An upper air low swirling over Arkansas, Tennessee and northern Mississippi kicked off a batch of showers Tuesday afternoon: some rain and some sleet mixed together!
Temperatures are far too warm at the surface for any major problem, but you may be wondering how in the world we can get frozen precipitation when we’re in the fifties (Huntsville’s high today was 53ºF).
The atmosphere is like ogres, who are like onions: it has layers (that’s a Shrek reference if you don’t get it).
It’s almost like what you see here:
The diagram above shows the surface at freezing. That’s not always true for sleet and snow!
On Tuesday, we started dry and cool. The surface temperature rose to the 50s, but that upper low to the west helped dynamically cool the next layer up (about 3,000 to 5,000 feet above our heads) enough to drop the temperature into the low-30s there.
Precipitation forming around 10,000 to 15,000 feet above the surface begins as snowflakes, then it starts melting as the temperature rises beneath the cloud. This little layer of colder air (30ºF to 32ºF) below that ‘warmth’ develops because of two things:
- Evaporational cooling (air naturally cools when water evaporates)
- Dynamic cooling (air naturally cools at a rate of 5.5ºF per 1000 feet when it is lifted)
When the melted snowflakes, now raindrops, fall through the cold layer some of them freeze again; since they’re falling quickly, they don’t all have time to melt for a second time before hitting the ground!
Brighter and milder days ahead
We’ve not had a sunny day in a while. It’s been nearly a week!
The sky becomes sunny again on Wednesday and Thursday, and that sunshine makes a huge difference in how it feels outside.
Expect highs in the upper 50s and some in the lower 60s Wednesday afternoon with a light north wind and bone-dry air.