I think I did not make some aspects of skywave clear enough so this article should clarify some things. It is going to get a little more technical than I had hoped, but I fear that is unavoidable in order to make some things clearer. I do not think this article is overly complex and should be understandable by most everyone.
There is a layer of the atmosphere that contains charged particles. This layer is called the ionosphere. The sun charges the particles in the ionosphere, when it is daylight they become charged, when the sun sets they start to lose their charge and continue to lose their charge until sunrise the next day.
The ionosphere is not just one layer, it is multiple layers which have their own properties. They all work basically the same way. The layers we will concern ourselves with are the D E and F layers (F actually has a couple layers itself).
When you transmit an HF signal there is a frequency range that will work with skywave. Too high and the signal will pass through the ionosphere, too low and the D layer may absorb it.
Because the D layer vanishes at night, the lowest usable frequency (LUF) will decrease at sundown and continue to decrease until it has gone. At sunrise the D layer will start to appear and get stronger throughout the day.
The E layer changes from day to night as well. At night the maximum usable frequency (MUF) will be lower than it is during the day. This means that some higher frequency signals can penetrate the E layer and make it to the F layer. This can alter the distance that a skywave transmission can make.
The ionosphere changes all the time, the output of the sun is not constant. While you can generally rely on lower frequencies working better at night, and can rely on longer range contacts after dark, exactly which ones, when and how much noise will be present is something you cannot count on.
The green line represents a frequency that is higher than the MUF, it passes through the ionosphere. This will generally be any frequency over 30MHz (sometimes 50MHz is below the MUF though).
The red line represents a frequency that is lower than the LUF, it is absorbed by the ionosphere. As previously stated, the D layer will disappear at night.
The blue line represents a frequency that is above the MUF for the E later, but below the MUF of the F layer, this offers the best range under normal circumstances.
The purple line represents a frequency that is between the LUF and MUF for the E layer. For added bonus I showed it reflecting off the ground and going back to the ionosphere.
As you can see the radiation angle of the signal will alter the range that you get, in general the closer to straight up and down the radio signal goes the shorter the range, this is called NVIS or Near Vertical Incidence Skywave. The lower the angle the further the range. As stated in a previous part of this crash course you can alter the radiation angle by raising or lowering the antenna. Of course the type of antenna is important, not all antennas work this way, but that is a different discussion.
If you want to talk to people that are nearby you can use a NVIS antenna. The typical range for NVIS is up to 500-1000 miles. If you want to talk to people further away you will want to use an antenna that has a lower radiation angle which will cause it to bounce off the ionosphere at a different angle. You can control the skip by altering the radiation angle of the antenna, generally by raising or lowering it. You can even do this from the same antenna if you rigged it in a way that lets you raise and lower it. This can be useful if you want to talk to a specific person.
A good way to see what frequencies are usable is to listen for a beacon that is near where you want to talk to. If you can hear the beacon you can usually transmit to that location as well. You can also just tune to the frequency and ask if anyone can hear you.