If I put on heavy clothes it will be very hot inside them after the warm-up. If I wear only a T-shirt I'll freeze.
What's the solution?
I ride around 20 km in a hilly city, with temperatures between 0 and 30 celsius.
I have a continuum of clothes that I add on as the weather gets colder.
During the summer, I ride like you do with shorts and a t-shirt. As things cool of a little bit, down below 60F/15C, I'll put on a windbreaker over my t-shirt and some full-finger gloves on my hands. The next step for me is to add an Under Armour Cold Gear shirt (many companies make equivalent tops) but to stick with naked legs, they're doing all the wok after all.
Once temperatures get down to the 45F/7C range, I'll add som snazzy leggings in a fabric similar to the Under Armour top mentioned above. When things start to freeze, my final move is a fleece vest over the long sleeve top, but under my windbreaker.
If things get even colder where you are, Icebike's clothing page is a good place to look for further advice. Keep two things in mind: you need a lot less clothing once you're moving and generating heat than you do standing around, but you also need to be prepared for a problem that might leave you standing around for some time. Dress light and carry extra clothes in a backpack.
Most rides in the cold I start out freezing. Once my body has generated some heat I'm great. Typically my outer layer is wind blocking followed by a jersey. Depending on the temperature I'll have either a long sleeve polypro shirt or just my arm warmers under that.
I have both leg warmers and fleece lined tights. I wear the tights if it's colder. If it's early then I'll opt for the leg warmers since they can be removed.
I also have windstopper gloves and when it's really cold I'll wear a set of Louis Garneau 2 finger (+ thumb) mitts.
It's also important to cover your ears. I have both a thin toque and a full balaclava. Again if it's really cold I'll wear the one that covers more.
As stated above the secret is layer. Many cycling specific jackets have zip off sleeves. They're great since you can easily remove them to cool down a bit if you dressed a bit too warm.
One thing I forgot to mention is shoe covers. I have a neoprene set of shoe covers. They keep out water decently but they are warm. You can make your own toe covers for the shoes with old socks and duct tape. Cut the old socs so that they extend past your cleat. Sew on a strap that goes around your heal. Cover them in the tape for wind. I've had a pair of those that lasted 3 or 4 years.
I do ride also about 24km per day in somewhat slightly hilly city where it's around 0°C in winter and 35°C in summer.
in winter time I wear this when it's not raining:
It's a bit sporty, but it's highly breath-able and keeps warm enough even at -6°C (the minimum temperature I had last year).
On rainy day I had on top a rain jacket or I replace the windproof one by the waterproof one, depends on how cold it is.
However, if I would do shorter commute, I would dress in winter more or less as I would if I go outside walking in the streets. Perhaps a rain trouser would be nice if it's really badly raining, or a poncho would do it. Cycle Chic ;-)
One of the best cycling investments I made has been a set of 'sleeves' and 'legs'. They are lycra with a very thin lining. I wear them when the temps are between 50 (which is as cold as I can comfortably stand to ride in lycra, even with a t-shirt under-layer) and 65 F or so.
They have extended my cycling season from just summer to summer +6 weeks or so on either side.
Wool is nice because it breathes better than warm synthetics, smells better when sweaty than any synthetic, and still keeps you warm when it's wet.
Advertising can compensate for a synthetic fabric's poor performance. Evolution forced wool to work.
The secret is layering. Read what the OneBag site has to say about it. This is not intended for biking, but the rules still stand.
If I'm comfortable temperature-wise when I leave the house, I will be too hot a few minutes later. I remind myself to be chilly as I leave, so that I can warm up and be comfortable on the bike.
When I'm too cold, I find that covering exposed areas, even lightly, helps more than adding layers where I already have some. So, gloves & hat before a heavy jacket.
For temperatures up to about +5 Celsius or so, I use a jacket (waterproof and/or lined and/or just fleecy): with a zipper down the front.
When I get warm, I more or less undo the zipper (keeping it shut at the bottom to stop it flapping).
Undoing the zipper spills a lot of heat from my core (body).
Keeping the zipper done up (to my throat) keeps a lot of heat in (especially when it's a long-sleeved jacket).
Being able to keep a lot of heat in, and/or being able to spill a lot of heat, means that I have a lot of control over how much I keep or shed, just by positioning the one zipper higher or lower.
I spent my highschool years on the wrestling team, which meant working out in multiple layers (2 sweatshirts was typical).
Its been several years and the habit is hard to kick. Unless the temperature is scorching I always wear a sweatshirt when riding. But then again, I'm not particularly fast, and I'm big enough that the sweatshirt adds negligible drag (great for my cycling group because on a windy day, my wake is wide enough for them to ride side by side behind me).
Of course, if its a pleasure ride then I wear comfortable clothing. The sweatshirt is something to maintain a good work out.
This is my first autumn. Here are my (quite immature, but nevertheless) thoughts.
When one is stationary and not carrying a backpack, heat loss occurs due to convection and radiation. If clothes are used, that are properly strapped at the wrists, waist and ankles, only radiation remains significant. Therefore, thick, layered clothing with homogeneous distribution over the entire body makes sense.
Quite different when riding. The torso, higher legs and the head generate the vast majority of heat (which is abundant, as we all know, when riding). For those areas something to brake the wind ought to be sufficient.
The areas that need warmth are the joints. This mandates the use of gloves and ,IMO , knee pads. Add to this that knee pads have other advantages, such as supporting the joint that we, cyclists, (ab)use the most, and physical protection.
That said, only the scarf remains. It doubles as throat insulation and as breathing aid in the cold air, that a physically active individual inhales rapidly.
Conclusion: wear light clothing at the torso and higher legs and pay special attention (i.e. a lot of protection) to the extremities and the (fore)head.