Heat is a quieter danger than cold. There is no dramatic forecast line, no gust number, just a warm day that turns into a dangerous one by the third hour of climbing. Understanding why comes down to one idea: the body’s only serious cooling system is evaporation, and evaporation depends on humidity as much as on temperature. Once that is clear, the wet-bulb temperature makes sense, the hard limits become obvious, and the planning follows a logic we can apply to any hot route, from a dry canyon at 38 C to a humid forest at 29 C, which is often the more dangerous of the two.
Where the heat comes from
A walker climbing at a steady pace produces 400 to 700 watts of metabolic power, and only about a fifth of that becomes mechanical work; the rest is heat that must leave the body. In cool air, radiation and convection carry most of it away. Above roughly 30 C air temperature, the air is warmer than or close to skin temperature (about 33-35 C), those routes shut down, and sweat becomes almost the only outlet. Evaporating one litre of sweat removes about 2,400 kilojoules, which is roughly one hour of hard climbing’s waste heat, and sweat rates of 1 to 1.5 litres an hour are normal for a fit person working in heat. The arithmetic is tight even in dry air.
Direct sun adds to the load: around noon a body in full sun absorbs several hundred watts more than one in shade, and hot ground radiates upwards, which is why the air above a rock slab at 15:00 is far hotter than the forecast for the nearest station, measured in a shaded screen over grass.
Why humidity is the real variable
Sweat only cools if it evaporates, and evaporation slows as the air fills with water vapour. At 40% relative humidity, sweat on the skin flashes off and the body copes with surprisingly high temperatures; at 80%, it runs down the arms and drips off, doing nothing. Meteorologists capture this with the wet-bulb temperature: the reading of a thermometer whose bulb is wrapped in a wet cloth and ventilated. It is the lowest temperature that evaporative cooling can reach in that air, and therefore the temperature our skin can hope to be. In dry air the wet-bulb is far below the air temperature – 38 C at 15% humidity gives a wet-bulb near 20 C – while in humid air the two converge: 32 C at 80% humidity gives a wet-bulb around 29 C.
The theoretical limit is a wet-bulb of 35 C, at which a resting person in the shade can no longer shed heat at all and core temperature rises until it becomes fatal within hours. Newer laboratory work suggests the practical limit for young, healthy adults is nearer 31 C, and lower again for anyone older, working hard or carrying a pack. A wet-bulb of 28 C is already an environment where sustained uphill effort is not sensible; it corresponds to something like 33 C at 70% humidity or 40 C at 40%. The “feels like” or heat index figure in ordinary forecasts is a related shortcut that combines the same two variables into a single equivalent temperature.
What goes wrong, in order
Heat illness is a progression, and the early steps are reversible. Heat cramps come first, usually in the calves and thighs, from salt and fluid loss. Heat exhaustion follows: heavy sweating, pale clammy skin, headache, nausea, weakness, a fast weak pulse, and often a core temperature near 38-39 C; the person can still think and walk, and the treatment is shade, rest, fluid with salt, and cooling with water on the skin and fanning. Heat stroke is the emergency: core temperature above 40 C, confusion or collapse, and in exertional cases the skin may still be sweating, so “hot and dry” is not a reliable sign. It is a medical evacuation with aggressive cooling on the spot, immersion in a stream if one exists, and it kills.
The trap is that the early symptoms are easy to attribute to the climb or a short night. In a group, the person who goes quiet and stops eating is the one to watch.
Water, salt and pace
Drinking cannot outrun a sweat rate of 1.5 litres an hour; the gut absorbs about a litre an hour at best. So on a genuinely hot day we start fully hydrated, drink to thirst plus a little rather than to a schedule, and replace sodium, since sweat carries 0.5 to 1.5 g of salt per litre and a person drinking plain water for six hours can dilute their blood sodium dangerously. Salty food does the job as well as any electrolyte product. Urine colour is a crude but honest gauge: pale straw is fine, dark amber means we are behind.
The other lever is pace, and it is the bigger one. Halving the climbing rate roughly halves the heat produced, so a slow, steady walk in the shade may be sustainable where a normal pace is not. Wetting a hat, shirt and neck gaiter at every stream turns the fabric into an evaporative cooler that works in dry air and, for the same reason as sweat, helps less in humid air.
A hot day, hour by hour
The forecast gives a maximum temperature and, if we are lucky, a humidity or heat-index figure; the plan is built around the daily curve. Air temperature bottoms out around dawn, climbs steeply until about 11:00, peaks between 14:00 and 16:00 and stays high until sunset. Here is a template for a route with, say, 1,200 m (3,900 ft) of ascent in a forecast of 34 C and moderate humidity:
- 04:30-05:30: leave. Head torch on, temperature near its minimum, most of the climb done before the sun reaches the slope.
- 05:30-10:00: the main ascent. Steady pace, drink and eat at every hour, refill at each stream.
- 10:00-12:00: summit or high point, then descend on the shaded aspect, east-facing in the afternoon if possible.
- 12:00-16:00: the hours to be off exposed ground. Long stop in real shade, feet in water if there is any, hats and shirts wetted; no climbing.
- 16:00-19:00: finish the descent as the sun lowers; the air is still hot but the radiation load drops fast.
Three details make this work. Aspect: a west-facing slope at 15:00 is a griddle, an east-facing one is already cooling. Water: we plan around known sources, carry at least 3 litres between them, more in desert country, and turn back if a source is dry. And the honest turnaround: behind schedule at 10:00, the high point is cancelled, because the second half of a hot day forgives nothing.
The humid-forest case
The most underrated hot-weather hike is the lowland one in humid air: a coastal forest, a river valley in late summer, a monsoon-season trail. The temperature looks tame, 28 to 30 C, but with humidity above 80% the wet-bulb sits near the mid-20s, sweat does nothing, and there is no evening cool-down. Here the rules tighten: shorter days, an earlier finish, and a lower threshold for stopping. In dry heat the danger is running out of water; in humid heat it is running out of cooling with a full bottle in hand, and no forecast line says so.