Scott's Note: This blog entry has been edited and updated since its original post.
One of the most remarkable facets from Sunday morning's snow was the unusually large size of the snowflakes. In fact, a reader from Shoreline described it as "it looked like someone was plucking chickens in the sky."
Here is video of really large flakes, shot by Dr. Dale Ireland of Silverdale (the one who does all our Time Lapse videos):
As to why they're so large, it's a factor of both temperature and intensity of the precipitation.
When snowflakes are on the fringe of freezing, or just above freezing and they begin to melt, they become a bit more slushy. When that occurs, the water droplets on the edge of the snowflakes make it so they can stick to other snowflakes, much in the same way that wet snow is sticky and is good for making snowballs and snowmen.
So you can get an aggregate effect of snowfalkes sticking together to make them even larger. But it's got to be the combination of a heavy snow shower and temperatures at or a touch above freezing, as the heavier the shower, the more force you have in smashing snowflakes together. You also need light wind to keep the flakes from being blown apart.
Most of the snow on Sunday was around 33-34 degrees -- good conditions for big snowflakes with the heavy squall moving through.
Note: In my original blog entry, I had also tied the size of flakes to updrafts in the cloud, much like how raindrops end up different sizes. I have since learned that these processes are not necessarily tied together -- the biggest factor in size of snowflake is this warming and sticking together process.
But while I've got you here, here is how you end up with big raindrops, versus the little drizzle we get on occasion here:
You've noticed most of the time around here, the rain is generally light and the raindrops are small enough that you won't get too wet if you're caught outside. But then there are sometimes when the raindrops seem considerably larger and can leave you soaked in seconds.
Large raindrops mean that there are strong updrafts inside the clouds above your head. These upward blowing winds can hold raindrops inside the clouds for a longer period, allowing them to continue to grow in size until they finally become heavy enough for gravity to finally win the battle over the updraft, allowing the raindrop to fall to the ground.
That's why thunderstorms tend to have large raindrops, as they tend to also have strong updrafts. It's also why rain in the Midwest and East Coast can quickly leave you drenched.