Lightning doesn't always strike the highest object

[Dan Robinson]

Not that we need more evidence, but this is probably one of the best examples I have seen illustrating the fact:

http://i51.photobucket.com/albums/f385/Can...er8-2-06022.jpg

Taken by a photographer on skyscraperpage.com.

 

[Joel Wright]

Wow. That's a pretty sweet shot there.

I remember seeing a picture in our local paper of a CG going right into the Mississippi River with the I-74 bridge about 100 yards away unscathed.

 

[Bill Schintler]

Well, there's often a simple explanation for this phenomena… In the case of this photo, the Sears Tower has dissipative lightning rods along the periphery of its antenna towers, which serve to keep charges drained from top of the structure, usually preventing direct lightning strikes. Meanwhile, local field strengths at surface structures may build to very high levels on structures a few hundred feed away – initiating a stroke at a much lower spot – which is what is seen here.

- bill

 

[Dan Robinson]

Lightning dissipation/elimination arrays and units have been found to be ineffective and the science behind them dubious at best. Most installations have been removed after receiving numerous direct hits (IE, such as systems at the Tampa and Orlando airports).

http://www.lightningsafety.com/nlsi_lhm/charge_transfer.html

If the one on the Sears Tower is still installed, it too is taking numerous direct hits on a regular basis.

The reason CGs don't always hit the highest object is because the descending stepped leader of the CG stroke is 'blind' to objects on the ground. Localized influence of ground-based objects doesn't occur until the stepped leader is very close to the ground, usually less than 100 feet. For most of a CG's trajectory, the earth in general is the only influence on the stepped leader path. The spot that is hit is always inside the very localized area that the stepped leader is already descending in. Anything outside of the narrow trajectory of the stepped leader (in this case, not even the Sears Tower) will not influence its path.

From the article linked above:

Drs. Rison and Krehbiel find that the negative cloud-to-ground lightning here in New Mexico, in western Kansas and over central Oklahoma typically starts at altitudes between 5 km (about 16,400 feet) and 6 km (about 20,000 feet) above sea level at heights where it is unaffected by objects at the surface of the Earth. It is now well established that surface conditions have little to do with the initiation of cloud-to-ground lightning discharges, high in thunderclouds.[/b]

and

In the years since there has been many incidents in which lightning has struck these "dissipation arrays". There are photographs of strikes to a tall tower at Eglin Air Force Base, to a 500 foot meteorological tower at Kennedy Space Center and to a "lightning eliminator" at Langmuir Laboratory here in New Mexico. [/b]