In winter the average speeds of many cyclists will be lower than in summer. What's the reason for this? According to many, it is mainly the fact that we're training a lot less. Myth? Or plausible? Let's dive in.
At low temperatures the body needs energy to stay warm. This translates into a higher heart rate. If you riding with a power meter you can see this quite easily. I'll take myself as an example: where I normally take it to HR 140 at 200 watts, I'm now only 5-10 beats higher. Now such a difference can of course also be caused by inferior shape, but when I take the data from winter rides from Gran Canaria or indoor rides in winter, I see the same difference.
It becomes even more interesting when the factor shape is taken completely out of the equation. And with a power meter this is also very easy. To achieve this we compared a number of rides of the same person. We selected a number of solo rides from both summer and winter with one thing in common: the average power and ride distance were more or less the same. What turns out? There is still a significant difference in speed. In summer: 190 watts on average is about the same as about 30 kph on average. In low winds a bit faster, in strong winds a bit slower. In winter the average speed was 10% lower at exactly the same average power, at about 27 kph average.
The most important factor is the higher air resistance, which is caused by the higher air density in winter. At lower temperatures, the air molecules have a lower average speed and therefore collide less hard against their environment. At lower temperature (and therefore less hard collisions) and equal pressure in the atmosphere means that more collisions have to take place, in short, that you have "thicker" air to cycle through. At 30 degrees difference in temperature, this can only make a difference of 10% - quite a factor.
Another reason is the fact that you have a lot more clothes on in winter, which makes for more air resistance. A spacious winter jacket, overshoes, gloves. It all contributes to it. Also, many of us will be riding on cheap non-aerodynamic wheels and puncture resistant, so not too fast rolling tires.
In winter, your riding style will be much more conservative when cornering. That doesn't directly affect average power - it does affect average speed. Finally, the trees have lost all their leaves in the winter, so you have less shelter as a rider. So the wind has more influence than in summer.
The inferior shape is only one reason why you ride slower in winter. If you ignore this factor, when it's cold, you'll still only drive 10% slower. The influence of temperature should not be underestimated.