4. Forecast Applications

Convection » Hail Hail/No Hail Nomogram

This figure plots the occurrence of hail versus no hail for 70 thunderstorm cases in the Midwest U.S. The horizontal position of each point shows the height of the freezing level. Hail is more likely to occur if the freezing level is relatively low. Thus, a position further to the right favors hail.

Hail prediction nomogram using cloud depth above and below freezing vs. height of freezing level

The vertical position of each point shows the cloud depth ratio, which is the ratio of the depth of the cloud above freezing to the depth of the cloud below freezing. A deep cloud layer that is below freezing will maximize the potential for hail formation and growth. Thus, a lower position favors hail.

Overall, positions in the lower-right corner indicate hail is likely, while positions in the upper-left indicate hail is unlikely.

The method outlined above considers the CCL (cloud base), the freezing level, and the equilibrium level (cloud top).

Procedure:

  1. Subtract the freezing level from the CCL. This represents the depth of the cloud from its base up to the freezing level. Call this a (hPa)
  2. Subtract the EL from the freezing level. This represents the depth of the cloud from the freezing level up to its top. Call this b (hPa)
  3. Determine the cloud depth ratio a/b.
  4. On the nomogram, find the cloud depth ratio on the vertical axis and the freezing level on the horizontal axis and plot the point. For points that plot above the diagonal line, hail is unlikely, and vice versa.