Cold, fear, and a piece of music can all stand your hairs up. On a dense pelt that motion matters. On a human forearm, the physics predicts almost no added insulation. Drag the coat away, then slow the clock down.
arctic peltcat-like coathuman forearm
cold: sympathetic signal
coat loft40.00 mm
retained-air factor70.0%
predicted insulation6.95 clo
1 clo reference
The hair rises the same way for every trigger. What changes the thermal result is not the feeling but the coat. A centimetre of perfectly still air has a theoretical resistance of 0.01 / 0.026 = 0.385 m² K/W, or 2.48 clo. Dense fur can retain much of that air; sparse human hair cannot.
The reflex has two clocks
A goosebump is the fast output. In mouse skin, the same sympathetic nerve is also part of the hair-follicle stem-cell niche. Operate the timescale below. This is biology in mice, not a hair-loss treatment for humans.
cold sensed
↓
sympathetic nerve fires
↓
arrector pili contracts
↓
follicle stem cells activate
Fast output: the muscle contracts and the hair stands. No new coat appears on this clock.
Mouse result only. The 2020 Cell study does not show that cold showers regrow human hair, and this page offers no treatment advice.
Music chills are reward-linked. Salimpoor and colleagues used [11C]raclopride PET, with fMRI timing, in a small selected sample. Binding changes were consistent with dopamine release in dorsal striatum during anticipation and ventral striatum near peak chills. That is an association measured under study conditions, not a settled evolutionary purpose for frisson.
The check
R = loft / kair; clo = R / 0.155; predicted effective clo = clo × retained-air factor.
At the dense-pelt end, the live bench computes 0.040 m / 0.026 / 0.155 × 0.70 = 6.95 clo. At the human end it computes 0.00030 m / 0.026 / 0.155 × 0.01 = 0.00074 clo. With air conductivity spanning 0.024 to 0.026 W/m K, the still-air ceiling is 2.48 to 2.69 clo per centimetre.
The measured anchor is Scholander et al. (1950): insulation in excised mammal skins correlated with fur thickness. The thermal calculation for a human forearm is a model prediction, not a direct measurement. Loft and retained-air factor are free choices in this demonstration. The endpoints (40 mm, 70%; 0.30 mm, 1%) are illustrative, deliberately exposed assumptions, not species measurements. Wind, convection, hair angle, skin blood flow, humidity, coat microstructure, and body geometry are omitted. The 1 clo line is the unit's conventional reference, roughly a typical business suit, not a T-shirt.