l If the temperature is:
0° to 7°C and the dewpoint spread is greater
than 2°C, there is an 80-percent probability of no icing.
8° to 15°C and the dewpoint spread is
greater than 3°C, forecast no icing with an 80-percent
chance of success.
16° to 22°C and the dewpoint spread is
greater than 1°C, a forecast of no icing would have a
90-percent chance of success.
Colder than 22°C, forecast no icing
regardless of what the dewpoint spread is with
90-percent probability of success.
l If the dewpoint spread is 2°C or less at
temperatures of 0° to 7°C, or is 3°C or less at 8° to
In zones of neutral, or weak cold air
advection, forecast light icing with a 75-percent
probability of success.
In zones of strong cold air advection, forecast
icing with an 80-percent probability of
In areas with vigorous cumulus buildups due
to insolational surface heating, forecast moderate icing.
Intensity Forecasts From Surface
If upper air data is not available, check the surface
charts for locations of cloud shields associated with
fronts, low-pressure centers, and precipitation areas
along the route.
Intensity Forecasts From Precipitation
Within clouds not resulting from frontal activity or
orographic lifting, and over areas with steady
nonfreezing precipitation, forecast little or no icing.
Over areas not experiencing precipitation, but having
cumuliform clouds, forecast moderate icing.
Intensity Forecasts Based on
Clouds Due To Frontal or Orographic
Within clouds resulting from frontal, or orographic
lifting, neither the presence, nor the absence of
precipitation can be used as indicators of icing. The
following observations have proven to be accurate:
. Within clouds up to 300 miles ahead of the warm
front surface position, forecast moderate icing.
. Within clouds up to 100 miles behind the cold
front surface position, forecast severe icing.
. Within clouds over a deep, almost
low-pressure center, forecast severe icing.
l In freezing drizzle, below or in clouds,
. In freezing rain, below or in clouds,
ICING TYPE FORECASTS
The following rules apply to the forecasting of icing
l Forecast rime icing when the temperatures at
flight level are colder than 15°C, or when between 1°
and 15°C in stable stratiform clouds.
l Forecast clear icing when temperatures are
between 0° and 8°C in cumuliform clouds and freezing
. Forecast mixed rime and clear icing when
temperatures are between 9° and 15°C in cumuliform
EXAMPLE OF ICING FORECASTS
USING THE SKEW T LOG P
It has been previously pointed out how the thickness
of clouds, as well as the top of the overcast, may be
estimated with accuracy from the dewpoint depression
and changes in the lapse rate.