Skip is caused by signals bouncing off what are termed "clouds" in the E-Layer of the atmosphere, about 50 - 100 miles above the ground. In continental latitudes, most skip (at least on TV and FM) occurs during the months of May, June, and July, with a peak (in my experience) around equinox, when it is a near-daily experience on TV. During a skip event, the maximum usable frequency or MUF (the highest frequency "bounced" by the clouds) rises progressively, often first identified on CB radio (27 MHz), then affecting the 10-meter ham band (28 MHz), certain highway patrol and other public safety frequencies (35 - 45 MHz), the six-meter ham band (50 MHz), TV channel 2 (54-60 MHz), rising to 6 (82-88 MHz), and finally up through the FM band, first affecting the non-commercial frequencies (88-92 MHz). The MUF of skip rarely rises much above the FM band, but on occasions (usually once or twice a year or so for a given location) it will reach the 2-meter ham band. On very rare occasions (perhaps every 10 years) it will soar past 170 MHz and begin propagating the higher VHF TV channels (7 - 13), and even the 1.25m (220 MHz) ham band.
The distances involved with skip are greatest near the maximum usable frequency. Looking through the 68 stations I have logged on FM via Es propagation, the average distance is 1,069 miles, with the shortest being 798 miles (to Lamar, CO) and the longest 1,382 miles (to Titusville, FL). On TV (44 Es loggings) the average distances is 1,043 miles, with a minimum of 750 miles and a maximum of 1,469. Short E-skip (under 600 miles on TV) is rare and indicates a high MUF.
There's little to predict skip other than the time of the year. In my experience, at least, skip affects the 6-meter band and TV channel 2 almost every day in late June, often twice or thrice daily.
Skip signals are distinguished by heavy amounts of fading, though they very often fade "up" to local quality, sometimes overtaking local FM and TV stations.
Tropo is a completely different phenomenon, residing in the lower atmosphere rather than in the ionosphere. One can basically divide it into 3 common types: troposcatter, tropospheric enhancement, and tropo ducting. One thing that is common to all forms of tropo (as far as I know) is that a much wider range of frequencies are propagated. For the TV and FM DX'er, the UHF TV channels are affected the most, followed by the high VHF channels (7 - 13), the FM band, and finally the low VHF channels.
Troposcatter is an everyday thing. Essentially it's when signals beyond the curvature of the earth (65 miles) come in on a daily basis, often weakly and with fading. Those with better setups (meaning high-end receivers, towers, high-gain antennas) may receive FM signals via troposcatter from well over 300 miles away.
Tropospheric enhancement is a common occurrence, particularly on summer nights. Signals within a 300-mile radius or so, such as those which a well-equipped DX'er might hear weakly via troposcatter, are "enhanced". Lesser setups receive signals they usually don't, often with good, stable quality. On many summer nights I can receive KFMC from Fairmont, MN, 241 miles distant, or watch most of the Minneapolis DTV's. This is probably via tropo enhancement.
Tropo enhancement is associated with a variety of weather phenomena, such as temperature inversions, fog, etc. and is most common in summer, but may occur any time of the year. Often on warm January days I can listen to stations from St. Cloud during my late morning commute. Signals propagated by tropo enhancement tend to be rather stable.
Tropo ducting is the exciting stuff. Essentially "ducts" form in the troposphere, bringing UHF TV stations (as well as VHF and FM radio, to a lesser extent) in from up to 500 - 700 miles distant, and at times farther (especially over coastal paths - for example, it isn't unusual for UHF TV stations in Texas to be seen in Florida, and vice versa). In my experience, tropo ducting tends to be rather fadey, though not nearly as much so as E-Skip. Via tropo ducts I have received UHF TV in Duluth from Springfield, Topeka, Omaha, Indianapolis, Detroit, etc.
