The FAA regulation describing SVFR weather minima for both fixed-wing aircraft and helicopters is FAR 91.157. Here are the highlights:
1. SVFR may only be conducted below 10,000 feet msl and to the surface within the lateral boundaries of an airport’s controlled airspace. An ATC clearance is required.
2. SVFR weather minima, except for helicopters (see the sidebar on page 19), are one statute mile of flight visibility and clear of clouds. “Flight visibility” includes the visibility from a Part 91 aircraft in takeoff position at a satellite airport lacking weather reporting capabilities.
3. SVFR is not available at night (or in Alaska, when the sun is six degrees or more below the horizon) unless both the pilot and aircraft meet IFR requirements.
4. Fixed-wing SVFR is not available at several Class B airports. (See FAR 91, Appendix D, Section 3 for a list, or use a VFR terminal chart to determine if the data for the airport in question includes the words “NO SVFR.”) Importantly, helicopter SVFR is allowed when “NO SVFR” is charted.
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… Best regard