Enroute Icing

### Produced by The COMET® Program

### Step 1: Diagnosing Locations of Icing » PIREPs and AIREPs » Description

Pilot Reports (PIREPs) and Aircraft or Air Reports (AIREPs) are arguably the most important indicators of current conditions. Forecasters use them with other observations to verify NWP analyses.

There are some limitations to be aware of in regard to these data. SInce aircraft tend to avoid areas with icing, there may not be enough reports where they are most needed along flight routes. Note, though, that icing cannot be avoided by commercial aircraft using high volume airports such as Chicago O’Hare (ORD) Thus, their icing reports can be used as a proxy for similar conditions in nearby smaller volume (General Aviation) airports.

Another common problem is that there are few to no PIREPS during the late night and early morning hours when few flights occur. As a result, the icing picture can change when the first PIREPs of the day come in.

PIREPs and AIREPs contain the following:

- Location of PIREP (relative to airports or other identifiers)
- Flight level of PIREP (in hundreds of feet above MSL, for example, FL180 is 18000 feet)
- Ambient temperature in degrees C at the PIREP location
- Type of aircraft
- Pilot-observed icing intensity (NIL, Trace, Light, Moderate, Heavy, or Severe) where heavy and severe icing have the same serious impacts on commercial aircraft

To encourage icing reports, CWSU service area forecasters prompt Air Traffic Controllers (ATC or TRAFCON) to request them from pilots where and when light or greater icing exists or is forecast. When the reports are received or come in unsolicited, the Air Route Traffic Control Center (ARTCC) enters them into the Aeronautical Information System Replacement (AIS-R). The icing reports are used by flight data personnel and the CWSU, and are available from the AIS-R, Flight Service Specialists, and Aviation Weather Center (AWC).

PIREPs and AIREPs are quality controlled before a CWSU issues a Center Weather Advisory (CWA), or a Meteorological Watch Office (MWO) issues an AIRMET or SIGMET. This is done by checking consistency with other observations and analyses. For example, moderate or severe reports in close proximity to similar reports are given more weight than those near nil, trace, or light icing reports. AIREPs and PIREPs from areas determined to be without clouds or precipitation are totally discounted.

Consideration is also given to the type of reporting aircraft and the length of its flight path through potential icing regions. Smaller aircraft often report more severe icing than commercial aircraft because they accumulate ice more readily and do not have icing equipment. Longer flight paths and extended holding periods in icing conditions result in more ice accumulation on all types of aircraft.