Sky transparency
Unless you’re extremely unlucky or are just starting out, you have probably experienced clear skies at one time or another. They can be breathtaking and highly addictive, refuelling your desire to be out under the stars. However, spectacularly clear skies are typically few and far between. Fully opaque skies, on the other hand, are all too common from wet countries such as the UK. Yet there is a huge diversity of weather that sits in between these extreme states, and although some of the intermediate conditions are not perfect for astronomy, you can still learn how to make the most of those nights when observing is a bit trickier.
If the sky is clear, the degree of clarity is quantified by a value called transparency. This may, for example, be recorded as a number ranging from 0 (opaque cloud) to 7 (perfect). It should be noted that transparency is not an observational indicator of cloudiness, but is intended to indicate the opacity of the atmosphere; in other words, how hazy it appears and how easily objects can be seen through it. Hazy skies can occur due to excessive moisture or because there’s dust in the atmosphere. Typically, hazy skies are a combination of both. Pollen is another contributor to poor transparency and it’s important to realise that this introduces an annual cycle that affects the quality of your sky. After heavy rain the atmosphere often exhibits good transparency due to the fact that much of the dust contained within it has been washed out. So even if a downpour stops one night’s observing, it means you could be due for a clear sky the next day.
Good transparency will give the best views, objects appearing brightest and faint wispy detail around nebulae and galaxies will be easier to detect. As the sky becomes less transparent, such detail is gradually lost. Although less affected than diffuse deep-sky objects, the view of a bright planet will still be degraded by poor transparency; contrast is diminished and small features become harder to discern and image.