Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
45 min.-1.25 hr
Hurricanes require certain conditions in order to form. First, the ocean water down to a depth of about 200 feet must be 80°F or warmer. For this reason, hurricane season in the north Atlantic runs mostly from June 1 through November 30. Secondly, there must be sufficient force from the earth's rotation (known as the Coriolis Force) to create the hurricane's spin. This will only occur at a latitude of 5° or higher. Once these conditions have been met, the atmospheric conditions must be right. The air needs to be cooler the higher in the atmosphere you go, and the wind speed and direction must be the same from sea level to a height of about 9 kilometers. If all these conditions are met, a hurricane may form.
Assuming the conditions are right, how does a hurricane actually develop? Warm ocean water heats the air above it adding moisture through evaporation. This warm moist air rises leaving a low-pressure area at the bottom of the air column. Trade winds rush into this low-pressure area, and become part of the column of rising warm air. This column of air twists because of the earth's rotation. In the northern hemisphere, the rotation is toward the right creating a counterclockwise circulation pattern. As the speed of the spiraling air column picks up, the winds and clouds are pushed to the outer edge of the column (due to centrifugal force) creating a tube with a fairly still center - this is the eye of the storm. The strongest winds are found as part of the eyewall. At the top of the hurricane, around 40,000 feet, the air begins to circulate in the opposite direction. This acts as a release for the storm and helps prevent it from petering out. Also at this height, some of the colder high air slowly sinks back down via the eye. As it sinks, it is compressed causing it to warm, perpetuating the cycle.
The energy in a hurricane comes from the heat and water vapor from the ocean water. As the air rises, it cools and the water vapor condenses forming clouds and precipitation. These processes release the stored energy and fuel the hurricane. In one day, the heat released by a mature hurricane is equivalent to about half the world-wide electrical generating capacity.
Select areas of the earth are more likely to produce hurricanes. In general, hurricanes move in a westerly direction. In the north Atlantic, many hurricanes form off the coast of Africa and head toward the Caribbean and United States. In the Pacific Ocean, hurricanes form in the eastern portion of the ocean and head toward Asia rather than threatening the U.S. This is why the U.S. attention is focused mainly on Atlantic storms and not Pacific storms. The National Hurricane Center has identified monthly hurricane zones. These show where hurricanes are most likely to develop and the tracks they are likely to take throughout the hurricane season.
Using archived hurricane data from the National Hurricane Center, we'll look at the hurricane seasons over a 20 year period to determine whether or not hurricanes are becoming more intense and/or more frequent.
Download the Bridge Hurricane Data Sheets. Form 10 student groups and assign each group a year. Please note: While archived National Hurricane Center data is available through 2016, our data sheets currently summarize the named Atlantic storms from 1995 through 2011. Data sheets include:
Each group of students should count the number of named storms, number of hurricanes, and number of category 3, 4, and 5 storms for their year; then calculate the average maximum wind speed and average storm duration. Record these data in the summary table at the end of the Data Sheets. To get a graphic image of the trends over the past decade, create bar or line graphs for each data category in the summary table (e.g., number of storms, hurricanes, categories, wind speed, duration). Note: for each graph, Years should be on the X axis and the variable should be on the Y axis.
Compare your data sheet answers to the Bridge's data sheets. We looked at data from the past decade only, but the National Hurricane Center's reliable Atlantic storm data goes back to 1943 (the beginning of routine aircraft flights into hurricanes). Recently, scientists have been conducting the same analyses of hurricane activity as we just did. Their findings indicate that hurricane activity cycles between high and low activity over the course of decades. So, although recent hurricane seasons may be getting worse, this is part of a natural fluctuation and not increased activity due to global climate change. See these related articles: