Weak signal operators use several propagation methods to get their signals out further. Line-Of-Sight As the name implies, basically working stations within line-of-sight from your antenna to theirs. A lot of rag chewing goes on with local stations using this method. Tropospheric Refraction or Super-refraction RF behaves a lot like light. We've all noticed that water will "bend" (refract) light a little bit. This occurs to some extent in the atmosphere. Usually the atmosphere bends our radio waves just a little bit, allowing us to make contacts just beyond line-of-sight. Super-refraction occurs during a temperature inversion. An inversion is where the air near the ground is cooled, while the air above it remains warmer. If conditions are calm enough, there becomes a distinct layer between the air masses that can bend radio waves a considerable distance. An excellent example of this is the annual "ducting" that occurs between California and Hawaii, allowing contacts over 4000km. Inversions usually occur under large areas of high pressure, although there are several other weather conditions that cause inversions. As you climb in frequency, you'll notice that refraction has a greater effect. There is very little effect at 50 MHz, when compared to 1.2 Ghz. At 1.2Ghz the effect is quite drastic, with little atmospheric disturbances causing a "mobile" like flutter sometimes. You'll notice the change in conditions while you're talking on 1.2 Ghz, causing conditions to vary from transmission to transmission. I've even had difficulties working a mountain station from a station down in a valley.  The mountain based station's transmissions hit a duct and were refracted to an altitude above my antennas while my transmissions were being ducted just below his.  At least that's our theory on the situation, we had excellent communications earlier that day, but then some ducting started working between stations at lower altitudes that appeared to interfere with us. Sporadic-E or "E" Skip This type of propagation refers to the "E" layer in ionosphere. "E" openings occur approximately 10 times more often on 50 MHz than on 144 MHz, and simply very rare on 220 MHz. Openings in the "E" layer refract your signal back down to earth. The higher in frequency you go, the more likely your signal will just go straight through and into space. The "E" layer is somewhere around 100km above the earth. "Skip" distances can vary from 400 miles to 1400 miles, with "multiple hop" increased distances even more. As the name implies, it is sporadic. Hams have tried to predict "E" openings with varying degrees of success for quite some time. An easy way to predict a 6-Meter "E" opening is to turn on your 6-Meter radio and listen, you'll hear it open and thus predict quite quickly. Seriously though, some people will listen for short "skip" on 10 and 11-meters, and watch it grow in strength, and hope 6-Meters opens up when it gets real strong. Others will listen to the calling frequency for a while, then tune to the beacon frequencies and scan for any beacons coming in, then go back to listening to the calling frequency. I've noticed a correlation between the occurrence of "E" openings, and the times I can't be at my radio. So if we all turn off our 6-Meter radios all at once, 6-meters will open up(is the refrigerator light on when you close the door???). Auroral-E or Aurora Aurora is one heck of a subject. One could write a book on the cause and effect of aurora on communications. I'll just try a couple of brief paragraphs. I'd recommend you try and read a few books if you're really interested in this subject. There are a couple of QST articles in "Beyond Line Of Sight". There are two types of propagation associated with Aurora Borealis(Australis in southern hemisphere), Aurora and Aurora-E. Both involve ionization of the upper atmosphere which allow for the bending of radio waves over distances as far as 800 miles. With aurora there is a great deal of fast QSB, or flutter of signal strength. This QSB gets faster the higher in frequency you go. Because of the QSB, CW is the most reliable mode of communications. Phone(voice) tends to get garbled, and sound "inverted", making phone contacts nearly impossible. CW gets turned into a "growl" or "hiss" that your BFO just can't keep up with. The further north(our south in the southern hemisphere) you are, the better chance you have of working aurora. The other propagation associated the aurora is auroral-E. Auroral-E is a lot like sporadic-E, with strong, clean signals. Auroral-E usually occurs during and after very intense Aurora openings. F.A.I. - Field Aligned Irregularity This type of propagation is a lot like Sporadic-E. It's probably been confused as being Sporadic-E on several occasions. FAI occurs towards the end and just after an E-layer opening, up to an hour after even. FAI is characterized by the fact that your beam heading just doesn't make any sense. To better understand this, get out a map of the U.S. Now, picture your in Atlanta Georga working a station in Minneapolis Minnesota on 6-Meters via an E-opening. The opening starts to fade away and suddenly you here a station somewhere northeast of Denver Colorado with QSB. You think, "the cloud has started moving west now". You move your beam, to the heading for the general area of the stations heard, and they disappeared. Move your beam back to the heading for Minneapolis, and you hear the Denver station again!? Lemme try to explain it now. While you were working the station in Minn., you're signal was being bent by an area of ionization in the "E" layer above St. Louis, and redirected back down towards the earth. Just as the opening started fading, an FAI developed in the region of the ionization you were just using. Not only did this FAI bend your signal back down to the earth, it also bend it a couple degrees off of its original heading. I just used these three cities as an example, because they were ones that stood out on the map next to me, also because they made a little sense with the beam headings. Beam headings during FAI do actually make sense. Picture the magnetic lines coming down across the area where the "E" cloud is. If Atlanta were say... 40 Degrees off of one of these line intersections, then Denver would be -40 Degrees off of the same intersection. My numbers may be off, but hopefully you get the general idea. Picture the FAI as a mirror at the region of E-Layer ionization, with the magnetic lines of the earth perpendicular to it's surface. Even looking directly into this mirror would cause you to see behind yourself (backscatter?). Now then, how often does this occur? FAI tends to be strongest during the summer Sporadic-E season. Happening, on 50 MHz, during about half of the "E" openings. With 144 MHz FAI occuring one third as often, during 50 MHz "E" openings. FAI tends to have a "flutter" QSB. Though, not as strong of a flutter as Aurora signals have. FAI openings tend to be weak, and requiring CW. Although some very strong FAI's can happen that will allow voice contacts from time to time. FAI occurs on 50 MHz and 144 MHz, with some thinking it might be possible on 432 MHz with EME sized stations.