cool it down, eventually the air will become saturated (100% relative humidity).  The temperature at which this happens is called the dew point.     

The more moisture in the air, the less you need to cool it down to get saturation.  

Thus, the dew point is a good measure of the moisture content of air.  Higher dew points mean more water vapor in the air.

When air is saturated, you can't cool off by sweating ---evaporating water from your skin.  And without that cooling mechanism, you feel warm when the temperature is high. That is that "sticky" feeling.

Most folks feel reasonably comfortable when dew points are under 60F.

They feel a bit "sticky" for dew points from 60-64.    Unhappy with dew points from 65-70.   And clearly uncomfortable for dewpoints above 70.    

The highest dew point I have ever experienced was 83F in Washington DC.    Just miserable.

So what were the dew points yesterday around the region yesterday?  

Below is a map of the dew point at 5 PM.  Every location shown was above 60F and many were in the upper 60s.  Some were EVEN IN THE 70s! (yellow colors).


If you look closely, the highest dewpoints  (70 and above, yellow colors) are on the western slopes of the Cascades and near Portland.

Around NW Seattle and Kirkland at the same time, many locations had dew points in the upper 60s.  Yuck 


The last few days have had the highest dew points of the year in our area.  The plot of dewpoint at the University of Washington documents the recent rise, and the much lower levels last week.


The "Trick" to Get High Dewpoints in Western Washington and Oregon

Believe it or not, it is quite difficult to get high dew points around here.   Why?

Most of the time our air comes from off the Pacific Ocean, which makes it difficult to get high dew points.   It is hard for air to pick up a lot of water vapor over the Pacific Ocean.

Are you shocked at this?   The reason is that the Pacific Ocean is cool (about 50F year-round in our area) and therefore the air can't pick up much water vapor above it.

Occasionally, such as during the most intense heatwaves, the winds are easterly (from the east) across the region.  This air starts above the warm interior and gets even warmer as it sinks on the western side of the Cascades.

Such air is dry because the region east of the Cascades is quite arid.

To summarize, below is a schematic of the situations described so far.  If air is coming from the west OR the east, it is hard to get high dewpoints.


So how can we get air with more moisture and thus higher dew point over western Washington?

I will describe the "trick".

If air comes over the moist vegetation of western Washington, western BC, and western Oregon during warm conditions, it is possible to pick up lots of moisture, driving the dew points into the 60s.

And the potential for high dew points is enhanced if it has rained recently.

And it DID rain a few days ago...mainly on July 27.  Here are some rainfall totals over the past week.  Substantial amounts, particularly over NW Washington and around Portland.


We can use model simulations to calculate three-dimensional air trajectories over time...in this case,  the 3-D path of air reaching the surface.  Looking at the air reaching SeaTac yesterday afternoon, it came from NW Washington (see below).


The air reaching  Portland was from the neighboring moist terrain.


I looked at large numbers of such trajectory figures...... they all agree..the uber-moist air was not from the ocean or the east, but had passed over the moist vegetated westside of our region.

The Future?

I have good news for those of you who don't like humidity:  the latest forecasts predicted a steady decline over the next few days into the mid-50s (see UW forecast below).


Of course, not everyone will be happy with this.



















11 comments:

  1. Isn't it high relative humidity that inhibits our sweat to evaporate, causing the air to feel muggy? Couldn't there be a "high" dew point temperature combined with an air temperature high enough to make the relative humidity low? In this case, the high air temperature might be uncomfortable, but the air would be relatively dry, and we wouldn't be covered in salty sweat.

    I always feel like just noting the dewpoint temperature alone misses an important part of the equation.

    ReplyDelete
    Replies
    1. You are correct....when the RH is at 100% sweating does not provide cooling anymore. The point is the high dew points make it more likely to have a higher humidity. Yes...if temperature is very high, even a high dew point will not prevent evaporatoin.

      Delete
    2. This is kind of an interesting comment. I'm a bit of a weather nerd and I don't think I've ever heard of an instance in which a "high" dew point temperature was associated with such a high ambient air temperature that the relative humidity was low. For instance, at an air temperature of 115F and a dew point of 70F, the relative humidity would be about 25%. I suppose perspiration would be effective at cooling us but I'd think most people would be quite uncomfortable and sweating buckets in order to keep their core temperatures down in such hot conditions.

      Delete
  2. On a micro scale, the top layers of large slow-flowing rivers may also warm up, providing a significant moisture flux. This may be the case for Portland along the Columbia River, and is likely a factor for Vancouver, BC, as Fraser River temperatures have exceeded 70F (21C); the warm fresh water easily floats above colder salty ocean water at the mouth of the river until dispersed by winds.

    ReplyDelete
  3. An off-topic note: I looked at the BPA balancing chart at 5:15 Friday. The wind contribution dropped mid-week as the heat built in the region. Sunnyside reports 0mph wind. Nuclear and thermal are steady. Hydropower is picking up as needed. Roll on Columbia!

    ReplyDelete
  4. Thanks for this informative and thorough post. You have discussed humidity and dew point very well in the past, but this current writing with illustrations makes everything abundantly clear. Much appreciated!

    ReplyDelete
    Replies
    1. My question is off topic, @GlacierBake, but I've been wondering where you get info for some of the comments that I see you post here. I'm always trying to find weather info for the Mt. Baker area (so we can go hiking up there when the weather is good) and since you live in/near Glacier, I thought you might know of some way to get more detailed info about Mt. Baker area weather (and a web cam link would be awesome).

      Last year, we spent an hour on the road before we realized that we couldn't see any mountains (where we would usually see them) because of smoke. Now, I know that I can find the Air Quality Index on Purple Air, but I had forgotten to check that before we left. I took a quick look at the PurpleAir app on my phone then and saw that the AQI at Artist Point was extremely awful. We turned around and headed back home.

      Anyway, it made me think about other ways to find conditions that would be relevant for hiking before we even get out of bed early in the morning, and I thought you might have some insight!

      Delete
  5. There seemed to be a temperature inversion yesterday: I went hiking along the 1-90 corridor, into the Alpine Lakes, but turned back at the first lake because of a thick layer of smoke at about 3000 feet. (Confirmed online, Alpental was over 100 AQI.) It was 61 in Snoqualmie Valley, then 63 at the trailhead about 2500 feet. Any connection to the high dew point?

    ReplyDelete
  6. Cliff, I am curious if you have any comments regarding the NWS change in zoning for western WA....pretty dramatic shift...might be worth a blog post: https://storymaps.arcgis.com/stories/ad23cf705e754f7ea7c6452e03a7d501

    ReplyDelete
  7. Awesome, this sort of article is why I keep coming back to learn more about the weather. It's helpful to have the science of this phenomenon given over so we can grok it.

    ReplyDelete
  8. Has the Pacific ever gotten warm enough offshore to add to the humidity?
    This doesn't seem that bad, just got back from Switzerland with temperatures of 87 degrees with 80% humidity from all the water saturated ground, that was really uncomfortable.

    ReplyDelete

Please make sure your comments are civil. Name calling and personal attacks are not appropriate.