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Chapter 9 - IFR Aviation Weather Theory 9-65

this category. This doesn’t mean, however, that airplanes Years ago Project Rough Rider (and subsequent govern-
in the pre- ’69 category can’t withstand vertical gusts of ment studies) established a correlation between the
greater intensity. This was just the minimum vertical gust amount of water suspended in a cloud and the vertical
requirement necessary according to the regulations at the gusts found within that cloud (measured in feet per sec-
time. ond) as shown in Figure 113. If we can equate rainfall
Now let’s talk about water. rates with radar reflectivity or dBZs, we can make some
As you’ve already learned, to keep large volumes of interesting assumptions using the results of the Rough
water suspended in the atmosphere it takes a lifting force, Rider study.
otherwise known as updrafts. The more water suspended It turns out that rainfall rates producing radar reflec-
within a cloud the greater the lifting force or updraft tivity ranging from 16 to 29 dBZs (known as a Level 1
that’s required to keep it there. When the amount of radar return) generate a 100% probability of light turbu-
water suspended exceeds the lifting force that suspends it, lence and a 10% chance of moderate turbulence. Light tur-
the water falls. We call this rain and measure the amount bulence is defined as vertical gusts from 0-19 FPS and
in inches per hour. moderate turbulence is defined as vertical gusts from 20
It doesn’t have to rain, however, for us to measure the to 34 FPS.
amount of water suspended inside the cloud. We can mere- Rainfall rates producing radar reflectivity ranging from
ly identify the amount of water in a cloud using radar and 30 to 39 dBZs (known as a Level 2 radar return) generate
say that if it did rain, then the rainfall rate would be a spe- a 100% probability of light turbulence, a 40% chance of
cific amount, calibrated in inches per hour. moderate turbulence, and a 2% chance of severe turbu-
Given this observation, it’s logical to say that a cloud lence. Severe turbulence is defined as vertical gusts from
with a rainfall rate (or potential rainfall rate) of .5 inches 35 to 49 FPS.
per hour has less updraft action than a cloud with a rain- Rainfall rates producing radar reflectivity ranging from
fall rate of 2 inches per hour. We can conclude that the 40 to 49 dBZs (known as a Level 3 and 4 radar return) gen-
larger the rainfall rate (or potential rainfall rate), the erate a 100% probability of light turbulence, a 90% chance
stronger the updrafts found inside a cloud. of moderate turbulence, a 10% chance of severe turbu-
As you undoubtedly know, radar energy is reflected by lence, and a 3% chance of destructive turbulence.
solid objects, not clouds themselves. The water suspended Destructive turbulence is defined as vertical gusts of 50
in a cloud is a solid object, but the cloud itself isn’t. The FPS and higher.
more water suspended within the cloud, the greater the Now you know the answer to the question, “What cri-
ratio of radar energy returned for a given amount sent out terion distinguishes a convectively harmless cumulus
by the radar unit. The amount of this reflected radar ener- cloud from one that can damage an airplane?” The answer
gy, referred to as reflectivity or Z, is calibrated in the form is: Any convective cloud with a radar reflectivity of 40
of a quantity known as dBZ or decibels of Z. dBZs (a Level 3 radar return) or higher needs to be avoid-
Now things are about to get really interesting. ed and treated as an immediate threat to your safety aloft.

Turbulence Probability
The turbulence probability
chart provides you with a
statistical means of estimat-
ing the probability of turbu-
lence based on the amount
of water in a cloud.
For instance, if a cloud
has sufficient water to reflect
radar energy to a value of
40 to 49 dBZs, then there is
a 3% chance of destructive
turbulence (at the higher 49
dBZ value), a 10% chance
of severe turbulence (at the
higher 49 dBZ value), and a
100% chance of light
turbulence in that cloud.
Because of this potential
for severe and destructive
turbulence, pilots always
avoid cloud areas/masses
containing 40 dBZ and
higher. Fig. 113