Pool swim season in Mountlake Terrace, WA
When can you swim in an outdoor pool in Mountlake Terrace, WA?
An unheated pool in Mountlake Terrace never reaches 78°F in a normal year. It peaks around 69°F in Aug. The air allows up to 4 swimmable months with a heater (Jun to Sep).
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What each heater buys you in Mountlake Terrace
| Option | Months | Swimmable | Energy/yr | All-in/yr |
|---|---|---|---|---|
| No heater | 0 | no month | $0 | $0 |
| Solar cover only | 0 | no month | $0 | $50 |
| Solar panels + cover | 2 | Jul to Aug | $0 | $283 |
| Heat pump + cover | 4 | Jun to Sep | $272 | $822 |
| Gas heater + cover | 4 | Jun to Sep | $654 | $1,079 |
For a 450 sq ft pool held at 80°F, at WA prices of 14.4¢/kWh electricity and $18/Mcf gas. All-in spreads the purchase over the equipment's life.
The cheapest way to the full season in Mountlake Terrace
Heat pump + cover: 4 of 4 possible months for about $822 a year all-in, which works out near $206 for each month it adds.
A solar cover alone adds no swim months here, so if you want a longer season you need a heater (why); still use one at night to hold the heat you pay for. If you get quotes, ask for a heat pump sized to your pool's surface area; here it beats gas all-in (heat pump or gas).
Weather station: Seattle Sand Pt NWS Office, 7.7 mi away (NOAA 1991–2020 normals). Water temperature is estimated from air and sunshine, not measured.
Mountlake Terrace month by month: air, sun and pool water
| Month | Air high | Air avg | Sun kWh/m²/day | Water, unheated | Water, covered | Heat pump $ | Gas $ |
|---|---|---|---|---|---|---|---|
| Jan | 47° | 42° | 1.0 | 40° | 46° | – | – |
| Feb | 50° | 43° | 1.9 | 42° | 48° | – | – |
| Mar | 54° | 47° | 2.8 | 45° | 51° | – | – |
| Apr | 59° | 51° | 4.2 | 50° | 56° | – | – |
| May | 65° | 57° | 5.2 | 56° | 62° | – | – |
| Jun | 70° | 61° | 5.7 | 62° | 68° | $106 | $248 |
| Jul | 77° | 67° | 6.3 | 67° | 73° | $55 | $137 |
| Aug | 77° | 67° | 5.4 | 69° | 75° | $39 | $98 |
| Sep | 71° | 62° | 3.9 | 66° | 72° | $72 | $171 |
| Oct | 61° | 54° | 2.3 | 57° | 63° | – | – |
| Nov | 52° | 46° | 1.2 | 48° | 54° | – | – |
| Dec | 47° | 42° | 0.9 | 42° | 48° | – | – |
Heating costs most in Jun (about $106 with a heat pump, $248 with gas). Sunshine peaks in Jul at 6.3 kWh/m²/day (NASA satellite data, 35.9 km grid cell). Highlighted months are swimmable with no heater.
How Mountlake Terrace compares with nearby cities
Mountlake Terrace ranks 41 of 91 Washington cities for unheated swim months, and no nearby city gets a longer unheated season.
| City | Distance | Unheated months | Heat pump months | Heat pump $/yr |
|---|---|---|---|---|
| Lynnwood, WA | 2 mi | 0 | 2 | $145 |
| Lake Forest Park, WA | 2 mi | 0 | 4 | $272 |
| Edmonds, WA | 3 mi | 0 | 2 | $145 |
| Shoreline, WA | 3 mi | 0 | 4 | $272 |
| Kenmore, WA | 4 mi | 0 | 4 | $272 |
| Bothell, WA | 5 mi | 0 | 4 | $272 |