For astronomers, clouds are both a blessing and a curse. When the weather clears, we see the stars and planets clearly. The lack of haze makes viewing the night sky easy and pleasant.
Unfortunately, when clouds cover the sky, conditions become difficult. Not only do objects appear dimmer, but stray light from streetlamps, city lights, and moonlight can make things look strange when viewed through dense cloud layers.
Additionally, observing instruments such as photographic plates or CCD arrays require relatively low levels of light.
Most astronomical observatories are far removed from urban areas, and the occasional passing airplane usually doesn't bother them. Clouds change these situations. They reduce the useful depth of field and contrast ratio.
By reducing usable light levels, clouds interfere with efforts to take pictures, measure spectra, or even read numbers off scales, and so have a detrimental impact on the quality of your stargazing.