Recently a question was posed on Quora: “How is the Tokyo summer heat?”. Here’s the bulk of my answer:
… June is the rainy season: not as hot, but, well rainy (though not every day). July is, apparently, the most humid Weather and Climate: Average monthly Rainfall, Sunshine, Temperatures, Humidity, Wind Speed. August is the hottest.
It’s been a long time – 35 years? – since I visited Arizona in summer, specifically Sun City West where my grandparents retired. What I remember, however, is the dry heat you mention. even coming from my hometown of Toronto, a place not known for being particularly hot at any time of the year, I was surprised that the 100-degree-plus days didn’t FEEL hotter than they did (seems to me the hottest temp during one of my visits was 116).
By contrast, a hot day in Tokyo reaches 35 degrees Celsius, sometimes topping 38 (100 degrees Fahrenheit). However, there are two additional factors to take into consideration: humidity, and the heat-island effect.
Offhand, I can’t find any useful stats on just how humid Tokyo can be in the summer, but I can recall days where the temp was around 38 and the humidity above 90%. According to the Heat Index calculator at the National Weather Centre, that works out to 178 degrees Fahrenheit. Now, that ain’t the average, as I say, and I’m not sure any days reached that high in the summer of 2016, but it gets there – and in the summer of 2015 there was a record number of straight days of temps above 35 celsius in early August – in fact I posted about it Big Sushi, Little Fishes: a japan blog
The other consideration is the heat-island effect. I don’t know of any way to calculate how much concrete can raise the ambient temperature of a city, but I do know that walking ib Shinjuku, say, on a hot summer day, with car and air conditioner exhaust, amidst crowds of sweltering people, and a hot wind blowing up the urban canyonland can be an overwhelming experience. Fortunately, there are bars and cafes and shops and department stores and the like, most with over-active aircon, in which to take refuge!
So yes, of course comfortable weather is a relative phenomenon, and coming from southern Arizona the temps may even be a little low compared to what you’re used to. But temperature is only part of the equation; people who know say that in summer Tokyo is a tropical climate, on par with Singapore and other hotspots in southern Asia.
Of course, life does go on – even in heat-island Shinjuku – and there are mountains nearby to escape to if the heat does get oppressive. So the heat and humidity is no reason not to visit in summer: it’s just a matter of adjusting your inner thermostat, so to speak!