Is your cadence and pedal stroke the same both into and with the wind?

I find it much harder to maintain the same HR and watts on flat or downhill sections that I can on climbs.

Gusts. If the wind were constant you'd like feel little difference. But winds that are 15 MPH sustained with gusts up to 20 or 25 are like a brick wall.

That was one thing I tried to keep an eye more today. Cadence data is pretty much the same. Actually 2rpm faster against the wind. Rid es almost all flat

That probably makes the most sense. Today actually wasn't terrible but noticeable. Other days there are this 15mph constant with gusts me those can be brutal. Feels great riding with it at your back and then you turn around and it's a battle.

Inertial load?

This. Same reason producing equal watts on the trainer is more difficult.

I used to think that, but riding into a headwind we have plenty of linear momentum. I think there is some more physics at play. When you push down with one leg, the other leg has to move forward against the wind. Let's say you are going 30 kph into a 20 kph, that's the entire surface of your other pushing 50 kph of wind plus an etra x kph of self generated "relative wind from leg motion" that should create an additonal work load. How much is X. Well if you are pedaling at 90 RPM on 170 mm cranks, this would be 34 cm x 90 /second ~3 m/s of additional wind load. 3 m/s x 3600 s is an additional 10.8 kph of wind load on the recovering leg. So add that to the 50 kph air flow and that leg is going at 60 kph into the wind, while our body is going at 50 kph into the wind. I believe this leaves less watts to push into the drivetrain, because you have more overhead watts just from moving the legs around to get into position for the downstroke.

Now someone will say you have that 10 kph of overhead windflow at all rider speeds, but since the work load is proportional to the cube of the velocity this addional amount adds more at higher relative wind velocities....at least it should.

When you are sprinting hard into a wind, you can really feel the air flow on your recovery leg as it pushes through the wind. It is why running shoe companies are making all those aero spikes. Its just not the wind flow over the body, that recovery leg has additional relative wind flow.

some good points,

into the wind you have to keep pushing, if you let off you slow down that much quicker, so I would say that the wattage is probably kept more constant into a head wind so it feels harder. simiar thing on a trainer... constantly loaded so no slacking off.

it's not constant if there's gust. think of it this way, one moment when there is gust, you are in a smaller gear, trying to turn over your legs. when the gust is gone, the additional increase in rpm is not enough to generate the same power at lower wind speed. conversely, when you are in a higher gear during low wind situations, a gust probably puts you below the bottom of your power band.

in this sense, what the poster above said about relative air speed is important. the delta goes from, say 35 mph during low wind speed (20 mph ground speed + 15 mph wind) to 45 mph at the instant the wind picks up to 25mph.

Conversely, when the wind is behind you, a 28mph ground speed and 15mph wind speed is 13mph air speed; when that wind gusts to 25mph, that air speed changes to 3mph. However, the difference btwn power to overcome 3mph-13mph is trivial compared to the difference btwn 35 mph to 45 mph.

Add to the fact that you lose a bit of power when shifting, it becomes a lot harder to maintain power into a headwind

i used to train on a wind-exposed park along the Potomac for a few years, and it was always easier to maintain power in the tailwind section.

My ride yesterday when I thought about it wasn't ridiculously windy but I noticed it felt harder than with the tailwind. I've experienced races where there is lots of wind and gusts and then you factor in other racers who get stacked up going into a headwind then I'm expending energy when in need a quick burst to get past. But it makes sense if there's some gusts and some shifting to deal with that which ends up with me working harder. With a tailwind I'm sure it was less shifting and just much more smooth

Well I'm no scientist but if you ride into strong headwind there is more force through the entire pedal circle.

Whenever you are not accelerating you are decelerating at a great rate vs no wind vs tailwind

Kinda like a trainer ... there is no rest throughout the entire pedal circle