What is occurring is a phenomenon called “ducting”.  This is something that is more common with higher frequencies, but can also occur at 500KHz (where we listen for lightning).  Ducting occurs when there is a thunderstorm or hurricane that creates a channel of moist warm air between cooler air above and below due to upward convection.  It is called ducting because it resembles your heating ducts in your home and directs the signal similar to the way your heating ducts direct the warm air in your home.  The diagram below shows this duct.  When the lightning signal occurs, it becomes trapped in this duct and bounces between the top layer and bottom layer until it can escape and reach your SFD1000 (much like the air coming out of a vent in your home). 

Since our thunderstorms are usually rather small, we can see this type of propagation for many miles, but typically less than 100 miles.  This condition is quite rare at 500KHz, so it won’t be something you run into on a regular basis.  We have only seen this once where we could verify it was ducting in four years of testing.

Even more rare at this frequency is ducting that occurs due to inversion layers associated with pollution, where warm air is trapped between colder air above and below forming a temperature inversion.

A temperature inversion is what causes a mirage as you drive down the road on a hot day. The temperature inversion creates a mirror that reflects the sky, which looks like water. A duct is the same effect with a “mirror” on the top and a “mirror” on the bottom.