Red sky at morning…

First of all, see James’ post below with all of the pictures of the red sunrise this morning.  You’ve probably heard the phrase “Red sky at morning, sailors take warning; red sky at night, sailor’s delight”.  Did you know that Jesus mentioned this phenomenon (see Matthew 16:2-3)?

There is a scientific basis for this.  When the sun is low on the horizon at a point on earth, like at sunrise and sunset, it is shining through a lot of substances (air, water vapor, ,etc.) that weaken, or attenuate, the intensity of its radiation before it can reach that point (shown below).

The view is looking down toward the north pole, and the earth is rotating counterclockwise.  At point 3, where it is noon, the sun doesn’t have to propagate through much atmosphere before reaching the ground, so all wavelengths can come through.  However, at point 1 (sunrise) and point 2 (sunset), the sun has to propagate through a lot more atmosphere before reaching the ground, allowing more attenuation of the sun’s energy.

The other factor here is attenuation as a function of wavelength (color) of the light.  The spectrum below shows that red light has the longest wavelength, and blue/violet light has the shortest wavelength.  Radiation with longer wavelengths (lower frequency, red) gets attenuated less than radiation with shorter wavelengths (higher frequency, blue).  So on a night with a clear sky, the sunset will be red or orange near the horizon, where the light is being attenuated most and only the red light makes it through, but still look blue up in the sky, where less attenutation is occurring because the sky is being illumanted by more sunlight from over the horizon, allowing blue to come through also.

(Wavelength increases toward the bottom in the spectrum on the right).

Now, if clouds, that generally move west to east, are moving into the area, they may obscure all but the eastern edge of the sky, where the sun is rising.  This means that, at sunrise, only the light lowest on the horizon will come under the clouds, and that is the red light.  Since clouds moving into an area often means rain is approaching, a red sky at morning often means rain is ahead.

If clouds are moving out of the area, they may obscure all but the western edge of the sky, where the sun is setting, allowing only the red light to come through at sunset.  When clouds are moving out, generally the weather should be improving.  Therefore, red sky at sunset often means good weather ahead.

(this is explained in the radiative transfer equation, for those interested in looking it up).

Category: Met 101/Weather History