This is due to the fact that there is little food (flying insects) for them to eat. If they stayed awake in a warm home, without hibernating, then they would starve. We can begin the bat exclusion process during the winter months.
Mean free path of molecules in a gas. The molecules in a gas are assumed to be in continuous random motion making collisions with each other and with the walls of the container. The mean distance that the molecules travel between one collision and the next is called the mean free path of the molecules.
For ionization of a diatomic molecule the only nuclear coordinate is the bond length. The lower curve is the potential energy curve of the neutral molecule, and the upper curve is for the positive ion with a longer bond length. The blue arrow is vertical ionization, here from the ground state of the molecule to the v=2 level of the ion.