Green, yellow, pink sky last night eastern BHM

Here in Trussville (TSV in radar picture), as the thunderstorms moved to our west just before sunset last night, the sky turned some interesting colors.  At first, it appeared to be green, then, as light faded, it went through yellow and pink.  Did anyone else see this?   Check out James’ post below “Stormy Summer Sky” for some pictures as the storm moved into BHM.

What was happening involved some interesting physics, and brings up the question as to why the sky sometimes appears green near intense storms.  I’ve heard several theories, but I’ll tell you about the one that makes the most sense to me (and is consistent with my observations over the years).

When the sun gets low on the horizon (near sunrise and sunset), it has to go through a lot more of the atmosphere before reaching your location than it does in the middle of the day when it is nearly overhead.  The sun emits light at all visible wavelengths, so during the day, the blend together to produce nearly white light.  However, near sunset, the sunlight gets attenuated (weakened) as it goes through more air, and near the horizon, only the longest wavelengths of light can make it through (shorter wavelengths/higher frequencies are attenuated more…just like in sound, you can hear bass a lot farther away than treble.)

Since only the longest wavelengths make it through to your location near sunset, the sky often appears red, orange, or yellow.  See the sunset picture with clouds I took last year over the Warrior River in September.  The yellow and orange shades are most apparent.

Then, remember that water is blue.  In small quantities (a glass) it looks clear, but in larger quantities (where you look through more of it) it appears blue. Think about a swimming pool with white sides, or what the ocean looks like from space.

So, near sunset, when yellowish light is shining through a large thunderstorm (containing millions of  gallons of water), the yellow and blue can combine to make green.  As the sun sets further, the remaining yellow or pink light may be reflected in the clouds.

To illustrate just how much water the sunlight was shining through around sunset last night, take a look at the radar reflectivity image below (from 0050 GMT, 7:50 pm CDT), and the cross section from the side along the 32 mile radius from the radar shown.

Category: Met 101/Weather History