Enroute Icing

### Produced by The COMET® Program

### Step 1: Diagnosing Locations of Icing » Skew-T/Log-P » Description

Skew-T/Log-P (Skew-T) diagrams provide vertical profiles of temperature and dewpoint from radiosonde launch sites usually every 12 hours, or from NWP model analyses and forecasts. They are used to find saturated or near-saturated layers with temperatures from 0° to about -22°C. The following general rules apply:

- Thicker, moist sub-freezing layers have higher moderate or greater icing risk. If a Skew-T shows a zero dewpoint depression, expect at least moderate icing.
- If the top of a moist layer is colder than -22°C, ice crystals or snowflakes may reduce the amount of supercooled water, reducing the icing risk.
  - Abundant moisture and upward motion located within the snowflake growth zone (Dendritic Zone, between temps -12°C to -18°C) can actually eliminate the amount of supercooled liquid below and thus reduce the potential for aircraft icing. This process is called scrubbing or the seeder/feeder process. Such is the case for some heavy snow storms.
  - Rarely, a separate moist layer above one producing snow may result in icing above the dendritic growth zone.
- Trace or light icing is still possible at temperatures as low as -40°C according to most jet aircraft manuals, particularly in convective clouds.
- Look for multiple layers with icing potential, especially in advance of warm fronts.
- Steep (equal to or greater than moist adiabatic) lapse rates in moist layers can increase icing severity. This results from convective motion embedded in stratiform clouds, resulting in the formation of supercooled large drops. If, however, convective motion becomes vigorous, chances increase for ice crystal formation and the subsequent “scrubbing” of supercooled water from clouds.
- Use Skew-Ts to supplement satellite data in areas of cirrostratus or altostratus. A smaller temperature-dew point spread indicates a greater threat for light to moderate icing.

The flowchart below, taken from the DoD Defense Technical Information Center Technical Note “Meteorological Techniques (2006),” is for use with observed or model soundings. It shows the expected icing type and severity for different temperature and dew point spread thresholds. The two predominant icing situations associated with synoptic scale low pressure systems are enclosed in the red dash. The flowchart for non-frontal or orographic situations, above the red dash box, can be used for situations that do not neatly fit such synoptic-scale situations. Note that moderate to severe clear or mixed icing is generally associated with freezing rain and freezing drizzle (usually found in advance of surface warm fronts), and with stratocumulus and cumulus in cold advection or near deep low pressure areas.

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