A freak hail event on February 26, 2007 resulted in extensive damage to the thermal protection system of space shuttle Atlantis while poised on the launch pad at Kennedy Space Center (KSC), FL. As a result, the launch of STS-117 to the International Space Station was delayed by almost three months while repairs were made due to hail of up to 35 mm diameter impacting the external tank. Hail size distribution data of this event was acquired by three monitoring stations configured in roughly an equal-lateral triangle surrounding launch pad 39-A. Each station consists of an active impact transducer, which processes the sound of hail impacting a metal plate using an on-board digital signal processor. The acoustic signature of hail is subtly but distinctly different than the sound of rain, allowing frequency analysis to discriminate the difference between the liquid and solid hydrometeor impacts.

The hail disdrometer or “hail monitor”, measures the maximum impulse amplitude generated by a hydrometeor impact on a 1 sq ft transducer sensing surface. Hail-rain discrimination is accomplished by real-time signal processing using a high pass infinite impulse response (IIR) filter. The electrical signal generated by the conversion of the mechanical/acoustic impulse to an electrical signal is converted to a 16-bit integer using an analog to digital converter. The magnitude of the impulse peak from a single hail impact is proportional to the product of the mass of the hydrometeor and the velocity component normal to the surface, raised to a power, where the exponent has been previously determined during a calibration procedure. Primary calibration was accomplished by dropping ice balls of known size from a 20 m tower at the Launch Equipment Test Facility, KSC. In situ calibration was also performed in 2005 and 2006 by volunteers of the Community Collaborative Rain, Hail and Snow Network, near Denver, CO.

After performing spatial and temporal interpolation/extrapolation based on hydrometeor fall dynamics, the hail disdrometer array data coupled with local WSR-88D radial velocity and reflectivity data are merged into a 4D (space and time) picture of hail size distributions. Hail flux maps can be generated and used for damage prediction and assessment over specific surfaces corresponding to structures within the disdrometer array volume, such as the space shuttle and its launch support facilities.