The Radar Looks Like Its Raining/Snowing, But Its Dry. What’s Happening?

By: STORMTRACKER 13 Chief Meteorologist Matt Meister

Occasionally you’ll see what looks like showers on MY HD DOPPLER when it isn’t raining or snowing at your location and in fact you may even have a clear sky. This is due to the sensitivity of our radar and the physical properties of a radar beam.

First, its important to know how radar works. In its simplest form, the radar antenna sends out an electromagnetic pulse (beam) that radiates away from the radar site. As this beam travels through the air and interacts with particles, it will scatter some of the microwave energy back to the radar location so that it can be measured and evaluated. While raindrops, hail stones and snowflakes will reflect energy back to the radar, ANYTHING the beam hits will also do this. That can and does occasionally include dust, bugs, birds and buildings. This means we need to be careful when analyzing a set of radar data as we want to be sure we’re detecting what we think we are detecting. The return from buildings obviously doesn’t move and we refer to this as “ground clutter”. The “clear-air” detection of dust and bugs allows us to see things like thunderstorm outflow boundaries that can be important in generating new thunderstorms during our summer months or decreasing the threat of a tornado from a particular storm, to the detection of an arriving cold front that promises a noticeable change in the weather.

There is another type of detection that will occur when temperature inversions (colder air settling to the surface with relatively warmer air aloft) exist, causing differing densities of air in the vertical that trap and scatter the radar beam. Temperature inversions are most common during the night hours and most radars in the central plains usually display this at night (see image 2 of SE US radars). This non-precipitation return of radar energy is called Anomalous Propagation, or AP. AP can also occur when significant moisture changes occur within a temperature inversion environment and when a cool and moist airmass at the surface is being over run by warm and dry air.

Inversions are so common that MY HD DOPPLER will show AP 8 or 9 out of ten nights, maybe more! Returns are usually on the order of 0 to 5/7 dbZ, the very lower end of our scale, but can occasionally get closer to 20dbZ or so. So no, the radar isn’t broken, it doesn’t need re-calibrated, but its likely that a temperature inversion is in place if you see radar return at night or early in the morning and nothing is falling from the sky!