In hot weather, planes may take a longer run to get off the ground, so when a plane is heavy, that weight has to be amended.
Thus London City Airport has been reducing the weight of their flights because of their 4,900 foot runway.
On Sunday, the fact that fifteen passengers were pulled from a Gatwick-Geneva flight made the news. The flight was overweight by a ton. According to Swiss Airways, a ground handler reported “It’s happening every day from London City because in bad weather the pilots need a lot of fuel, and can’t take off from City with a lot of fuel and a full plane of passengers”
Simultaneously, Geneva Airport was facing heavy thunderstorms.