Three Semi-Truck/Trailers collided with a school bus in fog in Richland County North Dakota just before 9 AM CDT (1400 UTC) on 25 September 2015. The GOES-13 Visible animation, above, shows an eroding fog bank over southeastern North Dakota. Richland County is outlined in the imagery. The school bus was traveling on Highway 46, which hugs the northern border of Richland County; the accident was about 3 miles west of Interstate 29 (Press Report 1 ; Press Report 2).
During the preceding nighttime hours, Suomi NPP VIIRS “fog/stratus product” IR brightness temperature difference images at 0756 UTC (2:56 am CDT) and 0937 UTC (4:37 am CDT), above, showed an increasing signal of fog/stratus – yellow to red color enhancement – over the region during that time period (when surface visibilities also began to rapidly decrease at Wahpeton KBWP, the county seat of Richland County).
GOES-R IFR Probability fields, above, around the time of the accident, showed a high probability of IFR Conditions over southeast North Dakota. High values were present over the area before sunrise (Sunrise in Fargo on 25 September is at 1318 UTC). Values at 1400 UTC show largest values over northern and western Richland County. The Low IFR Probabilty field for 1400 UTC, below, immediately after the time of the crash, shows a maximum across northern Richland County where the accident occurred.
July’s first Full Moon occurred at 0219 UTC on 2 July (a second full moon occurs later this month on 31 July). Strong illumination from the moon showed river valley fog in several tributaries of the Mississippi River (for example, the Wisconsin River in southwest Wisconsin; the Upper Iowa River in Iowa) across the Upper Midwest. The Suomi NPP VIIRS Day/Night Band also shows a plume of Canadian wildfire smoke aloft, stretching from central Iowa northwestward to western Minnesota. This smoke (visible on 1 July in Aqua true-color imagery from the MODIS Today site) is not apparent in the IR Brightness Temperature Difference field, although the river valley fog certainly is. Smoke is transparent to most infrared channels and detection at night is very difficult if visible information such as that provided by the Day/Night Band is not present.
The VIIRS Day/Night Band also enabled detection of the dense plume of Canadian wildfire smoke as it moved off the US East Coast and over the adjacent offshore waters of the western Atlantic Ocean at 0614 UTC (below). Again, note that the smoke aloft does not exhibit a signature on the corresponding VIIRS Infrared imagery.
The Suomi NPP VIIRS Day Night Band during a Full (or near-Full) Moon yields striking visible imagery at night because of abundant reflected lunar illumination. Sequential orbits along the west coast of the Pacific on the morning of 10 October showed the penetration of coastal stratus and fog inland at two times (click here for the images above without the surface observations). A similar case study from 9 October is shown here. The slow inland penetration of stratus/fog is captured by the scenes: tendrils of fog extend up small valleys along the edge of the Salinas Valley, for example, and the fog extends farther down the valley at 1048 UTC. Similar expansion of fog occurs over Sonoma Valley north of San Francisco Bay.
Careful inspection of the imagery shows parallel lines along the western edge at 0907 UTC and along the eastern edge at 1048 UTC. In addition, city lights and topographic features are displaced somewhat along the eastern edge of the 1048 UTC image. These are all artifacts of the VIIRS instrument viewing geometry (that is, parallax) and post-processing that is necessary near the edges to maintain high-resolution imagery there.
A toggle between the corresponding VIIRS 11.45 µm – 3.74 µm infrared brightness temperature difference images (commonly referred to as the “fog/stratus product”), below, similarly shows gradual expansion of water-based clouds between 0907 and 1048 UTC. There are also image features, color enhanced as black, that suggest very thin cirrus is moving over the coast. These clouds are thin enough that they cannot be discerned in the Day Night Band imagery, but their presence nevertheless inhibits the detection of low clouds in places, such as over the southernmost part of the Salinas Valley at 1048 UTC.
The image toggle above shows similar features over Washington State. Fog/stratus tendrils move up river valleys in the ~90 minutes between the two polar-orbiting satellite passes, and areas of fog increase in size. (click here for the same images without observations). Because the first image is very near the edge of the VIIRS instrument scan swath, there is also a shift in city lights and some geographic features, again an artifact of scanning geometry (parallax) and the post-processing to maintain high-resolution imagery at the scan edges.
The IR brightness temperature difference product over Washington, above, also shows evidence of a slow increase in the areal coverage of fog/stratus near the coast. The effects of limb brightening are also present in the first image. When a satellite scans near the edge of its domain, the path from the point on the Earth to the satellite traverses more of the upper atmosphere, and a colder sensed temperature results. This effect is wavelength-dependent. For example, at one point (47º N, 125º W) in the stratus (with fairly uniform temperature) off the west coast of the Washington, in the stratus (which should have a fairly constant temperature), brightness temperatures were about 1º C cooler in 11.35 µm imagery, but closer to 2.5º C cooler in the 3.74 µm imagery. Hence, the brightness temperature difference signal is larger at 0906 UTC.
Both brightness temperature difference fields show signals over dry land that are related to emissivity differences in the soils. These occur over central Washington, above and over Nevada in the images centered over California.