Upper level winds blow parallel to the contours, surface winds
blow across the contour lines always toward low pressure. So
this is an example of a surface wind and low pressure must be at
the bottom of the figure. The pressure gradient force always
points perpendicular to the contours toward low. Arrow c is
the only force pointing toward low pressure at the bottom of the
figure and it is perpendicular to the contour. Arrow b is
opposite the wind so it must be the frictional force. Arrow
a is perpendicular to the wind (it's slightly skewed in the
figure, but that's a drawing error) and is to the right of the
wind when you look downstream so this is a northern hemisphere
map.
