In article , Charles Oppermann
wrote:

This is all speculation, that is all true. Something that bugs me is that
the ILS for 31C requires RVR of 4000 or 3/4 of mile visibility. The METAR
from just before the accident pegged visibility at 1/2 mile. Seems to me
that the approach shouldn't have even started.

Well, for starters, prevailing visibility and a specific runway's RVR
can often differ by quite a bit. That's why RVR is controlling for an
airliner. It also changes minute by minute, and the hourly ATIS isn't
at all a good indicator of what the RVR was when that airplane landed.

Second, Southwest's 737-700 airplanes have a heads-up display that lets
them use 3000 RVR on that runway at MDW.