Wiz - the red lines are omega and the purple/yellow lines are for -10 to -20c and -12c to -18c, respectively. As the temperature profile changes those lines will move up or down. We care about -12c to -18c... it doesn't matter what height those are above the ground (doesn't matter if it's 6,000 feet up to 18,000 feet up if it's moist and you have vertical motion).