Convection » Hail Fawbush-Miller Method for Determining Maximum Hail Size
Having now determined that hail is likely to be observed in thunderstorms later in the day, we proceed to try and estimate the maximum hail size. There are several empirically-derived, hail-size forecast algorithms; we will focus on one here. The Fawbush-Miller method (Miller 1972) is a tried-and-true method that has been used by U.S. Air Force forecasters since the 1950s.

Procedure:
- Identify the CCL and the isobar where the sounding temperature crosses the -5°C isotherm. We will refer to this isobar as -5I
- From the CCL, proceed up the moist adiabat to where it intersects the -5I. This is point A.
- From -5°C at the -5I level (Point B), proceed down a dry adiabat to the level of the CCL. This is point C.
- Take the temperature difference A-B. On the nomogram, this is the base.
- Take the temperature difference C-B. On the nomogram, this is the altitude.
- Apply the Fawbush-Miller nomogram 1 to determine the maximum hail size.
- Check to see if the wet-bulb zero height is above 10,500 feet (3.2 km or ~670 hPa). If so, modify the maximum hail size from nomogram 1 using Fawbush-Miller nomogram 2.
In this procedure, the temperature difference A-B serves as a substitute for lower cloud buoyancy and hence, vertical velocity. The difference C-B serves as a substitute for cloud depth below the -5°C level.
The second nomogram provides a modification to the maximum hail size forecast to account for higher wet-bulb zero levels that are typically observed with summertime thunderstorms over the southeast U.S.