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CEE 1331 Extra Credit: Atmospheric Soundings and Severe Weather
Table 1: Upper Air Data for the DFW area March 24, 2019 at 7:00 PM CDT
Pressure Level (mb)
Dew Point Temperature (°C)
One of the ingredients for severe thunderstorms with strong damaging winds and tornadoes is the
presence of very dry air located in the mid to upper levels of the atmosphere (i.e., those levels in the
atmosphere that are above the 850 mb level). Extremely dry air in the mid to upper-levels of the
atmosphere encourages ENTRAINMENT which is the ingesting of dry air into a thundercloud. This
causes part of the cloud to evaporate forming cold, dense air pockets within that tend to fall and
enhance the downdraft. This enhanced downdraft can bring damaging winds to the surface and/or
cause a rotating updraft to descend to the ground in the form of a tornado!
Plot the temperature AND the dew point temperature data on the previous page taken from yesterday
weather balloon launch in the DFW area. Plot the data on the Stüve chart from the 1000 mb level to the
100 mb level.
1) Is the air very dry in the mid and upper levels of the atmosphere, and how can you tell?
Another way to estimate how severe a thunderstorm might be is by comparing the temperature of its
rising air bubbles to what the temperature of the bubble’s surrounding environment is at 500 mb.
Why 500 mb? Generally speaking, the 500 mb level (i.e, roughly 18,000 feet above sea level) is where
we typically find the strongest updrafts within a storm. Therefore, the bigger the temperature
difference between the rising air bubbles AND the environment at this 500 mb level, the faster the
storm updraft will be and hence the more intense the storm can be. And remember, updraft speed is
directly related to hail size. The faster the air goes up, the bigger the hail stones can be!
There is a simple parameter – called the LIFTED INDEX (LI) – – which can give us an idea of how strong
the updraft will be and ultimately how large a hailstone might get in any given thunderstorm. The
formula for LIFTED INDEX is:
LI = Temperature of the Environment at 500 mb minus the Temperature of Air Bubble at 500 mb
LI = TEnv500 – T bub500
Thus, for example, if the temperature of the environment at 500 mb were -10°C, and the temperature of
a rising air bubble were -4°C at that same 500 mb level, then the LIFTED INDEX would simply be:
LI = -10°C – (-4°C) = -6°C
(remember when you subtract a negative number, you add it!)
NOTE: The more negative the LI value is, the more unstable the atmosphere is. Why? Because the only
way you can get negative numbers for the LI is if the rising air bubbles are warmer than their
environment. And the warmer the bubble, the more negative the LI !
The table on the next page lists different values for the LIFTED INDEX and the hail size one might expect
from a thunderstorm developing in such an environment whose atmospheric temperature and moisture
profile has produced the given Lifted Index:
0 and higher
Possible Hail Size
No Significant Hail Expected
Pea to small marble
Pocket Change: Pennies, nickels, dimes, quarters
Canned Hams (> 5 inches in diameter)
It should be pointed out that these are estimates only. Other factors in the atmosphere could cause hail
sizes to be smaller or larger than the ones listed above!
2) What was the LI in the DFW area on the evening of March 24, 2019?
3) What would be the maximum size of hail stones that might be expected from a thunderstorm
that developed in this environment?
4) Why would a more negative lifted index value be associated with larger hail stones?
5) What does a positive value for the lifted index imply about the stability of the atmosphere, and
why would you NOT expect any hail if the lifted index were a positive number?
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