Severity of icing
The severity of icing depends on three factors:
- liquid water content (LWC)
- temperature
- droplet size
Of these, the most important one for the intensity of icing is LWC. It should be noted, however, that LWC can vary significantly both horizontally and vertically even in an apparently homogenous area.
Intensity classes listed by LWC (g/m³):
- light; 0,11 - 0,6
-
moderate; 0,61 - 1,2
-
severe; > 1,2
In short, in temperatures 0...-12°C icing is more severe the higher the LWC and the greater the droplet size.
Estimating the relevant factors for icing:
- LWC
Only a few of the operational NWP models include LWC as an output parameter.
Data of icing clouds has been collected with aircraft experiments in the mid-levels. In Figure 9 there is a compilation of over 1000 hours of data collected by the University of Wyoming's research aircraft. The vast majority of these cases have quite low LWC (less than 0.1 g/m3) with only about 17% of the cases having LWC greater than 0.3 g/m3. (MetEd)
Fig. 2.7. Frequency of occurrence of icing as a function of liquid water content /g/m3. The scale on the left: percent of cases, the scale on the right: cumulative percent of cases.
- Temperature
Temperature can be examined through soundings, infrared satellite images and NWP models fairly accurately.
- Droplet size
Cloud droplets have diameters less than 40 microns. In freezing drizzle the droplet size is 40 to 200 microns and in freezing rain greater than 200 microns (SLD). Droplet size is not predicted in the operational weather forecast models, but in some cases it can be estimated with radar observations.