The past week has seen a crazy set of storms in Australia’s cental east and southeast, heralding strong wind gusts peaking at 100km/h in regions of northern NSW, record rainfall along the NSW/Vic border and Tassie, millions of thunderstrikes across the country’s east and many cases of large sized hail (See our previous stories on the matter here). 

Now the common misconception is to think that hail is just frozen rain, however the way hail forms is much more complicated. It not only depends on the change of temperature in the atmosphere but also on the amount of energy available in the atmosphere which can be used to lift droplets back up into the atmosphere and the change in wind speed and direction. 

When rain falls through cold air and freezes as it descends, it’s called sleet. Sleet needs cold surface temperatures to be observed by us at the surface and is therefore most common in winter. Whilst as we saw this week, hail can occur on days with surface temperatures exceeding 30 degrees! 

Temperatures in the upper atmosphere are much cooler than on the surface and so water can be seen in the form of ice crystals (really high up) or supercooled droplets (liquid water below 0°C). When a storm comes through, it pushes these crystals/droplets higher up in the atmosphere through strong convection. The cooler it is, the more non-gas water particles there are. These water droplets/crystals bind together to form a larger droplet/crystal sizes and get dragged down by gravity. But wait it doesn’t stop there... 

Storms that have a good amount of energy for convection will act like a vortex, and suck these larger droplets/crystals up to the upper atmosphere once more. There, the droplet/crystal gets larger, solidifies into a hailstone and falls again with gravity. And if the storm has enough energy, the process above repeats again. 

Hail eventually falls to the ground when the weight of the hailstone becomes too heavy for the storms convective energy to suck it back up again. So, the size of a hailstone essentially depends on the time spent/number of times it’s gone through the storm’s updraft, which depends on the amount of energy the storm has, and the amount of melting which occurs to the hailstones outer layer before it hits the surface. 

The strongest storms we see in Australia’s east typically occur between the months of November and January, which actually matches perfectly with the months with the highest frequency of hailstorms! So in short, it is normal to have more hail in spring and summer than it is in winter.