Kids Health from Nemours - Thunderstorms
Parents Magazine article
Teach them the science...
Judging Lightning Distance by Thunder
Make a 'thunder and lightning kit' that may include flashlights, things to build a fort, special glow sticks, art, etc. Have the kids help decide what goes in it. Only get it out when there is thunder and lightning. My son is a Star Wars fan so calling him a Storm Trooper helped!
Have your child do creative art to help express his fears, free painting at an easel or on large paper taped to the wall is very therapeutic! Some kids like to talk about why they used certain colors or how painting made them feel. Putting together a social story about what is happening and what you are going to do when it happens is also a good strategy.
Try visualization techniques... have your child who is fearful close his eyes and pretend to put all his bad/scary feelings into a cloud, then take a deep breath and visualize blowing the cloud away. Deep breathing is calming.
Make sure to provide physical contact if your child wants it, even if you think they're too old to be scared of storms!