The second weather bar is the "forcasted" weather.
For example, you could set the top 2 bars for sunny (Top top Slider to the far right) and no fog (Bottom Top Slider to the far left). Then if you wanted it to rain and become foggy after a certain amount of time, you would set the bottom top slider to the far left, and the bottom bottom slider to the right accordingly. The farther to the right the Bottom slider(both top and bottom) is, the more fog you will have, in game and forcasted.
Hope this helps.
--Softball