Just a historical note, the basis for this rule and much of FAR 121 came out of a TWA DC-2 accident in 1935 that killed a senator. The Copeland Committe was formed by the US Senate to investigate air safety, recommendations included forming a separate agency to handle air safety matters, research on pilot fatigue and numerous rule making. The TWA accident was caused on part by rapidly changing weather that closed the destination (KMKC) and the alternate that also closed, the plane crashed in fog out of fuel.
GF