Read Radar And Satellite Images
Proper interpretation of imagery is critical to effective use of these resources.
On my way home from Oshkosh Air Venture last August, I observed the billowing anvils of thunderstorms south of my flight path. Both the radar and satellite pictures showed buildups advancing northward, which would cut directly through my route. So, I took the safe option and landed at Dalhart, Texas for the night. The ability to read effectively these images is critical to flying IFR—especially in today’s high-tech world of in-flight weather.
There is a difference between summer and winter in the ability to read and interpret radar and satellite imagery— the latter are not as easy to decipher. All the images shown with this article were captured at the same time. Viewing all these perspectives gives a broader understanding of what we could be facing. Let’s see what they say about a planned IFR flight from Salt Lake City to Minneapolis on January 10th. Assume the performance ceiling of your aircraft is 13,500 feet MSL maximum, and you are estimating seven hours of flight time.
An AIRMET exists over the central half of the flight for Icing from the freezing level (near the surface) to 11,000 feet. When you get briefings from Flight Service they are required by the National Weather Service to inform you of any AIRMETs within 3,000 feet altitude of your flight level, because their estimates are just that— estimates.
Winter Radar Returns
During winter, the biggest problem with individual radar sites, like the one shown based at Cheyenne Wyoming, is that levels of precipitation are deceptive in two ways. This picture (right) shows a large area of green with a small spot of yellow— that in the summer months is associated with light to moderate lower level precipitation, something most IFR pilots find easy to fly through. The problem with individual sites is that returns diminish with distance and mask the extent of the activity. In this picture it looks as though you could potentially circumnavigate to the north.
The Mosaic Summer radar picture combines all the data from various sites—giving you a greater understanding of the true coverage of weather systems. It is important to know where the holes in coverage are, because the individual sites are far enough apart that anything low-level can hide behind higher returns. One of these holes exists northeast of Cheyenne, so it looks like there could be an opening in far Northwestern Nebraska, which an aircraft might slip through.
Snow And Ice
Radar has a more difficult time picking up snow and ice than it does rain. During the winter time the thin pale blues and greys present a subtly sinister reading. Even a pale area of grey could mean there is light precipitation in the area—and flying through it could cause ice accumulation. A smudge of light to deep blue can mean more significant icing is possible or that a small intense snowstorm is blowing through an area. Just because it is small doesn’t mean it’s harmless.
If you are using WSI or other private vendor radar products, the colors may be altered by their computers—in most cases they will show winter radars that have green for rain, pink for a rain/ice mix, and blue for snow. The worst flight condition is a rain/ice mix—that’s where you can accumulate the heaviest icing on an aircraft. Become familiar with the variations of each vendor’s radar interpretations.
The Winter radar mosaic above shows the areas of precipitation as being more widespread than the summer weather radar mode, but they both indicate there may be a passage between the systems. The colors of the NWS winter radar give indications of cloud temperatures. Let’s go to the satellites and see if they can broaden our understanding.
Remember, radar gives you a picture of the sky from the ground upwards on a slant angle, while satellites look down from above. If there are layers between what is seen on the top and bottom, they could be hidden from view.
The visible satellite picture in winter lacks the brilliant white bubble tops of summer cumuliform clouds or the smooth thick creamy swaths of early morning fog banks with sharply defined edges. The challenge lies in determining whether the shadowy shades of gray are streaming at high or low levels and whether they hold moisture warm enough to constitute a problem for your aircraft.
If they are cold enough, the tiny ice crystals will bounce off the aircraft rather than adhering. The Visible Satellite shows the cloud activity is widespread and dense in the northern plains—including the area we were hoping to see a passage through. However, we get no sense of cloud heights or whether they contain dangerous icing.
The infrared picture on the right presents more dramatic tonal contrasts. The temperature grid across the top tells you how cold the clouds are based on their shade of white or gray. Now we can at least see lower layers in California, Nevada, Arizona and New Mexico, but the higher, colder clouds are obscuring what is going on in Colorado, Wyoming and the northern plains states.
Looping both these images showed the mass of clouds at upper levels generally stretching northwards while the entire area swiftly tracks from west to east with a slight counterclockwise rotation in the northern Texas panhandle.
Looping also gives a glimpse below the highest layer of clouds, which in this case shows a grey lower layer— the one you are really concerned with. It appears to be consistently the same shade of grey indicating one lower level below the more visible upper layer. Between these two layers could be altitudes free of cloud activity. Zoom in as close as possible to see if there are shades of gray distinctly different from the lowest and highest cloud shelves.
The Satellite picture contradicts the idea of a hole along the line of precipitation as it shows the clouds stretching in an unbroken line. You see no holes through the widespread cloud activity at the lower levels. METARs also indicate marginal VFR to IFR conditions at the surface.
Looping the images is integral for determining whether to go direct, take a routing to the south, or spend the night and go in the morning. In this case the sequence shows the system to be rapidly moving east. The next morning showed the area of freezing temperatures and precipitation had moved well into Minnesota.
By leaving Salt Lake City in the late morning and assuming a six-hour flight, the arrival at Minneapolis in the early evening was free of icing as we snuck in behind the frontal activity. Getting used to the seasonal differences in radar and satellite imagery takes a bit of practice.
Keep in mind that where summer images are stark and dramatically colored, winter’s colors are more subtle, but essential to safely planning long distance flights.
Rose Marie Kern was a certified aviation weather forecaster when she worked as a Flight Watch specialist. Her book, Air to Ground, gives pilots a good overview of aviation weather products as well as information about Air Traffic Control. www. rosemariekern.com.
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