The "critical angle" is the angle between the storm-relative wind at the surface and the 0-500 m AGL shear vector [(kt) displayed only for areas where the effective inflow base is the ground (SBCAPE 100 J kg-1 or greater, and less than 250 J kg-1 CIN].
A critical angle near 90 degrees infers streamwise vorticity near the ground, which favors stronger cyclonic rotation and dynamically forced ascent closer to the ground in a right-moving supercell (through the effects of tilting and stretching of horizontal vorticity). Critical angles in the range of 45 to 135 degrees suggest near-surface vorticity is more streamwise than crosswise, and values in this range are highlighted by the color fill. Large SRH colocated with a critical angle close to 90 degrees is most favorable for tornadic supercells.
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Additional information can be found here. (Please open this link in another browser window.)