|
Defining an icing environment
- role of temperature 
Now that we're into autumn and summer is
officially in the past, icing will become a very familiar phrase in our
preflight weather briefings for the next five or more months. As
discussed in detail in the Ice Is NOT Nice - Part 1 premium
workshop, an icing environment is defined by three basic
factors that include (1) temperature, (2) liquid water content and (3)
drop size. In this e-Tip, we'll briefly explore how
temperature plays a key role in an icing environment.
Temperature is perhaps the biggest
factor to determine if icing will or will not accrete on the airframe.
If the environmental temperature is warmer than 0°C,
structural icing isn't possible. If liquid water does accrete
at temperatures greater than 0°C, it likely means that your outside air
temperature (OAT) probe is not accurate. Consequently, it is
prudent when flying in visible moisture to prepare for a structural
icing encounter when your OAT gauge shows a temperature that is at or
below +5°C.
At the other extreme, it can be too cold for
structural icing. The theoritical limit is -40°C although
supercooled liquid water is extremely rare below -35°C. As a
result, we
rarely see AIRMET Zulu issued above FL300.
When the environmental temperature is this cold, supercooled
liquid water will homogeneously freeze into ice crystals.
Clouds with only ice crystals are called glaciated.
Ice crystals will not adhere to the airframe and do not pose
a structural
icing threat.
Given these two extremes, this leaves many general
aviation aircraft vulnerable to an encounter with structural icing.
To narrow this down a little further, icing is fairly common
in the temperature range from -15°C to -2°C which closely matches a
study of the pilot reports of icing versus environmental temperature
shown in the graph on the left. Click
here to view a larger image. If you are flying in
visible moisture with an environmental temperature in this range,
there's a very reasonable chance you'll encounter some kind of
structural icing. Don't
become complacent; there are still a lot of encounters with structural
icing when the temperature is well below freezing. While
the likelihood of icing doesn't disappear completely below -15°C, icing
occurrence drops off quickly at colder temperatures.
While there isn't a hard line in the sand when it
comes to predicting the possibility of structural icing as it relates
to the environmental temperature, cloud top temperature is one of the
best methods to determine the occurrence of structural icing.
As shown in the diagram on the right, when the cloud top temperature is
warmer than -12°C, the cloud below (that exists in subfreezing
temperatures) will likely be dominated by supercooled liquid water.
Click here to view a larger image.
Cloud top temperatures can be
determined by looking at several sources including radiosonde
observations or soundings as shown on a thermodynamic chart called a
Skew-T log (p) diagram. While these soundings represent good
observed data, they are only typically available at specific locations
throughout the country twice a day at 0000 UTC and 1200 UTC.
Perhaps a more practical source is the enhanced infrared satellite image.
Many of these images have a scale at the bottom that color
codes the temperature of the cloud top.
In a future e-Tip we will examine how
liquid water content and drop size play a role in defining an icing
environment.
|
