AUSTIN (KXAN) — Tuesday afternoon, a weak disturbance moving across Texas brought some much needed rain to the region, but also something a little unexpected – sleet!
You may be wondering how and why did this happen if it temperatures were above freezing while it fell.
Two important things to be mindful of for our particular sleet event. One, how does sleet even form. And two, we had very dry air at the surface.
As you see in the image above, sleet forms as water droplets refreeze after passing through the warm layer in the atmosphere and then form little sleet pellets before hitting the surface. This is how sleet forms 99.99% of the time.
However, in Tuesday’s sleet event, temperatures were above freezing by 10 to almost 20 degrees – in the 40s to near 50 degrees! So what gives?
A meteorological phenomenon called the “Wet Bulb Temperature” played a big part into why this happened.
The Wet Bulb Temperature is the coldest temperature air can cool to due to the evaporation of water. Remember the other thing to be mindful of from earlier in this article? Dry air at the surface was evaporating water at a rate that cooled the Wet Bulb to below freezing.
Essentially, as the weak system moved overhead, it interacted with moisture aloft to produce precipitation. But as this precipitation began to fall towards earth, it interacted with the very dry air at the surface and caused it to freeze into sleet pellets.
And thus, we have sleet in isolated areas across Central Texas.