Understanding Climate: Pacific Northwest Weather

Understanding Climate: Pacific Northwest Weather

The Pacific Northwest is well-characterized by rainy and overcast days. But as we look forward to summer, there’s a palpable expectation of sunny, dry, warm weather for the next few months. How is that the Pacific Northwest can expect dry summers and wet winters each year?

We can thank the ocean for the rain. In winter, westerly winds and low atmospheric pressure create an easy path for air to travel across the Pacific Ocean to the Pacific Northwest. These conditions are sometimes called the “Pineapple Express”, and they allow storms and moisture to build up over the ocean before they make landfall here.

As moisture-filled oceanic air is pushed east onto land, it brings us overcast skies and plenty of rain. Most of the precipitation occurs on the coast or in the mountain ranges. Low inland areas like Vancouver, Portland, Seattle, or the Puget Sound consistently get cloudy wet weather and a lighter amount of rain.

But why don’t we have wet and cloudy weather all year? To understand this, we have to add another layer of complexity. It turns out, we have a dry season because our summer air comes from the atmosphere rather than the ocean. In other words, our region circulates air differently during the winter and summer months. The change is striking.

In the summer, an atmospheric cell (which is an area in the atmosphere that circulates air) shifts northwards following seasonal ocean temperatures and the position of the sun relative to the Earth. This atmospheric cell, called the Hadley cell, circulates air from high in the atmosphere down to the Pacific Northwest during the summer, causing several things to happen.

Atmospheric air is generally warmer and drier than the moisture-filled air coming from the ocean, so the Hadley cell allows for warm and dry weather. Since that air is descending from the atmosphere, it creates high atmospheric pressure as it falls. This seasonal high pressure in our region breaks up oceanic storms and lessens the flow of air coming from the ocean.

So in the summer, the amount of moisture we get from the ocean is diminished, and at the same time, we receive dry air from the atmosphere. For those reasons, we can expect a relatively warm and dry season every year.

In terms of climate classification, a large portion of Western Washington is classified as Csb, labeled as a warm-summer Mediterranean climate. That means that whether you like the rain or the sun, there’s a mild-temperature season for you here. But next time you’re enjoying a sunny summer day, remember to be thankful for atmospheric circulation.

(And if you are curious what’s happening with climate change, you might be surprised to learn that the Hadley cell is expanding).