Jetstream cirrus — anisotropy shears the sky

A divergence-free helical field, evaluated per-pixel on the GPU from field.glsl(), drives an advected dye texture (image-based flow visualization). The one dial is anisotropy γ: at γ<0 the wavevectors avoid the wind axis, so the flow — and the clouds it combs — streak along the jet (fibrous cirrus); at γ>0 they stack into tight bands across it (billow layers). Same field, one number. Drag the sky to move the sun.