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!