So one might ask: do you really need A/C in Seattle and have our summers got "hotter and hotter"?
Let's check the facts. One of the key measures of the need for air conditioning is cooling degree days, which is based on the difference between daily average temperature and 65F. If the daily average temperature at a location is say 75F, then that day has 10 cooling degree days. Do this for all days of the year at a location and add them up, and you have annual average degree days. The idea is that once the daily average temperature reaches 65F (say a high of 80 and low of 50), you start needing A/C.
The map of average annual cooling degree days below shows that western Washington has the some of the lowest numbers of cooling degree days in the nation outside of high terrain. Much of western WA is white (low numbers) with only central Puget Sound entering the red colors (101-400). Seattle clearly has the lowest need for cooling of any major US city in the lower 48 states.
In fact, Sperling's Best Places has rated Seattle America's number one summer "Chill City" based on our temperature and humidity. Seattle Chill--I think I have heard this before.
The reason for Seattle's low number of cooling degree days is clear: we have some of the coolest summer temperatures in the nation, because western WA generally is covered with cool air from off the Pacific Ocean. The map of the normal high temperature for August shows this clearly, with our area the only coastal one with a lot of green. Seattle does better than Portland because we have cool Puget Sound next to the city and there is a sea level passage to cool Pacific air. Portland has much more need of A/C.
But temperature is only part of the story---humidity is also an important issue for summer comfort. Higher humidity reduces our ability to cool by sweating, which is a very effective way our species reduces our body temperature. A good measure of the water content of air is dew point, the temperature at which air becomes saturated (100% RH) when cooled.
Take a look at the dew point map of the US for August below. The lowland Northwest has LOW dew points (low 50sF)--low levels of moisture in our air. Which means our relative humidities end up quite low during the day. Your body can cool effectively from sweating (which may not even be apparent to you). Compare that situation to the eastern (and particularly southeastern) US where moist, humid air greatly interferes with our natural cooling mechanisms.
Why are our dew points low? Because of the cool Pacific Ocean....the amount of moisture air can pick up increases with temperature. Cool water...not much moisture in the air.
But wait...there is more! Our dry air allows temperatures to cool rapidly at night (water vapor is a powerful greenhouse gas), so our typical daily low temperatures even during a warm spell almost always drop in the 50s. Open a window or use a window fan, and comfortable sleep is almost always possible. Finally, even on the warmest days, cooler temperatures are generally very close....just head to the water, since Puget Sound is generally around 50F.
The bottom line of all this is clear: Seattle is one city where A/C is a luxury that is not particularly needed for buildings in which windows can be opened, except for the most unusual and exceptional days. Apartments with poor circulation and facing the sun can get warm (and could use A/C). Typically, Seattle has two days a year when the maximum temperature reaches 90F.
But what is the trend of Seattle temperatures and how about the future? Will A/C become a necessity for most, like in Houston?
To "warm up" the discussion, here are the average June to September temperatures over the Puget Sound lowlands (NOAA WA Climate Division #3) for the past 65 years. There is a small upward trend with a lot of variability. Roughly 60.5 F in the 50s to early 70s, and approximately 62F since then. The transition in the mid-70s may have been due to a mode of natural variability called the Pacific Decadal Oscillation (PDO), which transitioned from the cold to warm phase at that time. A small human-caused contribution is also possible (from urbanization of our region and increasing CO2). But the upward trend is modest and quite small for the last 30 years.
There was one crazy warm year recently (2015), which was associated with an usually persistent ridge of high pressure. Around a DOZEN days reaching 90F at Seattle. And you will notice some similar heat spikes early in the record. These transient features were probably the result of natural variability.
But what about the future? If A/C may not be necessary now, what about later in the century?
For the 1990s it shows about 50 per decade (4 per year). The model is overestimating the number of heat waves, probably because it does not have enough resolution to get Puget Sound correct. 2020s? Pretty much the same story. 2050s--a modest increase to 80 a decade. But look at the 2090s. Wow. A huge increase to 170 a decade (17 a year). You will probably want air conditioning in Seattle in the 2090s.
Global warming due to human-inspired emissions of greenhouse gases is probably having a small impact on our local temperatures now, but by the end of the century our climate will warm profound. But we need better information to get a better idea of the local effects. That includes running regional climate models at higher resolution and running them many times to get a handle on uncertainties. That is why we are trying to build a regional climate effort at the University of Washington.
And enjoy the perfect weather today...no A/C needed.