Probably the biggest challenge astronomers face is the weather, or more accurately the atmospheric conditions. Cloud clearly is a bad thing for astronomy but other phenomenon can be as bad. We are all familiar with typical weather forecasts but there are a couple of other factors that describe the conditions, relevant to astronomers.
‘Seeing’
‘Seeing’ is the definition of how stable the atmosphere is. The sky can be the clearest you have ever seen it but if the stars are twinkling wildly then there is not much useful astronomy that can be done. Poor seeing is caused by airmasses of different temperature making the gas in the atmosphere move around lots. The effect to the naked eye is gently twinkling stars but through a telescope the stars jump around in and out of focus. Planetary detail can be lost in conditions like this. Seeing is defined using a number of different methods but the most popular is the Antoniadi scale which was invented by Greek astronomer Eugene Antoniadi during the early 1900’s. The system is a 5 point scale;
I Perfect seeing, without a quiver
II Slight quivering of the image with moments of calm lasting several seconds
III Moderate seeing with larger air tremors that blur the image
IV Poor seeing, constant troublesome undulations of the image
V Very bad seeing, hardly stable enough to allow a rough sketch to be made
Other systems define seeing more accurately but this will serve the purpose for most occasions.
Transparency
Another key condition is transparency. As its name suggests, it defines how clear the air is and as a result, the faintness of objects that can be seen. Certainly high level cirrus clouds can turn a clear sky into a milky murk, through which stars can just be seen. Pollution, smoke and haze to name a few other atmospheric phenomenon can also affect it. Its defined by a scale 1 to 10, 1 equating to zero visibility with 10 referencing perfectly transparent skies.
Cloud
These conditions aside, the weather itself is the biggest enemy of astronomy. Cloud doesn’t have to be bad, if you are looking at bright objects, planets for example, then a high level haze can indicate relatively stable air and can even cut down the glare, giving you a good view. Thunder and rain storms whilst problematic, if you can move fast, then you can get some of the best stability and transparency in the clear spells between the storm clouds. Fog and mist too don’t necessarily mean you have to leave your telescope packed away. If its only a thin layer, then it might stop just above your head and whilst perhaps you can’t see anything in front of you, a clear sky may beckon above.
Wind
Wind can be an unexpected problem too. Don’t forget that your telescope not only magnifies the object under study but it also magnifies any movement of the telescope, including any buffeting from the wind. Windy nights aren’t great unless you can shield the telescope in some way.
Dew
One final thought on the weather is dew. Telescopes have big lumps of glass in them. Glass tends to hold its temperature and as soon as cold air hits a warm object, dew forms on it. A good way to try and avoid this is to set the telescope up before nightfall so the optics cool down before dew appears. Sometimes, this isn’t enough and a backup plan is needed. You can buy heater straps which wrap around the tube of the telescope outside the optics and they lift the temperature of them to just above dew point, stopping dew from forming. On occasions, even they aren’t enough and you may be forced to employ the services of a hair drier to get rid of the dew. 12V versions can be particularly useful if you don’t have mains power available. Typically dew is more of a problem for refracting telescopes as the lens is more exposed to the elements than a reflecting telescopes mirror. Either way, anything is fair game for dew, finder scopes, mirrors, lenses or even eyepieces so be aware of it and if the image starts to degrade, you may be getting thwarted by dew.