[kevin_thibedeau]

Cold air is denser. Pushing it out of the way saps more energy and drops the effective work done. You're oblivious to it because a motor vehicle has so much available power on tap. It is very noticeable on a bicycle at high speeds.

Conversely. Sailboats get more efficient in cold temperatures because more energy is imparted on the sail.

[bumby]

>Cold air is denser. Pushing it out of the way saps more energy

ICE mpg tends to drop in cold weather, but this isn’t why. It’s usually because it takes longer to reach optimal operating temp or due to changes in fuel composition. Cold air can produce more drag but that problem exists irrespective of ICE or EV.

The pumping efficiency is unaffected as any losses on the exhaust side will be offset by increased pressure on the intake side.

[scott_siskind]

> It is very noticeable on a bicycle at high speeds

Huh? I've been cycling for 15 years, many of those years doing 15k km a year, all year round. Mountain biking on mountains at -15C, down to urban cycling in 38-40C scorching summer heat. I've never noticed speed, effort, stamina changing with temperature as you suggest.

And I've done lots of road cycling at constant 35-40km/h, descents of up to 70+km/h.

[pastage]

I am not really contradicting you here. Just want clarification from GP. My reaction was Huh? As well.

With N=15 years of gps data and 40-70 of those days per year below zero very few below -10°C. I can say that I ride slower on cold days on avarage.

I am not fit and do not track that at all. I can barely keep my avarage above 25km/h in city traffic for 15km, very few conflict points maybe twenty but not comparable to road cycling. So a km/h decrease of avarage speed during winter is common but not a given. I do not count bad road conditions, but Stockholm has really good maintanance during winter, ice free asphalt all year.

[Lio]

I definitely ride slower in winter too but I always assumed that was due to my body having a harder time in the cold rather than air density changes.

That seems to be born out by using better (read more expensive) winter riding gear. I naturally run a bit cold so spending on low temperature tights, base layer and jersey/jacket has made a massive difference for me.

[LargoLasskhyfv]

More wind chill? More weight by thicker clothing?

[pastage]

I ride with heavy loads, 10+ kilos is common, no stats on that though. For my speed it does not matter much with 2 kg clothes.

[bennyelv]

Interesting! I’m also a long time high volume cyclist, and I see a clear effect of slower rides in winter. I’m talking about around specific routes and training sessions where the point is to go as fast as possible. With the same power I seem to go more slowly when it’s cold.

Obviously there could be many reasons for this: less aerodynamic clothing, more cautious cornering etc etc, but the effect is very noticeable to me.

[kevin_thibedeau]

Track your time on a consistent route on paved, snow free roads. I guarantee you'll see a dip in winter.

Scorching heat with high humidity has an opposite effect. H2O drops the density of air and you can go measurably faster than in drier air.

[Manuel_D]

Colder air's increased density and bigger temperature gradient also improves the efficiency of internal combustion engines.

Increased rolling resistance from displacing snow is a far bigger cause of winter driving inefficiency.

[trentgreene]

Hmm, I wonder if the increased resistance is overcome by the fact that ICEs can put out more power with colder denser oxygen rich air.

This is why performance cars sport cold air intakes. This is also why your car won’t hit its quoted 0-60 time in a warmer environment — those tests are normally performed or normalized to cold air conditions + 0% humidity. Just what counts as cold depends on manufacturer / reviewer.

Anecdotally, I track efficiency and drive consistent routes, and don’t notice a diff.

[bumby]

I wonder if the increased resistance is overcome by the fact that ICEs can put out more power with colder denser oxygen rich air.

Not likely, as an increase in air is going to be accompanied by a corresponding increase in fuel. Unless your fuel mapping is all messed up, it will give you more power, but not likely better fuel economy.

[eldaisfish]

this is the textbook definition of confidently incorrect.

Winter fuel efficiency is worse because of a winter fuel mix, not because of air density.

[dgacmu]

And if you don't pump up your tires to compensate, the lower pressure from cold temps will increase your rolling resistance. Not a 40% drop but enough to notice if you're tracking mpg.

[thaumasiotes]

> You're oblivious to it because a motor vehicle has so much available power on tap.

Dropping from 18 mpg to 10 mpg would increase your cost of fuel by 80%. It'd be hard to be oblivious to it.

[Gordonjcp]

> Cold air is denser. Pushing it out of the way saps more energy and drops the effective work done.

That makes absolutely no difference at all.

[sokoloff]

> That makes absolutely no difference at all.

That would be very surprising if it were true, given the very large effect density altitude has on airplane performance (engine and wing).

[maxerickson]

It's gonna make some difference, but air resistance goes with square of speed and the change in density due to temperature isn't that large.

[sokoloff]

It’s about -3% change in drag force per +10°C change in temp.

[maxerickson]

Which is like the same as 60 mph vs 59 mph.

[sokoloff]

In drag force, yes, but in this case, you are taking the 3% penalty and aren't getting the partially offsetting 1.7% increase in distance per unit time that 60 gives you over 59.

Note that it's 3% per 10°C delta so, for a 40°C swing (say 30°C/86°F summer to -10°C/14°F winter), that's a 12% difference in aero drag force.

[Gordonjcp]

Which is basically nothing.

Opening your window makes far more of a difference.

[Gordonjcp]

Over what sort of range of height and pressure?

It's just about observable with very light and very aerodynamic aircraft at ground level if you can get them cold enough.

[sokoloff]

It's well beyond "just about observable". Here's a sample landing distance performance chart: https://www.ascentgroundschool.com/~ascentgr/images/ct80802e...

At -10°C/14°F, with no other adjustments, in the hands of a test pilot, that aircraft will have a ground roll of around 1000' and clear a 50' obstacle and come to a stop in around 1500'.

At 30°C/86°F, with no other adjustments, in the hands of a test pilot, that aircraft will have a ground roll of around 1150' and clear a 50' obstacle and come to a stop in around 1750'.

Flying in and out of 2000'-2500' strips with trees, it's impossible to not notice the performance difference with temperature. Perhaps the pilots who can't notice those differences don't fly into anything other than very long runways, but I don't think very many pilots miss the differences.