Do the Tri-Cities really get 300 days of sunshine a year?

It seems like I’ve heard some variation of this phrase semi-frequently – the Tri-Cities gets 300 days of sunshine a year. I know I’ve found myself saying it when I tell someone I’m from Washington and they reply with the typical “oh, so you like the rain.” It’s even so common that the Tri-Cities Visitors and Convention Bureau (archive) has adopted it to advertise our city to people in other areas.

After repeating it a handful of times, I decided it was time to actually take a look at the climatology. For this project, I took a look at the 30-year average (30 years is pretty standard for weather averages) for the number of sunshine days in a year for the years from 1988-2017 at the Tri-Cities Airport.

My criteria for a “sunshine day” is as a day experiencing five or more consecutive hours of sky conditions being listed as sunny, mostly sunny, partly cloudy, or scattered clouds. Conditions such as fog, mostly cloudy, or partly sunny were omitted. For days with precipitation or fog, I looked at the hour-by-hour conditions to see if there were five hours meeting the criteria. Otherwise, I took the average conditions of the day.

Monthly sunshine days for 1988-2017 at the Tri-Cities Airport. (WeatherTogether graphic, click for larger image)

So, the short answer is no. The Tri-Cities does not see over 300 days of sunshine a year. In fact, the average from 1988 to 2017 was 238 days, almost two and a half months short of that number. Here’s a deeper look at the climatology.

Not only is our average extremely short of the 300-day threshold, there hasn’t been a single year over the last 30 years that even got close to that. The maximum for the time period was 266 days, which itself still has a month more of cloudy days than advertised. The least sunny days in a year was clear down to 194, back in 1993. That January didn’t even see a single sunny day.

That said, both the maximum and minimum sunshine days are outliers. It turns out 50% of the last 30 years fell between 230 and 250 days.

What’s holding us back?

Our summers are really sunny, with many months having every single day being a sunshine day. On average, July is our sunniest month with 29.1 sunshine days. August is super close, with 29.0 sunny days. August would likely have been the winner if 1989 didn’t have as many cloudy days as it did.

But our issue isn’t the summer, it’s the winter. Part of the problem is the strong inversions that often form during the winter months, giving us many cloudy or foggy days that would otherwise be sunny. Inversions form when there is a strong high-pressure system in place, trapping air in the basins.

Inversions don’t only happen in the winter, but as temperatures are cooler in the winter, it is much easier for the air to approach the dew point than during an inversion that forms in the summer. Often times during an inversion situation, areas in the Cascades and Blue Mountains will be sunny. During shallow inversions, even Jump Off Joe and Badger Mountain can see some sun.

Not all of our cloudy days come as a result of an inversion. Winter is our wet season, and as such, we see many cloudy days with rain. Unlike winter, when it rains in the summer, often times the remainder of the day is sunny. Sure, there are many times when it only rains west of the Cascades, but when that is the case, it is still common for high clouds to blanket the Mid-Columbia.

If you like sunshine, you don’t have to worry – 238 days a year is still a lot. On average, each month between May and October sees over 20 sunny days (and April is excruciatingly close to 20), so go ahead and enjoy the sunshine this summer!

If you would like to review the data yourself, feel free to head over to my contact page to send me an email.