By Scott Dennstaedt
The ground-based radar mosaic displayed on the Map view in ForeFlight Mobile combines radar data from the National Weather Service (NWS) and Environment Canada. Its primary purpose is to provide pilots with a good estimation of where precipitation is occurring and where it's not. While there are some holes in the coverage (especially in Canada) the radar mosaic is fairly accurate most of the time. Even so, non-precipitation returns generically called ground clutter can be displayed on the radar layer producing what looks like very real areas of precipitation.
Anomalous propagation, or AP, is perhaps the most annoying form of clutter. Essentially with AP, part of the side lobes of the radar beam are ducted or bent down toward the earth during certain atmospheric conditions. This causes it to strike objects on the ground (trees, buildings, cars, etc.) and some of that power from the beam is reflected back to the radar along the same bent path and gets recorded as areas of precipitation. When this occurs you might see on ForeFlight what looks like real precipitation. In fact, it can look remarkably like real convection at times fooling even the most seasoned pilot.
Anomalous propagation (AP) on the ForeFlight radar layer near Buffalo, New York.
What to do if you suspect AP
Since AP can look remarkably like real areas of precipitation (including thunderstorms), it's important to always examine the observational data in and around the area. This includes cross-checking surface observations (METARs) to see if precipitation or thunderstorms are being reported. Also, without clouds, it can't rain. So if clear skies are being reported all around the area, then either the precipitation shown on the radar is very isolated or perhaps it's erroneous. Keep in mind that automated reports only show clouds that exist below 12,000 feet AGL.… Read more ...