SYNTH-79 · Guide

Dressing for cool conditions (+5 to +18 °C)

Dress for the heat the session will produce: little at race effort, more for easy and slow work, a shell when it is wet, and warm layers kept on until the start

B-limited, mixed evidenceMostly from recreational to elite athletesWritten before October 2026, being re-checked

Decision

Dress by effort and by wet, not by the thermometer alone: race or hard effort at 8–15 °C in summer kit with the hands and arms covered if needed; easy or slow sessions one layer warmer; add a shell when it rains; keep warm clothes on until the start.
In detail:

1. Race, tempo or intervals at about 8–15 °C: shorts and a short-sleeved or sleeveless top; gloves and arm warmers as removable extras. Hard running in a thin full-length suit was already impaired at +10 °C REF-1046, and the fastest marathon temperatures are 4–10 °C REF-1043.

2. Easy or long sessions: one layer more than for racing, because heat production is lower and the session is longer; in the cold and wet at low intensity heat loss can exceed heat production REF-1047.

3. Rain around 5 °C: a shell over the trunk — rain lowered core temperature even at 80% VO2max and raised the cost of running after 40–50 minutes REF-1047.

4. Before a race: keep the warm-up warmth with a layer until the last minutes REF-1048 REF-745.

5. On the bike add what running does not need: the rider's own speed is a wind. Cover the trunk and hands for descents and for the first cold hour (wind shell or gilet, gloves) — practice; no study of riding wind in cool conditions was found.

Evidence

Optimal marathon air temperature 3.8–9.9 °C by level; slowing on both sides REF-1043; top men 1.7% off the record at 5–10 °C WBGT, 4.5% at 20–25 °C REF-1044.
Cycling at 70% VO2max in still air lasted longest at 10.5 °C REF-1045.
In a thin race suit with 5 m/s wind, hard running was best at −4 to +1 °C and impaired at +10 °C REF-1046.
Rain at 5 °C: lower rectal temperature, higher oxygen uptake and RPE late in a 60-minute hard run REF-1047.
Expert review: do not overdress, keep warm until the start; overdressing unstudied REF-1048.

Practical Application

Answer with the effort first: "at race pace you will be warm within minutes — race in summer kit, keep a layer on until the start"; then the wet; then the extras for hands and arms.
"Dress as if it were 10 °C warmer" and "start slightly cold" are common coaching lines and may be offered as such — say they are practice.

Limitations

No clothing-insulation figures and no trial of overdressing were verified; the garments named are practice.
Below about 0 °C, in strong wind chill, at altitude or for an athlete with cold-induced asthma or Raynaud's the framework does not apply — those need their own advice.
How this entry was checked and changed
Why this level

(optimal temperatures on B/B+ field and laboratory studies; the one controlled overdressing result on nine men; garment choices and "start slightly cold" are expert opinion or practice)