Windchill your friend in summer and just the opposite in winter.

Colder air is denser = more drag :)

---

"I'm thinking of a number between 1 and 10, and I don't know why!"

Wearing extra clothes makes it harder to move, 10% reduction in FTP.

Cold causes your body to use energy to heat itself, 5% reduction in FTP.

Snow wasn't there in the summer, its water, that has oxygen, which should be un the air! 5% reduction in FTP

Cold makes the ground harder, reducing your rolling resistance. 5% reduction in FTP.

This and less aerodynamic clothing.

you expend more energy thinking "WTF am I doing out here!"

Yep, 5% reduction in FTP!

Due to the colder temps, air density in most areas is higher in the winter than the summer, which directly causes an increase to aerodynamic drag. Increases of 5 percent or more between mid-summer and mid-winter are not unusual.

I'm not sure how precisely quantified it is, but tires should also be expected to roll a bit slower in the cold, since the temperatures stiffen the tire material.
More significantly, a lot of people switch to beefier tires with more protection during the winter. Depending on the particular tires involved, the performance consequences can be anywhere from subtle to astronomical, potentially up to a couple mph.
You could probably extend this concept to say that a lot of people switch to generally slower bikes in the winter in a lot of ways. Like fancy carbon wheels being swapped for cheap shallow aluminum so that the expensive carbon brake tracks don't get ground to a pulp in the mucky weather.

Winter clothing can be less aero. I say "can be" because this is very circumstantial. If you're someone who rides in classic highly-breathable road gear in the summer, and then throws on shoe covers and aero helmet and whatnot when the cold weather rolls around and the fear of getting too hot vanishes, it's entirely possible to be more aero in the winter.

Inclement weather and shorter days tend to slow people down for all kinds of circumstantial reasons, i.e. you're unlikely to take a twisty descent as fast as you would on a summer day if a nighttime downpour is preventing you from seeing clearly.
If the "inclement weather" starts to include snow and ice, then there's no telling how much people can get slowed by, or if they'll be able to ride their bikes at all.

Many folks also just plain lose fitness in the winter, due to not wanting to do as much activity. Although the rising popularity of quality indoor training setups is combating this.
But one thing that increasing indoor training does not combat very well is decreased speed in real outdoor rides due to reduced turnout. Reduce the amount of fast people in a paceline, and that paceline will tend to go slower.

Sometimes winter conditions directly make a person less powerful as a rider.

For me it always seems a whole lot easier. Not because of any aerodynamic aspect but because cold dry air is so much easier for me to breathe. In the summer I wilt

I say it about 10 times every ride I think !

Tire casings are less pliable at colder temps, so more .crr.

---

"They're made of latex, not nitroglycerin"

So when I ride when it's 20 degrees out compared to 80 in the summer. That's a huge difference! I feel like I work harder outdoors during the winter. I guess that's Minnesota

... and my old 2003 Spe Enduro with big studded tyres is less efficient than my GP4000s2 equipped P3...

Body composition and genetics have a lot to do with performance in the cold too. I'm scrawny and my body just likes it when it's above 60 degrees. I can't breathe right when it's cold and everything is just tight. Plus wearing all the layers constricts you. I make my best power when it's 85 -90 degrees and love riding when it's 95. Everyone is different......

Typically:
Higher air density => Slower for same output
Bulkier clothing => Slower for same output
If more wind => Slower for same output on a looped course
Slower tyres (for those who go that route) => Slower for same output
Psychological effect of bad weather => Can feel harder to produce same output
Worse form out of season => Feels harder to produce same output

i also think the cold weather affects anything greased like bearings and chain. perhaps friction increases on those lubricated parts as they get colder.

This.

Here is a calculator you can play around with to see the effect of temp on your speed: http://bikecalculator.com/

At TT speeds temperature alone can be up to a whole kph.

The calc spits out 44.1 k/hr at 20 deg C temp and only 43 at a typical winter temperature around here.

Now, you should in theory be able to generate a bit more power though given the air density. Runners go for records in stuff like the Berlin marathon, flat and cold. It still fails me why someone like Eliud would run in what he does versus a form fitting tri-kit kind of thing. Aero for the runner yo.

Cycling we move fast enough that the air density difference negates any power bump by the cold weather.

Go ride hills or gravel and don't worry about it.

It's harder for me because really really intense intervals in cold air tends to hurt the lungs and dry my mouth out really badly. Tempo or SS is fine. I tend to move intense stuff indoors if it's below 50 and over 85.

i definitely notice the impact of the body burning energy to keep itself warm on longer rides over winter - after a 2 hour ride it feels like easily 3 hours

personally, i find my legs just don't pedal effectively when cold - the suppleness that gives nice smooth circles just doesn't happen with cold legs. running it doesn't seem to be an issue despite wearing shorts rather than cycling leg warmers, i'm guessing mostly because the warmth generated is higher and the windchill is lower, possibly partly due to the different movement patterns too

Winter just sucks.

That's how I go about it during winter, just deal with it and go ride where I normally wouldn't!

Thermal contraction of pavement causes all the climbs to get at least twice as steep.

Gravity increases in Winter

(Only rational explanation I have to the yearly 10+ LBS increase on the scales this time of year)

Does the drive-train have more resistance in wniter ? Maybe cold will reduce the effectiveness of chain lube and of bearing grease ?

OK, I am just speculating with no real evidence at all. I used to ride through the winter, down to -20 C, but that was on clunker bikes, so it wasn't a valid comparison to a road bike in the summer. But what is the difference in lubrication effectiveness between -20 C and +20 C ?

---

run well, run happy
george