"When and how can a VFR pilot get a special VFR clearance to arrive at or depart a towered airport?" - Anonymous
“A Special VFR Clearance is an ATC authorization for a VFR aircraft to operate in weather that is less than the basic VFR minima. Basic VFR minima are 1,000-foot ceiling and 3 miles visibility. If the reported weather is less, a pilot can request a Special VFR Clearance.
Several conditions must be met; the visibility must be at least one statue mile, the pilot must remain clear of clouds and at night, the pilot must be instrument rated in an IFR capable aircraft.
The request for clearance can be made with either the Control Tower or Approach Control. It is not necessary to file a complete flight plan but the request should have enough detail to allow ATC to fit the flight into the traffic flow. The clearance will usually contain an at or below altitude to separate the SVFR from IFR traffic and still allow the pilot to maneuver clear of clouds.
SVFR can be an effective tool to allow a VFR aircraft to land or depart in conditions below basic VFR. On those days when there is a stubborn 900 foot cloud deck over the airport but you can see into the next county, a SVFR clearance can let you fly towards better weather. That is the most important point about a SVFR – always make sure you are flying towards better weather or have an escape plan. You never want to be trapped in marginal conditions.”