The enhanced reception distances you are seeing is the result of tropospheric ducting...it happens at all RF frequencies and results in varying conditions for both transmission and reception of rf. When you mentioned "something in the air" you were in fact right !!!
We have the same thing happen in ham radio with our repeater systems (Vhf and UHF) where all of a sudden we start hearing stations a long way away and getting interference from stations on the same freq that we don't normally hear....good example is all of a sudden somebody on one of the high hills or mountains around here (PHNL) can work somebody in California direct with a handheld radio....a distance of some 2300 miles...it might only happen for a few minutes...might last for an hour.
The frequency that ADS B operates (1GHZ) is less prone than some of the lower freqs but it does still happen resulting in reception over much larger distances.....then you look again 5 or 10 mins later and its gone....maybe even a few minutes later.... Tends to happen more at certain times of the year. Also happens with TV signals if you have an off air antenna rather than cable...all of a sudden you start seeing stations from a long way away that normally you don't see.
http://en.wikipedia.org/wiki/Troposp...pheric_ducting
This is a segment from the page below provided by a radio amateur working with ADS B propagation etc
Quote:
"In the spirit of Amateur Radio (well, one of the spirits) I decided I needed a bigger antenna, with more gain. I constructed a 16-element coaxial Co-linear (~12-dBd?), and mounted it 3 metres above the top of my 144 MHz array – about 14.5 metres above ground level. After a short (3.5 metre) run of SCF12-50 heliax, a VK5EME kit dual-stage Pre-amplifier (MGF1302 followed by an ERA-1) provides a boost before the signal hits the run of LDF4-50 to the receiver. Maximum range observed reliably so far is 225 nm (416.7km), with aircraft flying at high altitude (38,000-40,000 feet), though instances of greater distance have been observed as we enter late spring and come under the influence of tropospheric “ducting”.
I decided that wasn’t enough, so I built a 28-element T-boom Yagi. This has a calculated gain of 16.7-dBd. Admittedly I lose the omni-directionality of the co-linear, but I was more interested in maximizing my forward view".
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http://www.qsl.net/vk3bjm/ads-b.htm
Some good charts directly related to ADS B here
http://my.pinkfroot.com/forum/topics...ge-direct-adsb
Sorry to be so Geeky
tried to keep the explanation short but its quite a complicated and involved topic.
Regards
MS