Last Lab: (Hail formation)
Anthony R. Lupo
Atms 4310 / 7310
Lab 13
Last Lab: (Hail formation)



 Hail is of course fronzen precipitation that
forms within the updraft region of a severe
thunderstorm.
 A large droplet may be carried aloft into the
colder regions of the cloud and freeze, the droplet
may fall again and accumulate more unfrozen
droplets (may go up or continue down again).
This next coating of water may freeze and the
stone grows.
 The process repeats itself until the stones are
too heavy to be supported by the updrafts.
Last Lab: (Hail formation)



 Thus hail size is proportional to updraft
strength and instability
 Severe thunderstorm forecast: “….
these thunderstorms could be
accompanied by wind gusts of up to 80
mph and hail up to 2 “ in diameter…”!
 We’ve already examined an algorithm
for wind gusts.
Last Lab: (Hail formation)

Hail formation

Miller-Fawbush technique:


developed by Air Force Col. Miller. (of sounding
fame).
 Again, like some indicies and wind gusts, this is
an empirical relationship based on on solid
thermodynamic principles and using
thermodynamic quantities we’ve derived.
Last Lab: (Hail formation)

Recall: finding wet bulb temperature?

 To determine hail-size must first determine
wet-bulb zero. (WBZ)
-or-



C
determine height above the surface of Tw = 0
 This height is assumed to be a indicator of the
height of the 0 C level in a precipitating
convective storm.
Last Lab: (Hail formation)

Relationship to Hail formation:
 Empirical evidence suggests that 90% of hail
accurrences are associated with WBZ between 1.5
km (850 hPa) to 4 km (roughly 600 hPa) with a
maximum occurrence of roughly 3 km (700 hPa).


 If WBZ is below 850 hPa, the air is too cold or
dry for deep convection.
If above 600 hPa, air tends to be too warm and
hail will melt.
Last Lab: (Hail formation)



Use of CCL and Fawbush – Miller charts
(hail – size)
 Determine the CCL as the intersection
of the mean mixing ratio of lowest 150
hPa with the sounding.
 Then, trace the moist adiabat from the
CCL up to the pressure level at which T
sounding = -5 C
Last Lab: (Hail formation)

The Chart
Last Lab: (Hail formation)

 This forms a triangle bounded by the sounding,
the moist adiabat and the pressure level.

If the area is negative (-) stop!!

No hail formation!

 If we have a (+) area:

the –5 C level temp difference between the
sounding and the moist adiabat is identified as
the triangle base (B’ – B) = Base
Last Lab: (Hail formation)



A second dimension, the altitude:
 is the temperature difference along the
dry adiabat extending from the CCL to the
level of the triangle base and passing
through the triangle.( H – H’) = Height
 Enter the Fawbush – Miller chart values
chart 1 using the base and height!
Last Lab: (Hail formation)

The Chart
Last Lab: (Hail formation)




Determine WBZ:
 Pick any point above zero on the sounding,
determine the LCL and follow moist adiabat down
to that level.
 This is Tw, you may have to find wet-bulb-zero
iteratively.
If WBZ >= 10,500 ft (aaprox. 3.2 km or 680 hPa)
adjust size for melting using the second chart!
Last Lab: (Hail formation)

The modification
Last Lab: (Hail formation)

The End!
Last Lab: (Hail formation)

Questions?

comments:?

Critique?