Why’s it bumpy when it’s hot?
We get asked this question frequently and as the weather warms up coming into summer we will be flying in the cooler parts of the day because of it.
The answer is... convective turbulence.
The earth’s atmosphere is heated by the sun, but not directly. The light and heat we feel from the sun is solar radiation. Solar radiation is very energetic and has a short wavelength. It passes through the atmosphere without much heating on the way. When it meets the ground some of it is reflected back up into the sky and space, and some of it is absorbed which in turn heats the ground. The ratio this occurs at is called albedo and it varies depending on the surface. Snow has a very high albedo of up to 0.9. This means 90% of the solar radiation that reaches it is reflected back, only 10% is absorbed. A bitumen road can have a very low albedo at about 0.05, which means it absorbs 95% of the solar radiation reaching it. The earth’s land area has an average albedo of about 0.3 which is a lot of solar radiation absorbed by the surface.
As the ground is heated it radiates energy at a much longer wavelength than solar radiation and is easily absorbed by the atmosphere above. This combined with the heating of air directly in contact with the surface through conduction results in heating of the air near the ground. When air is heated it will rise, this is called convection and is a driving force for all weather in our atmosphere.
Dubbo is in a region called The Western Plains and it’s easy to see why. From the air you can see the countryside is made up of a vast flat patchwork of paddocks, fields, and forests of various colours. Dark green fields of wheat that become burnt yellow later in the season, lighter green pastures, brown or red paddocks freshly ploughed, bright yellow canola, bluish-green eucalyptus forests, darks blue lakes and dams. It’s this constant variation of the surface, this constant variation in surface albedo that gives rise to a huge amount of atmospheric warming at different rates.
The air above a ploughed field will heat at a different rate to air over a forest, which will heat at a different rate to air over a town, and so on. This means pockets of air are warming and rising at different rates over our patchwork countryside on a hot summers day.
This means turbulence! These pockets of air rising randomly as we fly through them disrupt the flight path of the aeroplane and make bumps. Hot days means more heating which increases the energy of the rising air pockets and increases convective turbulence.
On a hot day flying in the morning before the sun has had time to drastically warm the ground will avoid convective turbulence.
If you have any more aviation questions send them to firstname.lastname@example.org and I can answer them in a future blog.