The clouds tend to cluster particularly densely above a hilly region near the lakes, in keeping with the behavior of Earth's lake effect clouds.
"You never see them to be on the other side [of the moon] or [directly] over the lakes," he said.
He noted, though, that the Cassini spacecraft's flybys have tended to pass more frequently over the downwind region, so it is possible that clouds elsewhere might have escaped observation. And Brown admitted he hasn't proven that the clouds are undoubtedly created by lakes.
(Read about Cassini's trips to Saturn.)
But, he said, "I'm sure this is a plausible explanation." Proving it may take a few more years of observation, he added.
The find is another example of the many Earthlike processes that have been found to be operating on Titan, said Ralph Lorenz, a planetary scientist from Johns Hopkins University who was not involved in the study.
"The physical conditions and working materials are different, but the processes themselves and the phenomena that result—whether sand dunes, river channels, or rain clouds—are remarkably similar," he said in an email.
Ultimately, the clouds may also help scientists determine whether liquid methane or liquid ethane makes up Titan's lakes.
The apparent methane lake effect clouds detected under the cloud cap could be formed by either type of lake, experts say.
In the case of methane lakes, warm methane would evaporate into colder air passing over the lakes—exactly what happens on Earth.
But ethane lakes could also form methane lake effect clouds by warming the atmosphere as air blows across the lakes, causing air to rise until clouds condense.
(Related: "Titan Forecast: Cold, Drizzly Mornings" [October 11, 2007].)
Viewed over a short time frame, these two processes produce very similar looking clouds.
But as Titan's seasons progress through their slow, 16-year cycle, methane and ethane lakes would cool differently, producing different cloud patters as the seasons change.
"Over time, we'll be able to watch these things," Brown said.
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