The mystery behind the 'snow hole'

MINNEAPOLIS (KMSP) - Throughout the course of the cold season we get plenty of snow. In fact, the metro averages 54” a year.  Despite that fairly hefty total, you’d be surprised at how many people ask me why “their house” always seems to miss the snow.  In many of those cases, they are referring to what they see on radar versus what is actually happening.  Let me explain…

Just like anything else in the world of weather, there is no major outside force, land feature, random hill, river valley, death star, conspiracy, or voodoo that forces storms to go one way or another… hit one location over the next.  It’s all just weather patterns and sheer dumb luck.  Just because the last big snow storm missed your house and hit the next town over doesn’t mean the next one will or won’t do the same thing.  It’s all luck of the draw… at least there is no current scientific evidence to confirm otherwise.

Many of these so called “reasons” are either completely made up or just come from a misunderstanding of what you’re seeing.  For example, let’s discuss the “snow hole” effect.

I’d say most of us by now know what a radar is and generally what it does.  The radar is able to “see” precipitation falling out of the sky; rain when it’s warm enough and snow when it’s not.  Well, a lot of times we get what seems like a hole in the falling snow in the winter… and it always shows up right over the metro making it look like the metro is somehow escaping any and all currently falling snow.  There is a very good explanation for why you see this on the Fox 9 Weather App or on Stormvision… and no, it’s not aliens.

Doppler radar is a very complex piece of machinery that works solely to see things falling through our atmosphere.  In most cases that’s precipitation, but it has been known to catch meteors, bugs, birds, bats, and even airplanes from time to time.  That’s because the Doppler radar is essentially a radar beam that circles constantly looking up into the sky waiting to run into something.  When that beam runs into something (typically falling precipitation), some of that beam is reflected back to the radar.  The bigger the object, the more of the beam is reflected back and a stronger signal is seen at the radar site.  Computers then instantly interpret this data by adding a color to a map of the location that the reflection occurred.  The redder the color, the more of the beam came back.  This is why very heavy rain shows up as a red color on radar because most of that beam is getting reflected back to the radar site.  A red color would never get reflected back for snow because the crystal shape structure of a snowflake directly reflects far less of the radar beam than rain, no matter its size.  This is the same reason why it can be lightly snowing at your location and nothing shows up on radar.  In this case, the snowflakes or so small and/or so few and far between that as the radar passes through them, it can’t actually hit enough of them to reflect any of the beam back to the radar site so the radar thinks nothing is happening.

A radar beam is a lot like a laser beam, they are straight lines… there’s no such thing as a curved one… at least not yet.  So if you point that radar beam half a degree above the horizon what happens? It goes out in a straight line forever (in theory).  Well because the Earth is round and not flat, the curvature of the Earth will mean that the ground is slowly angling away from the radar beam, which means the beam is rising in elevation with distance traveled, meaning the farther away the radar beam is from the radar site, the higher in elevation it gets.  This is where the metro “snow hole” comes in because the Doppler radar site for the Twin Cities is in Chanhassen.

In the winter, snow rarely forms and falls from the low levels of the atmosphere.  Ice crystals have to form under certain conditions which are typically found between 7 ,000 and 14,000 feet above the surface.  So, when it starts snowing, the snow has to travel from potentially 10,000 feet above the ground or more before we see it here on the ground.  But just because we can’t physically see it doesn’t mean it’s not there… and the radar can certainly vouch for that.  As that radar beam travels a further distance away from the radar site, it’s height increases, eventually intersecting with the falling snow at 10,000 feet.  But if there is no precipitation falling before it reaches that point, then you won’t see anything on the radar.  But then all of a sudden it hits snow, part of the laser beam gets reflected back to the radar, and the computer incorporates a color onto the radar image at that location.  Because the radar beam travels in all 360 degrees to see in every direction, it can give the effect of a “snow hole” if the same thing is happening all around the radar… something that happens more often than you think.

As the atmosphere moistens up though, the hole will begin to disappear as it now takes less distance for the radar beam to get to the point in the atmosphere that snow is falling.  It will do this until the atmosphere completely saturates and you can see snow on the ground, in which case the “snow hole” will be gone altogether, or the falling snow stops completely and the radar image goes blank.

This happened on Saturday as some scattered snow showers were moving through.  Not all of it was reaching the ground because of low level dry air.  This is why nothing is simply black and white in the world of weather.  Just because you see it on radar, doesn’t necessarily mean it’s happening on the ground… especially the further away from the radar you go.  That said, this happens a LOT more in the winter months because it’s colder and dryer, forcing precipitation to fall from a much higher elevation than in the warm months.  As a reference point, thunderstorms in the summer season often have rain developing and falling out of the cloud under a thousand feet into the air… a far cry difference than from 10,000 feet.