Another Example of "Visible" Satellite Imagery At Night

Typically, when we refer to visible satellite imagery we are describing satellite pictures of the earth when the sun is fully illuminating everything. It's what you would see if you were "sitting on the satellite", looking down at the earth with a pair of binoculars.  At night, we are usually limited to infrared (IR) satellite imagery (there are other spectrums as well, but I won't get into that).  IR imagery looks at the temperature of the clouds and terrain.  One of the biggest limitations of this spectrum is if the clouds and terrain are very close in temperature (typical for low clouds), you will have a difficult time differentiating between clouds and terrain.   This can have a direct impact on accurate forecasting of the behavior of low clouds for aviation operations. 

On the Suomi NPP polar orbiting satellite, there is an instrument that can create what is an equivalent to a daytime visible image, but at night.  It is highly sensitive to light and can utilize the light of the moon to illuminate the clouds and terrain, similar to sunlight.   We call this the Day-Night Band.  We had a nearly full moon last night...perfect!

Here is an excerpt from NOAA's page about this instrument: Unlike a camera that captures a picture in one exposure, the day-night band produces an image by repeatedly scanning a scene and resolving it as millions of individual picture elements or pixels. The day-night band goes further, however, to ensure that each pixel collects the right amount of light. If a pixel is very bright, a low-gain mode prevents the pixel from oversaturating. If the pixel is very dark, the signal will be strongly amplified.

We have two basic types of satellites we use in weather forecasting.  One is a Geostationary Operational Environmental Satellite (GOES) located about 22,000 miles above the earth and is stationary relative to a point on earth.  The other is a Polar Orbiter located roughly 500-600 miles above the earth and it is in a constant orbit around the earth from pole to pole.  It takes about 90-100 minutes for one orbit. 

Okay, enough of that.  Below is an example of the difference between the new day-night band from the polar orbiter and the IR imagery we would typically look at from the GOES satellite.  It was taken at about 2 AM, Sunday, September 22, 2013.  Notice how easy it is to see the clouds over Lake Michigan and streaming off Lake Superior across the U.P. of Michigan and northern Wisconsin.  Look at the citiy lights defining the metro areas of Milwaukee and Chicago!  Even out in the less populated areas of Wisconsin and Minnesota, you can see how the smaller towns are located along the major roads and interstates.

The two image loop below shows two day-night pictures taken about 1 1/2 hours apart (remember, it takes the satellite about 90 minutes for one orbit!). 

This shows how we can watch the behavior of clouds over a period of time.  Notice how easy it is to see the extent of the fog in the Mississippi and Wisconsin River valleys.  Cool stuff.


NWS Milwaukee/Sullivan




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