January 16, 2013

Strange lines over the northeast Pacific

The visible satellite imagery over the northeast Pacific Ocean looks very strange today, with weird-looking lines and strange shapes in the clouds.  Take a look at image from 11:15 AM this morning from the National Weather Service GOES weather satellite:

You can also see the low clouds over Willamette Valley and the snow in the Sierra Mountains, as well as those infernal lines over the Pacific.  You can get another view of these clouds from the high-resolution MODIS satellite:

What are these lines?   They are commonly called ship track clouds and are produced by the particles emitted by ships that burn oil, diesel, or coal.    We often see ship tracks in the late spring and early summer, but they are fairly unusual this time of the year...and I will tell you why.

Right now, we have a BIG ridge of high pressure over the eastern Pacific, more like a summer pattern than a midwinter situation.  Here is the surface map at 10 AM to illustrate:

With high pressure we don't have storms or fronts, and certainly none of the clouds associated with them, since high pressure is associated with sinking air.   Sinking air produces a stable situation, often with an inversion (a zone of increasing temperature with height) above a shallow marine air.   Without a lot of mixing from the strong winds of storms, this shallow marine air layer gets saturated (it is in contact with the ocean!), with lots of thin stratus and stratocumulus clouds.

But where do the ships come in?  It turns out that Pacific air is relatively clean, often with only 10-20 particles per cubic centimeter.   Cloud droplets form on this particles and thus over the ocean we tend to have relatively few cloud droplets (but bigger ones) in some volume of air.  But combustion from ships produces lots of particles and can increase the numbers to hundreds per cubic centimeter (see graphic).  That results in a lot more cloud particles, but smaller ones.
Having a lot more particles, but smaller ones, reduces the ability of the clouds to precipitate, so the clouds last longer.  And it turns out that clouds with more small particles reflect much more of the sun's light, appearing brighter.    That is the origin of the ship tracks...the pollution of the ships enhancing the clouds and making them brighter.  The ship tracks tend to get wider downstream of the ship as the combustion particles mix into the environment.

We can look at super high resolution MODIS imagery and can almost see the ships.  Here is an example:

The U.S. Navy was very concerned about ship tracks during the cold war, since it was a way the Russians could spot our non-nuclear ships.  

Now could we use ship tracks to combat global warming?  Some people have suggested we do so.   Ship tracks produce clouds that reflect more of the solar radiation back to space (that is why they are whiter!) and thus help cool the earth.   So lets produce many more ship tracks...but we want to do it without burning fossil fuels.   To do so, it has been suggested we build a fleet of giant ships that inject salt water droplets into the atmosphere--the water would evaporate, leaving huge numbers of salt particles to waft into the atmosphere (see picture).   The result, a huge amount of particles and a lot more cloud droplets...super ship tracks and perhaps a bit cooler climate.


  1. Is there any credible research on possible negative effects of using ship tracks to combat global warming?

    One concern I have read about is 'global dimming'. As in dimming some of the light needed to grow plants.

  2. Cliff, how much of this soot from ships is being blown into the arctic, where its being deposited on the ice, increasing its albedo, thus its melting?

    1. Actually, a higher albedo would mean LESS melting. This is because the energy is being reflected rather than being absorbed and causing the ice to melt.

  3. NYT article on the global warming effects of soot:

  4. Off the topic, but there was a really nice effect earlier being on the edge of the fog coming in to the north end of campus seen from the upper floor picture window of Paccar Hall. Fog lifting up to go over and around the buildings in the quad and heading up the hill, not quite covering the sun.

  5. Solar powered, regulated (variable-controlled.) .. even.

  6. Why does king county get a pass on burning bans while the bans remain in place for both pierce and snohomish counties?

    Isn't the inversion the same over the entire area?

  7. We know that air travel is bad for the environment, and I've debated with friends whether travel by ship was more or less environmentally sound, considering there are no emission regulations for ships. What do you think would be the effect if there were fewer jets and more passenger liners between the US and Europe and the US and Asia???

  8. If you want to travel long distances in an environmentally sound way, there's sailing ships. Maybe too far for the Pacific, but the Atlantic used to be crossed pretty reliably in around three weeks. Maybe we could do better now.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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