Weather plays a huge factor in any investigation.  In the Summer, photographs taken when the camera is pointed towards the sun can create "orbs" in pictures giving you a false positive.  Other weather conditions which can produce orbs are rain, snow, sleet, fog, hail and even high pollen counts or dusty, dry weather on a windy day.

Winter conditions can cause misty apparitions to form in your pictures.  When you breath out on a cold day and the mixture of warm and cold air creates "fog," this can create misty apparitions in pictures or in video.  Always be sure to hold your breath when taking any photography at a site on cold winter days.

As stated in other places here, those who study the paranormal believe ghosts are comprised of electromagnetic energy and they "feed" from other sources of energy in their surroundings to manifest. During a thunder and lightning storm, there is more energy in the atmosphere; therefore, there is a possibility we can see an increase in paranormal activity.  Another theory also supposes low humidity and high static electricty, such as during winter, increases paranormal activity.

Be sure to note the weather conditions in your notebook prior to conducting any investigation and take into account your surroundings with any results you document.