July 23, 2018

Few Air Conditioners in Seattle (and western Washington): The Dry Facts

Today, one of my favorite Seattle Times writers (Gene Balk) had a fun story about the lack of air conditioners in Seattle.  Turns out that our ownership of air conditioners is noteworthy:  no major city has less AC per capita.

Much of the article is fine, noting our mild temperatures, proximity to water,  and cooling at night...all important factors in our modest AC count.

But there is an critical fact that he doesn't mention.   Our low humidities.   Whenever it is hot around here our air is dry.  Which makes us far more comfortable.

The relative humidity of the air is critically important for comfort because evaporation of sweat is an enormously effective way to lose heat.

Humans are unique in our ability to sweat from our skin.  No other mammal other than horse can do so effectively....and according to what I read, horses sweat differently than us (mainly in their armpit area).   Even our closest relatives (the great apes) use another mechanism...panting....as do our pets.   With millions of sweat glands and naked skin.....we are the superstars of sweat.  And evaporation of water requires huge amounts of energy (heat), and thus is an enormously effective cooling mechanism.  So if a human has a large supply of water, he or she can handle very warm temperatures....but there is a catch.

For sweating to be effective the water must evaporate from our skin and that only occurs when the relative humidity is below 100%.  The drier the air, the better we can evaporate and cool.  So 85F and 35% relative humidity is far more comfortable than the same temperature and 95% relative humidity.  The National Weather Service even has an equation to express this, which they use for their "Heat Index" (see below)

The southeast U.S. is miserable during the summer, not because they are crazy warm (it rarely gets above 100F there) but because the humidity is so high.  Why?  Because the air had been over the warm Gulf of Mexico and the amount of water vapor air can pick up depends on temperature of the water and the adjacent air.  And the Gulf is very warm (80s to 90F)

Here is Seattle, our summer air is nearly always quite dry.  Most of the time, our air is coming off the cool Pacific Ocean.  That keeps the temperatures moderate, but also prevents the air from picking up a lot of moisture--since cool air, even if it is saturated, can't pick up a lot.

The number one measure of moisture content of the air is dew point, the temperature to which air must be cooled at constant pressure to produce saturation.  Drier air has a lower dew point.   Our summer air typically has dew points in the lower 50s... over the eastern U.S. dew points in the 70s are common.

Dogs pant, we sweat
And when this relatively dry (low dew point) air moves inland it warms, and the relative humidity plummets.  Why?  Because relative humidity is the ratio of the amount of water vapor in a volume of air, divided by the max. water vapor that volume can hold--and that depends on temperature.  Warmer air can hold more water vapor than cooler air.  So warming air, allows it to hold more, which makes relative humidity plummet.

Our warmest temperatures...our real heat waves... occur when air comes from the east.  That is when we get into the mid-90s and more.  But that air is very dry.  First, there isn't much of moisture source east of the Cascades...the land is arid.  And as the air descends the Cascades, it warms by compression, which causes the relative humidity to fall.

Enough theory, let's check out what happened the last three days on top of my department!  (see plot below, time increases to the right).  The third panel shows temperature (black line) and dew point (red line).   Our dew point was very modest....around 50F...about the same temperature as the ocean offshore.   Temperatures swings up and down each day, with a slow overall rise as we have warmed.

But look at the fourth panel...relative humidity.   Huge gyrations... very low (about 30%)when it is hot later in the afternoon , but 70-80% when the air temperature is lowest at night (55-60F).

And the low humidities have another benefit...they allow our nights to cool.  Water vapor is the most potent greenhouse gas, which means it traps and reradiates infrared energy....slowing down the night time cooling.  That is why deserts can get cold at night, even when they are hot boxes during the day.

Want proof?...Here are the minimum temps last night. In some lowland locations temperatures dropped into the upper 40sF!  Those folks needed a blanket.

So sweating is virtually always effective around here, unlike for those poor folks in the SE U.S.  Just make sure you stay hydrated.   Don't sweat enough?  Take a cold shower or run through a sprinkler.  A wet tower in front of a fan can cool things down.  And air out your home or apartment at night to prevent the place from progressively warming up.

"The S is for SuperSweat"


  1. The challenge around here is often the process of getting hot air out of a home and replacing it with cool air at night/early morning.

    Our condo is a good example. In our living room, we have huge picture windows - but just one smallish window that only opens (at an angle) a foot or so. We have bigger window openings in our bedrooms, but all of the windows in our unit face west, which makes it difficult to get a cross-breeze going that can help to push out the hot air.

    We've got a protocol that we follow every night involving cranking up our bathroom and kitchen range fans, and helping things out with a couple of portable AC units. But we'll definitely think twice about buying another condo with a "modern" design that treats windows and air circulation as afterthoughts.

    I'd like to meet somebody who designs these kinds of buildings for a living and ask them: What are you thinking? Eventually, given hotter summers in the future, buildings like ours will have to permit residents to install ductless AC units - in our case, a $20K investment. A less brain-dead design approach wouldn't eliminate that need completely, but it would make it far less common, and probably buy a few extra years for those folks who will still need to install permanent AC units.

  2. "Humans are unique in our ability to sweat from our skin. No other mammal can do. Even our closest relatives (the great apes) use another mechanism...panting....as do our pets."

    That's not even close to accurate. All primates sweat for cooling (particularly from the apocrine glands in the armpits), although yes, among primates humans are most efficient at it and are unique in our abundance of eccrine glands. However, apes do not pant to cool, that's just flat wrong. All mammals sweat in some way, but among other mammals, horses also rely on evaporative cooling and can be nearly as efficient as humans. This is because of the presence of the protein latherin in the sweat, which acts something like a detergent to move the sweat through the hair to the surface.

  3. Frankly, I am stunned that horse people are not trolling you into the ground over your error about the human uniqueness of sweat thermoregulation. :-)

  4. "Humans are unique in our ability to sweat from our skin. No other mammal can do."

    That's not true. Horses (for one) sweat from their skin.

  5. Horses sweat too, although I think that's it in Mammalia.

  6. Cliff,

    How about an explanation why the current heat wave, 7 days or so, is occurring. As a lifelong area resident I am used to the natural "air conditioning" kicking in after two or three warm days. I've heard this was due to a surface heat low forming in the interior which allows offshore surface high pressure to invade Puget Sound through the gaps. What's going on this week? Are these extended heat waves becoming more frequent?

  7. Folks...I have a lot of comments about horses sweating. OK... horses do sweat...but in a different way from humans and much less effectively through humans...at least from the stuff I read, like this:


    Horses sweat from glands in their arm or leg pits, according to these references, and still use panting for much of their heat loss. OK...I am no horse expert. But one thing is for sure...HUMANs depend on sweating and are the best at it of any animal...cliff

  8. It seems like if properly designed attic fans sucking hot air up and out of the house would bring in cool evening air. It always cools down here in the evening. That way even on "hot" nights, really hot in the house and cool outside you could cool your house without AC and it is cheaper too and more environmentally friendly.

  9. Don't know about lack of breeze in the South Sound, but we haven't had any doldrums or stagnation in the high country near Mt Baker. We've only had 13 days since measurable precipitation, and overnight lows are still pretty chilly and damp (with humidity at 98%). Lows in Glacier beginning 7/11: 47.6, 49.4, 49.5, 50.1, 48.6, 49.6, 51.5, 50.9, 52.2, 53.5, 43.0, 47.1, 51.9, 53.4 F

  10. Horses sweat from everywhere, not just arm/leg pits. I was riding horses last weekend in the heat, and my horse's neck was drenched from sweating. They may pant too, but it's not the main way they cool down (I didn't notice any of the horses panting).


  11. Having spent most of my childhood and adult years in the mid-Atlantic and South, I can attest to the fatiguing effects of wilting humidity. That said, when we moved to Portland, Ore., and experienced some upper 90s and low 100s for days on end without A/C, even in low humidity and drinking plenty of water, the heat can still be debilitating. A whole-house fan in the attic was crucial for drawing in cooler air at night, if you hoped to get any sleep, that is. On a related note, I traveled (and camped) one scorching summer through the Southwest during a 110-120 degree heat wave, with no car A/C, and the "cool" tip was to soak our towels and wash clothes (at every gas station) and drape them over our arms and legs. Instead of letting the blast furnace of hot air bake us in the car, we rolled the windows up almost all the way. The controlled evaporation in the car dropped the temps into the 80s, if I had to guess -- our DIY version of a swamp cooler :)

  12. This is why an evaporative cooler (AKA swamp cooler) makes so much sense here. They use a lot less energy. My guess is there are lots of people in this town spending money (and a lot of energy) on air conditions that are unnecessary. For the type of dry heat we have, they are the wrong choice. I wish that Seattle City Light tried to inform the public about this -- I think it would save a lot of energy (which in turn would save everyone a bunch of money).

    Of course once in a blue moon we actually do get hot, humid weather. A few years ago that was the case. But even then it lasted all of couple days at most. Weather like we are having now (hot and dry) is a lot more common.

  13. Some of us in the Northwest have this problem every summer where people tell us we're not actually uncomfortable from the heat and we don't actually need the AC we're saying we wish we had.

    A wise person once told me, "Don't let anyone else tell you what you like." I would say a corollary to that is, "Don't let anyone else tell you the weather isn't too hot for you when you know perfectly well that it is."

    I honestly don't like it above 80, ever, humidity or no. I keep explaining this and people keep not believing me and telling me about humidity and insisting the hot weather isn't actually adversely affecting me. Yet I know it is. Some of us are ferns this way. We'll be over here, in the shade, under the sprinklers.

  14. I agree, Molly. When December rolls around, and it's dark, wet and 40 degrees for days on end, I'm perfectly content. I probably spend MORE time outside during the winter, and I'll actually get annoyed if the sun pops out unexpectedly.

    In other words, it's safe to say that I'm not on board with the whole anti-AC thing. We've got two portable units running right now, and there's a reason why our friends refer to my home office as "the meat locker." :-)

  15. Same, Foo! I recognize that many have seasonal affective disorder that makes winter hard to get through, but for me our "longest, wettest winter ever" wasn't so bad, while last year's two months of heat and drought and wildfire smoke were hell on earth. Some of us have the reverse of S.A.D. and can't take the summer! Oh well; they're saying back to the 70s and partly cloudy by Tuesday. Let's hope! :)

  16. It's a form of natural selection. While many people move to the PNW for work, some of us moved here precisely because of the prevailing weather, in my case to live out in the Sequim area. I enjoy warm days, and sun in the winter is fine when we get it, but all day sun and 75 crosses my boundary into too hot. Sitting in the sun? That's completely nuts. That stuff will kill you.

    Turns out my location on the top of a thousand foot hill less than two miles from the Strait creates a cap of warmth that lasts until 3 or 4 am on each of these recent nights, a trend that repeats every night during summer high-pressure periods until we get a really good marine push. It'll be in the 50's 500 feet below me and a half mile away, and 70's on the hilltop. Every night. We are also hotter in the peak of the daytime.

    I'm guessing the cool onshore flow that floods into the lowlands pushes the hot air up and keeps the hilltop warm. A different kind of heat island. Happens every summer when we have high pressure with mild onshore pushes at night. This is a piece of micro-climate that I hadn't considered (being cut-off from the milder marine pushes) so I'm glad I've got AC to allow me to sleep.

    But I'm envious of the people half a mile away and a little lower who are naturally cool at night while I need to burn electricity to stay cool.

  17. Dr. Kamberov (in the PRI story) is flat-out wrong and is perpetuating an inaccurate myth, one that can endanger the lives of horses whose riders may not know how to help them cool down under heat or exercise stress.

    Horse sweat itself of different composition than humans (more electrolytes, in general) - but its function as a coolant is the same as humans. Unlike humans, horses start by dissipating heat via panting and skin radiation, but quickly change to whole-body sweat cooling as exercise becomes vigorous and sustained.

    In spite of Dr Kamberov's incorrect assertion, horses sweat everywhere. Go visit an endurance horse ride camp and watch the totally drenched horses coming into the vet checks. Or on cool days or from slower riders, note the sweat marks anywhere that is covered (can't evaporate).

    As a mental exercise, think about how freaking big horse muscles are. I mean, they are silly huge. Then think about those muscles operating near peak capacity for a while. All that generated heat is going to be dissipated via ... panting out a relatively teeny mouth? Riiiight.

    If you still are skeptical of this experience and reasoning, Google "horse sweat for cooling". Every single result talks about horses using sweat to cool, and they quote various equestrian experts (no offense to a DNA expert, but that's not their area of expertise). For example, this article that quotes a horse veterinarian, and explains both the difference in composition and similarity in function:


    As I mentioned, I'm only being this assertive because this myth may harm horses. The myth that horses do not substantively cool via sweat needs to die.

  18. Clif, you said "For sweating to be effective the water must evaporate from our skin and that only occurs when the relative humidity is below 100%."

    Because our body temperature (although not skin temperature) is constant, wouldn't the effectiveness of sweating be more closely related to the dew point temperature than to relative humidity?

    For example, wouldn't sweat evaporate better from our skin if the air is 80F and 100% relative humidity than if the air is 100F and 80% relative humidity (aprox 93F dew point)?

  19. An interesting post relates to sweating when the relative humidity is below 100% but for those with air conditioners installed always need to take care of ac maintenance for the longer lifespan of the air conditioners.


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