GLACIER NATIONAL PARK, Mont. â Isaiah Sullivan can measure the intensity of this yearâs hot summer by the lines of visitors outside the boat rental business he manages on the shores of Lake McDonald. Once the afternoon heat intensifies, the line to paddle on cool water fed by melting snowpack doesnât dissipate until he puts out the closed sign at 7 p.m. Many visitors would rather just cool off or wait for smoky haze from forest fires to lift, rather than brave slopes of the surrounding peaks in unusually high heat and limited visibility. The temperature Sunday at Lake McDonald was in the 90s, as yet another searing heat wave settled over the northern Rocky Mountains.