In Alaska conditions can change quickly. Fog, snow, ice and clouds all impact transportation, on the ground and in the sky. Pilots rely on weather forecasts and reports to determine if it’s safe to even take off and to stay safe once in the air. Ground-based weather observations show conditions at a specific point and time, but satellite observations can show the extent and evolution of low clouds and fog over a large area. These two types of observations are then used to make short term predictions about how these conditions will change at the surface.
An example from April 3, 2026 shows how satellite imagery can be used in tandem with ground based observations any time of day. A series of images, shown as an animation below, captured by NOAA satellites pictures low clouds and fog moving across the North Slope of Alaska and the Arctic Ocean, first in the darkness of the early morning and then in the light of day. Two satellite products are used to accomplish this around-the-clock coverage. The animation begins at 11:30 UTC, 2:30 am Alaska time, first using the VIIRS Nighttime Microphysics RGB to see cloud progression in the dark, then switching to the VIIRS Day Snow/Fog RGB at 16:30 UTC as the sun comes up.
The VIIRS Nighttime Microphysics RGB is a product designed specifically for night observations. Low clouds and fog take on a dirty yellow to light grey color, while high clouds appear magenta to red depending on thickness, while the snowy ground and sea ice take on a pink to orange appearance. Low clouds and fog are generally lumped together when describing satellite imagery as it is very difficult to distinguish them without the help of ground observations. In this RGB from the polar orbiting satellite the green component is assigned to the common fog channel difference of 10.8μm – 3.7μm, and the red component is assigned to the channel difference of 12.0μm – 10.8μm to assess cloud thickness. While the blue component is assigned to the 10.8μm to enhance areas of warm (i.e. low) clouds where fog is more likely.
One of the challenges faced in winter daytime satellite observations is to distinguish white “reflective” snow from white “reflective” clouds on visible imagery, this is when the VIIRS Day Snow/Fog RGB works its magic. Fog and low clouds appear in a bright lavender or white color, while snow covered ground and sea ice appear in a contrasting bright red. The reflectance of snow, water, and ice clouds varies across the visible, near infrared, and infrared spectrums. The channels which bring out the distinguishing differences are combined in the Day Snow/Fog RGB to show greater contrast between snow and cloud than is generally possible with a single channel.
Together these satellite products provide an excellent overall picture of what low clouds and fog look like and how they are moving. Ground based observations can then confirm what is occurring at several locations along the north gulf coast. These tandem observations can help assist forecasters particularly when it comes to writing Terminal Aviation Forecasts (TAFs).
Table 1 shows ceiling and visibility observations from different ground stations at corresponding times to the animation. Red highlighted cells indicate IFR, Instrument Flight Rules, conditions and blue cells indicate MVFR, Marginal Visual Flight Rules, conditions. In aviation the IFR category is used in conditions with a visibility of 3 miles or less and ceiling of 1000 ft or less. MVFR is for a visibility of 3-5 miles and/or a ceiling of 1000 to 3000 ft. When weather conditions improve beyond MVFR, pilots can operate under VFR, Visual Flight Rules.
Under IFR pilots rely on the navigation instruments in their airplanes to safely fly and land. Not all aircrafts are equipped to fly in those conditions, making accurate forecasting even more important. Monitoring the evolution of stratus and fog in satellite imagery combined with observations made on the ground help forecasters make short term predictions about when IFR conditions might improve, keeping pilots informed and safe.
Written by Abigail Haas and Carl Dierking