People sleeping cold outdoors usually blame the sleeping bag. The pad underneath is more often responsible, and the physics of the two situations are genuinely different.
Conduction moves heat far faster than air does
A sleeping bag manages heat lost to the air, which is slow because air conducts poorly once it is held still by the insulation.
The ground is a solid in direct contact with the body, and solids conduct heat quickly. The earth also acts as an effectively unlimited heat sink.
So the same temperature underneath removes warmth much faster than it does above, which is why the surface below matters disproportionately.
Bag insulation compresses to nothing under body weight
Down and synthetic fill work by holding loft, and the fill beneath a sleeper is crushed flat by their own weight.
Crushed fill traps no air and provides almost no insulation, meaning roughly half the bag stops functioning the moment it is used.
The pad exists to replace that lost insulation, which is why a warm bag on a thin pad performs poorly regardless of its rating.
R-value describes resistance to heat flow
Pads are rated by R-value, a measure of how strongly the pad resists heat passing through it, with higher numbers indicating more resistance.
The scale is additive, so stacking two pads gives approximately the sum of their values, which is how people extend a summer setup into colder conditions.
Values are produced under a standardized test, which makes them comparable across brands in a way that temperature ratings on bags are not always.
Construction determines warmth and weight
Closed-cell foam pads insulate through trapped bubbles in the material and cannot fail, but they are bulky for the warmth they provide.
Air pads use internal baffles and often reflective layers or fill to slow the air circulating inside, since an empty air chamber convects heat away efficiently.
Self-inflating designs combine open-cell foam with air, landing between the two on both weight and reliability.
Site selection changes the requirement
Snow, rock and saturated ground pull heat away much faster than dry duff or sand, so the same pad performs differently across surfaces.
Choosing a dry, insulated site does real work, and a layer of dry leaves or a foam pad under an air pad addresses a cold night without new equipment.
Which is why experienced campers upgrade the pad before the bag when a system is not keeping them warm enough.