Just to be literal about it, that was almost certainly sporadic-E skip.
Of course, these signals were almost certainly from Mexico. (there's no full-power analog left in the U.S.; I count 13 low-powered analog stations left in Utah, but they all relay Salt Lake network affiliates (usually KUTV) in English.)
Due to terrain and the dry climate, tropo is fairly rare in the interior West. I would be VERY VERY surprised to hear of tropo from Salt Lake City to any point outside Utah, with the possible exception of southern Idaho.
A few rules of thumb to tell the difference:
Tropo:
- Can affect any channel (including FM radio). Often better at UHF than at VHF.
- No minimum distance. Tropo can strengthen semi-local (~30mi.) signals.
- Maximum distance usually on the order of 300 miles. Long-haul openings do happen but are quite rare.
(exception: over warm water, longer paths are possible. New Orleans<=>Tampa, etc.)
- More likely in the warm season, and with stable weather.
E-skip:
- DOES NOT HAPPEN on UHF.
- EXTREMELY rare on channels 7-13.
- ALWAYS starts at channel 2* and, in more intense openings, moves up through 3, 4, 5, 6, and into FM.
* there may however be no channel 2 station in the area the opening is reaching..
- Has a MINIMUM distance on the order of 500 miles. Rare shorter openings but nothing less than 300 miles. If you're receiving DX from 150 miles or so, it's NOT E-skip.
- Maximum distance on the order of 1,400 miles.
- Typical distance on the order of 1,000 miles.
If you have 1,000-mile +/- DX, it is almost CERTAINLY E-skip.
- More likely from mid-May through early July. Not really tied to weather, at least not at the endpoints.
