Fog Season

You know when the fog season is for the western WA interior? Right now. But the foggiest periods differ at other locations in our region.

Check out this figure, which shows the number of days of heavy fog (1/4 mile or less visibility) by month for a variety of locations.For Seattle, October is the foggiest month and the same is true for Olympia. You will notice that Olympia has much more dense fog than Seattle.

On the coast the foggiest month is August.

East of the Cascades at Spokane and Yakima, December.

Some people are surprised by all this...wouldn't the foggiest period be in the middle of the winter..during the rains of November through February? And for many locations the LEAST foggy period is not in the middle of summer, but in spring.

But if you think about it, it all makes sense. Most of the fog we get around here, and particularly the dense type, is radiation fog...which is generally produced by relatively cloud-free conditions in the middle and upper troposphere. Such conditions are often associated with high pressure and little rain. Long nights help it along, as does a moist surface. On relatively clear nights the earth can radiate heat to space and temperatures can cool to saturation...which leads to fog.

Fall is really good...the nights are getting longer, there is some occasional showers that moisten the surface, but there are still plenty of cloud-free nights to allow good radiational cooling to space. West of the Cascades, October is a sweet spot for all this.

Sunday Morning Satellite Photo (10:45 AM)--You can see the fog over the lowlands

On the coast they get some of this fall magic, but they also get some of the low clouds that hang out along and offshore of the coast during the summer.

Eastern Washington tends to be relatively warm in the summer and early fall and late spring, with little fog. But during the winter, the "bowl" of eastern Washington fills up with cold air and fog.

During the next few days, we will have warm and dry weather, but watch out for fog in the morning, particularly in river valleys. And as it get colder REALLY watch out for fog, since fog is often associated with roadway icing. The biggest weather killer in the Northwest...it is not wind or flood or storms...it is ice on the roadways....but more about that some other time.

The Meteorology of the Chilean Mine Rescue

Now that all of the miners are safe, it is interesting to think about some of the meteorological aspects of the disaster--and of the misinformation provided by the media.

Some of the media suggested that the miners could get the bends from "rapid decompression", but this is nonsense!

The level of the mine entrance, where the drilling is taking place, is at roughly 2400 ft above sea level, and the miners were trapped at a level a few hundred ft above sea level. Clearly, the pressure was higher where the miners were--roughly 8 % higher. And it took them about 15 minutes to make the change as they were lifted out. This is nothing to worry about! Virtually all of you experience this change many times each year. An example: drive to Snoqualmie Pass (elevation roughly 3200 ft) from the west. The last fifteen minutes you gain roughly the same elevation in the same amount of time! Or when you take off in a plane, take a long lift ride while skiing, take that gondola ascent on vacation, etc., you experience the same or worse!

Then there was the "steam" coming out of the hole (click on picture to see video):


Why the cloud coming out of the hole? The temperature down in the mine was very warm (90-105 from various reports) and there were sources of moisture down there. The air had sufficient water content that when it hit the cool nighttime air, it was cooled to saturation--thus the fog. It looked to me that temperatures were fairly cold during the evening rescues---40s F perhaps from the jackets people were wearing. A small contribution could also have come from the expansion cooling of the air as it rose.

The region surrounding the mine is very, very dry--in fact one of the driest places on earth...the Atacama Desert (see map). Some locations have never observed rain, and in others they receive perhaps a few hundredths of an inch per year. Why so dry? Sinking air from a subtropical high, the high Andes preventing moisture form moving from the east,

Mars or the Atacama?

and the coastal mountains preventing moisture movement from the west. Plus an inversion aloft that stops the air from moving over the coastal mountains.

And talking of media issues...which country is wrong in this map?

The mine site from space


LOCAL WEATHER ALERT: Looks like dry conditions for the next 4-5 days with considerable sunshine!