The full moon has always been associated with ghost, paranormal, supernatural etc. Many believe that during the full moon is the best time to hunt ghost. Contrary to popular beliefs the actual best time to conduct an investigation would be two to three days before or after the full and new moon. During the new moon there is hardly any light whatsoever. This makes for an extremely dark investigation.
Solar flares from the Sun can cause effects on the geomagnetic field. This can cause geomagnetic storms. One theory is that spirits can draw energy from this magnetic field. The storms can cause high levels of geomagnetic energy which could help out during an investigation. When the geomagnetic energy starts to change paranormal activity may increase.
Some paranormal researchers believe that thunderstorms will help spirits manifest. Thunderstorms cause the air to become ionized which increases the amount of energy that spirits may use to manifest from. I would not recommend investigating outside during a thunderstorm as it could be very dangerous.