Serious Ghost Hunters always check these conditions before an investigation.   The reason for that is these conditions can affect the amount of ghost activity.   Ghosts & spirits draw their energy from many sources.   The Earth's Geomagnetic Field can be a very strong energy source for them. 

During times that Geomagnetic Field is not as strong,  spirits can tap into other normal energy sources, such as batteries & electric appliances.   This can often be witnessed as a sudden drain of  batteries, an appliance acting  oddly or malfunctioning, or lights flickering.    Energy can also be harvested from the air: This often causes sudden spikes or drops in temperature in isolated areas.   Spirits are often stronger near sources of water, & at times they can even cause faucets & toilets to run on their own.    When humidity is low -think static electricity!   This is another energy source.   Energy can also be tapped from living beings, especially if they are afraid or angry.   People can feel literally 'drained' after a spirit encounter. 

The Geomagnetic Field changes under different conditions.  Solar Flare activity causes surges in the Geomagnetic Field.    M class and X class Solar Flares can cause Geomagnetic Storms within days of their occurrence.    This surplus geomagnetic energy has a history of increasing the amount of ghost activity.    M class and X class flares are  stronger sources of energy for spirit manifestation,  but any spike in the field helps.     It also important to realize that this increase in the Geomagnetic field can also cause false reading on E.M.F. meters, especially with the more sensitive ones. 

The moon can also make a difference in the amount of energy ghosts & spirits have at their disposal.   The Earth's Geomagnetic Field is always stronger during the full & new moon cycles.    Think of how the moon affects the tides.  (Or some of your fellow human beings!)