Most Seattle summers are mild. The weather station at Seattle-Tacoma International Airport averages fewer than four days a year at or above 90°F. Even so, the June 2021 heat dome sent Sea-Tac to 108°F, and Washington's Department of Health counted 100 heat-related deaths between June 26 and July 2, 2021. Whether you need air conditioning depends on how you want to handle those rare extreme days, who lives in your home, and whether you are replacing your heating system anyway. This guide covers Seattle's normal climate, the 2021 heat dome, state heat guidance, and how a heat pump can add cooling.
How hot does Seattle get in a normal summer?
Degree days measure how much heating or cooling a climate calls for. NOAA's 1991–2020 normals for Sea-Tac show 265.0 cooling degree days and 4,375.8 heating degree days a year. That works out to more than 16 times as many heating degree days as cooling degree days.
Very hot days are rare. The same set of normals shows Sea-Tac averages 3.8 days a year at or above 90°F and 0.1 days at or above 100°F. In August, the normal average high is 77.6°F, and the month averages 98.1 cooling degree days.
Seattle-Tacoma International Airport climate normals, 1991-2020
| Cooling degree days per year | 265 degree days |
|---|---|
| Heating degree days per year | 4,375.8 degree days |
| Days per year at or above 90°F | 3.8 days |
| Days per year at or above 100°F | 0.1 days |
| August normal average high | 77.6 °F |
The June 2021 heat dome in Seattle
Averages do not show how extreme a single event can be. In late June 2021, NOAA's daily records show Sea-Tac reached 102°F on June 26, 104°F on June 27 and 108°F on June 28. The lowest temperature recorded on June 27 was 73°F.
Daily high temperatures at Sea-Tac, June 26-28, 2021
- June 26, 2021102 °F
- June 27, 2021104 °F
- June 28, 2021108 °F
A NOAA State Climate Extremes Committee memo, dated February 3, 2022, says Seattle broke its all-time record high by 5°F during this event. The memo says the heat peaked west of the Cascades on Monday, June 28, with highs of 108°F in Seattle, 116°F in Portland and 117°F in Salem, Oregon. Across Washington, 128 all-time maximum temperature records were broken or tied.
How many people died in Washington's 2021 heat wave?
The Washington State Department of Health's Heat Wave 2021 page says the National Weather Service in Seattle reported a long, unprecedented heat wave from June 26 to July 2, 2021. DOH says the event created a high to very high risk of heat illness, especially for people who are heat sensitive and those "without effective cooling and/or adequate hydration."
The department reports two death counts, and each covers a different time period:
- 100 heat-related deaths between June 26 and July 2, 2021, from the Heat Wave 2021 page.
- 157 heat-related deaths reported from June 26 to August 31, 2021, according to DOH's September 2023 report on excess deaths.
DOH's extreme heat page calls extreme heat "the deadliest weather-related hazard in the United States." It also says Washington summers are getting longer, hotter, and potentially more dangerous.
What the Washington Department of Health says to do in extreme heat
DOH's guidance explains where cooling without air conditioning stops working. Its July 2022 handout, Cooling Indoor Spaces Without Air Conditioning, says fans only provide cooling when the air temperature is below body temperature, about 99°F. Sea-Tac's highs on June 27 and June 28, 2021, were both above that mark.
The same handout is written for facilities. It says portable room air conditioners are a preferred alternative to opening windows and using fans when outside air quality is poor.
DOH's hot weather precautions add one more step for homes without cooling: "If air conditioning is not available, stay on the lowest floor out of the sunshine."
Can a heat pump add air conditioning to a Seattle home?
Yes. Several local utilities and the City of Seattle describe heat pumps as both heating and cooling systems:
- Seattle City Light says heat pumps "move heat from the outside air during heating mode and remove heat from inside your house during cooling mode."
- Puget Sound Energy says heat pumps "also offer cooling, humidity reduction, and increased air circulation and filtration."
- Tacoma Power says that in warmer months, the system keeps your home cool while also dehumidifying the air.
- The City of Seattle says heat pumps "also provide air-conditioning, making them a great solution for year-round comfort."
Seattle needs far more heating than cooling. A heat pump can serve as the home's main heating system and also handle the few hot days each year. Rebates exist, but each program has its own rules on who qualifies. For example, Snohomish County PUD says customers with electrically heated homes are eligible for heat pump rebates. The City of Seattle's Clean Heat program is aimed at homes that heat with oil.
Deciding whether to add cooling in Seattle
Based on the normals, a Seattle home needs cooling only a few days a year. The 2021 heat dome showed that those days can be extreme, and DOH says the risk was especially high for people without effective cooling. If someone in your home is heat sensitive, or if you are replacing an electric or oil heating system anyway, a heat pump is one way to add cooling. If you do not add cooling, DOH's guidance on fans, low floors and staying out of the sun covers what to do on hot days.