May 06, 2010

Strange Ship Tracks and Good News for Mom

This time of the year we sometimes see strange, bizarre lines in the visible satellite pictures. Want to see for yourself? Below are two examples apparent over the weekend.

Pretty creepy! A signal from aliens? Meteorological versions of crop circles? Nope. Such lines are often apparent during the spring and early summer in low clouds over the Pacific. And we know what causes them....ships...and thus they are known as ship tracks.

It turns out that such lines are produced by ships powered by internal combustion engines, ships that eject particles into the atmosphere that help produce more cloud droplets in low clouds such as stratus and stratocumulus. But let me explain in more detail.

Nearly all cloud droplets form on little bits of dust, dirt, or some other type of particle. In the biz they are known as cloud condensation nuclei (CCN). Now the ocean environment is relatively clean and there are a limited supply of CCN. Small numbers of CCNs result in only a modest number of relatively big droplets.

But what happens when a big ship moves through, injecting a huge number of particles from its engine stacks? LOTS of CCN and instead of a small number of big droplets the same amount of water is shared among a large number of small droplets. Thus, the clouds are modified by the passage of the ship. It turns out that even with the same amount of total water, a large number of small droplets reflects considerably more of the sun's rays (visible light) than a smaller number of big droplets. So ironically, the gunk coming out of ship smokestacks make the clouds whiter!

Thus, as ships move through the relatively thin low level clouds there is a line of whiter clouds left behind...ship tracks! And these lines get distorted by the the winds. On some days you see lots of ship tracks, crossing each other and leaving lines like some kind of ethereal tic-tac-toe board. The U.S. Navy was concerned about these lines during the cold war period...the Russians could see where our ships of the line were located! However, nuclear ships leave no ship tracks, so the big nuclear carriers remained invisible. Interestingly, ship tracks help combat global warming (since they reflect the sun's rays better than regular clouds), but only to a small degree.

Finally, I have good news for Mom. The latest forecast model output indicates that our stormy, cold period is over for a while and that the weekend should be sunny, with highs in the 60s west of the Cascades, and near 70F in the Tri Cities. So you can take Mom for that spring walk at your local park or travel to mother's day brunch in comfort. But don't be tempted to plant your tomatoes yet!


  1. Pretty Cool.
    Was thinking in the middle of the night the % of minimal viability (low clouds,mountain uplift etc) around Mount St. Helens this time of year. What would have the May May 18 eruption looked like during a socked in day?

  2. I see no reason not to plant tomatoes on Saturday (or today), assuming they are already hardened. Is there some crazy storm brewing that isn't obvious to those of us non-meteorologists?

  3. I was thinking about planting tomatoes and peppers this weekend...maybe not quite yet! The temp. dropped 15 degrees in about half an hour this afternoon and got quite breezy with these "passing" thunderstorms we're having! Maybe I'll wait until the end of the month to finish planting my garden!

  4. Ok, we'll take this weekend for what it us and know it will get cold again :-( But tell us about these awesome thunderheads that are visible over the Cascades today. Makes me think I'm out on the Plains. Just beautiful and unusual.

  5. Planting the tomatoes depends on where you are to plant them. It isn't rocket science. It depends on experience,attention and preparedness.Generalizations are general. Duh.

  6. Well, my own limited experience with tomatoes has suggested to me to hold off until the nighttime minimum temp gets to 50F...and we are not there yet. Perhaps one of the expert gardeners who read the blog can provide some help!...

  7. My old copy of Growing Vegetables West of the Cascades (Steve Solomon, founder of Territorial Seeds) says you want nighttime temps above 50 for peppers, above 40 for tomatoes, assuming they have been hardened. Of course, the warmer the better, but you could be in for a long wait if you hold out for lows in the 50s.

    And, of course, temps could drop below 40 in some areas of Western Washington. Probcast says it could maybe get below 40 Saturday night in my zip code, so maybe I'll plant the ones near the house first.

  8. I really appreciate your mystery weird weather blogs, like the one above because there's always something new to learn.

    You gotta love the high res. visable weather pic. Today was cool. You could see the eastern outflow of cascade storms and the marine push on both sat. and radar, then match it up to local observations. Yeah, thats my nerdy hobby.

  9. In Naval ROTC at UW, one of my profs was a Naval Aviator. He used to talk about looking for the aircraft carrier under a cloud, where it always seemed to hide. Several years later, as a Naval Aviator, I learned what he meant, but I noticed that the cloud was from the carrier. The carrier's captain often wants to stay in a particular area, so they steam "up and down," laying ship tracks side by side, which from the air appear as a rectangular cloud bank. I always thought, though, the condensation was from the moisture in the ship's exhaust. In any case, I too, learned to look for the cloud to find my ship.
    Technically these ships use external combustion engines, as the oil burns to produce steam to drive the turbines.

  10. I noticed those clouds and wondered if you were going to tell us how they form, and how far away they were from my vantage on the I5 bridge. thanks!

  11. there is a lot of dis-information floating around about these ship tracks. claiming they are "...pollution rivaling a nuclear war..." etc. one article was passed on to me, by a usually sensible person. i emailed her this blog entry about ship trails, and she amended her article and back-linked to the information that cliff provides.


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