Conditions

Heat & Humidity

Heat is the condition most likely to hurt you, and the one most often judged by the wrong number. Air temperature alone predicts very little; the combination of temperature, humidity, sun and exertion predicts almost everything.

Instrument

Heat index & hydration

Apparent temperature, risk band, hourly sweat rate and sodium replacement for your body mass and intensity.

Scope

What this section covers

Heat index, wet bulb temperature, sweat rate, acclimatisation and heat illness recognition.

Field notes

Reference

HeatThe Heat Index Is a Model of Your Skin, Not the AirWhere the heat index comes from, the assumptions baked into it, and the conditions under which it understates the real danger.
HeatHow Much You Actually Sweat, and What to Replace It WithSweat rate ranges, how to measure your own, the sodium question, and why both under- and over-drinking carry real risk.
HeatHeat Acclimatisation: Two Weeks That Change Your PhysiologyThe adaptations that occur with repeated heat exposure, the timeline they follow, how quickly they decay, and how to induce them deliberately.
HeatThe Heat Illness Continuum and the One Sign That Changes EverythingHeat cramps, heat exhaustion and heat stroke distinguished, the central nervous system sign that marks the emergency, and why cooling comes before transport.
HeatHumidity Is the Variable That Decides Whether Sweating WorksVapour pressure gradients, why dew point beats relative humidity as a planning metric, and how to adjust effort when evaporation stops working.