Enroute Icing

### Produced by The COMET® Program

### Step 1: Diagnosing Locations of Icing » METAR Surface Observations » Description

METARs are coded surface observations. They include:

- Cloud cover
- Ceiling height
- Surface temperature, dewpoint, and wind
- Mean sea level pressure and pressure tendency
- Precipitation and precipitation type
- Visibility and visibility obstructions, such as fog, mist, and rain

METARs can be used to quality control PIREPs and estimate the flight levels of clouds. With no precipitation, in-cloud icing may take place from the cloud base upward, given favorable conditions. Where METARs report freezing precipitation, there will be icing from the cloud base to the surface.

Data from METAR plots can also be plotted on a surface chart. An icing risk exists where surface charts and surface weather depictions indicate the following.

Freezing precipitation |
Light rain | Symbols: ∙ for very light rain or – –RA in METAR; or ∙∙ for light rain or –RA in METARs
Light snow | Symbols: * for very light snow or – –SN in METAR; or ** for light snow or –SN in METARs). Note that if moderate to heavy snow (SN to +SN) is reported in a METAR, supercooled water aloft will be removed and icing minimized.

What are the advantages of using METARs and their depictions in surface charts? They are compact descriptions of aviation-relevant ground conditions, and are useful for identifying icing conditions at ground locations. However, when they are reported from automated stations, freezing drizzle or very light snow may not be detected, with mist or haze reported instead.