#viirs

Our Sun provides the light and heat that energizes our plants, our solar panels and our weather, among other things. But its influence stretches over the horizon into our nights, as well. As our nearest star, the Sun bathes Earth in a steady stream of energetic particles, magnetic fields and radiation that can stimulate our atmosphere and light up the night sky, like the aurora borealis, or northern lights.

At 3:46 a.m. Eastern Time on April 21, 2018..  VIIRS

Just as aircraft can leave a condensation trail (or “contrail”) as they zip across the sky, some ships can leave a stripe in the clouds revealing their location and course.  

The GOES-16 geostationary satellite cannot see Alaska, but here is a nice movie loop over the north Atlantic showing some ship tracks at night and then during the day after the sun has risen.  Special thanks to the crew at the Cooperative Institute for Research in the Atmosphere (CIRA) for posting this loop!

http://rammb.cira.colostate.edu/ramsdis/online/loop.asp?data_folder=loop_of_the_day/goes-16/20171031000000&number_of_images_to_display=200&loop_speed_ms=100

Just like not every airplane leaves a contrail, not ever ship leaves a ship track.  The determining variables are the ambient atmospheric conditions: what is the temperature, what’s the humidity, what are the winds?  When atmospheric conditions are right, the addition of comparatively small amounts of heat and water vapor from an internal combustion engine can tip the balance and yield a distinct ribbon of cloud.  

Below is a longwave thermal infrared (IR) image over Alaska as taken by the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument on the polar-orbiting Suomi National Polar Partnership (SNPP) satellite taken at about 4:30am on November 2nd of this year.   When you look at a thermal IR image like this, you see temperatures, either the temperature of a cloud top or the temperature of the land or sea surface in places that are free of clouds.  Warm temperatures (comparatively warm, that is) appear as dark gray, and the color scheme changes to whiter temperatures as cool, eventually showing as yellow or even red for very cold cloud tops or very cold inland valleys where clear skies allow a temperature inversion to develop.

It’s tough to see here in this IR image, but there is a mix of cloudy and clear areas over the Gulf of Alaska and over the north Pacific south of the Alaska Peninsula and eastern Aleutians.  Look closely, and you can see slightly different shades of gray in this area.  The reason it’s tough to discern which regions are cloudy and which are clear is that the temperature of the clouds and the temperature of the sea surface are almost identical.  Infrared imagery only reveals temperatures.  If two objects have similar temperatures, like the clouds and the open ocean in this case, then those objects will appear very similar in the thermal infrared imagery.  

This problem is easily solved during the daytime by also consulting visible spectrum imagery, the wavelengths that the human eye sees, where cloudy skies and clear skies look very different even if their temperatures are similar.  But at 4:30am in Alaska in November it is quite dark out, so visible spectrum imagery will be of no help.  Or will it?  The amazing Day Night Band (DNB) on the VIIRS instrument senses every last bit of moonlight or starlight to produce visible spectrum imagery even in the middle of the night.  

The above DNB image is from exact time as the longwave thermal IR image at the top of this blog post.  Note how in this DNB picture the distinction between cloudy areas and clear areas is easy to make.  And getting back to the theme, there are some ship tracks in the clouds south of the Alaska Peninsula and eastern Aleutians.  A zoom into this area will highlight the ship tracks more clearly:

To learn more about ship tracks, consult this nifty Wikipedia page:  

Source: These shallow Arctic waters frequently produce phytoplankton blooms spanning hundreds of kilometers in the summer.

Image of the Day for August 13, 2024