This video was recently posted on YouTube by Steve Nichols, G0KYA and it gives a good introduction to understanding the basics of propagation on the HF bands.
Here are some of the bullet points...
- Solar Flux Index
- Sunspot Number (which is not the number of actual sunspots!)
- SSN - Smooth Sunspot Number
- MUF - Maximum Usable Frequency
- Solar Flares
- Coronal Mass Ejections
- A & K Index
- Critical Frequency
- LUF, MUF & FOT
- VOACAP online
- Seasons & MUF
1) VOACAP website... https://www.voacap.com/
2) G0KYA's website... http://g0kya.blogspot.com/