Many times I am caught in a room on a cloudy or overcast day. The light from the window is just not enough.
To make the above photograph I put an Alienbees B1600 monobloc light with a white shoot through large umbrella just outside the professor's window. To trigger the light I used the PocketWizard Plus II receiver and the PocketWizard Plus II Transmitter.
Inside, the light needed just a little help to keep the shadows from going too dark. To soften the shadows I put another Alienbees B1600 with just a bare-bulb on a light stand and put it as close to the ceiling as I could.
The outside light was on full power and the bare-bulb light inside was at 1/8 - 1/16th power.
Sometimes the available light need a little help.