Originally Posted by rickair7777

1) In addition to what Joe said, a strong X-wind can be a factor.

2) Generally you want less-dense air (less drag), which always means higher altitudes. But the tradeoff is that it at takes a lot of gas to climb to higher altitude, so on a shorter flight it may not be economical to burn the gas to get to F390 only to have to immediately start your descent. Also high-altitude winds play an important role. You may better of at F290 with a 30 kt headwind than that at F390 with a 130 Kt headwind.

These are typical CRJ altitudes (westbound, no unusual wind):
15 min flight: 16,000
30 min flight: F240
1 hour flight: F320
> 1 hour flight: F380