Why can a
constellation x not be seen in the
summer, but only in winter?

Because you do all of your stargazing at the same time every night, but the
stars and constellations reach the same place in the sky about 30 minutes
earlier every week. 

Pick any constellation that interests you.  If you're willing to look for it
at different times of the night, you can find it SOMEWHERE in the sky
at SOME time of the night, during as much as 11 months of the year.

Answer volcanic outgassing;