The Thiokol engineers had also argued that the low overnight temperatures of 18 °F (−8 °C) projected on the day prior to launch would almost certainly result in SRB temperatures below their redline of 39 °F (4 °C).
Ice had accumulated all over the launch pad, raising concerns that ice could damage the shuttle upon lift-off.
The Kennedy Ice Team inadvertently pointed an infrared camera at the aft field joint of the right SRB and found the temperature to be only 9 °F (−13 °C).
This was believed to be the result of supercooled air blowing on the joint from the liquid oxygen (LOX) tank vent.
It was much lower than the air temperature and far below the design specifications for the O-rings.
The low reading was later determined to be erroneous, the error caused by not following the temperature probe manufacturer's instructions.
Tests and adjusted calculations later confirmed that the temperature of the joint was not substantially different from the ambient temperature.

The temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28.0 to 28.9 °F (−2.2 to −1.7 °C); previously, the coldest launch had been at 54 °F (12 °C).
Although the Ice Team had worked through the night removing ice, engineers at Rockwell still expressed concern.
Rockwell engineers watching the pad from their headquarters in Downey, California, were horrified when they saw the amount of ice.
They feared that during launch, ice might be shaken loose and strike the shuttle's thermal protection tiles, possibly due to the aspiration induced by the jet of exhaust gas from the SRBs.
Rocco Petrone, the head of Rockwell's space transportation division, and his colleagues viewed this situation as a launch constraint, and told Rockwell's managers at the Cape that Rockwell could not support a launch.
Rockwell's managers at the Cape voiced their concerns in a manner that led Houston-based mission manager Arnold Aldrich to go ahead with the launch.
Aldrich decided to postpone the shuttle launch by an hour to give the Ice Team time to perform another inspection.
After that last inspection, during which the ice appeared to be melting, Challenger was cleared to launch at 11:38 am EST.
The Thiokol engineers had also argued that the low overnight temperatures of 18 °F (−8 °C) projected on the day prior to launch would almost certainly result in SRB temperatures below their redline of 39 °F (4 °C). Ice had accumulated all over the launch pad, raising concerns that ice could damage the shuttle upon lift-off. The Kennedy Ice Team inadvertently pointed an infrared camera at the aft field joint of the right SRB and found the temperature to be only 9 °F (−13 °C). This was believed to be the result of supercooled air blowing on the joint from the liquid oxygen (LOX) tank vent. It was much lower than the air temperature and far below the design specifications for the O-rings. The low reading was later determined to be erroneous, the error caused by not following the temperature probe manufacturer's instructions. Tests and adjusted calculations later confirmed that the temperature of the joint was not substantially different from the ambient temperature.
The temperature on the day of the launch was far lower than had been the case with previous launches: below freezing at 28.0 to 28.9 °F (−2.2 to −1.7 °C); previously, the coldest launch had been at 54 °F (12 °C). Although the Ice Team had worked through the night removing ice, engineers at Rockwell still expressed concern. Rockwell engineers watching the pad from their headquarters in Downey, California, were horrified when they saw the amount of ice. They feared that during launch, ice might be shaken loose and strike the shuttle's thermal protection tiles, possibly due to the aspiration induced by the jet of exhaust gas from the SRBs. Rocco Petrone, the head of Rockwell's space transportation division, and his colleagues viewed this situation as a launch constraint, and told Rockwell's managers at the Cape that Rockwell could not support a launch. Rockwell's managers at the Cape voiced their concerns in a manner that led Houston-based mission manager Arnold Aldrich to go ahead with the launch. Aldrich decided to postpone the shuttle launch by an hour to give the Ice Team time to perform another inspection. After that last inspection, during which the ice appeared to be melting, Challenger was cleared to launch at 11:38 am EST.
Space Shuttle Challenger disaster