I received this question by e-mail today:
"I just saw something weird today that I hadn't thought about before.. I know there can be sun-showers, rain on a mainly clear day, but walking to work this afternoon it was snowing even though the sky was pretty much blue and clear, except at the horizon.
I hadn't really thought about it before, but I wonder how that happens.. if it's clear, I'd assume high pressure, but for it to snow, even flurries, I'd expect lower pressure...
Just wondering if you know how this happens.. "

My response:
Here's what I think you might have seen taking place:
The terminal velocity of a raindrop is much greater than that of a snowflake. Therefore, a snowflake may reach the surface well after the cloud that produced it has passed away. Also, wind can easily waft snowflakes a significant horizontal distance. This is especially true of small snowflakes that have a low density (the kind that you likely saw this afternoon).
Clouds that produce snowfall can dissipate rapidly and/or move downwind after producing snow. This can cause snow to be observed at the surface while the sky is partly cloudy or clear.