John Wheeler: When snowflakes fall from an apparently clear sky
This has to do with the speed at which snowflakes fall.
FARGO — When a summer shower passes you, it is sometimes possible for the sun to come out from behind a cloud while it is still raining. This is more likely in the morning or later in the afternoon when the sunlight is coming in from the side while the cloud making the rain is still overhead. Sometimes in winter, it can be snowing with virtually no evidence of clouds at all. The difference is the speed at which snowflakes fall compared to raindrops.
Raindrops fall at a rate of approximately 20 to 25 miles per hour, but the wispy, flat shape of snowflakes causes then to fall at only one to two miles per hour. This means if a snowflake is falling from a cloud a thousand feet up, which is fairly common, it will take ten to twelve minutes to reach the ground. This is plenty of time for the cloud of a smaller snow shower to move beyond most of the horizon.