Occurrence if icing conditions in or below clouds is a very important aviation impact variable. A small group of meteorologists in a joint project between the Swedish Armed Forces and the Swedish Meteorological and Hydrological Institute has made a study on aircraft icing from the forecasting point of view. The output from that project, which ended in 1999, was an updated forecaster's guide to be used in Sweden when forecasting icing on aircraft and helicopters within and below clouds.

To estimate icing conditions in clouds, an icing index from a numerical model may be a useful forecast tool although you have to be aware that such an index will not solve the whole forecast problem. A new icing algorithm was developed by the Swedish group. The algorithm takes into account, directly or indirectly, all the main meteorological variables for icing; temperature, amount of liquid water, size of the droplets. The vertical velocity was chosen as a substitute for the droplet size, which is not available from the numerical models. The index has been operational as a post-processed product from the Swedish HIRLAM model since 1999. It has proved to be a good forecast tool for icing in clouds, especially in situations with lows, fronts and orographical updraft.