Beyond Clothing

A working field guide to living and adventuring in heat. The right clothing is one part of it. Hydration, sun, physiology, and a few habits matter as much or more.

Approx. 10 min · Hydration, sun, physiology, safety

This is the reference we wish we had when we started testing gear in the desert. It is not exhaustive. It is the set of things that have kept us comfortable, and occasionally kept us safe, across dry canyons, humid coasts, and the cities in between. Clothing recommendations appear where they belong, in context, and we name other companies' products freely, because pretending to be the whole answer would not help you.

In this guide:

1. Hydration: how much, what to drink, what to watch for
2. Sun protection: sunscreen, eyewear, what fabric does and does not replace
3. Heat physiology and acclimatization
4. When heat becomes dangerous

Hydration

The advice to drink eight glasses a day was never written for someone working in extreme heat. Active hot-weather exertion can require 1 to 1.5 liters per hour, sometimes more. The body cannot replace what it does not have, so pre-hydrating before a long day matters as much as drinking during it.

Water alone is not enough over long durations. Sweat carries sodium, potassium, and other electrolytes out with it. Replacing only water dilutes blood sodium and can produce hyponatremia, which mimics dehydration but is worse and more dangerous. For sustained effort beyond two hours in heat, add electrolytes. Liquid I.V., LMNT, and Nuun all do the job; so does eating something salty at a rest stop.

Thirst lags behind need. In dry heat, by the time you feel thirsty you are often already 1 to 2% dehydrated, enough to measurably dull both thinking and physical capacity. The habit that works is small amounts often, not large amounts occasionally, with urine color as your gauge.

[Insert image here: fig6-hydration-scale.svg · upload from the figures folder, alt text in the README]

Figure 1: Pale yellow is the target. Dark amber means drink more. Persistently clear can mean you are over-diluting and should add electrolytes rather than water.

In humid heat the picture shifts. Because sweat evaporates poorly, the body sweats more to compensate, and total fluid loss can exceed a dry day even at a lower temperature. The feedback is misleading: dry heat feels dry, humid heat looks drenched. The dry scenario is the more dangerous of the two, because the loss is hidden.

Sun protection

Sunscreen and UPF clothing are complementary, not interchangeable. Sunscreen protects exposed skin; clothing protects covered skin. Neither replaces the other.

For exposed skin, mineral sunscreens built on zinc oxide or titanium dioxide sit on the surface and reflect UV. They work on contact and stay stable over time. Chemical sunscreens absorb UV through reactions that consume the active ingredient, take 15 to 20 minutes to bind, and degrade with exposure. For multi-hour days in the sun, mineral formulas are generally the more reliable choice. Reapply every two hours, more often when sweating heavily or swimming.

The ratings matter less than people think above a point. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%. Above SPF 50 the added protection is small. It is not more coverage so much as slightly longer coverage.

Eyewear is underrated. UV exposure to the eyes accumulates and contributes to cataracts and macular degeneration. Sunglasses rated UV400 or 100% UV are the standard, and wraparound styles earn their keep in deserts where UV reflects off pale sand, rock, and salt. Lips are the most forgotten surface of all: lip skin has no melanin and burns fast, so an SPF balm belongs in every kit.

Heat physiology and acclimatization

The body adapts to heat over 7 to 14 days. The process raises your sweat rate, lowers the sodium concentration of that sweat, increases plasma volume, and lowers your heart rate at a given workload. Someone two weeks into a hot climate tolerates it far better than someone who flew in yesterday.

[Insert image here: fig7-acclimatization.svg · upload from the figures folder, alt text in the README]

Figure 2: Save the hardest days for the back half of a trip. Acclimatization improves with light exercise during the adaptation window, not passive sitting, and it raises fluid demand while it builds.

If you are flying from a temperate place into a hot one, plan for the first two or three days to be limited and save the hardest hikes for later in the week. Acclimatization also decays on a similar timeline once you return to cooler weather, so someone who lives in San Diego and visits Big Bend each July re-acclimatizes every trip. A gentle hike on day one prepares you better than shade does.

When heat becomes dangerous

Two heat illnesses are worth knowing by sight. They sit on a continuum, and the first becomes the second if it is ignored.

Heat exhaustion

Looks like: heavy sweating, weakness, fatigue, dizziness, nausea, headache, cool and clammy skin, fast pulse, body temperature normal or slightly raised (below 104°F / 40°C).

Do: stop, move to shade or a cooler space, remove excess clothing, drink water with electrolytes, and rest. Recovery is usually quick with the right response.

Heat stroke · medical emergency

Looks like: body temperature above 104°F / 40°C, hot skin that may be dry or still sweating, confusion or altered mental state, rapid pulse, possible loss of consciousness or seizures.

Do: call 911, move to shade, and cool aggressively by any means available, cold-water immersion if possible, wet cloths to the neck, armpits, and groin, and fanning. Do not wait. Heat stroke can cause organ failure and death within hours.

The signal that matters most is mental state. A hiker who is dizzy and hot but lucid is in exhaustion territory. A hiker who is confused, slurring, or acting strangely is in stroke territory and needs immediate cooling and care. When in doubt, treat it as the more serious version. Watch your partners for the early signs, irritability, clumsiness, reduced urination, headache, and remember that prevention through water, electrolytes, pacing, and a willingness to turn around is worth far more than any rescue.

The wisest people we know in heat are not the ones who push through. They are the ones who read the day correctly and choose to come back to it.

Heat is not adversarial. It is a constraint, and it rewards respect. Carry the water. Watch the color. Save the hard miles for the afternoon you have earned them.

Continue: Heat Guide index · The Science of Staying Cool