In my French learning book À Paris published by Cideb, p.126 we have
J'espère qu'à Noël, il y aura de la neige
Le principal, c'est qu'il n'y ait pas de pluie
Why is the definite article present in the first case but not the second? Is it context or is some rule at play here? I know there are some irregularities when reporting the weather, so perhaps this is another?