Two pictures hang on the lab board: a thin, twisting funnel under a cloud, and a huge cloud spiral seen from space.
"Same thing, different sizes?" Rocket guesses.
Raven measures each with her thumb. "What do you notice about where each one lives?"
"The funnel hangs from one thunderstorm," Rocket says. "The spiral covers a whole ocean."
Nova lights a ring around the spiral's center. "One is a column, one is a system," she says. "Both have rules that keep people safe."
A new message blinks on the screen: Watch. Rocket reaches for his shoes, then stops. "Watch means possible. Warning means now. Right?"
"Right," Nova says. "So a watch means we get ready, not that we run."
A tornado is a violently rotating column of air extending from the base of a thunderstorm down to the ground.
Around 1,200 tornadoes form in the United States in a typical year. Most last less than 10 minutes.
They are most common in the Central Plains and the southeastern United States, but they have been reported in all 50 states.
They can occur at any time of day or night and at any time of the year. The peak of the season is April through June.
A tropical cyclone is a warm-core low pressure system with no front attached. It forms over tropical or subtropical waters and has an organized circulation.
It needs warm ocean water of at least 80 degrees Fahrenheit through a depth of about 150 feet. It also needs a start at least 300 miles from the equator.
When its sustained winds reach 39 miles per hour it becomes a tropical storm and gets a name. At 74 miles per hour it is a hurricane.
A typical hurricane is about 300 miles wide. Its calm, clear eye is 20 to 40 miles across, and the eyewall around it holds the heaviest rain and strongest winds.
| Storm | Size | Lives in | Lasts |
|---|---|---|---|
| Tornado | A column under one thunderstorm | Over land, most often in the Central Plains and Southeast | Usually less than 10 minutes |
| Hurricane | About 300 miles wide | Over warm tropical oceans, then sometimes onto the coast | Days |
The Storm Prediction Center in Norman, Oklahoma issues tornado and severe thunderstorm watches. A watch covers a large area, often many counties or even states.
A watch means severe storms or tornadoes are possible in and near the area. Review your plan, check your supplies and check your safe room.
Your local National Weather Service forecast office issues warnings. A warning covers a smaller area, about the size of a city or small county.
A warning means the hazard has been sighted or shown by radar. Move to your safe place right away.
For hurricanes, the National Hurricane Center issues a watch 48 hours before tropical storm winds are expected. A warning comes 36 hours before.
| Watch | Warning | |
|---|---|---|
| Means | Possible | Happening or about to happen |
| Area | Large, many counties or states | Smaller, a city or small county |
| Who issues it | Storm Prediction Center (tornado, severe storm) | Local forecast office |
| You | Get ready: review the plan, check supplies and the safe room | Act now: go to the safe place |
| Statement | True or false? |
|---|---|
| A watch means a tornado has been seen. | ? |
| A warning covers a smaller area than a watch. | ? |
| Tornadoes have been reported in all 50 states. | ? |
| A hurricane watch is issued 36 hours ahead and a warning 48 hours ahead. | ? |
Well done. Tomorrow is Station Lab: you build a spinning column of water and time your family's walk to the safe spot.