  We ask what determines the (small) angle of turbulent jets. To answer this
question we first construct a deterministic vortex-street model representing
the large scale structure in a self-similar plane turbulent jet. Without
adjustable parameters the model reproduces the mean velocity profiles and the
transverse positions of the large scale structures, including their mean
sweeping velocities, in a quantitative agreement with experiments. Nevertheless
the exact self similar arrangement of the vortices (or any other deterministic
model) necessarily leads to a collapse of the jet angle. The observed (small)
angle results from a competition between vortex sweeping tending to strongly
collapse the jet and randomness in the vortex structure, with the latter
resulting in a weak spreading of the jet.
