Learn Polish: Tam zawsze jest śnieg, ale dzisiaj nie ma śniegu. - There is always snow there, but today there is no snow.

This is the main rule for expressing "lack" or absence in Polish. When something exists, as in jest śnieg (there is snow), we use the basic dictionary form (the Nominative case). But when we say something is missing using nie ma (there is no), the noun that follows must change to the Genitive case. For the masculine noun śnieg, that means adding an -u to make śniegu.

Wait, doesn't ma mean "has"? Why does nie ma mean "there is no" here?

You are right that ma normally means "he/she/it has". However, the fixed phrase nie ma is the standard Polish way to say "there is no", "there isn't", or "is not present". Just remember: jest means "there is", but its exact opposite is always nie ma, never "nie jest".

For the Genitive case of śnieg, why is the ending -u instead of -a?

For masculine non-living things (inanimate nouns), the Genitive ending can be either -a or -u. There is no perfect rule, but the -u ending is very common for uncountable things, substances, and weather words. That is why words like śnieg (snow), deszcz (rain), and cukier (sugar) all take -u when they follow nie ma.