Running in Japan's Summer: Eleven Years of Yokohama Humidity Data

Japan doesn't have a heat problem the way a desert city does. Air temperatures in Tokyo and Yokohama rarely crack 35°C even at the height of summer — genuinely mild compared with Riyadh's 43°C Julys or Dubai's 41°C. What Japan has instead is a moisture problem that simply never lets go: a coastal, humid subtropical climate sitting under a semi-permanent Pacific high through July and August, where the air is carrying so much water that your sweat has almost nowhere left to evaporate into.

This guide uses Verus's own eleven-year climate archive — hourly Open-Meteo records for Yokohama, 2015-2025, sampled at the hours people actually run rather than averaged across a whole day that includes hours nobody's out in — to put real numbers on what that actually feels like, month by month. Yokohama is the anchor because it's the actual race city for T100 Yokohama and a thirty-minute train from central Tokyo; the broad shape of the year applies to both cities, though inland, more built-up Tokyo can run a touch warmer on any given day thanks to urban heat island effect on top of the same regional humidity.

Why dew point, not temperature, is the number to watch

You don't slow down in the heat because the air is warm. You slow down because your body can't shed the heat your muscles are producing fast enough — and at running effort, that happens almost entirely by evaporating sweat off your skin. Dew point measures how much moisture the air is already holding, and therefore how much room is left for your sweat to evaporate into. Below about 10°C you'll barely notice it. Above 20°C you'll feel it inside the first mile. Above 24°C, evaporative cooling is badly compromised no matter how fit you are.

A 25°C morning in April, with a dew point around 8°C, and a 25°C morning in August, with a dew point above 23°C, are not the same run — even though a weather app would show an identical air temperature for both.

What a Yokohama morning actually looks like, month by month

These are averages across 2015-2025 from the Verus climate archive, at 6am local time. The index is air temperature plus dew point in Fahrenheit — the standard coaching rule of thumb for how heat and humidity combine into a single number.

The shape of the year is the real story. December through March is genuinely cold by running-comfort standards — 6am temperatures at or below freezing for two straight months, an index in the 60s-80s. April and May are a fast, pleasant transition. Then June through September is a four-month block where the index never drops below 133, peaking at 151 in August, before October snaps back down to a comfortable 116 almost as fast as it climbed.

The number that captures August specifically: mornings that never cool down at all

The 6am index above is an average, and averages can flatten out the worst mornings. So the Verus archive also tracks something blunter: across each month, what share of days had a low temperature — the coolest point in the entire 24 hours, usually just before dawn — that stayed at or above 24°C. That's not a dawn snapshot; it's asking whether the day ever genuinely cooled off at all.

• January through May: 0% of days.
• June: 2% of days.
• July: 45% of days.
• August: 74% of days — the day's low itself averages 24.8°C, meaning the "cool point" of an August day in Yokohama is, on average, already above the threshold where evaporative cooling stops working properly.
• September: 22% of days, as the humid block starts breaking down.
• October through December: 0% of days.

Rainy season vs. the actually wettest month — they're not the same

Japan's tsuyu (rainy season) is popularly understood as June-July, and the Verus data agrees that's when rain is most frequent — 14.4 rain days in June, 14.2 in July, out of roughly 30-31. But by the archive's own day-count, September edges both of them: 15.4 average rain days, driven by typhoon season rather than the seasonal tsuyu front. If you're planning a training block or a trip around avoiding rain specifically, don't assume tsuyu's end in mid-to-late July means the wet weather is over — September is, in this archive, Yokohama's single wettest month by day-count, even though it isn't the one with the cultural name.

Why Tokyo moved its own Olympic marathon over this exact problem

This isn't a theoretical concern. The 2020 Tokyo Olympics marathon and race walk events were relocated to Sapporo, roughly 800km north and expected to run 5-6°C cooler, specifically over heat and humidity risk to elite athletes — and even Sapporo recorded its hottest 97-year stretch during the actual Games, with consecutive days above 30°C. Before the move was confirmed, a group of Japanese doctors publicly urged organisers to start the marathon as early as 5:30am, warning that anything later could contribute to deaths from heatstroke. That's the scale of seriousness Japanese sports medicine attaches to a Tokyo-area summer race — worth knowing before you assume a mild-looking air temperature means a comfortable run.

It's also, not coincidentally, part of why T100 Yokohama is scheduled for 15-16 May 2027 rather than mid-summer: per the Verus climate archive, May sits at a 6am index of 118 with zero days recording a low above 24°C — a genuinely different proposition from the July-August block above.

Training and racing through it

• Japan's humid heat behaves like Jeddah's, not Riyadh's — you will feel yourself sweating heavily, so the risk isn't a hidden dehydration signal, it's genuine electrolyte and fluid loss on top of impaired cooling. Drink and fuel electrolytes on a schedule, not just when you feel thirsty.
• There is no reliably cool window in a Yokohama or Tokyo July-August day, per the data above — dawn is still the best option, just not a guaranteed escape the way it is in a dry desert climate.
• Heat acclimatisation still works exactly the way it does everywhere else: 10-14 days of genuine heat exposure meaningfully expands plasma volume and brings sweating on earlier. If you're arriving from a cooler climate for a summer trip, expect your first several runs to feel disproportionately hard regardless of home fitness.
• Indoor options are genuinely normal here in peak summer, not a compromise — see Verus's guide to running culture in Japan for gyms and run stations that make that an easy fallback.

When it stops being about pace

Everything above is about running slower and more carefully in the heat. Exertional heat illness is a different thing entirely — it develops faster than people expect and is a medical emergency, not a pacing problem.

Verus is a running site, not a medical one. If you have any condition that affects how you handle heat, or any doubt at all, ask a doctor before you rely on anything in this guide.