Looking at this Ariane 5 launch, I notice that:
- the speed continously increase (as I expected)
- the altitude first increase as I expected, then decrease (I was not expecting that) and then increase again.
When looking at the graph on the right upper corner, it looks like a totally calculated maneuvre. The slight altitude decrease is obvious around 30 minutes in the linked video.
Why do the trajectory of this rocket include a temporary loss of altitude for several minutes before increasing again the altitude?