Science Q & A

Why do clouds look white?

New York TimesApril 28, 2013 

Q: What makes clouds white? Is this a property of light or of moisture?

When clouds are white (and many are not), “it’s because light from the sun is white, a combination of all the colors of the rainbow,” said Michael Kaufman, professor of physics and astronomy at San Jose State University in California.

As light moves through the clouds, it is dispersed by water droplets. These are comparable to or bigger than the wavelengths of light, Kaufman said, and there are lots of droplets in the average cloud.

The result is that all the wavelengths are scattered more or less uniformly; to our eyes, no color predominates.

“Looking at a cloud from any other angle, you see a mixture of all the scattered colors, so it looks white,” Kaufman said. “White light in, white light out.”

This effect is called Mie scattering, he added, “and is what you get when the wavelength of light and the particles it hits are comparable in size. It also works when particles are bigger than wavelengths of light.”

When light is moving through cloudless atmosphere, on the other hand, our perception of it is very wavelength-dependent. The small gas particles in the atmosphere scatter blue light about 16 times as efficiently as they scatter red light, an effect called Rayleigh scattering. That is why the sky is blue.

Storing medical images

Q: When doctors order X-rays, CT scans, PET scans, MRIs, ultrasounds and mammograms these days, how are the images stored and transmitted?

All kinds of images can be captured, reviewed, stored and transmitted digitally, using a standard format called Dicom, for digital imaging and communications in medicine, said Dr. Keith Hentel, executive vice chairman of the department of radiology at New York-Presbyterian Hospital/Weill Cornell Medical Center.

Dicom files contain not only the images but also information about the patient and how the images were acquired, along with the radiologist’s annotations, Hentel said.

The viewer can manipulate Dicom data in ways as simple as making standard measurements or as complex as producing detailed three-dimensional representations of the anatomy.

The images may be printed on film or even on paper, Hentel said, but most are interpreted by radiologists electronically and stored on systems called PACS, for picture archiving and communication systems. For those without access to such systems, the Dicom files can be put on a CD-ROM along with a viewing program.

Patients can increasingly expect to have their images securely archived and easily accessible on Web-based or cloud-based services, Hentel said, making them available wherever and whenever needed.

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