the.com/heat spell

means A stretch of unusually hot days in a row, long enough to earn its own name and ruin your sleep schedule.

from Straightforward compound: 'heat' plus 'spell,' where spell means a continuous stretch or period of time — the same sense survives in 'cold spell' and 'dry spell.' That use of spell for a duration is distinct from the magic-incantation spell; it likely traces back to an old sense of 'a turn at doing something,' which drifted into just meaning 'a while.' No sorcery involved, though it can feel like a curse.