For the discussion below, only the interaction between visible light and our atmosphere will be considered.
Cloud droplets are very strong scatterers of visible light. They can also weakly absorb visible light. Because of this, cloud coverage is one of the most important factors to consider in ground-based observational astronomy.
Perhaps obviously, if the sky is covered in clouds … you won’t be able to get much visible light from space. Absolutely clear skies are ideal, but it can be difficult to discern with the unaided eye exactly how clear the skies above you actually are. In particular, we need to watch out for cirrus clouds or high, thin clouds. These are wispy clouds that are nearly invisible with the unaided eye, but will still scatter light and therefore disrupt astronomical observations. This is why it is important to rely on weather forecasting instruments and other data sources.
[Image from NOAA satellites showing nearly clear skies in visible, but obvious thin clouds in IR]
Even with cloudless skies, the Earth’s atmosphere is made of mainly nitrogen and oxygen. Both of these scatter visible light very effectively. In general, this “Rayleigh Scattering” is responsible for the reds, oranges, and yellows of our setting and rising Sun.
In addition to the nitrogen and oxygen, there is water vapor and other particles in the atmosphere that can further disrupt observations. In general, these other particles are called aerosols. Some good examples are smoke, pollution, and wind-blown dust. These aerosols also tend to absorb incoming light and so contribute heavily to light pollution. Light pollution is the result of light scattering strongly off of aerosols, and it makes it challenging to tell the atmosphere apart from the object you’re attempting to view.
Transparency is a general measure of how well we can view distant objects in space through the water vapor and aerosols in the atmosphere. The lower the humidity and particulate pollution, the higher the transparency.
As light travels through our atmosphere, it encounters regions of the atmosphere with slightly different temperatures. These slight changes in temperature cause the light to change directly slightly by a process called “refraction”. If light must travel through many such different regions, all of this refraction causes the light that reaches the ground to appear to be jumping around or maybe flickering. We all know about this process – stars twinkle!
The regions with different temperatures are always present in our atmosphere, but how much the different regions mix around is caused by atmospheric turbulence. With very heavy mixing, the light we get on the ground jumps around constantly, obscuring an otherwise perfect image. A measure of how perfect an image (e.g., a star) appears through a telescope is called the “astronomical seeing”. As described above, the seeing is a factor of atmospheric turbulence and it ultimately depends on the temperature, wind shear, and water vapor content in the air.
The snapshot on the far left would be referred to as “good seeing”. This means that the level of atmospheric turbulence is low and the light moving through the atmosphere ends up not jumping around as much. The image on the far right shows an example of “bad seeing”, indicating very high atmospheric turbulence. As a result, the light moving through the atmosphere jumps around constantly and the image of the star (in this case) is smeared out into what is called a “seeing disc”.