September is a month of radical improvement in high band conditions. The ionosphere is cooling off from the overheated conditions of summer, bringing higher daytime MUFs. Recent advances in solar activity also mean higher MUFs, and set the stage for our first great fall season of the new cycle. 10 Meters is opening up big time. It's time to kick the CB'ers off the CW band, or at least get 'em to move above 28.3 and pay their 10-10 dues.
The 10 Meter DX season runs from October through about March with reliable openings to Europe almost daily (except in geostorms). Some parts of the world will remain difficult to reach on 10: those involving high latitude paths over the pole. In winter this path is in darkness. Sunset at the north pole is later this month, and the northern darkness will expand throughout the fall. Thus, polar daypaths begin to close just as 10 is opening up. Opportunities to work these paths are limited, and are at their best early in the fall while most of the Arctic still receives daily exposure to sunlight. For this reason, I have always felt that October was the best month for worldwide propagation on 10. To catch the openings at high latitude, get on the band shortly after sunrise - or watch for polar signals as the band closes in the evening.
The 10 Meter season is not at peak as I write this because it is still September - late summer. Summer is the WORST season for 10 Meter work - even a high solar flux doesn't help much. However, the transition from these summer doldrums to spectacular fall conditions in October is very dramatic and occurs mainly in September.
15 Meters has already returned and will continue to improve. The fluxes are high enough now that polar paths should not be a problem. The fall season on 15 is not as interesting as the peak season in late spring, because the spring season involves paths that span both day and night. Nighttime MUFs have begun their decline that will continue until January. 15 does NOT stay open late in the fall, so DX is workable primarily over daytime paths just as it is on 10. Night paths in the southern hemisphere do open, so 15 should be watched for long path opportunities which many DXers are unaware of.
20 is still open all night, so excellent conditions should continue through October. The Antarctic paths are open again, and daily long path openings to southern Asia are back. Worldwide DXing on 20 is almost too easy, but lowband season approaches with declining QRN levels.