While most of the hazards below are highly unlikely to cause any serious injury or death, astrophotography in less-than-ideal conditions can lead to you not getting the results you desire from your photographs, and can even damage your camera equipment. As astrophotography can take place almost anywhere (such as on mountains, in the forest, in snowy conditions and at sea), it may also be necessary to exercise additional caution as there could be other risks present, such as avalanches on mountains.

For most situations, there are three types of conditions that can be detrimental to astrophotography, precipitation, obstructions in the sky, and extremely cold temperatures.

• Precipitation: any form of precipitation, whether it be rain, snow or otherwise, is detrimental as astrophotography would be almost impossible during these conditions. Most importantly, the clouds would restrict view of the night sky, however the falling particles of precipitation would also further hamper any visibility in the air, and make it impossible to get a clear long exposure photograph. Additionally, precipitation is harmful and can cause damage to camera equipment that gets wet.
• Clouds: while astrophotography is still possible with a small amount of cloud cover, you still need at least half of the sky to be visible in order to create decent photographs of the night sky. Most of the time, an absolutely clear sky is preferred for astrophotography, as you are able to get maximum coverage of the sky. High pressure systems in the forecast are a good sign as they mean that the likelihood of clouds is low. If a low pressure system or cold front is approaching, it would be wise to reschedule your photography to another day. 
• Fog: similar to clouds, fog can completely obstruct your view of the sky as well. Since cool air is denser than warm air, cool air often settles in valleys, causing valley fog to form. Additionally, fog is more likely to form in more humid conditions. Therefore, during situations where fog is likely to form or has formed, it is best to go to a higher elevation that might be clear of any fog. 
• Any sort of obscuration: other than clouds and fog, other forms of obscurations such as smoke, volcanic ash, haze and dust can hinder your ability to take good photos. Sand or dust flying through the air can also damage your lens and camera equipment. 

Photo credit: Alirod Ameri. Astrophotography is indeed possible in partly cloudy conditions, and it can produce an interesting effect. Despite this, you would usually want at least half of the sky to still be clear and visible. In overcast or foggy conditions, astrophotography is simply impossible. 

• Extremely cold temperatures: cold temperatures are often preferred for astrophotography as there is less atmospheric distortion. However, extremely cold temperatures can cause camera equipment and batteries to stop working. Most cameras are rate to work in temperatures ranging from 0˚C to 40˚C. With caution, most cameras can work down to temperatures as low as -20˚C though. Furthermore, since astrophotography can take a long time, standing still in the cold outdoors can also be dangerous for you as well. 

Photo credit: Philippe Roberge. The Milky Way as seen from Northern Ontario.