July 27, 2022

Humidity Storm

Yesterday on the way home from the UW, I stopped at the local supermarket to buy some food and some cold beer.

But I could hardly see into the refrigerated cases:   they were completely covered with a thick veneer of condensation.  

I mean a LOT of water (see below).

Why was there so much water vapor in the air?  

Normally, the initial stages of major heat waves are associated with downslope flow on the Cascades that is relatively dry.

Then I got a few emails from folks in northwest Washington complaining about the humidity.

OK, I had to figure out what was going on.

The best measure of the absolute amount of water vapor in the air....the "stickiness" and discomfort.. is the dew point temperature.

This is the temperature to which you have to cool air down to for saturation (100% relative humidity).  For dry air you have to cool it down a lot to get it to saturation.

So dry air has low dew point temperatures.  And moist air has high dew point temperatures.  Here in western Washington, the dew point temperatures in summer are generally in the lower 50s.  Even lower dew points are typical in dry eastern Washington.

To figure out the situation, I plotted up the dew points today around the region and was shocked (see below)

For example, the dew points around 11 AM this morning were 70F in Bellingham and around Vancouver.  And well into the 60s around Puget Sound.

These are very high dew points for our region.  The only time they were higher in recent memory was during the big heat wave last June.    Very warm temperatures can evaporate a lot of water and cause high dew points, but our temperatures today, although warm, were not even close to last summer.

The highest dew points were over the relatively cool northwest WA/Vancouver area.

 Interesting, high-resolution numerical weather prediction models, like the UW-WRF model, predicted the high dew points over NW Washington (see below for 11 AM Tuesday, light green colors indicate higher dewpoints.)

So what was the origin of this humidity storm?

First, this heat wave did not have strong flow from the east like many heat wave periods.  Such flow would be drying, with an origin east of the Cascades and lower humidity from the warmth associated with sinking air on the western slopes of that mountain range.

But then I looked at the atmospheric moisture coming off the Pacific:  a plume of very high moisture values was circling around the high pressure offshore.  This is shown by the moisture and pressures/heights near 10,000 ft on Sunday as shown below (red and grey indicated high moisture values).  You can see the moisture moving around the high on the north side.  

This moisture rotated around the high and moved into Northwest Washington (see the 10,000 ft moisture Tuesday morning, below)

The air that reached the surface near Bellinghan and Seattle originate northwest of those cities and descended out of the moisture patch down to the surface, warming as it descended.    We can see that from the 3-D path of air reaching Seattle this morning (see below).  The top panel shows the spatial path and the bottom one shows the elevation of the air (later to the left).  The air was descending rapidly on July 24-25.

In summary, a  plume of moisture from deep over the Pacific rotated around the high and then descended to the surface.  And during the final few hours, it was over the moist western side of our region.

Well, after exploring this situation, I decided that I deserved some ice cream to help me cool off a bit.

I open my freezer....and I was stunned by what I saw....something that is associated with moist air being chilled to below freezing.    Life has its frustrations....


  1. Thank you for answering the weather question that has been on people's minds. I'm very sensitive to dewpoint and dislike when it exceeds 55F. Yesterday morning, I was out in the pre-dawn hours and felt that the DP was well over 60. This puzzled me, since I had assumed that this heat wave like most was associated with an inland High pushing desert air downslope into a thermal trough parallel to the I-5 corridor.

    Perhaps the spring was so wet on the Palouse that the air had some stickiness to it? But no, that didn't seem right. A July version of the Pineapple Express inherently makes a lot more sense. I appreciate you showing how that oceanic moisture blew around the High and hit us from the north.

  2. Just 5 minutes north of the border in Abbotsford, BC. Monday morning had a very heavy dew and was also extremely muggy as the day warmed up. Perhaps this was the effects of leading edge of the moisture plume before the moist air worked its way further south for Tuesday morning?

  3. Warm, moist air sounds like thunderstorm ingredients, but no storms. What more is required?

    1. You have to have a mechanism to start moving that moist air upward into an unstable atmosphere. At least for the time being, there are none around.

  4. Thanks. That is interesting stuff.
    KELN hit 103° with DP ~50 and RH ~16
    Herrbrahms mentions “the Palouse”.
    You have previously mentioned the wet spring in eastern WA.
    Two thoughts:
    I suspect the area has already dried; wheat harvest is underway.
    The topography and distance (~250 mi) between the Palouse and Puget Sound is not conducive to the former affecting the later.
    And to Franz: “What more is required?”
    I'll guess “lift”. If the air was “descended out of the moisture patch down to the surface” then the Convective Available Potential Energy – CAPE - measure would be low. (I used the NWS Glossary) I think that is how that works.

    1. I mentioned the Palouse because I was puzzled about the source of all the humidity and started brainstorming. I could have just as easily mentioned the irrigated fields around Moses Lake. Sure, there's some distance out to the Idaho border, but if the High was over the Canadian Rockies providing us easterly flow, the distance wouldn't have been too much of an impediment. Think of how mobile wildfire smoke can be in similar situations.

      I dismissed the notion after considering it because even with the wet spring and present irrigation, there's just not that much water in play. The reality of course is that the High is not in the right place to provide that easterly flow. It's on top of us like a clamp preventing uplift, or frankly much atmospheric mixing of any sort. I'm sure we'll start seeing NOX haze locally as the heat wears on.

  5. Wait, shouldn't the investigation have been about the atmosphere inside the store instead? After all, what kind of air conditioning system could they have had to allow that to happen? I've never seen a humid air conditioned environment. It's usually quite dry, since most of the moisture is now dripping off the AC unit outside. Was it on the fritz? Had there just been a power outage?

    1. I once worked in a small office building which had a central AC system which had been undersized to save money when the building was first constructed. Under high outside temperature and humidity conditions, if the temperature control was set below 82, the coils in the air handler would freeze up solid with ice. Both the room temperature and the room humidity would then rise as a consequence.

      It became necessary to put a lock on the AC thermostat box to keep the staff from setting the temperature below 82 whenever the outside temperature rose above 95. At least we had portable fans available to keep us cooler than we might have been otherwise.

    2. Depending on the temperature of the CW running through the cooling coils in an air handler, it's quite possible to have 'comfortable' temperatures, with uncomfortable humidity. At least in commercial applications, thanks to [needed limits] on energy usage, gone are days when HVAC systems might have both a 'Cold Deck' & a 'Hot Deck' - running simultaneously - with the cold deck, perhaps, supplying air with a temperature in the 40's-! and the Hot Deck tempering the air back to a temperature making the correct setpoint in the space. That massive cooling, coupled with re-heating is capable of removing vast quantities of moisture from the air, producing cold and very dry air - often too dry.

      But, oh man oh man, for those of us old enough to remember it... on a very hot day, walking into a building employing this system (energy usage be damned) and to feel that incredible wall of exceptionally cold and dry air - it was as if you walked off the Sahara Desert and onto an ice floe somewhere in the Arctic Ocean-!

      Yes, less sophisticated systems simply working at "100% output" can produce cool, dry air - but, they are becoming more and more the odd man out these days and going forward.

  6. I just got back from a couple of days of hiking at Nason Ridge which is to the east of Steven's pass. As I was driving through Monroe early Monday morning, It was fairly foggy, which seemed unusual for late July. It was quite warm (90 -100) where I was hiking, but not too oppressive. When I got back to my car at the trailhead, the thermometer registered 102, which while hot didn't seem that bad. When I got back to Seattle however, the temperature was about 95 and oppressive, due in no small part to the greater humidity. This posting affirmed what I observed with your dew point measurement.

  7. Looking at the sea surface temperature plot from U of Maine it looks like the north Pacific is above average with a big hot spot just about where the moisture is coming from with deviations above 5 degrees C. I'm assuming that's the source?

  8. Wouldn't the Wetbulb Temperature be more appropriate as a measure of discomfort for humans since it is more analagous to the sweating and how sweat cools the human body? I know it is less common to have the value included in a weather briefing (and somewhat similar to dewpoint) but they are not exactly the same.

    1. the dew point temperature is a measure of moisture in the air. The wet bulb brings in temperature as well. Thus, I like dew point for its simplicity.

  9. I'm writing from Winthrop (Methow Valley, east side of the Cascades). The air around us is warm and fairly dry so we seem to be insulated from the moisture plume your post describes. But the air is very hazy. I am not aware of any fires around. I know that the ridge of high pressure over us traps the air near the surface so that particulates in the air accumulate rather than getting blown away, but can you tell us more about the nature of the haze and where the particles that cause it come from? Maybe something for your Friday topic of the week? Thanks!

    1. You can get large-fire information from the link. There are no large fires in WA at this time.

    2. https://wasmoke.blogspot.com/
      You can adjust the visible layers setting in the upper right corner to see the current smoke plume. Today it's covering at least half the U.S.

  10. Wasn't a great time for the condensate drain on my air handler to become plugged up. What a mess!

  11. Does the high dew point pressure explain why the air feels so heavy and it feels so hot even though the temp right now is only 83. It feels much hotter to me. I just got home from running errands on foot and could barely make it out there. Even earlier in the day when the temps were still in the 70s it felt much hotter than a typical 75 degree day. All I know is this is feeling almost as unbearable as the heat dome, not quite but still pretty bad.

  12. Wondering how much tides play into this? We live on the coast & watch winds change direction during tidal change. So fascinating. Incoming tides often bring fog with it. When it is warm inland we often get heavy fog.


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