  Far Ultraviolet radiation has been recognized as the main source of heating
of the neutral interstellar gas, and, as a consequence, it determines whether
the thermal balance of the neutral gas results in cold ($T\sim 50 - 100 K$)
clouds (CNM), warm ($T \sim 10^4 K$) clouds (WNM), or a combination of the two.
High FUV fields convert the neutral gas to WNM, while low fields result in CNM.
The sources of FUV radiation are the short-lived massive stars that generally
originate in associations that form in Giant Molecular Clouds present in the
galactic disk. Using McKee & Williams' (1997) distribution of birthrates for OB
associations in the Galaxy, we determine the expected behavior of the
time-dependent FUV field for random positions in the local ISM. The FUV field
is calculated in two bands (912-1100 \AA and 912-2070 \AA) and at the
wavelength 1400 \AA. Our median value for the [912-2070 \AA] band is $G_0=1.6$
times Habing's (1968) value for the radiation field at the solar circle in this
band, and quite close to Draine's (1976) value, $G_0=1.7$. Due to attenuation
by dust, only associations within about 500 pc contribute significantly to the
energy density at a given point. The FUV field is asymmetric at a given point,
and the asymmetry grows for higher fields. The FUV field fluctuates with a
variety of amplitudes, the larger ones being less frequent. The mean field is
about twice the median field because of these fluctuations, or spikes, in the
radiation field. These spikes, which last $\sim 30$ Myr, are caused by the
infrequent birth of nearby associations. We also model shorter duration spikes
caused by runaway OB stars. The presence of a fluctuating heating rate created
by the fluctuating FUV field converts CNM to WNM and vice versa.
