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said, turning to the news director. “Have I ever told you that we were going to have a horrible tornado outbreak?”
    â€œNo.”
    â€œWell, today is that day. Get ready for mass casualties if these things hit cities.”
    â€œWhen does it start?”
    â€œAn hour or two.”
    â€œWhere do you think the first one will be?”
    â€œIt could be anywhere.”

CHAPTER 6
GROUND TRUTH
    It could be anywhere.
    In an age when we can map the human genome, gather dust from a comet hurtling through space, and engineer synthetic DNA, science cannot predict exactly when and where a tornado will form. A radar cannot “see” tornadoes. It can only detect conditions known to be present when they form, such as signs of strong rotation. And the presence of those radar features does not prove a tornado exists. The only way to confirm a tornado is for a human to lay eyes on it. This, in meteorology, is called ground truth. It is one of the most valuable tools meteorologists have today.
    Ground truth is the difference between probability and reality. It is a key factor in the false alarm rate—the percentage of warnings issued by the National Weather Service based on conditions seen on radar when in fact no funnel has materialized on the ground. The problem is, supercells that bear tornadoes look almost exactly like supercells that do not. So when a radar signature glows with the inflamed comma of a hook echo—a sign of strong rotation—but no ground truth ensues, a warning coordination meteorologist at the NWS must decide whether to issue a tornado warning based on circumstantial evidence alone.
    A tornado warning triggers a ripple effect of communication and action, beginning with countywide sirens. Broadcast meteorologists such as Spann have no authority to issue a warning, but they shoulder a great responsibility to disseminate it quickly and broadly to the public. They are the town criers, informing the legions who still do not gettheir warnings directly from the NWS via weather radios, websites, or smartphone apps.
    Herein lies the great dilemma: Warnings issued without ground truth are often false warnings. The sirens sound. People run to their basements. Then nothing happens. After a few such occasions, people stop running. They grow complacent. They stop trusting meteorologists or, worse, accuse them of crying wolf. Then, when they hear the next alarm, they go outside to scan the sky for confirmation. Or they simply ignore it.
    To warn or not to warn? That is a loaded question. In weighing the consequences of being wrong in either case, it would seem logical to err on the side of warning. But overwarning has dire ramifications, too. At the time of the outbreak, the false alarm rate at the NWS office in Birmingham—and nationwide—was hovering around 80 percent.
    Ground truth is one of the reasons James Spann spends a great deal of off-the-air time teaching children and adults about storms. The best storm chasers may be able to put themselves in the right place at the right time to catch a twister—sometimes. But storm spotters, who stay put, are everywhere. The 290,000 volunteer spotters across the country scan the skies above trailer parks, small towns, rural farms, and other places the SkyCams cannot reach. Most of them have attended two-hour classes that teach them how to tell a violently rotating column of air from a harmless funnel-shaped cloud. Spann’s classes are eight hours and often draw four hundred people of all ages and walks of life. When something wicked comes their way, they photograph it, film it, e-mail it, tweet it, and post the valuable real-time information in private chat rooms where Spann and others can harvest info from trustworthy sources. They are one of the most valuable tools meteorologists have today.
    Education is, as Spann sees it, a critical part of his duty, not only to enlarge his network of ground truth but to teach people how to save