All tall steel structures, particularly radio towers and 'cell' towers, lean away from
the sun slightly.
The reason is the uneven heating of the structure. The sunny side becomes
heated and expands slightly, while the shaded side remains cool.
The effect is that during the course of a clear sunny day, the direction toward which
the tip of the tower points describes a part of a small circle in the sky.
This effect can become troublesome when a large, high-gain antenna with
a very narrow beamwidth is mounted on a tall unguyed 'monopole'. The pole
can deflect so much during a day that the antenna wanders away from its best
pointing direction, and its transmitted and receive signals decrease or "fade"
substantially.
