The wet-bulb zero, or WBZ, is the height above ground where the wet-bulb temperature first drops to zero degrees Celsius as you move up through the atmosphere. It is read off a sounding and is usually reported in feet above ground level or above mean sea level.
Why wet-bulb, not dry-bulb
A hailstone melting as it falls cools the air right around it by evaporation as well as by direct heat transfer. The relevant temperature for that combined process is the wet-bulb temperature, not the ambient dry-bulb temperature, so WBZ marks the level below which a falling hailstone starts to lose mass to melting.
Because wet-bulb temperature is always less than or equal to dry-bulb temperature, the WBZ height sits at or below the freezing level. In a dry environment the gap between the two can be sizable, and any forecast that leans on the freezing level alone will overestimate how far a hailstone must fall before it begins to shrink.
How forecasters use it
The traditional guideline cited in NWS guidance, including the Louisville severe weather tip sheet, is that WBZ heights roughly between 7,000 and 10,500 feet above ground level tend to correspond with environments most favorable for large hail reaching the surface. A very low WBZ implies the whole column is cold and updrafts are usually weak. A very high WBZ implies a deep warm sub-cloud layer that melts hailstones on the way down.
Forecasters read WBZ off the daily sounding along with CAPE, freezing level, and hail growth zone thickness. It is one line item on a hail checklist, alongside deep-layer shear, updraft strength, and expected storm mode.
What WBZ does not prove
The Edwards and Thompson study in Weather and Forecasting compared WBZ across hail size categories and found substantial overlap between marginally severe hail and giant hail. The authors concluded that WBZ, along with several other single-variable indices, should not be used on its own to forecast hail severity.
WBZ is best treated as a first cut. It flags whether melting will be a limiting factor, and it screens out environments where large hail is unlikely to survive to the ground. It does not by itself say whether an updraft in a given storm will grow hail large enough to matter.