def invalid_sn_vsby(i, v):
    """Checks if visibility is inconsistent with SN or DZ
Returns 0 if consistent, -1 if too high, +1 if too low"""
    if i == '+':
        if v > 0.25:
            return -1
        else:
            return 0
    elif i == '':
        if v > 0.5:
            return -1
        elif v <= 0.25:
            return 1
        else:
            return 0
    elif i == '-':
        if v <= 0.5:
            return 1
        else:
            return 0
    else:
        return 0