It is up and down only. Actually depends on how things were when you zeroed, I guess. Surveying vid says the bend is always down and from their perspective, I can see why. Litz says it is only up and down, but he was probably starting his observations from somewhere in the middle. They actually put a video camera on a scope positioned on a distant target and recorded the results through out the day.
Indeed, under the 'wrong' conditions, the effects are pretty easy to see even at 200-300yd. Just shoot a group at first light, with the sun at your back (a common hunting situation) and then shoot another at high noon. My POI moves around the most on cold sunny days. I can't say it's all due to refraction, but with snow on the ground I've had shifts approaching 2moa in -20C/-5F.
It was very frustrating chasing my zero around. I spent a lot of time and resources re-zeroing my scope and freaking out thermal expansion coefficients. Different stocks, bedding, scope mounts.... until someone keeping more disciplined records than I suggested optical phenomenon. Now I just do my best to zero under diffuse lighting (overcast conditions) and am very reluctant to change my zero without multiple data points.