Notice in Figure 2 that the plenum temperatures stay at 32° even though ambient and parallel plate temperatures drop far below freezing. It must be that ice was forming in the plenum, releasing the huge heat of fusion.
Source: Zomeworks technical note, October 2002. Text extracted by OCR from scanned document. For full "Work Leading Up" document see 2002-10-01-cool-cell-work-leading-up.
PDF: 2002-01-01-cool-cell-work-leading-up-2.pdf