Here is a list of conditions for ghost hunting including: moon phase, weather forecast, and solar activity. These are all commonly believed to be factors which help predict spiritual activity:
The theory about how the moon phase affects paranormal activity states that spiritual activity peaks during the full moon phase, new moon phase, first quarter, and last quarter phases. This is based on the Solunar calendar which predicts the best times for hunting and fishing. Therefore, it is believed that if the moon affects the feeding habits of animals, it may affect spiritual activity as well. The common consensus states that paranormal activity peaks on the full moon; however, we seem to find that we get more activity on the new moon.
Solar activity is not a direct factor which influences paranormal activity because when a solar flare erupts on the sun, the expelled particles, which are electromagnetically charged, must travel an average of 92.96 Million miles until it reaches earth. Therefore, it could take a couple of days for these charged particles to reach earth and affect the natural magnetic field of the earth. Hence, a solar activity rating of "X-Class Flare", for example, might not affect us on earth for a day or two, and thus it would have no effects for the spirits to draw from until it reaches the earth.
This is where the "Geomagnetic Field" indicator comes in. A Geomagnetic Field reading of "quiet" means that the geomagnetic field is within normal parameters. A geomagnetic field reading of "unsettled" means that there is some disturbance within the earth's magnetic field which could mean an increase in paranormal activity. Finally, a reading of "storm" means that the earth's magnetic field is experiencing a lot of disruption and there is an excessive amount of electromagnetically charged particles in the atmosphere which may cause interference with radio transmission, power lines, and electronics. In theory, this would be a "buffet" of energy for the spirits to feed off of and paranormal activity may be at a peak.
We do believe that spirits are affected by weather conditions. Based on experience, active times for spiritual activity occur during cloudy, overcast conditions, windy conditions, and the best time is when lightning is occurring.