Low-Earth Orbiting and Ground-Based Perspectives on the Ongoing Air Quality Event
The dominant weather story across much of the central United States remains the continuing impact of the Minnesota and Ontario wildfires on the air quality in the upper Midwest. You can see just how impactful the smoke was by looking at this slider of two images from the Blog’s home office on the campus of the University of Wisconsin-Madison. These two iamges were taken 24 hours apart, with the one on the left showing Madison’s isthmus and downtown on the hot and humid morning of Wednesday 15 July, while the one on the right shows the smoke-filled skies of 16 July. Watch as the prominent State Capitol, just 1.1 miles (1.9 km) away, disappears under the cloak of smoke.
Here’s a map of PM2.5 from Airnow.gov valid for 1900 UTC (2 PM CDT) on 16 July 2026. Hazardous air levels stretch all the way from north central Minnesota all the way to central Pennsylvania. The challenge of monitoring this outbreak of hazardous air was complicated by the fact that the day before, GOES-19 experienced an anomaly and was unable to transit observations until the early afternoon of the next day. In this post, we’ll explore a few of the alternatives for keeping track of what the air was doing until the workhorse geostationary platform could be returned to service.
Here in the Blog’s Headquarters City of Madison, the air quality degraded in the hours after sunset. While it’s easy to follow smoke during the day using satellites, tracking smoke at night can be challenging because the smoke particles are generally not significant sources of infrared radiation. However, there are other tools at our disposal that can be used to monitor this situation. Among the most important tools are ground-based air quality networks, such as those operated by the EPA or PurpleAir. Here’s a time series of a PurpleAir sensor in downtown Madison showing an initial increase in particulate matter around 10:00 PM CDT (0300 UTC) where the Air Quality Index (AQI) plateaued close to 200, then another increase around 4:30 AM (0930 UTC) to truly astonishingly high levels.
We can actually see the atmospheric current that brought the flow in. Here’s a loop from the Milwaukee/Sullivan NEXRAD radar showing the inland penetration of a lake breeze well into central Wisconsin. That lake breeze brought some relief from the humid conditions that had been dominating Madison’s weather for the previous few days, but it also cleared out air that originated from the south and instead replaced it with smokey air from the north. It’s also interesting to see how the lake breeze impacted the propagation of the radar beams. Lake breezes create shallow inversions as the cool air undercuts and lifts warm air. In this case, we see that shortly after the lake breeze front passed over the radar, the ground clutter increased significantly. This is because the lake breeze passage fostered an inversion over southeastern Wisconsin, refracting the radar beams in unexpected ways and fostering anomalous propagation.
The lake breeze can also be tracked with surface weather sensors. The Reliable Automated Instrumentation Network (RAIN) suite of sensors on the rooftop of Blog HQ provide a multiyear archive of weather conditions on the scale of a minute. Here’s a meteogram of 24 hours of conditions on 15-16 July. The arrival of the lake breeze can be seen right at 0300 UTC (10:00 PM CDT) with the rapid drop in temperature, increase in wind speed, and sharp change in wind direction from northwest to east.
The same cameras that we looked at above can also be used in the overnight hours. Here’s a movie between 8:00 PM and midnight CDT that shows the arrival of the smoke. See how the clouds disappear and the lights on the far shore of the lake vanish as the smoke arrives. While not appearing as dramatic as it would if the
The Space Science and Engineering Center (SSEC) is, along with NOAA, a parent of CIMSS. Researchers at SSEC are renowned experts in instrument design and deployment, and one of the highlights of SSEC’s work is the High Spectral Resolution Lidar, a laser-based instrument designed specifically to look at how aerosols like smoke are distributed through the atmosphere. This is a time-height cross section of aerosol backscatter running from 0000 to 0600 UTC (7:00 PM to 1:00 AM). We can see the smoke as the bright layer that first appears right after 0300 UTC (10:00 PM) and is about 1 km thick.
All of these things: the radar-indicated lake breeze front, the surface weather conditions, the HSRL cross sections, and the visible camera movie; all of these show significant events that are coincident with the sharp increase in AQI measured by the surface station.
The polar orbiting satellites operated by NOAA’s Joint Polar Satellite System (JPSS) also provide some unique perspectives on the event that we can’t get from geostationary view. This VIIRS true color image from NOAA-20 at 1756 UTC (12:56 PM CDT) perfectly encapsulates how widespread the smoke is. Nearly the entire states of Wisconsin and Michigan are severely affected, with additional impacts stretching from the Red River Valley to the Acela Corridor.
The VIIRS true color product has a spatial resolution of 750 m, which is slightly better than the 1 km resolution of its GOES counterpart. However, the practical resolution is a bit better because the GOES resolution is defined at the equator and it degrades the further poleward you go. By contrast, the VIIRS resolution doesn’t have a latitudinal dependence. That enables us to zoom in a little more than we might for a geostationary product. When we do, we see yet another interesting phenomenon.
Do you see how the cumulus clouds are almost exclusively outside the smoke, while the smokey regions are otherwise generally cloud free? This is likely because the smoke is attenuating enough of the incoming solar radiation to prevent surface-based convection from starting. When we talk about “fair weather cumulus,” we should make sure that the definition of “fair weather” includes no smoke.