Tornadoes, like the one pictured here, are very powerful storms that can cause incredible destruction. Their winds may exceed 300 miles per hour, and they can completely level towns and neighborhoods in just minutes. Because tornadoes can be so serious, they are closely monitored, measured, and counted. As a result, there are a lot of data on tornadoes. One way to present these data is with graphs.

### Using Graphs in Science

Graphs are very useful tools in science. They can help you visualize a set of data. With a graph, you can actually see what all the numbers in a data table mean. Three commonly used types of graphs are bar graphs, circle graphs, and line graphs. Each type of graph is suitable for showing a different type of data.

### Bar Graphs

The data in **Table** below shows the average number of tornadoes per year for the ten U.S. cities that have the most tornadoes. The data were averaged over the time period 1950–2007.

Rank | City | Average Number of Tornadoes(per 1000 Square Miles) |
---|---|---|

1 | Clearwater, FL | 7.4 |

2 | Oklahoma City, OK | 2.2 |

3 | Tampa-St. Petersburg, FL | 2.1 |

4 | Houston, TX | 2.1 |

5 | Tulsa, OK | 2.1 |

6 | New Orleans, LA | 2.0 |

7 | Melbourne, FL | 1.9 |

8 | Indianapolis, IN | 1.7 |

9 | Fort Worth, TX | 1.7 |

10 | Lubbock, TX | 1.6 |

Bar graphs are especially useful for comparing values for different things, such as the average numbers of tornadoes for different cities. Therefore, a bar graph is a good choice for displaying the data in the**Table** above. The bar graph below shows one way that these data could be presented.

**Q:** What do the two axes of this bar graph represent?

**A:** The x-axis represents cities, and the y-axis represents average numbers of tornadoes.

**Q:** Could you switch what the axes represent? If so, how would the bar graph look?

**A:** Yes; the x-axis could represent average numbers of tornadoes, and the y-axis could represent cities. The bars of the graph would be horizontal instead of vertical.

### Circle Graphs

The data in **Table** below shows the percent of all U.S. tornadoes by tornado strength for the years 1986 to 1995. In this table, tornadoes are rated on a scale called the F scale. On this scale, F0 tornadoes are the weakest and F5 tornadoes are the strongest.

Tornado Scale(F-scale rating) | Percent of all U.S. Tornadoes |
---|---|

F0 | 55.0% |

F1 | 31.6% |

F2 | 10.0% |

F3 | 2.6% |

F4 | 0.7% |

F5 | 0.1% |

Circle graphs are used to show percents (or fractions) of a whole, such as the percents of F0 to F5 tornadoes out of all tornadoes. Therefore, a circle graph is a good choice for the data in the table. The circle graph below displays these data.

**Q:** What if the **Table** above on tornado strength listed the numbers of tornadoes rather than the percents of tornadoes? Could a circle graph be used to display these data?

**A:** No, a circle graph can only be used to show percents (or fractions) of a whole. However, the numbers could be used to calculate percents, which could then be displayed in a circle graph. If you need a refresher on percents and fractions, go to this URL: http://www.mathsisfun.com/decimal-fraction-percentage.html.

### Line Graphs

Consider the data in **Table** below. It lists the number of tornadoes in the U.S. per month, averaged over the years 2009 to 2011.

Month | Average Number of Tornadoes |
---|---|

January | 17 |

February | 33 |

March | 74 |

April | 371 |

May | 279 |

June | 251 |

July | 122 |

August | 57 |

September | 39 |

October | 65 |

November | 39 |

December | 34 |

Line graphs are especially useful for showing changes over time, or time trends in data, such as how the average number of tornadoes varies throughout the year. Therefore, a line graph would be a good choice to display the data in the **Table** above. The line graph below shows one way this could be done.

**Q:** Based on the line graph above, describe the trend in tornado numbers by month throughout the course of a year.

**A:** The number of tornadoes rises rapidly from a low in January to a peak in April. This is followed by a relatively slow decline throughout the rest of the year.

### Summary

- Graphs are very useful tools in science because they display data visually. Three commonly used types of graphs are bar graphs, circle graphs, and line graphs. Each type of graph is suitable for a different type of data.
- Bar graphs are suitable for comparing values for different things, such as the average numbers of tornadoes for different cities.
- Circle graphs are used to show percents of a whole, such as the percent of all U.S. tornadoes with different strengths.
- Line graphs are especially useful for showing changes over time, such as variation in the number of tornadoes by month throughout the year.

### Explore More

When you make a line graph, you need to locate x and y values on a set of axes. This lets you plot the points that will be connected to create the line. Do the frog-and-fly activity at the following URL. See how many flies you can catch while you practice plotting points.

http://hotmath.com/hotmath_help/games/ctf/ctf_hotmath.swf

### Review

- What is the advantage of displaying data in a graph rather than just listing data in a table?
- Explain what a circle graph shows.
- Examine the data in
**Table**below. Which type of graph would you use to display the data? Why would you use this type of graph?

State | Average Number of Tornadoes |
---|---|

California | 4 |

Idaho | 2 |

Kentucky | 10 |

Michigan | 18 |

Montana | 5 |

North Carolina | 14 |

North Dakota | 20 |

Tennessee | 12 |

- Using a sheet of graph paper, create a graph of the data in question 3. Use the type of graph you identified in your answer to question 3.