Instrument 03
Computes National Weather Service apparent temperature from air temperature and relative humidity, then estimates hourly sweat rate and sodium loss for your body mass, working intensity and acclimatisation state.
Acclimatised means 10 to 14 consecutive days of exertion in comparable heat. Adaptation decays within two to four weeks away from it.
Shade value. Direct sun adds up to 15°F to effective load.
Drink about — every 15 minutes to match it.
Fluid 0 L (0 oz)
Sodium 0 mg
Apparent temperature uses the Rothfusz multiple regression published by the National Weather Service, including the standard adjustments applied below 13 percent and above 85 percent relative humidity. Below 80°F the simplified linear form is used, as in the NWS implementation.
Sweat rate is estimated from body mass and metabolic intensity, scaled by a factor derived from apparent temperature. Acclimatised individuals are modelled with a higher sweat rate and substantially lower sweat sodium concentration, which reflects improved aldosterone-mediated sodium reabsorption in the sweat duct.
The underlying Steadman model assumes a person in shade, in a light breeze, walking at about 3 mph, in long trousers and a short-sleeved shirt, hydrated and acclimatised. Every departure from that raises the effective load. The two that matter most are direct sun and exertion above walking pace — both common, and both absent from the published number.
Gastric emptying and intestinal absorption cap fluid uptake at roughly 1.0 to 1.2 litres per hour for most people. If the estimated sweat rate exceeds that, some deficit is unavoidable and attempting to drink past the ceiling produces gastrointestinal distress rather than hydration. Reduce intensity or duration instead.
Over-drinking carries its own risk. Exercise-associated hyponatraemia is produced by fluid intake exceeding losses, and its early symptoms overlap with dehydration, which makes the instinctive response the wrong one. Finishing heavier than you started means you drank too much.
General education, not medical advice. Suspected heat stroke — any confusion, disorientation or altered behaviour in someone exerting in heat — is a medical emergency. Cool aggressively and call emergency services immediately.