The margin nobody states between the forecast and your sleeping bag

Ask any guide what temperature rating you need and the answer is the forecast. That is not what the planner asks for, and the difference is a single constant that almost nothing published states out loud.

It plans to 5 °C below the night low you describe. Not because the forecast is wrong, but because the number a rider gives is a typical night rather than the coldest one, a comfort rating and a limit rating are different claims about the same bag, and a bag that is exactly adequate has nothing left when any of that goes against you.

Worked against the four answers the interview offers: “Below freezing” is -5 °C, so the planner asks for a bag rated to -10 °C. “Near freezing” is 2 °C, so the planner asks for a bag rated to -3 °C. “Cold but not freezing” is 8 °C, so the planner asks for a bag rated to 3 °C. “Mild” is 15 °C, so the planner asks for a bag rated to 10 °C.

Two things follow. A rider who reads the forecast and buys to it has bought one margin short of what the plan asks. And the figure moves in steps rather than smoothly, because the interview offers four answers rather than a slider — so the honest question is which of the four your ride is, not what tonight will be.

What this page does not have is a bag to sell you. Nothing in the planner holds a weight, a price or a maker, because none of those were measured. It holds a figure to hit, and the figure is above.