As the aircraft began taxiing to the departure runway, the rain intensified and visibility at the airport fell below the three mile minimum required for VFR flight. Cheyenne's control tower advised the Cessna about the reduced visibility and that the "field is IFR". In general, when an airport is officially IFR (normally because of reduced visibility or low cloud ceiling), only IFR or Special VFR operations are allowed. Reid then requested and received from the control tower a Special VFR clearance to allow him to exit the airport's control zone visually, despite the reduced visibility.