PORTLAND, Ore. (KOIN) – It is very dry outside right now. A good portion of the Willamette Valley is observing dewpoint values in the single digits this afternoon. While Pearson Airport is actually coming in with a negative dewpoint!
First off, what is dewpoint? It is the measure of moisture in the atmosphere. It’s not often that locations west of the Cascades experience dewpoints this low. This is definitely the case in October when we typically start to see the moisture increase.
In fact, the dewpoint has dropped 25 to 35 degrees from Troutdale to the Oregon Coast in the last 24 hours. That tells us that dry air has moved in very quickly! According to the National Climatic Data Center, the mean dew point in October for Portland is 45.3 degrees. We are currently in single digits!
What does a negative or subzero dewpoint mean? Well, you can also think of the dewpoint as the temperature to which air must be cooled to saturate. When there is a negative reading, that is what the temperature would have to drop to for saturation to occur. When we have a temperature at 45 degrees and a dewpoint at -1 (like Pearson Airport was reading at 3 p.m.), that means we would have to drop the temperature all the way to -1 or increase the moisture.
Imagine a water balloon that is filled halfway with water. You could think of that as a moderate dew point somewhere around 50 (in this particular example and depending on temperature). If you then take a water balloon of the same size and only fill it with a tablespoon or splash of water so it’s barely full, then you could think of it as having a very low dewpoint.