thunderstorms and hail

see also:

• climate - water and rain
• climate: air and wind
• how to reduce your risks in lightning storms
• photographing storms and lightning

• thunderstorms are common in most parts of Australia, particularly in the tropics during the wet season
• they can cause damaging winds, hail, flash floods and lightning

• these are the common thunderstorms seen:
  • in the tropics during the wet season
  • those arising over hot land due to convective uplifts and developing in the afternoons
  • those preceding cold fronts
• these tend to be associated with intense rainfall dumps, flash flooding, severe wind gusts, hail and lightning

• require a deeply unstable atmosphere, characterized by a significant temperature difference between the warm surface air and the very cold air above it
• these are mainly in the southern states of Australia in winter, often associated with Low pressure systems to the south of the continent bringing fast moving cold air masses from the Antarctic over warmer land or sea surfaces
• these storms tend to be fast-moving due to being in a strong wind shear environment
• hail and bursts of heavy rainfall are typically observed in Victoria, however, rarely, thundersnow may occur in alpine areas

• hail generally forms when there are these four factors present:
  • deep moist convection
  • adequate updraft to keep the hailstone aloft for an appropriate amount of time
    • high convective available potential energy (CAPE), especially > 1000J/kg as these lead to high upward velocities within a thunderstorm
    • lower precipitable water values have the potential to produce large hailstones when significant CAPE is present as there is less gravity water weight loading to oppose upward convection
    • high wind shear such as in a supercell allows CAPE to be maximised by separating the updraft and downdraft
  • sufficient supercooled water near the hailstone to enable growth as it travels through an updraft
    • high altitude regions will be closer to cold layers of upper atmosphere and storms there are more likely to produce hail
    • low freezing level (the height of atmosphere above which it is freezing)
      • at low elevation, if the freezing level is closer to the surface than 650 millibars, strong thunderstorms have a good probability of producing hail that will reach the surface
      • freezing level can be found by examining the morning or afternoon Skew-T Log-P plot or forecast sounding
      • dry mid-level air entrained into the storm is evaporatively cooled and can lower the freezing level as well as create strong surface wind gusts
  • nucleation: a piece of ice, snow or dust for it to grow upon
• hail falls when the thunderstorm's updraft can no longer support the weight of the ice
• nearly all severe thunderstorms probably produce hail aloft, though it may melt before reaching the ground
• multi-cell thunderstorms produce many hailstones, but they are not usually very large as the mature phase is short and not enough time for large hail to form
• supercell thunderstorms have sustained updrafts that support large hail formation by repeatedly lifting the hailstones into the very cold air at the top of the thunderstorm cloud
  • hail 2 inches (5 cm) or larger in diameter is generally associated with supercell thunderstorms