Editing Hail (section)

=== Short-term detection ===
[[Weather radar]] is a very useful tool to detect the presence of hail-producing thunderstorms. However, radar data has to be complemented by a knowledge of current atmospheric conditions which can allow one to determine if the current atmosphere is conducive to hail development.

Modern radar scans many angles around the site. Reflectivity values at multiple angles above ground level in a storm are proportional to the precipitation rate at those levels. Summing reflectivities in the [[Vertically Integrated Liquid]] or VIL, gives the [[liquid water content]] in the cloud. Research shows that hail development in the upper levels of the storm is related to the evolution of VIL. VIL divided by the vertical extent of the storm, called VIL density, has a relationship with hail size, although this varies with atmospheric conditions and therefore is not highly accurate.<ref>{{cite web|url=http://www.srh.noaa.gov/hgx/projects/hail_study.htm|title=VIL density and Associated Hail Size Along the Northwest Gulf Coast| first1= Charles A. |last1= Roeseler | first2= Lance |last2= Wood |publisher= National Weather Service Southern Region Headquarters| date= 2006-02-02 |access-date= 2009-08-28|archive-url = https://web.archive.org/web/20070818231127/http://www.srh.noaa.gov/hgx/projects/hail_study.htm |archive-date = August 18, 2007 }}</ref> Traditionally, hail size and probability can be estimated from radar data by computer using algorithms based on this research. Some algorithms include the height of the freezing level to estimate the melting of the hailstone and what would be left on the ground.

Certain patterns of reflectivity are important clues for the meteorologist as well. The [[three body scatter spike]] is an example. This is the result of energy from the radar hitting hail and being deflected to the ground, where they deflect back to the hail and then to the radar. The energy took more time to go from the hail to the ground and back, as opposed to the energy that went directly from the hail to the radar, and the echo is further away from the radar than the actual location of the hail on the same radial path, forming a cone of weaker reflectivities.

More recently, the [[Dual polarization|polarization]] properties of weather radar returns have been analyzed to differentiate between hail and heavy rain.<ref>{{cite journal |first1=K. |last1=Aydin |first2=T.A. |last2=Seliga |first3=V. |last3=Balaji |date=October 1986 |title=Remote Sensing of Hail with a Dual Linear Polarization Radar |journal=Journal of Climate and Applied Meteorology |volume=25 |issue=10 |pages=1475–14 |doi=10.1175/1520-0450(1986)025<1475:RSOHWA>2.0.CO;2 |issn=1520-0450|bibcode = 1986JApMe..25.1475A  |doi-access=free }}</ref><ref>{{cite web| url= http://www.chill.colostate.edu/w/Hail_signature_development|title=Hail Signature Development |publisher= CHILL National Radar Facility, [[Colorado State University]]| date=2007-08-22|access-date=2009-08-28|archive-url= https://web.archive.org/web/20090107012923/http://chill.colostate.edu./w/Hail_signature_development|archive-date=2009-01-07}}</ref> The use of differential reflectivity (<math>Z_{dr}</math>), in combination with horizontal reflectivity (<math>Z_{h}</math>) has led to a variety of hail classification algorithms.<ref>{{cite web|url=http://www.chill.colostate.edu/w/Hydrometeor_classification_example|title=Hydrometeor classification example| publisher= CHILL National Radar Facility, Colorado State University |date=2008-08-25|access-date=2009-08-28|archive-url= https://web.archive.org/web/20100624185402/http://chill.colostate.edu/w/Hydrometeor_classification_example|archive-date=2010-06-24}}</ref> Visible satellite imagery is beginning to be used to detect hail, but false alarm rates remain high using this method.<ref>{{cite journal|doi=10.1016/S0169-8095(96)00032-4|date=1998-07-25|title=Satellite data based detection and prediction of hail|last1=Bauer-Messmer| first1= Bettina| last2= Waldvogel| first2= Albert|journal=Atmospheric Research|volume=43|issue=3|page=217|bibcode = 1997AtmRe..43..217B }}</ref>