Last Updated on July 10, 2020 by Tom Shu
I moved to Seattle from Minneapolis, Minnesota, and have loved it so far! Other than the many outdoor activities I now have access to in and around Seattle, one of my favorite things is the cooler, crisp summer months here.
To my surprise, though, I found out that Seattle is actually one of the most humid cities in the US.
In fact, according to one online publication, Seattle is ranked as the 38th most humid city in the United States.
This little weather-related fact surprised me so much because the weather here never feels humid – at least to me.
Coming from Minnesota and Taiwan where the summers are hot and humid, I didn’t believe Seattle was humid until I started to research a little more.
Here are 5 interesting things from my research about Seattle’s humidity and humidity in general that I think you need to know about!
1.) Seattle Maintains a Steady Humidity Throughout the Year
Many cities have a steady humidity throughout the year like San Diego, California, and Honolulu, Hawaii. You would expect this considering how nice the weather is in these two places year-round.
I mean that’s part of the reason why they are such popular places to visit, especially in the winter months.
Surprisingly though, Seattle’s average humidity does not change too much throughout the year when you compare its summer months to its winter months with an average humidity around 66% in the summer and 79% in the winter months
After living here for a year, I thought the difference would be much larger considering how much more it rains in Seattle during the winter months.
Just take a look at these average rainfall numbers for Seattle and tell me if you would think the same.
Average Rainfall by Season in Seattle
- Spring (Mar 1 – May 31): Average precipitation is 2.28 inches
- Summer (Jun 1 – Aug 31): Average precipitation is 0.93 inches
- Fall (Sep 1 – Nov 30): Average precipitation is 3.31 inches
- Winter (Dec 1 – Feb 28): Average precipitation is 4.84 inches
Of course, average humidity isn’t the only variable that causes it to rain, but I’m no meteorologist or scientist and majored in finance, so the fact that Seattle’s average humidity remains about the same even though summers and winters are so different was surprising to me.
2.) St. Paul Island, Alaska is the Most Humid City in the US
According to BestLifeOnline.com, a lifestyle, travel, and wellness publication, St. Paul Island in Alaska is the most humid city in the United States with an average relative humidity of 86%.
This was crazy the first time I read it! An island in Alaska, with its cold snowy winters and long summers, is the most humid place in the US?
In comparison, Seattle is the 38th spot on the list with a relative humidity of 76%. Unlike Seattle though, St. Paul Island is located in a much harsher polar climate and is known to have a constant summer fog with an average temperature range around 19°F – 51°F.
I have always been interested in visiting this part of Alaska and hopefully one day I can.
If you have never heard of St. Paul Island before, it is located in the Bering Sea between Alaska and Russia and it’s the largest island of the 4 Pribilof Islands.
The main reason why I want to visit is that I found out that the Island has so many birds.
In fact, the Island is claimed to be the migratory bird capital of the world! In total, they have recorded at least 313 different species of birds who have visited the island.
Most importantly, though, my favorite bird of all time the Puffin is also found on the Island!
If I do visit, I plan to go during late July or mid-August as the weather is bearable and its when the island bird activity is at its highest.
3.) The Relative Humidity Usually Increases at Night
Many people wonder why relative humidity is usually higher at night, even in Seattle. To help you better understand why this happens I did a little more research for you.
So, let’s all travel back to high school science class where many of us probably first heard about these concepts and have long since forgotten them. (That is unless you’re a weather reporter now. If that’s the case, then please let me know if I missed anything here.)
The first thing you need to understand is that relative humidity is the amount of water vapor in the atmosphere.
During the day, the sun heats the water molecules forming steam or water vapor.
The steam then rises and forms clouds.
In total, this process tends to reduce the amount of water vapor in the surrounding during the day.
Ok, that’s cool and all but how does this increase the relative humidity at night then?
At night, there is no heat to evaporate the water molecules that are in the air.
This means that they will remain in the atmosphere for as long as possible and since there is no sun to heat the water molecules, the air starts to become more humid and dense.
That is why the relative humidity is usually higher at night and why it tends to feel that way too.
Take Seattle for example.
December and January usually have higher relative humidity than the other months in the year and they are also usually the coldest months too.
Since it is colder, it means that there is no heat to evaporate the water molecules in the air. So at night, relative humidity increases given that fewer water molecules have evaporated during the day.
4.) Even Though Seattle is One of the Most Humid Cities in the US, It Doesn’t Feel Humid
When you see Seattle as the 38th most humid city in the United States, you would think that the Seattle air would be quite muggy, right?
For those of you who have been here, then you know this is far from the truth!
Going back to that high school science class, this is all because of the relationship between the temperature and the dew point.
Temperature is the amount of heat in the atmosphere while the dew point is the specific temperature at which the air becomes 100% humid. So, the dew point is what determines whether you are comfortable or not regardless of the humidity.
Let’s now take a look at the relationship between humidity, dew point, and temperature.
The bigger the difference between dew point and temperature, the lower the humidity while the smaller the difference, the higher the humidity.
For example, when the dew point is around 55 in Seattle and the temperature is in the 60s, you will experience higher humidity than when the dew point is around 55, and the temperature is in the 80s.
In Seattle, the dew point and temperature range are more like the dew point of 55 with temperature in the 60s example, which is why it tends to stay more humid here.
5.) The Geographic Location of Seattle Produces a Predictable Climate
Maybe this is obvious to those of you with a science background, but I was surprised to find out that one of the main reasons why the climate is so mild here is because of Seattle’s geographic location.
The city sits within the mountains, and it also borders the Pacific Ocean. On one side of Seattle, you’ll find the Olympic Mountains and on the other side, you’ll find the Cascade Mountains.
I had to dig a little deeper into this information, but this is why it matters.
You see, the ocean is like a giant heat absorption device, which absorbs heat much better than the land or the atmosphere!
When the sun hits the ocean, the water molecules are heated up and evaporates into the atmosphere which increases both the surrounding area’s temperature and humidity.
Then the warm air flows along with an ocean current and distributes heat around the world.
This warm ocean air that flows into Seattle in combination with how the mountain ranges protect the city is why Seattle has warmer winters and cooler summers when compared to other cities at the same latitude.
So yes, to answer the question once and for all, Seattle is surprisingly one of the most humid cities in the US!
But even though it’s ranked as one of the most humid cities in the US, it doesn’t mean that it feels humid when you’re here.
Now you know that because of the city’s geographic location and how the relationship between temperature and dew point works, the climate here is relatively nice all year round.
Of course, the city has a reputation of being one of the grayest, rainiest cities in the US, but when you look at the actual amount of rainfall, Seattle gets less rain in a year than cities like New York and Atlanta. (maybe that can be a blog article for a different day 😉 )
Did it surprise you to hear that Seattle is actually one of the most humid cities in the US?