Nov
15
2006

Storm chasers know that puffy cumulus clouds often cause sudden rainstorms, while storms associated with stratus clouds form more slowly. Now physicists at England’s Open University have finally found an explanation.

They propose that neighboring water droplets in a stable stratus cloud don’t crash into each other because they’re all moving at about the same speed. But fast-forming, turbulent cumulous clouds contain water droplets moving at many different speeds. They crash into each other and form larger drops. As the turbulence grows, the drops grow quickly and fall as rain within a few minutes.

Cumulous cloud: These puffy clouds are associated with sudden rainstorms. Scientists are beginning to understand why.
Cumulous cloud: These puffy clouds are associated with sudden rainstorms. Scientists are beginning to understand why.

Sun and rain
Ever noticed the bright, moving lines on the bottom of a stream, bathtub, or swimming pool? They’re called caustics, and they’re caused when ripples on the water’s surface focus sunlight. (Caustics form whenever light rays are bent by a curved surface or object and then projected onto another surface.

Caustics have a characteristic shape. Physicists can graph the phenomenon mathematically, and the graph also describes other phenomena, such as particle motion or the movement of raindrops within a cumulus cloud.

Caustics: What do the rippling patterns on the bottom of a swimming pool and cumulous clouds have in common? (Photo by R. Motti)
Caustics: What do the rippling patterns on the bottom of a swimming pool and cumulous clouds have in common? (Photo by R. Motti)

Atmosphere to outer space
The researchers say their finding won’t have any impact on weather forecasting. But particle collisions in turbulent gases must have been involved in planet formation. Perhaps the same theory can be applied?

If you're at the museum on Saturday afternoon (11/18), the MakeIt team can help you play with caustics. Does bending mylar in a different direction produce a new pattern? Does using a different color flashlight or a brighter or dimmer light affect the design?

You can also play with caustics at home.

Your Comments, Thoughts, Questions, Ideas

Anonymous's picture
Anonymous says:

You could give more information

posted on Thu, 11/30/2006 - 2:07pm
Liza's picture
Liza says:

So could you!

If you know something about caustics, or rainclouds, or anything else that's relevant to the article, and you want to share, the Science Buzz staff want you to post it. That goes for anything on the site.

Information gets deeper, richer, and better as more people contribute.

posted on Mon, 05/07/2007 - 5:09pm
Anonymous's picture
Anonymous says:

this is cool

posted on Thu, 03/22/2007 - 9:03pm
Anonymous's picture
Anonymous says:

thats horrific

posted on Sun, 04/27/2008 - 8:56am

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