As you may or may not be aware, fluctuations of temperature and humidity can cause timber (as a natural living material product) to react in different ways.
In extremely hot weather, for example, you may see cracks and splits start to appear as the moisture level within the timber goes down.
Please do not be alarmed.
This is not a fault in the timber itself and will not effect the strength and durability of the item in question.
Once colder/wetter weather returns these cracks and splits should close up.