I would like to know the difference between marginal VFR (MVFR) and Special VFR (SVFR), particularly when flying in the US airspace. What does each of them mean exactly?
Straight out of the Aviation Weather Services advisory circular (AC 00-45G with changes 1 and 2):
Marginal Visual Flight Rules (MVFR) indicated on the Weather Depiction Chart represents ceiling 1,000 to 3,000 feet and/or visibility 3 to 5 statute miles and VFR operations can take place. MVFR areas are outlined with a solid line, but the area is not shaded. MVFR areas are typically shaded blue in colorized versions of the chart.
For all regulatory purposes, this has no rule changes from VFR. I believe the reason it exists is for pilots to more easily identify areas where the weather conditions are approaching IMC.
Special VFR in the US
Special VFR allows a pilot to fly within the lateral boundaries of controlled airspace to the ground with at least 1SM visibility and clear of clouds.
Special VFR is something that can be granted by ATC upon request by a pilot. It may only be granted within the lateral boundaries of controlled airspace that goes to the ground.
If the special VFR request is being made at night, the pilot and airplane must be IFR legal.
JO 7110.65Y Section Regarding Special VFR
There is no requirement that the weather conditions being reported are below basic VFR.
MVFR is an advisory term.
Special VFR is an, on request by pilot only, "required weather minimums" change that may be approved by ATC.
For the purposes of special VFR in the US, here is the FAA regulation. (14 CFR 91.157)
4$\begingroup$ Special VFR exists across the world: skybrary.aero/index.php/Special_VFR. I've never heard "marginal VFR" being used as an official term, though, so I'm guessing that's FAA specific. $\endgroup$ May 27, 2016 at 17:25
$\begingroup$ Just curious, where exactly in the FARs does it say that the airport must be reporting IFR conditions at the surface for Special VFR to be granted? I'm imagining a case where the conditions are just barely above IFR-- say, there is an overcast layer at 1100' AGL-- and a pilot wants to request Special VFR as a precaution in case the conditions deteriorate while he is still in the airspace in question. $\endgroup$ Dec 4, 2020 at 14:01
$\begingroup$ Also, there seems to be some ambiguity as to whether not Special VFR may be granted in E4 Class-E-to-Surface "extensions". For more, see related answer aviation.stackexchange.com/a/75022/34686, from the beginning of the answer through the end of the quote "Our opinion is that E4 airspace is not..." $\endgroup$ Dec 4, 2020 at 14:02
$\begingroup$ @quietflyer I think the matter of class E is referring to those little squared-off pieces of class E for the purpose of protecting instrument approaches that step outside of the primary airspace ring for the airport. $\endgroup$ Dec 5, 2020 at 3:57
1$\begingroup$ From an ATC perspective, MVFR conditions (ceiling 5000 or below, visibility 5SM or below) are one of the triggers for soliciting PIREPs. But that isn't a regulatory thing, just part of our orders. $\endgroup$ Mar 2, 2021 at 0:33