crack the flags

See flag (“a flagstone”); the idea is that the extreme heat might cause paving-stones to split.

crack the flags (third-person singular simple present cracks the flags, present participle cracking the flags, simple past and past participle cracked the flags)

1. (northern England, colloquial) Of the sun or sunshine: to beat down very strongly; to be extremely hot.
   • 1998, Julia Wild, Blue Silk Promise, page 80:

     Early summer sunshine threatened to crack the flags outside.

   • 2005, Andrew Porter, Suicide Squad: The Inside Story of a Football Firm:

     I was apprehensive, feeling butterflies, pissed, the sun cracking the flags.

   • 2008, Dave Spencer, A Smudge on My Lens, page 276:

     […] plus the sun was still cracking the flags as we headed back to Geneva.

   • 2010, Maureen Lee, The House By Princes Park:

     The sun's hot enough to crack the flags and there's not a cloud in sight.

   • 2012, Audrey Howard, There is No Parting:

     'Tis fit to crack the flags so you'll not be wantin' your brolly.