How much will the heat slow me?

Running is a furnace. Only about a quarter of the energy you burn becomes forward motion; the rest comes off as heat. To hold your core temperature steady, your body sweats, and as that sweat evaporates off your skin it carries heat away. Sweat that drips off provides much less of that evaporative cooling.

Air temperature, humidity, sun and wind all affect heat strain. The dew point describes moisture in the air, while WBGT combines several environmental inputs when measured on site. As humidity rises, evaporation becomes less effective, which reduces evaporative cooling. Heart rate and core temperature can then rise at a pace that felt comfortable in cooler conditions.

The performance response is not linear or the same for everyone. Population marathon studies show an association between weather and finishing times, but they do not produce a personal seconds-per-degree rule. The tool therefore shows a broad Clinic sensitivity range rather than assigning a precise penalty from air temperature or dew point alone.

Longer races generally create more opportunity for heat strain to accumulate, so the Clinic model applies a larger distance factor to a marathon than a 5K. That factor is a coaching heuristic, not a validated individual dose-response. Even a small modelled percentage can materially change a marathon planning range, which is why the calculator shows both finish-time edges instead of one midpoint.

Many race organisers use on-site environmental measurements, including WBGT (wet-bulb globe temperature), as one input to their own protocols. Their thresholds and decisions vary by event, population and medical plan. The calculator estimates a shade form from temperature and humidity only. It cannot measure solar radiation, globe temperature or wind. The labels below are Clinic planning bands, not organiser cut-offs, medical risk categories or clearance to race:

Heat acclimation can improve tolerance. When practical, at least two weeks of progressive, controlled exposure is a useful target for more complete adaptation. Shorter blocks can still provide some benefit, so a late forecast does not mean preparation is pointless. Responses vary, and any heat exposure adds training strain.

Common adaptations include changes in plasma volume, sweating response, heart rate and thermal strain. The size and time course vary, and some adaptations decay after exposure stops. If your race is hot and your training has been cool, use the conservative edge of the modelled range and respond to effort.

• Reset the goal before the gun. Use the modelled range as one planning input, start toward its conservative edge and keep checking effort. Chasing a cool-weather number can make an already demanding day harder.
• Bank nothing early. Start at the easier end and let the race come to you. An aggressive first half can add avoidable thermal strain.
• Drink to thirst. Use a rehearsed fluid plan and avoid gaining weight during the race. Sodium can replace some sweat losses and make a drink more palatable, but it does not prevent hyponatraemia if you over-drink.
• Cool from the outside. Pour water over your head, neck and forearms; take the sponges; run the shaded line. External cooling can support heat loss, but it does not make an unsafe effort safe.
• Know when to stop. Confusion, collapse, loss of coordination, altered behaviour or reduced consciousness can signal exertional heat stroke and need immediate emergency help. Dizziness, severe weakness, vomiting or chills are also reasons to stop and seek medical support. Sweating can still be present in serious heat illness.