The death toll from the May 22, 2011 tornado in Joplin – now 122 — is the latest tragedy of a horrific year for tornadoes. On April 27, twisters in Alabama and nearby states killed 314, the fourth highest in U.S. history. The 480 deaths in 2011 are already the highest number since 1953, and tornado season continues through mid-August.

The Why Files asked Jonathan Martin, an expert on the large atmospheric disturbances that form tornadoes, some questions about tornado prediction. We edited the answers of Martin, a professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison, after the interview.

The Why Files: What must we know to make a good tornado prediction?

The Why Files: Are predictions getting more accurate?

The Why Files: Why so much damage and death this year? Is this a result of climate change?

Thunderstorms can be classified by severity or structure. For example, the National Weather Service defines a severe thunderstorm as one that produces one or more of the following: wind gusts of at least 58 mph, hail at least one inch in diameter, or a tornado.

The structure tells us how thunderstorms, severe and otherwise, are created, and how dangerous they may be. Thunderstorms are composed of basic building blocks called cells, which are compact regions of a cloud with a strong vertical updraft.

Thunderstorm cells come in two basic flavors: ordinary cells and supercells. Ordinary cells are a few miles in diameter and exist for less than an hour, whereas supercells are larger and can last for several hours. The supercell thunderstorm is a single-cell storm that almost always produces dangerous weather.

Single-cell ordinary thunderstorms are short-lived storms that are common in Florida but do not produce severe weather. Groups of thunderstorms often join into larger storm systems. Multicell storms are composed of lines or clusters of thunderstorm cells. A squall line is a line of thunderstorms in satellite or radar images.

Although they are less common than ordinary-cell storms, multi-cell and supercell storms cause the vast majority of severe weather associated with thunderstorms. The severity of these storms is primarily a result of the structure of the environment in which the storms form, with plenty of energy that can be stored as heat or water vapor.

Wisconsin gets hit by lightning about 300,000 times a year; most of that during the spring and summer. That’s about five flashes for each square mile in the state.

For about 20 years, the continental states have had a national lightning detection network. The network indicates that there are about 25 million cloud-to-ground lightning strikes a year. This type of lightning accounts for only about 10 percent of total lightning flashes; most occur within clouds or from clouds to the air.

Nationally, approximately 300 people are injured by lightning each year, and 62 are killed by a strike. On average, lightning causes about one death in Wisconsin annually. Your chances of getting hit by lightning are approximately 1 in 1 million.

Lightning is dangerous, but you can decrease your risk of getting hit by taking proper precautions. If you are outside and hear thunder, you are at risk and should seek shelter in a large building or closed vehicle. Avoid being in or near high places or in open fields. Stay away from metal objects like fences, golf carts, and farm equipment or, if you are on a boat, any of the boat’s metal parts. In a forest, seek shelter in a low area under thick growth. In open areas, go to a low place and crouch down on the balls of your feet. Don’t lie down–if lightning strikes nearby, you minimize your risk of getting burned by crouching instead.

Thunderstorms, which can be the home of tornadoes, usually happen later in the day, when the sun is approaching the horizon. That creates a reddish tinge in the sky, as any fan of sunsets knows. But light under a 12-mile high thundercloud is primarily blue, due to scattering by water droplets within the cloud. When blue objects are illuminated with red light, Bachmeier says, they appear green.

Green is significant, but not proof that a tornado is on the way. A green cloud “will only occur if the cloud is very deep, which generally only occurs in thunderstorm clouds,” Bachmeier says. “Those are the kind of storms that may produce hail and tornadoes.” Green does indicate that the cloud is extremely tall, and since thunderclouds are the tallest clouds, green is a warning sign that large hail or a tornado may be present.

If this explanation is confusing, Bachmeier offers some alternative folk wisdom for the color change: that tornadoes sucked frogs and grasshoppers into the sky.