def set_zenangle(profs, zen=None):
    """Set zenith angles to a new value. If zen is missing, angles cycle in
       steps of 10 degrees between 0 and 70 over the profile set."""
    if zen:
        zenlist = [zen]
    else:
        zenlist = range(0, 71, 10)

    for p in range(profs['nprof']):
        profs[p]['zenangle'] = zenlist[p % len(zenlist)]

    return profs