Weather: Many passengers are aware that weather conditions may cause a delay on a rainy day however in some situations, sunny or windy conditions can also affect the performance of the aircraft. When the temperature is higher the aircraft has less air which means less lift or if there are strong headwinds predicted, more fuel is required to perform the flight.
Special Airports: Some smaller airports are based in beautiful surroundings but are affected by special conditions which require a steeper takeoff or decline. This is due to weather and shorter runways. An example of this would be Lugano, if the weather is over 20 degrees celcius, we would need more fuel to depart due to the runway length and a fuel stop would be required in Milan.
Routing: When flying in certain directions, an aircraft is able to fly at different altitudes. Depending on the routing, an aircraft is required to operate at a certain level. An aircraft will fly faster at higher altitudes because lower air density makes it easier for them to do so. When flying from the South (E.g. Spain) to the North (E.g. UK), the aircraft will be at a lower altitude affecting the fuel performance and a fuel stop would be required to reach the final destination.