For a complete explanation of special VFR procedures, see 14 CFR 91.157 and Aeronautical Information Manual (AIM) chapter 4 section 4. This page will only give an overview of the procedures.
One of the first rules you should know about special VFR clearances is that we cannot suggest that you ask for one. You, the pilot, have to initiate the request. If you request to enter or depart a surface area that is IFR, we will advise you that an ATC clearance is required, but it is up to you to then request a special VFR clearance if that is what you want.
The next thing you should know is that you must have at least 1 Statute Mile (SM) ground visibility officially reported at the airport in order to land or depart the airport unless you are flying a helicopter. In most cases there will be an official weather report available, but if not, then all you need is 1 SM flight visibility. Also if you are not landing or departing an airport that reports weather, for exampleyou are flying through the class E surface area but are not landing anywhere in the surface area, or you are landing at a location in the surface area other than an airport where weather is reported, then all you need is 1 SM flight visibility.
For example, let's say you are flying a float plane from Whitehorse, Yukon to Yarger Lake which is inside the Northway, AK class E surface area. You monitor the Northway ASOS and it is reporting 3/4 SM visibility and ceiling 600 overcast. You call Northway FSS and report that you are inbound from the southeast, have 1 SM flight visibility and are requesting a Special VFR clearance to land at Yarger Lake. The clearance can be issued, but you must maintain at least 1 SM flight visibility at all times. However, in the same situation, if you requested a Special VFR clearance to land at Northway, we would have to say that the Northway visibility is 3/4 SM, A-T-C unable to issue entry clearance unless an emergency exists.
In some cases there will be personnel on the airport reporting the weather. In other cases there will only be an automated system on the field, in which case it will be the pilot's responsibility to monitor the automated broadcast to see if 1 SM or more is being reported. In those cases where there is only an automated system, unless the controller issuing the clearance is located on the field, he or she will only have access to the most recent weather information that has been transmitted into the national database, which very well may be different from the current 1 minute weather observation, which is what matters.
For example, let say you want a special VFR clearance to land at Tanana airport. Fairbanks FSS is the only ATC facility that has communications in that area so you would call us for a clearance. First you should monitor the Tanana ASOS broadcast on frequency 135.1 and let's say they are reporting 1 1/2 SM visibility. You then call Fairbanks Radio and advise us that the current weather at Tanana is visibility 1 1/2 SM and ceiling 800 overcast. We may check the Tanana weather from our system and lets say we show 3/4 SM visibility, but that is irrelevant. We can still issue the clearance because you have told us that the visibility is now 1 1/2 SM.
Always keep in mind that if you have an emergency situation, declare an emergency and we can issue a clearance no matter how low the visibility is. Don't fly around until you run out of fuel or fly into some mountain because of low visibility.