really?... i see chinese are very good at checking weather in their mobile always and decide what to wear.. even my in- laws do that..

Agree that they wear before we think it is necessary... but its from their experience with cold climate... now a days i am getting used to it as well... other wise every winter why should we wait to catch cold then wear long johns

'Cause it is freakin' cold!!!!

Habit, just force of habit, I guess, another one of those local customs.  It is the same where I live.  I usually wait until the beginning of November however, when the temperature during the daytime is only + 6 or 7, and even colder in the morning.

Quick temperature drops cause colds, I think. When the temperature dropped here, I got sick for a damn month.

But I'd say it's also for the same reason my wife won't allow me to expose any skin through the blanket at night.

I think it is habit because in the past when there was no such thing like global warming..first week of October use to start with cold..so Beijingers are carrying legacy with them..

GRaf said it - because it's COLD!

My Xingyi teacher recommended to me that I should start wearing longjohns as soon as the first cold day came along in Autumn. When Spring comes along he said I should try to keep wearing them as long as possible too.

I don't know if there is any 'tradition' behind this (I mean come on, have longjohns been around for 5000 years?) but the intention here is more or less the same as Zhuge Liang mentions above - to minimise the risk of getting ill.

The (twisted) logic he argued was that putting longjohns on early not only means you won't feel the cold so much (duuurrrr) but it also fools your body into thinking it is still warmer than it is, so it stays in 'Summer' mode for longer. This results in your body easing into 'Winter' mode, rather than crashing as it tries to adapt to the 20 degrees C temperature change that tends to happen in a week or so here in Beijing.

And the converse was his argument for taking the longjohns off in Summer... to help your body ease into Summer mode.

I have to say I was skeptical about this, as it sounded like a classic piece of Chinese 'logic'. I've done it for a few years now though, and it has helped with the transition out of Winter which was when I always used to get a cold. But putting thermals on after the first cold day in September? Nope. Couldn't take it, sweated buckets - granted I didn't get ill the couple of years I managed to endure this, but I sure as hell must of looked ill!

Despite persistent urban myths to the contrary, the rhinovirus ( the common cold) is not more contagious in the cold weather, it's an old wife's tale!

For example, people in Alaska don't get more colds than people anywhere else. We do have more colds in winter than in summer, but not because of the cold.  One of the expert scientists (Bill Nye) said that you can not catch a cold from being cold; you catch a cold from germs and being cold has nothing to do with it. Plenty of tests have been conducted proving this.

The old belief that freezing temperatures cause illness started before people knew about germs; however, it continues to be passed along to others as a legend today and is not an evidence based finding from studies - just based on anecdotes and incorrect associations.

Then why do we get more colds and flu in winter? It had been long held that this was most probably due to school children returning to schools and people being in closer proximity indoors in winter where they could pass all their germs around more easily. One of the most commonly cited studies used as a basis for this hypothesis was the "Seattle Virus Watch", done by John Fox, Carrie Hall, and friends.

Source - http://wiki.answers.com/Q/Can_you_catch_a_cold_from_being_in_the_cold

completely in agreement with Mr Rection. 

To add to that http://www.medhelp.org/common-cold/slideshows/Fact-or-Fiction-The-Common-Cold/286?camp=nl_2012_11_19

I often hear people confuse "being cold" with "having a cold", not sure if there is anything in the Chinese language that would lead to this, e.g. if the two sounds similar then they must be the same. 

Putting on longjohns early before it is really cold would for me result in, getting soaked with sweat, followed by getting frozen by being outside in damp clothing getting chilled off. 

The guy from my elementary school who was always sick was the guy who's mum forced him to wear winter outfit as soon as the leaves fell of the trees. (not saying he got sick from how he dressed, just that it didn't help that particular person)

It is worth considering, that when you are cold, the body will spend more energy trying to keep warm, this will cause some basic maintenance tasks to be down prioritized, this the immune defense will dip in effectiveness during and in the time after having been cold. Therefore if someone sneezes you in the face at this time, the chances of the "cold" catching on is higher. 

I think it's just common in China for people to wear certain clothing according to season, not the daily temperature. I didn't really notice it in the fall, but in the spring, it would be a warm day, and everyone but me would still be wearing long sleeves. Obviously some Chinese people don't do this, but I'd say quite a few do.

Yes, agree with above jnusb, Happen t ma many times same. Also when i let my wife dress our son more according to the weather, as its only 8-10 degrees now usually, she say its only November now  hard to talk about it.

   And do not ask me, how we argue about using at least for few hrs the AC or oil-heater , just to keep a bit warm for the walls before the freezing come and you can not do really nothing at that time. coz above you, bellow you, and left and right side neighbors, they will not help you keep walls warm , they rather wear 6 layers cloth and breath the freezing air.  oooh, happy day