First, let's classify to judge: the influence of the rainy season on pre-verification before booking is not the same for all situations.
We need to distinguish two main categories: leisure trips and business trips.
For a leisure trip to Majorca, yes, the risk of rain is a major factor for the average traveler wanting to enjoy the sun, directly influencing the verification.
However, a business traveler going to Frankfurt for a crucial appointment does not care at all about the weather, focusing instead on schedules and logistics, because professional imperatives take precedence.