The daytime problem is the D-layer. It absorbs lower frequency HF signals (often well into 11m) and prevents them from reaching the E and F layers where they would normally bounce back to Earth at some distance. The D layer is awake during the day and gobbles up most of the signals lower than 20MHz to the point they never come back down.
The nighttime problem is that the sun isn't there to make an E layer (a very unappreciated layer) or the two distinctive F layers (that combine into one F layer at night), so signals that do hit them don't refract as well as they would during the day. It becomes a balance between low bands and high bands as the sun rises and falls.
VHF, on the other hand, cares very little about the ionosphere (ie, the sun) and cares more about local weather conditions such as temperature inversions common to the troposphere.
The magic number of the
day moment is the MUF. Maximum usable frequency. And that changes from hour to hour. Depending on the ionospheric conditions, there is a frequency and angle at which the ionosphere cannot refract it back to earth due to insufficient ionization.
The reason 11m goes to shit at night is because the ionosphere cannot bend the signal back towards Earth enough and the signal is lost to space (hence the defined "skip-zone" with no signals at all, like the 11m guys in the state next to you). Then there is the gray line
the time when the sun is setting but still illuminating the upper ionosphere enough to give you the higher E and F layers, but no D layer to absorb your signals... Fun times!
Part of getting licensed is the pleasure of discovering the intracacies of it all and learning how to make a specific contact in a specific set of conditions. Ham operators enjoy learning these intracacies and making use of them. This is why they have awards for working all countries or all states. They don't tell you what band to use, thats for you to figure out!
And don't assume all the fun is limited to HF, the troposphere can do some cool stuff at VHF too, and meteors and airliners can do amazing things at UHF and up! Its all about having fun learning how to use what mother nature has to offer!
