I took this picture on my morning run. It got me thinking about the old saying “red sky in the morning, shepherds’ take warning, red sky at night, shepherds’ delight” and wondering if it applies here in Colorado.
The basis behind this little rhyme is to do with aerosols and high and low pressure systems, and winds. Let’s think about the pressure first.
If you have a region of low pressure, it will have on both sides regions of high pressure, since the overall pressure cannot be altered. In a low pressure region, air converges from both sides to fill-up the space. It comes in at a low level and as it converges it is pushed upwards. Upwards motion of air like this causes clouds to form, and the precipitate out.
In a region of high pressure, air in the lower levels of the atmosphere is moving out, towards the lower pressure areas on either side. Therefore air from higher up is sucked downwards. As the air moves downwards it heats up (since the atmospheric pressure lower down is higher, so the air is pressurised as it moves downwards, and thermodynamic teaches us that when you increase the pressure of something you also increase its temperature). The fastest movement of air occurs at the tropopause, and so this is where the greatest heating occurs. This causes what we call a temperature inversion, where the air higher up is warmer than the air lower down.
So far so good, and now we need to add in the aerosols. Most aerosols come from lower down in the atmosphere, from pollution, fires, sea-spray, dust and other sources. In a low pressure region, where are is moving upwards, these are swept up and away with the air and dispersed, leaving the air lower down much cleaner. In a high pressure region, the air cannot move upwards because of the temperature inversion, and the aerosols are trapped lower down.
When sunlight passes through air, it is scattered by the molecules of the air, and by any aerosol particles present. The aerosol particles are much bigger than the air molecules. Light from the sun contains a all colours of light, and different colours of light have different wavelengths. Blue light has a shorter wavelength and can be scattered by smaller objects, red light has a longer wavelength and is scattered by larger objects. So a clear sky looks blue and a sky filled with aerosols can look red. It only looks really red when the sunlight is passing through a LOT of atmosphere before reaching your eye. This happens at sunrise and sunset, when the sun is lower in the sky.
And then, the last piece in the puzzle, why does it matter whether it’s sunrise or sunset. Well if you live between about 30 and 60 degrees latitude in either hemisphere then the dominant winds are westerlies (moving from west to east). So if you’re watching the sunrise, you’ll be looking east, and if it’s red then there is a high pressure region to east of you. This is moving away from you, and so what is moving towards you is a low pressure region, meaning clouds and rain. If you’re watching the sunset, you’re looking west, and if it’s red that means there’s a high pressure system to the west of you, and because of the wind direction, it’s headed your way, bringing cloudless skies along with it.
Now Boulder, Colorado lies within this band of westerlies, being situated at 40°N, so the saying applies here as much as it does anywhere. That’s not to say it’s always true, winds can still be in a different direction to the dominant one, and other factors can determine the concentration of aerosols in the boundary layer, but its still something I notice when I see a red sky.