Heat in Japanese Summers: Heatstroke Alerts, Cooling Tips and Planning

Japan publishes heat alerts and a color-coded heat index every day in summer, and knowing how to read them helps you plan sightseeing around the hottest hours.

Published October 20, 2026 at 12:49 PM JST | By Tokyo Paul

Quick answer: Japan's Ministry of the Environment publishes a heat index called WBGT, which combines air temperature, humidity and radiant heat, plus Heat Stroke Alerts and stronger Special Heat Stroke Alerts. Visitors can check the color-coded level before heading out, schedule outdoor sightseeing for cooler hours, drink fluids regularly, and rest in air-conditioned places.

Why summer heat needs planning

Summer in Japan is hot and humid across much of the country. Summers across much of Japan are hot and humid, especially after the early-summer rainy season ends, and temperatures of 35 degrees Celsius or above occur in some places. Humidity matters as much as the number on the thermometer, because damp air makes it harder for the body to shed heat.

For that reason, Japanese agencies do not rely on air temperature alone. They use a heat index and a system of public alerts that tell residents and visitors how risky the day is for outdoor activity. Once you know what the index and the alerts mean, a hot day becomes something to schedule around rather than a surprise.

What WBGT is and how to read the levels

WBGT stands for Wet Bulb Globe Temperature. The Ministry of the Environment explains that it is a heat-stress index defined in the international standard ISO 7243, reported in degrees Celsius but not the same as air temperature. It combines humidity, radiant heat from sunlight and surrounding surfaces, and air temperature, since all three affect how the body holds or loses heat. The ministry's guidance becomes noticeably stricter once WBGT reaches 28.

The ministry's English site shows five color-coded levels: under 21 (blue, almost safe), 21 to 25 (caution), 25 to 28 (warning), 28 to 31 (severe warning) and 31 or higher (danger). Its exercise guidance is to replenish water often at the lower levels, rest often at the warning level, avoid heavy exercise at severe warning, and avoid exercise at the danger level, so on those days it is wise to switch to indoor plans.

Heat Stroke Alerts and Special Heat Stroke Alerts

On top of the daily index, Japan issues two kinds of public alert. A Heat Stroke Alert (netchusho keikai alert) is issued for a region when conditions are hot enough to carry a real risk of heat illness. The Ministry of the Environment's Japanese-language alert page advises people to use air conditioning indoors, take regular breaks, drink water and take in salt, keep a close eye on older adults and children, and stop exercise or outdoor activity in places that are not cool.

A Special Heat Stroke Alert is a stronger warning reserved for dangerously hot conditions over a wide area. The ministry says ordinary precautions may not be enough on such a day, and it asks facility managers, such as schools and event organizers, to consider canceling, postponing or changing activities. The alert maps are updated three times a day: the map for the current day at 5:00 a.m., and the map for the next day at 2:00 p.m. and 5:00 p.m.

Cooling spots and where to rest

The Ministry of the Environment's site links to a list of designated heat-refuge facilities, commonly called cooling shelters (kuringu sheruta in Japanese). These are places designated so that people can step in out of the heat. Availability and opening hours vary by municipality, so the list on the ministry site and the local city or ward website are the places to check for the area you will be in.

For ordinary sightseeing, the simplest cooling spots are the indoor places already on most itineraries: museums, department stores, station buildings, libraries and cafes. Building the day around a rhythm of outdoor time followed by an air-conditioned break is an easy habit. Many parks and temples have shade and benches, and some stations and shopping streets have covered walkways that take the edge off the sun.

Hydration, clothing and parasols

General guidance on preventing heat illness is consistent across sources. Drink fluids regularly rather than waiting until you feel thirsty, since thirst is not a reliable guide. The Wikipedia article on heat stroke lists wearing light, loose-fitting clothing and light-colored, wide-brimmed hats, avoiding strenuous exercise in peak sun hours, and staying out of enclosed spaces without ventilation, such as parked cars.

Many people in Japan use a folding parasol on sunny days, and a compact one packs easily into a day bag. A small towel for wiping sweat, a hat and a refillable bottle round out the kit. Convenience stores and vending machines are widespread, so drinks are usually easy to find along a route, though it is still sensible to carry a bottle on days with long gaps between stops.

Planning an outdoor day in summer

Start early. Shrines, gardens and hiking areas are noticeably more pleasant in the morning, and the hottest part of the day is a good time for a museum, an aquarium or a long lunch indoors. If an outing involves a lot of walking, the ministry's guidance for the higher WBGT levels is to cut back intense activity and take frequent rests.

Check the day's forecast and the index the evening before and again in the morning, and keep a flexible plan with an indoor option nearby. Festivals and fireworks nights bring crowds and standing time, so allow extra rest and fluids. Watch over children and older travelers in particular, since the ministry's guidance singles them out.

Checking conditions and staying flexible

The Ministry of the Environment's English heat illness prevention page shows the current WBGT readings for major cities and whether an alert has been issued, and the Japan Meteorological Agency publishes weather forecasts and warnings in English. Because alerts and thresholds can be revised, treat any summary, including this one, as a starting point and read the official page on the day of travel. Hotel staff and local tourist information desks can also point you to the nearest cool indoor spots.

This guide shares general cultural information, not professional advice; for health questions, talk with a doctor. If someone shows signs of serious heat illness, such as confusion, the Wikipedia article notes that heat stroke is life-threatening, so seek local emergency help rather than waiting.

Frequently Asked Questions

What does the WBGT color mean on Japanese weather sites?

The colors show how much heat stress the environment puts on the body. Blue means almost safe, while the levels move up through caution, warning and severe warning to danger at 31 or higher. The Ministry of the Environment pairs each level with advice, such as taking frequent rests or avoiding exercise, so check the official English page for the current status.

What is the difference between a Heat Stroke Alert and a Special Heat Stroke Alert?

A Heat Stroke Alert tells people that a region has a high risk of heat illness and asks them to take precautions. A Special Heat Stroke Alert is a stronger warning for dangerously hot conditions over a wide area, when ordinary precautions may not be enough and organizers are asked to consider canceling or changing activities.

When is the best time of day for outdoor sightseeing in Japanese summer?

Early morning is usually the most comfortable, and late afternoon can also be easier. The middle of the day is better used for indoor stops. Because humidity and sun vary by place and weather, check that day's index and forecast, and keep an indoor option close to your route.

Want to go deeper? Tokyo Paul's book Tokyo, Made Simple is available on Amazon.

Keep Exploring

More guides from Tokyo Paul: Day Hike Packing List for Japan: Layers, Rain Gear and Trail Essentials, Ganbanyoku: Bedrock Bathing and Warm Stone Spas in Japan and Fever at Home in Japan: Cooling Sheets, Medicine and When to Call.