Hole punch clouds over the Upper Midwest

November 7th, 2021 |

On the morning of Sunday, November 7th, numerous elongated hole punch clouds were visible over the Upper Midwest, including parts of Wisconsin, Illinois, Iowa, and Minnesota. Also called fall streak clouds, these are a relatively rare phenomenon that form because of the unusual properties of cloud droplets.

Photo of a hole punch cloud and the associated fall streaks, taken on the east side of Madison, WI, at 11:20 AM CST on Sunday, November 7th. Photo by the author.

While most people know the freezing temperature of water is 0 °C (32 °F), that’s only true when dealing with a flat surface.  A curved droplet has more energy in it due to surface tension squeezing the droplet together, and so the air temperature has to be colder in order to make the droplet cold enough to freeze.  As a result, clouds of liquid water below freezing are relatively common, especially in the spring and fall when temperatures at cloud level are just below freezing.  These are called supercooled clouds.

Another commonly-known fact about water is if the relative humidity of the air is less than 100%, liquid water will evaporate.  Again, that’s not necessarily true for cloud droplets. What is especially interesting is that the relative humidity required to support growth is bigger for a cloud droplet than it is for an ice crystal.  Given an environment with both cloud droplets and ice crystals, the droplets will evaporate and the ice crystals will grow.  This is known as the Bergeron-Findeisen process and is a key part of forming precipitation from cold clouds. 

Both cloud droplets and ice crystals require a nucleus to form. Dust, pollen, and other aerosols are common nuclei.  While water can condense on many different aerosols, ice crystals are much more selective. Due to the rigid crystal shape of ice, it can only form on aerosols that have a similar structure. This is, in part, why supercooled clouds are relatively common: there’s just not enough ice nuclei around for ice crystals to form.  

That brings us to Sunday morning: a rather large altostratus deck was present across the upper midwest. Even though the surface temperature was approaching 16 °C (60 °F), the clouds were high enough above the surface that their temperature was below freezing.  The morning sounding from Davenport, IA, showed that the freezing level was around 3300 m (11,000 ft) above sea level, but airport observations around the region showed that cloud bases were around 5100 m (17,000 ft). Without a sufficient amount of ice nuclei present, they stayed in the liquid phase and were thus supercooled clouds.

1200 UTC (6 AM CST) sounding from Davenport, IA, showing the freezing level was approximately 3300 m (11,000 ft) above sea level. Image from the University of Wyoming sounding archive.

However, numerous aircraft were flying through those clouds as they ascended from or descended into airports across the region.  The moisture-rich exhaust from the planes was deposited into the low-pressure wake behind the airplane, where it cooled very quickly and formed ice.  Normally, this would form the classic contrails seen behind many aircraft in the sky.  However, in this case the contrail served as a nucleation site within the supercooled cloud.  The droplets near the ice rapidly evaporated and the ice crystals generated by the airplanes grew even larger. In some cases, the crystals grew so large that they could no longer be supported aloft, and they started falling to the ground as snow.  They didn’t reach the ground because the air was warm and dry beneath the cloud, and so the ice crystals either melted and evaporated, or they sublimated (going directly from solid to vapor).  

The Terra polar-orbiting satellite happened to be passing overhead at the right time to capture this phenomenon while it was happening around 10:30 AM CST.  Almost-clear holes are seen in northeastern Iowa and southeastern Minnesota, while in northern Illinois they appear as elongated ice clouds surrounded by a clear region embedded within a larger cloud.  

MODIS True-color image from the 10:30 AM CST overpass showing hole punch clouds, circled in white.

The loop from Band 2 (0.64 micron) from GOES-16 also shows these clouds propagating through the region. This view, over Dane County (Madison) Wisconsin, shows one hour of visible-wavelength satellite imagery. The embedded ice clouds are clearly visible as structures that propagate from the west to the east. While the airplanes that created these structures have long since departed to other locations, their impact remained for some time.

Animation of GOES-16 Band 2 reflectance over south central Wisconsin. Dane County, home of Madison, is outlined.

Other blog posts showing examples of hole punch clouds can be found here.

Unstable Air Comes to Madison

June 8th, 2021 |

Most people in south-central Wisconsin have noticed how uncomfortable the weather has been over the past week. The unseasonably cool air from a week ago has been replaced with a hot, sticky airmass that feels more at home in late July than in early June. And while precipitation has been sorely lacking in parts of the area, at least the atmosphere is now more supportive of rain and numerous short-lived storms have been popping up (and quickly dying out) since the weekend.

The thermodynamic profiles observed by NOAA-20 and processed by NUCAPS help give insight into the impact of the atmosphere’s change from cool and pleasant to hot and humid. Three profiles are shown here: from Thursday the 3rd, to Sunday the 6th, to Tuesday the 8th.  All three are taken in the early afternoon, and all are in the general vicinity of Madison.  The surface temperatures on all three days are generally the same.  However, the surface dew points show a steady increase from one day to the next.  The conditions on the 3rd were warm, but they were not nearly as unpleasant to experience directly as they were on the 8th thanks to the substantial increase in surface moisture that took place during that time.

Three skew-T plots from around Madison, Wisconsin, showing an increase in instability.

Of course, surface conditions don’t tell the whole story, and this is where the NUCAPS profiles prove their worth. The dashed line on each sounding represents the temperature of the near-surface mixed layer air parcels as they ascend. These air parcel trajectories were calculated by the SHARPpy package, a free and open-source tool designed to analyze atmospheric soundings.  As air rises, it cools, but it cools at different rates depending on if the air is saturated or not. If air from the surface is warmer than the air that surrounds it, it will continue to rise all on its own, but air that’s cooler is going to sink. Note that the 3rd and the 6th  are stable days. The dashed line is to the left of the red temperature line. On those days, an ascending air parcel is always colder than its environment and since cold air sinks, no surface-based air is going to rise high enough to make deep convective clouds and the rain that accompanies them.  However, on the 8th the situation is reversed: the dashed line is to the right of the temperature sounding which means that air from the surface is warmer than its environment and will rise on its own.

The instability isn’t that large on the 8th; rising air parcels aren’t substantially warmer than their environment and so there’s not a lot of buoyant energy.  Still, it has been enough to contribute to some localized showers.  That’s evident on the CMORPH satellite-based precipitation estimates over the midwest (available on SSEC RealEarth), which shows virtually no rain on the 6th but the results of some scattered showers in south-central Wisconsin on the 8th.  It’s not much, but right now the area needs every drop it can get.

CMORPH rain estimates for 6 June 2021.

CMORPH rain estimates for 8 June 2021.