As Houstonians wake up to a rare winter storm blanketing the area, many are experiencing unfamiliar weather phenomena. Sleet and graupel, two types of icy precipitation, are often confused for one another.
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Here’s how to tell them apart and what they mean for the Bayou City.
What is sleet?
Sleet, or ice pellets, forms when raindrops freeze into small, solid particles before hitting the ground. According to the National Weather Service (NWS), sleet begins as snowflakes in the upper atmosphere. As they fall, these snowflakes pass through a layer of warmer air, melting into rain. When the rain encounters a deep enough layer of below-freezing air near the ground, it refreezes into tiny, hard pellets.
Sleet is typically accompanied by a tapping sound as the pellets strike windows or roofs, and it’s more common during storms with temperatures hovering just below freezing—a scenario Houstonians are facing today.
What is graupel?
Graupel, also known as “soft hail” or “snow pellets,” is formed differently. It starts as a snowflake but collects supercooled water droplets on its surface as it falls through the atmosphere. These droplets freeze, creating a fragile, icy coating. The result is a small, white pellet with a soft, crumbly texture.
Graupel tends to occur when the atmosphere is unstable and there’s enough moisture for snowflakes to encounter supercooled droplets. In Houston, this phenomenon is especially rare but can happen during powerful winter storms like today’s.
How to identify sleet and graupel in Houston
If you’re curious about what’s falling outside your window, here’s how to tell sleet and graupel apart:
- Texture: Sleet is hard and solid, like tiny ice cubes, while graupel is soft and fragile, resembling Styrofoam balls.
- Bounce: Sleet bounces when it hits the ground; graupel does not.
- Appearance: Sleet is clear or translucent, while graupel looks opaque and snow-like.
Picking up a pellet can help confirm what you’re seeing. Sleet will feel like a tiny ice pellet, while graupel will crumble easily in your hand.
Why it matters for Houston
Sleet often results in icy, hazardous roadways, prompting residents to stay off the streets whenever possible. Graupel, on the other hand, is less likely to create significant ice accumulation but still serves as a reminder of the unusual weather conditions.