On a summer camp, what gets you isn't the rain or the wind — it's the heat. You check a forecast of 30°C, head out, and only realize you misread it when the tent turns into a furnace in the early afternoon. A single number — air temperature — can't tell you how brutal the heat really is. WBGT (the heat index) reads humidity and radiant heat as well, and shifting your destination up in elevation is a countermeasure that actually works in the field. Let me be honest up front. I can't field-test heatstroke on my own body, and I won't pretend to. So in this piece I do only what I can do honestly: verify the primary sources — the Ministry of the Environment, the Japan Meteorological Agency, the Japanese Association for Acute Medicine — one at a time, and digest the official protocols into something you can act on. Read it not to be scared, but to have something to do before you leave. Note that the 2026 figures here were confirmed as of this writing (June 2026); always check the official source for the latest.

Air temperature and WBGT are not the same thing — humidity is 70% of it

First, one definition. WBGT (Wet Bulb Globe Temperature, the "heat index" / 暑さ指数) is an indicator of heatstroke risk. It's expressed in the same unit as temperature — degrees Celsius — but the value is not air temperature. Per the Ministry of the Environment, WBGT combines three things: (1) humidity, (2) solar and radiant heat (including glare bouncing off the ground and your tent fabric), and (3) air temperature.

The weighting is the key. Outdoors (in direct sun), WBGT = 0.7 × wet-bulb temperature (which reflects humidity) + 0.2 × globe temperature (which reflects radiant heat) + 0.1 × dry-bulb temperature (air temperature). So air temperature itself carries only 10% of the weight, and humidity carries 70%. That's why "it's only 30°C, why is this so punishing?" happens. In a humid riverside valley or a still, windless stand of trees, WBGT spikes even at the same air temperature. The day I misjudge most often in the field is exactly this kind — "normal temperature, but muggy."

WBGT values are split into four bands in the daily-life guidance from the Japanese Society of Biometeorology, listed below. Roughly: once it's above 28, take it seriously; above 31, don't push it as a rule. Camping is exertion in the sun — pitching, cooking, playing — so read it on the safe side, a notch harsher than sitting still at home.

You can see local WBGT, both current readings and forecasts by location, on the Ministry of the Environment's Heatstroke Prevention Information site. Build the habit of judging by this number rather than by air temperature, before you go and once you're there. One bookmark you can open on your phone is enough.

  • Danger (WBGT 31 and above): as a rule, stop exercise and hard work. Break up pitching and cooking often and rest in the shade.
  • Severe caution (WBGT 28–31): the band where heatstroke cases climb sharply. Keep daytime activity minimal; rest and drink frequently.
  • Caution (WBGT 25–28): rest and hydrate proactively. Children and the elderly especially.
  • Attention (below WBGT 25): risk is lower, but don't let your guard down during hard exercise or work in full sun.

The Ministry's heat alerts — the 2026 thresholds and how to receive them

When WBGT is forecast to climb past a set level, the Ministry of the Environment issues a Heatstroke Alert (熱中症警戒アラート). The 2026 (Reiwa 8) operating period runs from April 22 (Wed) to October 21 (Wed). The whole summer-camp season falls inside it. The figures below were all confirmed against the Ministry's announcements as of this writing (June 2026); check the official source for the latest.

There are two tiers. The standard Heatstroke Alert is issued when the next-day or same-day maximum WBGT is forecast to reach 33 or higher at any one point within a forecast region (the country is divided into 58 such regions). It's announced at 5 p.m. the day before and 5 a.m. the same day. The higher tier, the Special Heatstroke Alert (熱中症特別警戒アラート), is issued when the next-day maximum WBGT is forecast to reach 35 or higher at every point within a prefecture; it goes out at 2 p.m. the day before, by prefecture. The Special Alert was newly created in April 2024 to flag broader, disaster-level heat.

Receiving it is something to set up before you leave. The moment I book, I make sure alerts for the destination area will reach me before I head out. There are three ways, below. Because alerts are issued by destination (prefecture / forecast region), the trick is to set them up for the campsite's area, not your home base.

To be honest, an alert is a broad warning that "this area is dangerous today" — it won't tell you the difference between the valley floor and the ridgeline at the scale of your own site. Use the alert to decide whether the dates are go or no-go, and use the live local WBGT to decide the day's moves. That two-step approach is the realistic one.

  • Ministry of the Environment Heatstroke Prevention Information site (wbgt.env.go.jp): check live and forecast WBGT by location, plus alert status. Open it the night before and the morning of.
  • Personal email subscription service (mail_service.php on the same site): receive alerts for your registered area by email. Register the destination's area.
  • Ministry of the Environment official LINE account (LINE ID: kankyo_jpn, account name "環境省"): add it as a friend and set your area to get alerts as notifications.

Altitude as a lever — a guide of about 0.6°C per 100 m, but not a promise

Against heat, what works before any gear is where you pitch. Go up in elevation and the temperature drops. The lapse rate runs about 0.6°C per 100 m, roughly 6.5°C per 1,000 m. By simple arithmetic, a highland at 1,500 m comes out about 9°C cooler than the lowlands. If you want to knock the daytime WBGT down a notch, this is the most direct lever.

But don't treat the number as a promise. That rate is an average guide, and it shifts with humidity, weather, and terrain. On a damp day when water vapor condenses, the rate eases off (the moist adiabatic lapse rate is about 0.5°C per 100 m); on a clear, dry day, or higher up, the drop can be larger. You can't claim "1,500 m, therefore exactly 9°C cooler." Treat it as a probability — "likely cooler than the lowlands" — and use it as one vote in choosing where to go.

And here's the part you only learn in the field. The price of altitude is the night chill. A highland that's cool by day cools far more than you'd expect on a clear night, from radiative cooling. The altitude drop stacks with the radiative cooling, so even in midsummer you can be shivering before dawn. On a highland in July I once spent the day sweating in short sleeves, then lay awake at night, cold, with only a thin sleeping bag. If you're going up in elevation for the heat, add a notch of warmth to your sleep gear — the two come as a set.

Two more costs. One is weather — higher up raises the risk of thunderstorms, strong winds, and sudden changes. On mountains where afternoon storm clouds build, finish pitching and striking camp early. The other is access — narrow forest roads, a remote site, a slow escape (bailout). In exchange for the cool, the nearest hospital down in the valley is far. Factoring in the evacuation if someone goes down with heatstroke, that's not a light cost. Don't choose on coolness alone; decide with the exit route and travel time in the picture too.

A heat-beating camp routine — shade, airflow, fluids, timing

Once the site is set, the way you pitch shaves off more heat. The order is simple: block the sun, move air through, never run out of fluids and electrolytes, and don't move during the hot hours. Sort the gear by who needs what, and why.

Here's one number from the field. On a midsummer afternoon, ground in direct sun can get too hot to touch. Even when the air is around 30°C, the glare off asphalt, sand, or rock is higher still. A tent interior climbs above the outside air from the radiant heat of the fly and the ground — a tent pitched sealed in full sun becomes a steam bath. So the daytime tent is a place to sleep, not a place to spend the day. Make your daytime spot under a separate patch of shade (a tarp). That's the backbone of heat management.

Creating a path for the wind helps too. Pitch both tent and tarp so the windward side is open and air can flow through. Mesh wide open, fly propped up. In my experience, a sealed, windless tent and a tarp with a single thread of breeze passing through feel completely different even in the same shade. A tarp is a tool for making shade and a tool for catching wind. Before pitching, check which way the wind is coming from that day, then set the orientation. A spec sheet is not a promise — you only really learn what a piece of gear does once you've pitched it in the wind.

Fluids and electrolytes: before you feel thirsty, and often. Sweat costs you not just water but salt (sodium and the like), and drinking only large amounts of water can actually upset your internal balance. Pair it with oral rehydration solution, salted drinks, or salt candies. Apart from the cooler, I keep one room-temperature rehydration bottle within arm's reach — so hydration doesn't stall just because getting up for something cold is a hassle. Then, timing. During the high-WBGT window, roughly noon to mid-afternoon, avoid the "moving" tasks — pitching, cooking, play — and use that time to rest in the shade. Shift activity to the cooler morning and evening hours. This is just scheduling, and it works more reliably than altitude or gear.

  • Tarp (makes shade): essential for everyone, every site. Your daytime spot lives under it. Roughly a few thousand to the low tens of thousands of yen; families want one large sheet.
  • Mesh / fully-openable tent (moves air): essential. A structure that opens front, back, and sides stays cooler at night. A full-close winter tent steams in summer.
  • Oral rehydration solution / salt tablets / salt candy (electrolytes): everyone. Don't rely on water alone. Carry extra if children or elderly are along. A few hundred to about a thousand yen.
  • A small meter with a thermometer (ideally a WBGT readout): speeds up on-site judgment. Simple WBGT meters start in the low thousands of yen; if you don't have one, the Ministry site's live readings stand in.
  • Handheld fan / wet cooling towel: supplementary. For comfort once you're already out of direct sun. Don't over-rely on them — shade, airflow, and fluids come first.
  • Sleep gear that's warm despite being light (for altitude): a required add-on if you go up in elevation. For the night chill — pick a sleeping bag rated a notch warmer than the expected overnight low.

The stages of heatstroke and first response — "cool, chill, drink," and when in doubt, get care

After prevention comes response. Heatstroke severity is classified I–IV in the Japanese Association for Acute Medicine's Heatstroke Clinical Practice Guidelines 2024 (a most-severe Grade IV was newly added in the 2024 edition to the earlier I–III). You don't need to make a definitive on-site diagnosis as a layperson — but being able to sort "is this a rest-and-recover stage, a go-to-the-hospital stage, or a call-an-ambulance-without-hesitation stage" is what divides outcomes. The stages and responses are below.

The basics of on-site first response, in an easy phrase: cool, chill, drink. Move the person somewhere cool (shade, airflow, air conditioning if possible); chill the body (loosen clothing, cool the large blood vessels at the neck, armpits, and groin with ice packs or wet towels, and fan them); and give fluids and salt (if they can drink on their own). Do the three in parallel.

The dividing line is consciousness and "can they drink water on their own." If they can take fluids themselves and rest brings them around, that's in the Grade I range. If there's nausea or headache, they're limp and can't drink on their own, or they just seem off, get them to a medical facility without hesitation. And — if there are signs of impaired consciousness (slow to respond when called, can't walk straight, convulsions, the body abnormally hot), this is life-threatening. Keep cooling and call 119 immediately. The 2024 guidelines' most-severe Grade IV is gauged by "a core body temperature of 40.0°C or higher plus severe impaired consciousness," and a quick field criterion (qIV: the body surface is clearly hot plus strong impaired consciousness) was added to catch it fast on scene or in transit. You don't need to measure core temperature. "Hot, and consciousness is off" — suspect the worst and act.

Finally: this is a digest of public guidelines, not numbers I've verified on my own body. For specific medical decisions, follow medical professionals and official information. Even so, just having this triage in your head changes the one breath you take in the field. Read the cool with altitude and WBGT, shave it with shade and wind, never run out of water and salt. Get that far in your routine and a summer camp is plenty enjoyable. What changes is the bit of prep before you go, nothing more.

  • Grade I (manageable on site): dizziness, light-headedness, muscle cramps, heavy sweating. Consciousness normal. → Rest in a cool place, give fluids and salt. If it doesn't improve, treat as Grade II.
  • Grade II (get care promptly): headache, nausea, vomiting, fatigue, reduced concentration. Can't drink on their own, or symptoms don't improve. → Keep cooling and get to a medical facility promptly.
  • Grade III (intensive care): impaired consciousness, convulsions, can't walk straight, organ damage (liver, kidney, brain, etc.). → Call 119 at once. Keep cooling the body until help arrives.
  • Grade IV (most severe, added in the 2024 guidelines): gauged by core temperature 40.0°C or higher plus severe impaired consciousness. On site, suspect it from "body surface clearly hot plus strong impaired consciousness" (qIV). → Time-critical. Keep cooling and call 119 immediately.