Being a radio pirate isn't easy. Going on the air without a license means having to evade the FCC and there's often the need to broadcast "on the run" - moving from location to location, trying to stay clear of the law.
Shortwave pirates face additional challenges that unlicensed FM stations do not - the range of an FM station's signal is much, much shorter. FM signals travel in a "line-of-sight" manner - the only receivers that can pick up an FM signal are those that can "see" the transmitting antenna. It takes thousands of watts to cover a few dozen miles on the FM band.
Due to the tendency of shortwave signals to propagate through the atmosphere, it is possible for a 10-watt signal on the shortwave band to travel hundreds - even thousands - of miles.
You can find a good example of this kind of signal "skip" by tuning through the AM band at night - often times you can pick up stations from distant parts of the continent if the conditions are right.
However, because of the longer range shortwave allows, it gives the FCC better chances to find and locate pirates that operate on the band. The FCC has a network of automated "listening stations" around the United States that it uses to monitor the shortwave bands - once one listening station finds a pirate, others tune in to pin down its location.
Therefore, shortwave pirates do a lot more broadcasting on the run than FM stations. Many broadcasters even tape their shows in advance, haul their transmitting gear to the most remote location possible, start the tape, and leave.
Warmer weather makes it more conducive to use this tactic - and the activity among shortwave pirate stations should increase appropriately.
Unfortunately, because the range of shortwave broadcasts is so dependent on the state of the atmosphere, the summer months also tend to work against the shortwave pirate. Solar energy interacting with the atmosphere causes seasonal changes to those bands. The more solar energy, the less chance for "skip."
Thunderstorms are also a big enemy: with their large discharges of electricity (lightning), they can wreak havoc on the reception of shortwave signals (you can hear this on the AM band, too, when it's stormy in your area).
Because of this, some of the frequencies shortwave pirates use during the winter months tend to lose their effectiveness during the summer. 6955 kHz has been a popular frequency for many pirates for more than two years now; expect that to continue, although the "noise level" in this part of the band may increase.
Where else to listen? 7415 kHz, an old standby, may see some more activity, as well as 7450 kHz. Others may consider moving farther up the band - between 13900-13950 kHz and around 15050 kHz are good bets, because the increased solar radiation doesn't degrade propagation potential as much up there.
Remember to tune around these target frequencies - shortwave pirates will sometimes select frequencies slightly higher or lower, and shortwave transmitters are more prone to "drift" slightly up or down the dial.