James Pembroke, age 9, of Philadelphia, Penna., for his question:

Is it ever too cold to snow?

A winter's day in Alaska may be colder than 50 degrees below zero. But that is no guarantee that snow will not fall. Actually, it’s never gets too cold to snow. It is true, however, that the lacy white flakes are more likely to form when the winter air is warm ‑ warm for the season, that is.

Snow forms when water vapor in the air crystallizes at temperatures below freezing point. The feathery flakes are composed of tiny fragments of ice and pockets of air. In order for snow to form there must be vapor in the air and the air must be cold enough to crystallize this vapor to ice.

The warmer the air the more vapor it can hold and the more vapor it is likely to hold. Cold air must give up its surplus vapor. The cold air of winter has usually shed its surplus vapor. It is usually dry air. That means that it has little vapor with which to make snow flakes. It is less likely to snow in real cold weather, but snow is never impossible. For, no matter how dry the air becomes, there is always a little vapor willing to turn into snow flakes.