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 cooling, high wind shear situation (supercell), relatively low PW (reduces water/ice loading thus allowing for a stronger updraft). |