Have you ever wondered why sometimes in the winter there is what appears to be a ring of precipitation echoes surrounding the radar, and why even under these rings of radar return that it may not be snowing? Here is an example from the Doppler radar this morning at the NWS Northern Indiana. The following radar image is from around 5am.
You may notice this donut hole appearance preceding some winter events when there is a very dry initial low level air mass in place in advance of a weather system. Even where the radar returns are noted, there are no reports of any snow at this time. This is because the radar is detecting snow aloft. Once the radar beam is sent from the NWS Northern Indiana Doppler radar, it will get higher in elevation at greater distances from the radar. In the above image, radar is detecting snow at elevations of about 4,000 feet above ground level, but this snow is evaporating before it reaches the ground which is why there are no radar returns noted closer to the radar. As the low levels of the atmosphere continue to moisten in advance of a weather system, you will notice often times the edge of this ring getting closer and closer to the radar as snow reaches closer to the surface.
An alternative way NWS meteorologists view this process is through a tool called the VAD wind profile. In addition to the intensity of radar returns, the NWS Doppler radar can detect the velocities of meteorological targets. In the below VAD wind profile display, you can see that the radar provides a vertical profile of wind. However, this is only possible where there is something to reflect a portion of the radar beam back to the radar, in this case, snow. The image below shows the VAD wind profile from about 320am on the left to about 5am on the right (the same time as the radar image above). Note how the radar is detecting the snow at lower elevations through time indicating that the very dry air mass is beginning to moisten up. Thus, one might expect snow should soon be observed at the surface.
The upper air sounding from Lincoln, IL (shown below) from last evening really shows this dry low level air mass nicely. The left line in the below sounding from 7pm last evening represents dew point, or a measure of moisture. Note the very dry pocket of air on this sounding. It is this dry air which explains why it takes some time for snow to begin at the surface.