Heat

Have I mentioned that I slow down about 1 min/mile for each 10 degrees the temp goes up from 60 degrees F?

That's about it.

Maybe not a whole minute, but pretty close.

60-70 degrees is not such a big change.
80 makes a difference.
90+ is a whole different situation.

Fitting in 3 cool (60s) weather workouts a week in the Bay Area is not a problem. Upping that to 5 or 6 will eventually run into trouble when there is a hot spell.