Although temperatures have been above normal in Milwaukee for much of the month, snow pellets have actually fallen on the city twice in the past week.
On Saturday and again on Tuesday, the Milwaukee-Sullivan National Weather Service station recorded a "trace" of frozen precipitation. Low temperatures on those days hit 46 and 40 degrees, respectively.
However, this recorded precipitation was not snow, local NWS meteorologist Taylor Patterson said. It was a mixture of hail and graupel -- two kinds of small, pellet-like frozen droplets.
So, what is graupel, and what makes it different from hail? Here's what to know.
What is graupel? How is it different from hail?
Graupel and hail look very similar, so, you might not even realize it's "graupeling" outside. However, the process of making graupel is different from hail, and graupel also has some subtle physical differences.
"The best way to describe the difference is graupel tends to be a little smaller ...," Patterson said. "Graupel tends to have a softer appearance, something you can kind of crush between your fingers. So, it's pellet-like, but it's not necessarily a hard sphere like a hailstone."
According to AccuWeather, graupel is also known as "snow pellets." Graupel pellets typically have a diameter of 0.08 to 0.2 inches, while hailstones can range in diameter from a common, pea-sized 0.25 inches to a dangerous, softball-sized 4 inches, AccuWeather says.
How does graupel form?
Graupel forms when extremely cool water droplets at the top of a cloud interact with snow crystals, Patterson said.
"Much of the precipitation that falls throughout the year begins as snowflakes high in the clouds," AccuWeather explains. "The snowflakes form as air rises, cools and condenses, usually around an area of low pressure." Whether precipitation remains snow or becomes rain, freezing rain, sleet, hail or graupel by the time it reaches the ground is determined by the temperature fluctuations the snowflakes encounter as they fall through the layers of the atmosphere.
Graupel forms when snowflakes become coated with ice. Like hail, graupel can occur at any temperature, Patterson said. However, because part of the cloud needs to hit freezing for graupel to form, the colder it is, the more likely it is that the necessary cold water will be present, she said.
Graupel and hail often fall in the same event. Graupel is more likely earlier in a storm; Patterson described it as "pre-hail."
How is hail formed?
The process of creating hail is similar to the process of creating graupel. According to the National Weather Service, hailstones are formed when raindrops are carried upward by thunderstorm updrafts into extremely cold areas of the atmosphere, causing the raindrops to freeze. Hailstones then grow by colliding with liquid water drops that freeze on the hailstone's surface.
Hail falls when the thunderstorm's updraft can no longer support the weight of the hailstone, which can occur if the stone becomes large enough or the updraft weakens, NWS says.