METEOROLOGIST JEFF HABY
Most precipitation within a
thunderstorm in the middle and upper levels of the atmosphere is in the form of
ice (snow and hail). The
PBL tends to have the warmest temperatures in the troposphere. When ice begins to fall
from a typical thunderstorm toward the surface, it must endure a lengthy ride from the
freezing level to the
surface. The freezing level associated with most mid-latitude thunderstorms is above the 700-millibar level.
This means ice has more than 3 kilometers to melt before reaching
the surface (in a low elevation regions).
High elevation regions have the freezing level located closer to the surface. This is one reason
hail is more
common in high elevation regions. Compare Denver's elevation to that of New Orleans. A hailstone falling over
New Orleans will have over 1,500 meters MORE in distance to fall before reaching the surface. Ice and hail
begin to melt rapidly once they fall into the low levels of the atmosphere where temperatures are above freezing.
It is like putting a blow drier to the ice. Warm temperatures around the ice and the velocity of the ice through
the warm air, melts and strips mass from the hailstone.
Most of the mass of ice and hailstones reach the ground
in the form of rain. Hailstones that reach the surface, especially in lower elevation regions, were truly
large pieces of ice when they were in the middle levels of the atmosphere. If a hailstone is huge, the melting in the
lower levels will not be able to melt the hailstone away. Large hailstones have a small surface to volume ratio and
thus do not melt as fast as small hail stones. Hailstones have a greater chance of reaching the
surface in: A high
CAPE situation, high elevation region, low freezing level, high potential for evaporational
wind shear situation (supercell), relatively low
PW (reduces water/ice loading thus allowing for a stronger