A two-valued qualitative variable can be represented by a single 0-or-1-valued "dummy" variable. If a qualitative variable has three or more possible values (e.g., make-of-car, or marital-status), ...

this result is identical to the kitchen sink model in Excel (R 2 = 0.8271). AirEntrain is a categorical (yes/no) variable and cannot be used in regression without transforming it to a numerical {0,1} ...