Adam Clarke on Exodus 16:14

Behold, upon the face of the wilderness there lay a small round thing - It appears that this small round thing fell with the dew, or rather the dew fell first, and this substance fell on it. The dew might have been intended to cool the ground, that the manna on its fall might not be dissolved; for we find from Exo 16:21, that the heat of the sun melted it. The ground therefore being sufficiently cooled by the dew, the manna lay unmelted long enough for the Israelites to collect a sufficient quantity for their dally use.