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 –15°C: — In   zones   of   neutral,   or   weak   cold   air advection,  forecast  light  icing  with  a  75-percent probability  of  success. — moderate 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 Chart Data 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 Data 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 Lifting 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, severe icing. .  In  freezing  rain,  below  or  in  clouds, severe icing. ICING TYPE FORECASTS vertical forecast forecast The following rules apply to the forecasting of icing types: 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 precipitation. .  Forecast  mixed  rime  and  clear  icing  when temperatures are between –9° and –15°C in cumuliform clouds. EXAMPLE OF ICING FORECASTS USING THE SKEW T LOG P DIAGRAM 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. 5-31