[ccp4bb] frosted crystals during storage in pucks

[<Rain Field>]

Hi All,
We found if the crystals are storaged in pucks for 3-4 days in shipping dewar 
(with liquid nitrogen), they are almost frosted.
Although I can wash them with liquid nitrogen, but it's not convenient to do 
that for each crystals.
I doubt it's because the humid air in North West America.
Does anyone has an idea how to avoid this?
Thank you!