The hardest to pack for; the most draining of conditions
Certainly the image people will have of the climate in southern China, Taiwan, and the southern Japanese islands is hot, humid, and tropical. For most of the year this is true. But East Asia is a vast territory, with great variety in landforms, seasons, and weather.
In central and south China during December - February, and for much of the year in Sichuan and the highlands of Yunnan, as well as the southern Japanese main islands, humidity is high with frequent misty rain showers. Temperatures are moderate to cool - not usually freezing, but awfully chilly by Asian standards. Snow is not uncommon in the west, but very rare in the far south, where everything will shut down even if just 1 or 2 centimeters accumulates. In short, think Seattle, Copenhagen, or Auckland during a cool spell and over the winter...
In the north of course, snow and freezing temperatures are more common, and some of the coldest winter conditions can be experienced in the cities of far northeast China, like Harbin (home of an amazing winter festival) and Shenyang. Korea could get snow nearly anywhere; Honshu in the mountains and from Osaka north certainly sees snow, and the island of Sapporo becomes a winter wonderland, with its own famous festival, too.
But at least it’s not hot
The problems with this kind of weather are:
Your sweat doesn’t evaporate when you’re exerting yourself
The cold works its way through your clothes and into your body’s muscles and joints
Most buildings - including your hotel, unless it's a brand-new skyscraper under a Western brand - are not well-insulated and ventilated, so you may be in for clammy, stale air and cool, damp sheets when you’re trying to sleep
How to prepare
Watch the news and weather forecasts for the cities you’ll be visiting in the days before your trip; fine-tuning what you pack will save you pounds, strain, and stress.
Wearing multiple layers of thin clothing helps keep your body heat in, and lets moisture migrate out. It’s also easier to keep comfortable by removing a layer if you need to, and thinner clothes are much easier to pack in your suitcase than puffy, bulky gear. "Puffy coats" are not a traveler's friend!
Cotton is soft and light, but may retain too much moisture to use as the layer closest to your skin. Look for polypropylene / “performance wear” for underwear and light shirts.
A lightweight windbreaker or raincoat is good to have with you; keep it thin for easy packing and look for a design that keeps water off but lets moisture pass outward. Umbrellas are always appropriate - you can pick one up for cheap almost anywhere in East Asia at any time, just pop into the convenience store on the corner...
That’s why the food is spicy
Traditional Chinese and Korean medicine and folk wisdom has many ways to battle the chills, and recognizes that environmental stress leaves you susceptible to disease. No wonder that Sichuan cuisine is known for its heat - it’s a great way to get hearts pumping and blood flowing. Don’t forget about hot tea, steamed buns and stir-fried foods, too.
And while a crowded, lively restaurant might be a place to catch a cold from all the people gathered, it’s also likely to be a warm, comfortable, and friendly spot to forget the conditions outside for a while.
Depending on where you're visiting, another traditional activity is a visit to a hot spring; a long soak with friendly conversation - often preceded by a filing meal - can do wonders for one's joints, muscles, and mood. And if you can find a winter festival or street food gathering, join in on the fun!
Staying comfortable takes some forethought and appropriate packing, a good attitude, and good food. With these you’ll be ready to brush off the elements and concentrate having fun with your family and learning more about East Asian society.