Hole punch cloud features over Wisconsin and Illinois

November 22nd, 2021 |

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images [click to play animated GIF | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (above) revealed the formation of several “hole punch” features across southeastern Wisconsin, northeastern Illinois and Lake Michigan on 22 November 2021 . These cloud features were caused by aircraft that were either ascending or descending through a relatively thin layer of clouds composed of supercooled water droplets — cooling from wake turbulence (reference) and/or particles from the jet engine exhaust acted as ice condensation nuclei, causing the small supercooled water droplets to turn into larger ice crystals (many of which then fall from the cloud layer, creating “fallstreak holes“). The ice crystal clouds appear as darker shades of gray on the 1.61 µm Snow/Ice images.

The GOES-16 Cloud Top Temperature derived product (below) showed that values were generally in the -30 to -35ºC range.

GOES-16 Cloud Top Temperature product [click to play animated GIF | MP4]

A toggle between 250-meter resolution Terra MODIS True Color and False Color RGB images from the MODIS Today site (below) provided a more detailed view of the numerous hole punch features at 1730 UTC, including a better depiction of the glaciated fallstreak clouds (shades of cyan) within the middle of each hole punch.

Terra MODIS True Color and False Color RGB images [click to enlarge]

Other blog posts showing examples of hole punch features can be found at this link.

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.

Early November with little snow

November 7th, 2021 |
BRDF Imagery from MODIS, 7 November 2021 (Click to enlarge)

A MODIS-based true-color cloud-free image, above, from SSEC’s Real Earth (link) shows a distinct lack of snow cover — for early November!!! — over the USA and Canada. These BRDF (Bidirectional Reflectance Distribution Function) fields account for sun angle, viewing angle and surface type; data over the past 16 days are used in this computation. Monitor these fields at the RealEarth link in the coming weeks to see the inevitable (albeit delayed!) increase in snow cover over North America!

Blowing snow across the Upper Midwest

February 6th, 2021 |

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 (GOES-East) Day Snow-Fog RGB images (above) showed widespread horizontal convective rolls (HCRs) which highlighted areas where blowing snow was more concentrated across parts of southern Manitoba and the Upper Midwest on 06 February 2021. Snow cover (and glaciated clouds) appeared as shades of red, with bare ground exhibiting lighter shades of green and low-level water droplet clouds appearing as brighter shades of white.

Closer views of the northern, central and southern portions of the region where blowing snow was most prevalent are shown below. The HCRs were evident during the early to late morning hours across southern Manitoba, far eastern North Dakota and northwestern Minnesota — and then became more apparent across western/southern Minnesota extending into far northern Iowa as the day progressed. Surface reports showed that the visibility fluctuated dramatically at some sites as HCRs moved through.

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

GOES-16 Day Snow-Fog RGB images [click to play animation | MP4]

Terra MODIS True Color and False Color RGB images [click to enlarge]

Terra MODIS True Color and False Color RGB images [click to enlarge]

In comparisons of MODIS True Color and False Color RGB images from Terra (above) and Aqua (below), the areal coverage of HCRs could be seen in the False Color imagery.

Aqua MODIS True Color and False Color RGB images [click to enlarge]

Aqua MODIS True Color and False Color RGB images [click to enlarge]

Farthest to the north, one cluster of HCRs appeared to originate over Lake Manitoba — as seen in 30-meter resolution Landsat-8 False Color imagery from RealEarth (below).

Landsat-8 False Color RGB image [click to enlarge]

Lansdsat-8 False Color RGB image [click to enlarge]

Two notable pilot reports across southern Minnesota (below) showed that flight visibility was restricted to 4 miles at an elevation of 3000 feet, and the tops of HCRs extended to 5000 feet.

GOES-16 Day Snow-Fog RGB images, with plots of Pilot Reports [click to enlarge]

GOES-16 Day Snow-Fog RGB images, with plots of Pilot Reports [click to enlarge]

GOES-16 Day Snow-Fog RGB images, with plots of Pilot Reports [click to enlarge]

GOES-16 Day Snow-Fog RGB images, with plots of Pilot Reports [click to enlarge]

Additional material on satellite identification of blowing snow is available here and here.