requirements and certification required of the pilot, and increased equipment requirements for the aircraft. Additionally, an IFR flight plan must usually be filed in advance. For efficiency of operations, some ATC operations will routinely provide "pop-up" IFR clearances for aircraft operating VFR, but that are arriving at an airport that does not meet VMC requirements. For example, in the United States, California's Oakland (KOAK), Monterey (KMRY) and Santa Ana (KSNA) airports routinely grant temporary IFR clearance when a low coastal overcast forces instrument approaches, while the rest of the state is still under visual flight rules. For pilots without an instrument rating who cannot legally fly by instrument flight rules, the restrictions of VMC minima can be troublesome in locations where weather conditions can change suddenly and unexpectedly or when weather events are highly localized. For instance, a small cloud forming over the airport at less than 1000 feet technically requires the airport to allow only IFR flights using instrument approaches/departures. A VFR flight intending to land there would normally be denied clearance, and would either have to divert to another field with VMC, or declare an emergency and override the denial of clearance, which can prompt an inquiry and possibly result in adverse consequences for the pilot. To avoid these scenarios, VFR flights intending to land at or take off from an airport experiencing localized conditions marginally below VMC minima may request Special VFR clearance from the tower. SVFR flight is only allowed while within the portion of an airport's controlled airspace (class B, C, or D) that extends to the surface, and it must be explicitly requested by the pilot and granted by the tower (some airports, primarily large Class B facilities, do not allow SVFR operation in their airspace at all). Visibility on the ground must still be greater than one statute mile, but most other VMC minima such as ceiling are waived. The pilot is required to maintain VFR separation distances from other aircraft and, by requesting SVFR, asserts that they can do so despite the marginal conditions. For departing flights, the pilot must be flying in VMC by the time they leave the surface footprint of the controlled airspace (otherwise they would be flying VFR in IMC which is illegal). VFR flight is not allowed in airspace known as class A, regardless of the meteorological conditions except after failure of two way radio communications or