in Thoreau’s Journal:

What produces the peculiar softness of the air yesterday & today—as if it were the air of the south suddenly pillowed amid our wintry hills— We have suddenly a different sky—a different atmosphere.

It is as if the subtlest possible soft vapour were diffused through the atmosphere. Warm Air has come to us from the S. But charged with moisture—which will yet distill in rain or congeal into snow & hail—