Why are there echoes on the radar, but no snowfall?

Have you ever seen a radar image that suggested it should be raining or snowing at your location, but it wasn't? The radar isn't lying, rather, the the rain or snow is not hitting the ground. If you have a dry air mass in place in the low levels, sometimes rain cannot completely penetrate that dry layer before it evaporates. Rain that doesn't make it to the ground is called virga.

Below is a graphic showing a cross section of a saturated environment with rain intersecting a dry air mass. The deeper into the dry air mass you go, the less chance the rain has of reaching the ground. The radar will still detect the rain aloft, but you may not see it at the surface. This scenario is one reason why it is hard to break drought conditions once they set in. Rain that you do get is reduced because of the dry air in place.

Below is an image of what virga looks like as it is falling out of clouds.

virga