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 can do.   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"

from Cliff Mass Weather and Climate Blog https://ift.tt/2JPqbo0