November 07, 2020

Steam Fog Season

 That time has come again.   The time when our lakes occasionally appear to boil with tendrils of vapor reaching skyward.  

Surreal, but beautiful.  Northwest steam fog season is here.

Saturday Morning From Magnuson Park, Seattle

Close up, steam fog can be subtle, like this image taken around 7:50 AM on Monday:


But at other times it is simply stunning, especially when illuminated by the sun:


Steam fog occurs when cold air passes over warm water-- usually water that is at least 10-15F warmer than the air right above it.  20F differences usually produce a very good show.

Why steam fog?   

To get fog you need two things:  bountiful water vapor and then cooling the water vapor laden air to 100% relative humidity (called saturation).

The most typical fog around our region is radiation fog, and in that case the atmosphere starts with a certain amount of water vapor and then the land cools (by emitting infrared radiation), which in turn cools the air near the surface to saturation.

Steam fog is a bit different (see figure).   You start with a relatively warm water surface, which in turn warms and moistens a shallow level near the water.    That alone is not enough to give us much fog at all.   Then we move in some cold, but relatively dry air above it.   If there is some wind, turbulence and mixing between the cold/dry air and the warm/moist air occurs, and the MIXTURE is both sufficiently moist and cool... it is saturated and fog forms.  


The air above supplies the cooling and the water supplies the moisture.  

Neither can produce a saturated layer by itself, but together they can.  Sort of like epoxy cement, in which only when the contents of two tubes are mixed can the real action occur.


So what was the temperature situation this week?

The air temperatures dropped into the thirties around Lake Washington  on Monday morning and today, as illustrated by the temperatures this morning at 8 AM (see below)


And what about water temperatures?  

King County has a buoy right in the middle of Lake Washington, between Magnusson Park and Kirkland and measured a water temperature of about 56F, while the air temperature at the buoy was 39F  (4C) at the same time (see below).  A big enough difference for steam fog!


Tomorrow (Sunday) morning may be even cooler, so you might consider looking for some steam fog by Lake Washington and Lake Sammamish if you get up early.    But bring a warm drink.


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3 comments:

  1. I have to think we get as much Advection Fog around here. I’m a local helicopter pilot. Keeps us grounded all the time.

    ReplyDelete
  2. Hey Cliff -- you said "Then we move in some cold, but relatively dry air above it" -- why is the dryness of the air useful to creating steam fog? Wouldn't cold/moist air be even more effective, because the resulting mixture would be that much closer to/past the dew point?

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  3. How does steam fog differ from Tule fog?

    ReplyDelete

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