Bolts From the Blue
The photos below are classic examples of 'Bolts from the Blue". A "Bolt from the Blue" is a cloud to ground lightning flash which typically comes out of the back side of the thunderstorm cloud, travels a relatively large distance in clear air away from the storm cloud, and then angles down and strikes the ground. These lightning flashes have been documented to travel more than 25 miles away from the thunderstorm cloud (see the "LDAR" discussion below). "Bolt from the Blue" lightning flashes are a very dangerous type of cloud to ground lightning flash, as they "appear" to come out of the clear sky.
This is why it is still dangerous to be outside when thunderstorms are in the region, the lightning can, and does, strike many miles away from the thunderstorm cloud itself. This is why it is a good idea to wait 30 minutes or more after the rain ends before resuming outdoor activities.
Special thanks to Robert Prentice and Al Moller for allowing me to use their photos.
Bolts from the Blue as seen by lightning detection devices
The image below is a "bolt from the blue" detected by a lightning detection system which observes lightning in 3 dimensions. (This 3D lightning detection system is called "LDAR"; see below).
The colored dots above represent the lightning channel of a cloud to ground lighning flash which struck in east central Florida. Note how this flash travelled to the east 40 KILOMETERS (~25 miles) in less than 1 second, and then struck the ground! (note distance on horizontal axis in upper left box, time is upper right box). This flash struck very close to the National Weather Service office in Melbourne, Florida. At the time of the flash, the skies where mostly sunny at Melbourne. The storm itself was located in northeastern Osceola county, about 35 kilometers to the west of Melbourne (Note: ~1.6 Kilometers = 1 Mile).
Below are two other examples of "Bolt from the Blue". The first image showed a bolt from the blue which occurred near the Kennedy Space Center, Florida. This flash is overlayed on radar data which was collected near the time of the flash. Note how the flash travels away from the radar reflectivity into clear air. This flash travelled about 6 kilometeres in clear air before hitting the ground. The small white "x" marks the location of where the cloud to ground lightning flash hit the ground.
The second flash is similar to the the KSC data, but was collected from a storm which formed in NE Colorado during the STEPS experiment.
Lightning and radar data for 11 July 2000 for a storm on the Colorado and Kansas border during the STEPS experiment. Image courtesy of Dr Bill Rison of New Mexico Tech and Daniel W. Breed Project Scientist at NCAR's Research Applications Laboratory.
Bolts from the Blue are obviously very dangerous to the general public as they can strike many miles away from the thunderstorm. More information about LDAR and "Bolts from the Blue" can be found here.