Basically, as I read in the NYTs, rain hits the tank and causes the foam to break off.
Correction - the shuttle is not transported through a rainstorm, as the impact of the falling drops
will cause the Shuttle Protective Tiles to crack and break-up, serial shedding causes additional damage.
Ice, ice, baby:
When we built the SLC-6 Launch Complec for the Shuttle operation at Vandenberg, icing was more of a problem that at KSC.
When filled with the cryogenic fuels - Liquid Ozygen (LOX) & Liquid Hydrogen it ices up adding several hundred pounds
per hour of frost and then compacted ice rind buildup.
The Western Range hasd to build a stationaty launch mount where hot high speed airflow was directed upwards
to keep a dry-flow barrier between the external tank & the ambient atmosphere.
The ISS (Ice Supression System) was a pair of B-52 class jet engines running in a containment vault
& mixing the exhaust gasses with de-humidified air and feed into the launchmount base where it
would flow upward at a rate to keep the local ambient air containting moisture off the tank.
SLC-6
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Several other operation issues remained unresolved. Because it appeared that tank icing would be much more severe at VAFB than at Kennedy, an additional $12.8 million was spent on ET Area icing-protection.(41) The Air Force added two jet engines near the launch mounts, hoping that the warm exhaust flowing through ducts will sufficiently warm the tanks. But as one Air Force Officer reported, "We don't know if it will work or not."(42) A 1986 Senate report warned that the icing problem remained "very significant."(43)