Snow on the radar, but no snow falling?!?!

January 25th, 2013 at 10:57 am by under Weather

Sometimes the radar can be deceiving.  Like early Friday morning.  Here is a look at the radar from about 5 am Friday morning.

Radar Image – about 5 am Friday morning – NWS IWX

Notice the doughnut shape of “precipitation” around the radar center (red circle).  At this time, the closest snow report was in Wisconsin and Michigan.  We didn’t even have reports of snow in Fort Wayne until after 8 am.  So whats going on here?

Well, the radar is detecting snow that is falling from the clouds, but is evaporating before it hits the ground.  During the winter, this is actually fairly common.  Here is how it happens.

In this and most cases, very dry air is near the surface as the snow moves in.  Because the Earth is curved, the radar beam actually gets higher and higher off the ground the further away from the center it is.  So, as the snow falls from the clouds, it hits that mass of very dry air and evaporates.  But because the radar beam is now above the dry air, it actually sees the falling snow before it evaporates.

In most cases when you get this “doughnut” effect, as time goes on the hole will get smaller and smaller.  This means that the dry air is moving out, and the snow is making it to the ground.

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