My question is are we supposed to treat thunder (loud but no visual) the same as lightning?
Lightning causes thunder so when you hear thunder lightning must be present. We just had this discussion tonight at our association meeting. We were told that
any sighting of lightning or sound of thunder, regardless of how far away we think it may be, should result in the field being cleared. Our state FED association requires that we remain off the fields for 30 minutes from the last flash of lightning or rumble of thunder. Lightning can travel sideways for up to 10 miles. Even when the sky looks blue and clear. At least 10% of lightning occurs without visible clouds overhead in the sky.
http://www.lightningsafety.noaa.gov/outdoors.htm