Hail Scars Visible On Satellite Imagery

On Friday July 24, 2009, multiple significant hail storms moved southeastward across northeast Iowa, southwest Wisconsin, and northwest Illinois. These hail storms produced extremely large hail, and copious amounts of hail, which led to some concentrated swaths of damage to vegetation. In some areas, most of the crops were severely damaged or destroyed. For a complete write-up on the situation, click here.

With a relatively clear day today, some of the scarring is visible on satellite images. First, the MODIS Vegetation Index which is a 1km resolution product designed to pick up on areas of greenness in the vegetation:

 

A minimum of about 28% greenness is evident just south-southeast of Belmont, which is not surprising given that is where some of the worst crop damage was observed. Corn stalks were completely stripped and sheared off to a height of less than 2 feet. These damaged areas of vegetation now absorb more radiation from the sun, thereby allowing the surface to heat faster. This phenomenon is evident in the MODIS 250m resolution satellite image from below. Cumulus clouds fired in greater abundance on the Wisconsin hail swaths, which makes them less distinguishable than the Iowa hail swath.

 

The below image is from a few days later, a little earlier in the day so fewer cumulus clouds.  The hail scars are more clearly visible over southwest Wisconsin as well as in northeast Iowa.

In western Clayton County, the surface skin temperature (temperature of the Earth's surface, rather than the air temperature), as estimated by MODIS 1km resolution satellite imagery, had a differential of about 22 degrees between the hail swath and the green vegetation remaining outside of the swath at 11:58 AM CDT. In northwest Lafayette County, the differential was around 15 degrees. This proves that the areas with vegetation damaged by hail tend to heat up much more quickly than surrounding areas, and it also explains why the diurnal cumulus would develop there first.



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