Compressional heating: How we generate triple-digit temperatures in the Willamette Valley

KOIN 6 Weather Kids

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PORTLAND, Ore. (KOIN) – Summer can bring a variety of temperatures to the Pacific Northwest. During one July afternoon, you could see 60s for the Oregon coast, while we’re sitting in the 80s for the Willamette Valley and if you were to travel east, it’s soaring to the triple-digits for Baker City.

The summer may bring a fairly large temperature gradient from the west side of the state to the east. However, there are times when the Willamette Valley is actually the warmest slice of Oregon.

Weekend Meteorologist Joseph Dames

It just doesn’t happen very often. You see, we don’t tend to collect many 100 degree days in Portland. The maximum temperature in July will hit 100 in Portland 1% of the time. It is slightly higher in August, coming in at 1.5% of the time. You may be asking what the warmest temperature on record is for Portland, and it is 107. If we are to hit the triple digits, we don’t tend to get much warmer than the lower 100s.

With all that in mind, there is a reason that we hit 100 degrees in Portland, and it’s because of the east wind. If 100-degree temps start popping up on the seven-day forecast, we tend to have what is called downslope warming or compressional heating in the works. Now, I must add, this isn’t always the case. It is most likely the case and more common than not when we hit the higher temperatures.


Now is a good time to check out the graphic below. Compressional heating is the result of downsloping wind coming off the mountains. That air is compressed, which will lead to an increase in temperature and a drop in relative humidity. This is one of the main reasons for the warm and dry weather east of the Cascades. The wind is mainly coming out of the NW from April to September.

You may hear the term subsidence when referring to compressional heating. This is just another term to define the descending motion of air. When those air parcels are heating up, the relative humidity will decrease. You can go back to this lesson for a reminder about relative humidity.

KPDX Weather Conditions July 26, 2020

Above is a snip of the afternoon weather conditions coming out of the Portland International Airport weather station on July 26, 2020. This was a day that we hit 100 degrees in Portland. During the afternoon an east wind, running around 10-20 mph, boosted our temperatures to the triple digits. At this time the relative humidity was coming in at 13%. A good example of the temperature going up and the relative humidity going down.

Now, I know that a lot of folks out here are avid bike riders. Have you ever pumped up your bicycle tire and felt the heat from the pump? This is the act of compression that is increasing the temperature.

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