When we have one of those really icy-feeling days, we could end up with wintry hail. But what exactly does that mean?
Wintry hail is not the like the warm, summer thunderstorms that bring large hailstones. Those stones are produced by raindrops going up and down in the storm cloud, growing in size every time they ride the updraft.
In fact, wintry hail doesn't necessarily come from thunderstorms.
All winter precipitation starts out as snow but how it looks when we see it at the ground all depends on what temperatures it falls through.
Generally, in most towns in Australia, we will have raindrops reach the ground.
Rare for Australia is freezing rain. Snowflakes turn to raindrops that partially refreeze in cold air, coating the ground with ice. The temperature at the surface is close to zero.
Ice pellets are the wintry hail that I mention. BoM use the term "possible hail" or "local hail" in the forecast, and separate it from thunderstorms.
- They are small (pea-sized), translucent balls of ice, that bounce when they hit the ground and make a high-pitched tapping noise.
- They fall within a mass of raindrops, but my theory is that they have fallen faster, so each pellet is still frozen by the time it reaches us.
- The temperature at the surface is still well above freezing, so its not snow.
Snow is precipitation that remains frozen as snowflakes all the way to the ground. The temperature at the surface is in the negatives, 0C, or up to 2C - and very rarely as high as 4C (it would need to be a very low dew point to still remain as snow).
We can also add sleet to that list. This one remains in cold air for much of the trip down, warming up just enough when it nears the ground. That is a wet snow mix, or snow that is melting by the time it reaches us.
When the weather map shows a long southwesterly fetch - ie the lines on the map form a 'road' coming straight up from Antarctica - with a cold pool (thickness dropping below 5400, the blue lines), then we have the chance of wintry showers (ice pellets within raindrops, in showery activity that stops and starts).
This is what a cold pool looks like as it approaches the country:
And this is what it delivers - little balls of ice, not snow (its still +10C!):
This may help explain further:
