Upper Tropospheric Conditions Associated with
Recent Clear-Sky Ice Falls
Kevin Brink1, David Travis1, and Jesus Martinez-Frias2
1
Department of Geography and Geology, University of Wisconsin-Whitewater,
Whitewater WI, 53190, USA
2
Centro de Astrobiología (CSIC-INTA) Associated to the NASA Astrobiology Institute, Instituto Nacional
de Técnica Aeroespacial Ctra de Ajalvir, km 4 28850 Torrejón de Ardoz, Madrid (Spain)
The occurrence of clear-sky ice events has sporadically been reported throughout
human history. These events are characterized as hailstones striking the earth’s surface in
non-thunderstorm environments. Currently, there is no accepted explanation regarding
the physical processes associated with these events. However, in recent years there has
been a noted increase in the occurrence of such ice events, leading some to speculate that
they could be related to changes occurring in climate (i.e. global warming). This study
provides a “first look” analysis of the upper tropospheric conditions associated with the
19 most recent, and well-documented, events. The 19 events have all occurred during the
past 5 years, with a majority reported in western Europe and North America.
We analyzed NCEP/NCUR reanalysis data of the upper troposphere to determine
conditions occurring in the atmosphere on the day of each event and compared these to
climatological “normals” for that same day and location. Particular attention was given
to conditions near the tropopause (i.e. 200-300mb) because geochemical analysis of some
of the ice samples suggests that they likely originated at these levels.
We report that the mean upper tropospheric temperature gradient for all cases
combined was significantly greater than climate normals. This supports previous
findings that have suggested that these events may be associated with tropopause
undulations, where strong temperature and pressure gradients typically exist near the
tropopause. The well-documented recent cooling of the upper troposphere may be
providing a more favorable environment for these events to occur when tropopause
undulations exist.