4. Forecast Applications

Icing » Icing Forecasting Forecasting Icing Occurrence

Plot showing frequency of icing type versus temperature as reported by PIREPS

In making an icing forecast, one must evaluate observed and model-derived soundings to look for factors favorable for ice formation. These include:

  • Existence of clouds, including coverage, type and depth, always taking into account local effects
  • Subfreezing areas or layers where there are clouds and, possibly, precipitation

Over the years, empirical forecast methods and various forecasting heuristics related to icing have been developed. These studies have found icing is likely:

  • if the dewpoint depression in a layer is less than 2°C for temperatures of 0 to -7°C.
  • if the dewpoint depression in a layer is less than 3°C for temperatures of -8 to -15°C.

Icing tends to be less likely at temperatures below -15°C. For instance, for temperatures of -16 to -22°C with dewpoint depressions greater than 1°C, there is a 90% probability that icing will not occur, and for temperatures colder than -22°C, there is a 90% likelihood that icing will not occur no matter the dewpoint depression.