An astute reader from Renton observed last week that light fog was rolling in around his area, but a check of the current weather conditions at Renton Airport showed a temperature of 48 degrees and a dew point of 44 degrees -- a relative humidity of only 86%.
It turns out it was very light fog -- visibility only dropped to 6 miles -- but he brought up an interesting point: Can it be foggy without the relative humidity being 100%?
I think the answer is yes and no. Fog is, by definition, saturated air at 100% humidity. It's no different than a cloud, it's just an elevation issue -- fog is a cloud on the ground. But then how can we have fog when the dew point and temperature don't match?
The most likely scenario is that the air is not totally saturated on the ground where you and the weather instruments are, but *is* saturated not too far off the ground -- maybe even as little as 50-100 feet -- giving appearance of being surrounded by fog, even if the saturated air isn't right next to you. Thus, if the humidity is 100% where you are literally standing, the fog could appear to be quite dense.
Speaking of fog, take a look at this time lapse video of fog moving into Seattle's Elliott Bay early Wednesday morning. (It happens right at the end):
Why does fog sometimes wait until sunrise to form? The sunlight's warmth will sometimes trigger enough evaporation to add *just* enough moisture to punch the dew point up to the temperature and create fog.
Over water, the moisture source is obvious, but over land, this process is really noticeable on a mainly clear morning after a decent rain the day or night before. The clear skies allow the temperature to drop, while that initial burst of sun evaporates water from the wet ground adding moisture to the air, and now you've put the two ingredients together for fog. (Although if the temperature drops far enough, you don't even need the added evaproative mositure to get to fog and the fog can form before sunrise.)
That kind of fog is called "radiation" fog -- where clear skies allow warm air to radiate back into space, allowing temperatures to drop toward the saturation point. The other type of fog we see quite often around here is "advection fog", or imported fog, if you will. This is common with marine pushes, where the fog is formed out over the Pacific Ocean or other areas and then blown into the interior.
How about some pictures from way above the fog?
As we wrap up the week, I usually like to toss a few great photos into the blog to hold over the weekend. These next few photos are from one of the blogs' favorite sources: NASA Astronaut Douglas Wheelock, who is up on the International Space Station and frequently Tweets Photos from Space via his Twitter Account @Astro_Wheels.
The first one though is a mystery for you to solve. Can you figure out what this is? You can guess in the comments below. (But if you want to cheat or just can't figure it out, you can find the answer here -- no spoiling it, though!)
And back down here on Earth, Cindy Anderson snapped a photo of a lovely fall scene in Renton's Coulon Park:
Have a great weekend!