Weather school: Why is the sky blue?

Insolation is the driving force behind all our weather

At sunrise and sunset, the angle of the sun is such that the insolation has to travel through much more atmosphere than during the day.

After taking a bit of a break from Weather School, it’s time to get right back at it. With all the craziness going on in the world, hopefully a little bit of learning from the comfort of home or the office is just what you need. Plus, with really mild spring weather still a couple of weeks away, it’s a good time to dig into this bright topic.

In our last class we looked at the composition of the atmosphere, breaking it down into the heterosphere and homosphere. Then we looked at the atmosphere from a temperature point of view and proceeded to break it down into four regions or layers: the thermosphere, mesosphere, stratosphere and troposphere. We finished off by saying one of these layers is responsible for most, if not all, of our weather. This week we will get back on track and extend our understanding of weather and the atmosphere by beginning our look at surface energy balances.

To understand how solar energy is spent as it reaches Earth’s surface, and thus understand our surface energy budget, we need to look at the pathways that solar energy can travel once it reaches Earth’s surface.

The Earth receives energy from the sun in the form of shortwave radiation. When this energy is turned into heat, it takes on the form of long-wave radiation. A good portion of both of these types of radiation passes through our atmosphere in the process known as transmission. When we are looking at shortwave radiation reaching the Earth’s surface, we call it insolation, and it is this insolation that is the driving force behind all our weather. Insolation is comprised of shortwave radiation that is transmitted directly to the ground, along with diffused or scattered radiation (indirect radiation). As shortwave radiation travels through our atmosphere, some of it interacts with gas, dust, pollutants, water droplets and water vapour, changing the direction of the shortwave radiation — or scattering it. This scattering is what causes the sky to be blue during the day (it is not the reflection of the oceans) and why sunsets and sunrises take on a reddish hue.

Blue waves

The principle behind why we see these colours is known as Rayleigh scattering, named after the English physicist Lord Rayleigh, who came up with this principle back in 1881. The principle relates wavelength to the size of the particles that are causing the scattering. The general rule is: The shorter the wavelength, the greater the scattering; the longer the wavelength, the less the scattering. Small gas molecules will scatter shorter wavelengths (remember, with visible light, blues and violets have the shortest wavelengths, while oranges and reds have the longest wavelengths). So, since short waves are scattered the most and the molecules in our atmosphere scatter short waves, we end up having the lower atmosphere dominated by scattered blue waves.

At sunrise and sunset, the angle of the sun is such that the insolation has to travel through much more atmosphere than during the day. The short blue wavelengths are still scattered, but now they encounter so much scattering, only the longer orange and red wavelengths are left to reach our eyes — so we tend to see these colours.

Another thing that happens to shortwave radiation as it enters the atmosphere is that it refracts. Refraction is the bending of light as it passes from one medium to the next. In this case, it is passing from the virtual vacuum of space to our dense atmosphere. We have all seen examples of refraction. Rainbows are created when light passes through dense water drops, causing the different wavelengths of light to refract at different rates. Mirages are another example of refraction. Most of us have experienced mirages on warm days along a highway when you stare down the highway and see what appears to be something floating above the road. In this case, it is the hot air above the highway that causes the light to be refracted.

One interesting note about refraction is that without it, the amount of daylight we receive would be about eight minutes less each day. When the sun sets or rises, the light refracts as it passes from space into our atmosphere. This refraction allows us to “see” the sun when it is actually below the horizon. In the morning we see the sun rise four minutes before it actually moves above the horizon, and at sunset we continue to see the sun for four minutes after it has actually dropped below the horizon!

Next class we will continue our lesson by looking at reflection of insolation, or albedo.

About the author

Co-operator contributor

Daniel Bezte

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.



Stories from our other publications