Keeping your cool is more than just an attitude; it's a necessity
Most families traveling to East Asia in northern summer will not have had much experience working in a tropical climate over several weeks. Americans from the Deep South or Gulf Coast may be already acclimated; for everyone else, this article should be helpful.
So, how rough does it get?
In the southern and central provinces of China - Guangxi up through Anhui and Henan - summer temperatures can routinely reach into the low 40s Celsius (low 100s Fahrenheit), with 100% relative humidity. The hot season generally begins in late April-early May and lasts through the end of typhoon season in September.
Korea - and Japan from Tokyo southbound - will also experience high humidity from May through August. While air temperatures won't usually get as extreme as in central China, in combination with the saturated air, conditions will still have potential to be rather uncomfortable.
Weather systems in central China can become locked in place between oceanic tradewind belts on the east, the Himalaya Mountains on the west, and high pressure coming down from Siberia. (This also traps air pollution, too.)
While the northern and northwestern provinces can get hot, too (after all, the northern border with Mongolia is mostly desert), they tend to get more attention from the jetstream and see more frequent weather changes.
Japan and Korea likewise can get locked into ocean-weather patterns from the south for weeks at a time in the summer; rather like how in North America persistent rain will funnel up from the Gulf of Mexico onto the Midwest.
Can I just stay inside with the air conditioning?
The TV programming will get old after a couple days. Hotel food is ridiculously overpriced and not that good (and if you're going to get sick, it's from hotel food.) All those attractions you paid good money to come see are still sitting there, waiting for you. The locals are still going about their daily business. Finally, the air conditioning at your hotel isn’t going to be as strong as you’d care for. No, you’ll have to go outside for a while...
Preparation means more than just packing the right things
If you anticipate traveling during the hot season, before you even have a travel date you could start acclimating your body - yes, even if you live in a cold climate. As you gradually increase your exercising, put on an extra shirt or sweat pants to simulate the hotter, more-humid conditions. This must be done gradually, though; don’t overdo it. (And check with your physician ahead of time.)
As you start packing, look for clothes made of light, breathable fabrics that wick moisture away from your body. “Performance gear” - underwear and shirts especially - is now widely available from many clothing manufacturers online and at retail.
Remember, East Asian social norms frown on excessive exposed skin (unless you're an old man; they get away with everything, but everyone knows they look ridiculous.) Practically, this also cuts back on sunburn risk.
Men - go for lightweight full-length pants
Women - capris, lightweight full-length pants, and long but light skirts
Short sleeves are perfectly fine for both sexes, but keep your choices neat, simple, and tasteful
Pack several handkerchiefs (wash them several times before packing to make them nice and soft)
Watch the weather forecasts for the cities you’ll be visiting on your trip as the date grows near, to help you fine-tune your clothing choices
On the trip, take care of yourself and your child:
Stay hydrated - load up on bottled water and always keep a bottle with you
Quick showers twice a day
Use an umbrella for the sun. If you buy a stroller for your child, look for one with a sun-shield
Keep your shirt untucked to promote airflow. (Sweat wicks down ... and then out through the front of your pants. Seriously, we’ve seen too many Westerners who look like they wet themselves.)
Find the air-conditioning where you can. The tour bus is usually a good bet. Stores and shopping centers, surprisingly, are seldom cooled; plus there is no cross-breeze indoors. Buildings with good a/c often leave their doors open and blast it out onto the street - as advertising.
Take it easy when walking; don’t over-exert yourself. Take frequent breaks and drink water along the way.
Eat smaller snacks more frequently instead of taking just a few big meals. Sugar, fat, and alcohol keep your body from absorbing fluid, so look for light and fresh foods.
Try to go out more at night. In the east, the sun goes down early - and the cities stay up late.
Avoid consuming cold (freezing) water in extreme heat conditions, as this can cause cramping.
Chinese folk wisdom leans toward consuming hot beverages in hot weather; the Centers for Disease Control do not agree. In most cases it’s a moot point, since stores usually keep bottled water and soda displayed at room-temperature. Big coolers for beer and beverages aren’t common features at Chinese retailers. In Japan, the ubiquitous vending machines will give you hot or cold beverages. Convenience stores everywhere will have frozen treats, and the Japanese/Korean/Taiwanese chains will have cold beverage coolers, too.
Common-sense precautions while you’re in-country and gradually incorporating hot/humid simulation to your pre-trip conditioning will go a long way to making you a little more comfortable during your journey, so you can concentrate on getting out and enjoying your family's experience there.