Kevin Myatt is The Roanoke Times' weather columnist.

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@roanoke.com
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Weather with Kevin Myatt

Recent columns

• We got graupel, but not on official record
• Moisture could get caught up in cold blast
• Forecast for Weather Journal: Partly print, with frequent Internet
• Column archive

Read the Weather Journal blog

• Sprinkles or flurries possible Tuesday, but maybe something bigger for the weekend?
• For now, it looks like a quiet, mostly mild week ahead for SW Virginia
• Coldest morning of winter so far likely across much of Southwest Virginia; Tuesday precipitation looking doubtful
• Weather Journal blog

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Reader question: What causes clouds to appear different shades of gray? What about the greenish color the sky/clouds can take on prior to a tornado?

Sunlight and the density of moisture are the two most important factors in cloud color.

The darkness of a cloud is determined by how much sunlight can get through it, which is largely determined by how dense the water droplets are within the cloud and how thick the cloud is.

The denser the cloud or the taller the cloud extends into the atmosphere, the more light it is likely to block, giving it a gray or even black appearance. Thinner clouds, with less dense moisture and/or less thickness, block less light, and so can appear white as more light penetrates.

Early- and late-day sunlight often adds a red or orange cast on clouds. I have seen deep blue, green and yellow shades in and around violent thunderstorms. Various theories have been advanced to explain the phenomenon, but the common idea is that it has to do with what wavelengths of light both reflected away by and refracted through the larger raindrops abundant in thunderstorm clouds that can tower up to 8 to 12 miles high.

If red light waves are reflected away, that leaves blue and green colors as the primary light waves getting through the clouds.

Keep in mind that whatever color you see in any cloud may appear differently to someone else viewing the same cloud from a different angle, as sunlight would reflect off it or refract through it differently.

Kevin Myatt's column runs Monday, Wednesday and Friday.