Jonathan Sachs wrote:
There's a light on the the landing outside my back door (second floor
of a three-flat) which is useful for lighting the steps when I come
home in the dark. Since I must turn the light on before I leave so
that it will be on when I return, I would like to save electricity by
replacing the incandescent bulb with a screw-in fluorescent tube. I
wonder whether those tubes are safe and effective to use at very low
temperatures. I live in Chicago, where the annual low temperature is
typically -15 to -20° Fahrenheit.

In addition to what the other poster said, I would recommend looking
into one of those screw-in light sensing sockets. That way it won't
burn all day, only when it gets dark out.

-Nathan