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Cliff Mass Answers Your Questions, Including Why Seattle Is Rainy And L.A. Dry

Seattle Municipal Archives
"Man with umbrella, 1980s"


Engaging with his audience is something KPLU weather expert Cliff Mass very much enjoys.

Mass says the significance of public interaction was one of many lessons he learned while studying with Carl Sagan at Cornell University many years ago. (Sagan hosted the original version of Public TV’s "Cosmos" series and was an early popularizer of science.)

“He really impressed upon me the importance of scientists directly interacting with laypeople out in the community. And I’ve tried to continue that myself," Mass said.

In keeping with that sentiment, we’re finishing out the year with another round of answers to listener questions.

Is Wind Power A Good Option In Anacortes?

A listener named Andrew wants to know where he can find good data on wind averages where he lives, in Anacortes. “I'm trying to size up whether a wind turbine to help power my house is a cost-effective option,” he wrote.

Mass says this could be challenging to assess. “It’s very hard to get good wind statistics,” Mass said, because they are only available at a few locations. The National Weather Service site has some information for a limited number of local stations, which can be found via your local weather service office. There’s also something called the National Climatic Data Center in Ashville, North Carolina, which offers a wealth of climatological statistics on its website.

Why Is Seattle Rainy And L.A. Dry?

Another listener wanted to know: “Why does Seattle get so much rain and Los Angeles so little, relatively speaking, when both are next to the ocean?”

Mass says the reason is our northern location, which puts us in the path of the jet stream. That’s the current of strong winds that move roughly from west to east and produces most of our storms.

“The jet stream hangs out north of 40 degrees north, generally,” Mass said. “And so we often get the jet stream in the winter, but it rarely gets down to Los Angeles,” which is in the subtropics, where there are substantially less precipitation and storms.

How Is Accuracy In Local Forecasting Measured?

Clark wrote in to ask: “How is (should) the accuracy of local weather predictions be measured? I assume this is a complex question. What counts? Temp, precipitation, big/small area? … How good are the local forecasts and what index tells you this is so?”

Mass says local forecasts have gotten better and they are evaluated on all of these parameters. “It really depends what you’re interested in,” Mass said. The National Weather Service and his department at the University of Washington verify all the major forecast parameters: temperature, humidity, wind, pressure and precipitation are all evaluated independently.

And there are a number of measures, some fairly statistical and complex, that they use to see how well they’re doing.

“We have a lot more local data now, so we’re able to verify at many more locations,” Mass said. 

Exactly how it’s done “gets very technical very fast,” Mass said. They look at things like “mean absolute error” and bias (whether you’re warmer or colder than normal) as well as some more complex statistical measures. “So we get into fairly sophisticated statistics, quite quickly, because it is not trivial to verify. There’s a lot of issues to do it right,” he said.

How Far Ahead Can A Forecast Really Predict?

"SH" wrote in wondering about the 45-day forecasts put out by Accuweather, asking: “Are they worth anything?”

“The answer is no,” Mass said, adding that such long-term weather forecasting is extremely controversial. “What you should know is that weather forecasts two or three days out tend to be highly skillful,” he said.  That skill fades by six or seven days and there’s a modicum of skill seven to nine days out. But after you get past eight or nine days, he says the skill drops off to virtually zero.

“So anybody who’s giving you 45-day forecasts, or there’s one group giving one year forecasts, they’re not giving you a product that you can trust,” Mass said.

Why Does the Wind Pick Up In The Afternoon?

A listener named Dana wrote in wondering about the wind where she lives in Skagit Bay: “It almost invariably kicks up in the afternoon. Why?”

Mass says that’s a phenomenon seen in many places around the Puget Sound area. Anywhere in the north Sound or the Strait of Juan de Fuca, winds tend to pick up later in the afternoon.

“This has to do [with] the difference in temperature between land and water,” Mass said. “The oceans cool and as the interior of Puget Sound on land warms up, the pressure tends to fall, because warm air is less dense,” Mass said. That causes a difference in pressure to develop during the afternoon. And the winds from the west rev up, pushing into our region during the afternoon. “So it is very typical to see increased winds somewhere after 2 or 3 o’clock.”

This tendency is much more pronounced in summer, where there are bigger differences in heat between land and ocean. These so-called “diurnal winds are much weaker in the winter time,” Mass said.

What Is Mass' Favorite Kind Of Clouds?

Credit Stormloverswin93 / Flickr

Sorchia Zilavy wanted to know: "What is your favorite cloud formation?”

Mass says he loves big cumulous clouds that bring thunder and lightning. “Cumulonimbus — that’s probably my number one,” he said.

“They’re tall clouds. They have big anvils. They can be extraordinarily pretty,” he said, adding the weather they produce can be “phenomenally dramatic. And they can produce very intense precipitation and winds, so it’s good every way you look at it.”


The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU Environment Reporter Bellamy Pailthorp. Cliff Mass is a University of Washington Professor of Atmospheric Sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of “Weather with Cliff Mass” shows.

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to