Friday, February 4, 2011

Seeing the Snow on Satellite

Just a quick post tonight showing how much of the central part of the country is covered by snow--and we can see it on satellite images.  Snow is usually very bright, and because of this we say it has a high "albedo".  The albedo of an object is how effectively it reflects any incoming radiation.  The higher the albedo, the more reflective the surface.  Snow and ice, being bright white (usually), have rather high albedos, particularly when compared to vegetation or even the open ocean.  Clouds can also be very bright white and consequently have high albedos.  (We're actually rather uncertain as to the true effect albedo differences have on how much radiation is absorbed and emitted.  This is one of the biggest question marks in today's climate models.)

What we see in our "visible" satellite images is a good measure for albedo.  Since clouds and snow are both white (they both have high albedos) they look similar on a visible satellite image:
Fig 1 -- GOES-E Visible Satellite image for 1945Z, Feb. 4, 2011.
A lot of that white stuff is snow--not just clouds like we usually see.  It's pretty easy to guess that the white splotches all over New Mexico are snow in the mountains.  There was also additional snowfall in Texas today, so much of the white over Oklahoma, Arkansas, East Texas, Louisiana and the southeast really is clouds from that particular shortwave.  There are also some high cirrus over the Dakotas and Montana.  But everything else across the northern tier of the US?  That's all snow.  Here's an outline of the approximate edge of the snow cover:
Fig 2 -- Same as in figure one, but with line showing the approximate extent of snow cover.
That's a huge area of the country.  It's a lot easier to differentiate snow from clouds when looking at an animation.  In simple terms--snow doesn't move, but clouds do.  So it's somewhat easier to tell if what you're looking at is snow or a cloud.  You can also sometimes see snow melting on the time lapses as the edges of the snow pack slowly disappear over time...

We can also look at snow on the infrared satellite imagery.  IR satellite images retrieve temperatures.  Here's one from later yesterday afternoon:
Fig 3 -- GOES E IR image from 2145Z, Feb. 4, 2011.
Cloud tops are very high in the atmosphere where it's much colder.  As such, the bright colors (indicating very cold temperatures) can show us where the highest cloud tops are.  All those yellows and reds--those are clouds.  Even some of the dark greens are clouds.  However, notice that swath across the same area of the country as we were looking at before.  A lot of the area is showing slightly colder temperatures (more muted greens and cyans), particularly when compared to places in the southwest.  This color swath even has a pretty sharp edge in western Kansas and central Texas:
Fig 4 -- Same as in figure 3, but annotated to show the approximate extent of the snow cover.  The red arrows point to places where the edge of the snow can be easily seen.
So what we're observing in the infrared image is the thermal characteristics of the snow.  That entire area of the country is showing as slightly cooler because there is snow on the ground.  Since snow has a high albedo and is very reflective, not much sunlight is able to be absorbed by the ground and later re-emitted as long-wave radiation.  As such, all that reflection off the snow keeps things colder wherever there is snow cover.  But anyone who has lived in areas with snow knows this--it gets much colder out when there's still snow on the ground.

So there's a quick look at the snow on yesterday's satellite images.  With so much snow (and such cold temperatures...) that snow will be sticking around for a while.  I invite you to look at loops of visible and infrared satellite images over the next few days to see the snow pack (Remember...clouds move.  Snow doesn't...but it can melt around the edges...).  The links take you to the HOOT website where there are loops of both satellite images.

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