What's a Weather Spotter?
A good Samaritan with a ruler.
In a news item on severe storms in Iowa last week, a local newspaper noted that "trained weather spotters" had observed two tornadoes in the area. This morning, a newspaper in Buffalo, N.Y., mentioned that a "trained spotter" recorded 1.75 inches of rainfall during a hailstorm. What's a trained weather spotter?
A diligent amateur who does grunt work for the National Weather Service. Local NWS offices periodically conduct free training sessions on storm structure. In principle, anyone can go to these sessions, but most participants are already public servants—firefighters, EMT workers, and so forth. Attendees learn how to scan the horizon for conditions that might lead to "funnel clouds," and how to differentiate between "wall clouds" and "shelf clouds." There are no tests, just instruction; after two to three hours of training, each volunteer receives an NWS identification number, plus information on how to contact the office if he or she happens to witness a severe weather event like a tornado, a strong thunderstorm, or a flash flood. (Click here to find out about training sessions in your area.)
When any of the 280,000 registered spotters scattered across the United States see something, they'll call in with a description of the event and where and when they noticed it. These reports are usually unsolicited, but sometimes the NWS e-mails a group of spotters to gather information about a particular area.
The NWS needs human observers because radar imagery provides little information regarding actual conditions on the ground. For example, a Doppler radar can't distinguish between a funnel cloud—a rotating column of air—and a full-blown tornado. (Most tornadoes begin as funnel clouds, but funnel clouds don't always become tornadoes.) Spotters can also fill in details about the strength of a storm by counting, for example, how many trees have fallen in a particular area. This kind of information can be used to make an estimate of wind speeds. Knocked-over signs and broken branches correlate roughly with 40 to 70 mile per hour winds; uprooted trees and heavy cars lifted off the ground suggest 160 to 200 mph winds; and Wizard of Oz-like conditions—strong-frame houses lifted off their foundations—indicate speeds of 260 to 320 mph.
In sparsely populated rural areas, local NWS officers ask spotters to record temperatures and provide rain and snowfall totals. To measure rainfall, spotters use NWS-supplied rain gauges—glass or metal buckets with a funnel and a built-in ruler. To measure snowfall, the NWS recommends placing two flat boards painted white (preferable since white doesn't absorb sunlight and therefore keeps melting to a minimum) side by side in a backyard. The spotter lets snow pile up on one board through the course of a storm and measures its total height at the end. Snow on the other board is measured and wiped clean at regular intervals to determine changing precipitation rates throughout the storm.
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Explainer thanks Brian Lasorosa of the National Weather Service. Thanks also to reader Ryan Seal for asking the question.
Juliet Lapidos is a former Slate associate editor.
Photograph of the tornado from the National Oceanic and Atmospheric Administration.