Due to fog, less flights can land and depart. Therefore, some flights will be operated on time, while others will be delayed and have to wait for their turn to depart or arrive. When a flight is delayed more than 2 hours, passengers have the right to care, for example food and refreshments have to be available.
There is no right to financial compensation, because bad weather conditions are an extraordinary circumstance. The airline cannot prevent the delay and thus does not have to pay compensation.