Hurricanes
Haunted by Hurricanes
Written by: Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science
Summary
Determine if the frequency and intensity of hurricanes are changing using data from the National Hurricane Center.
Objectives
- Describe the conditions that can lead to hurricane formation.
- Explain how a hurricane forms.
- Judge trends in hurricane patterns over the past 10 years.
Vocabulary
Hurricane, Coriolis force, Evaporation, Centrifugal force
Introduction
In 2004 a rash of category 3, 4 and 5 hurricanes
battered Florida one right after the other, with Hurricane Ivan even looping around to hit the
U.S. a second time. In 2005, four category 5 hurricanes formed in the Atlantic including one the United States will never forget — Hurricane Katrina. Hurricane Ike tore through Cuba in 2008 before hitting the United States becoming the third costliest hurricane. With these recent intense hurricane seasons, many questions come
to mind. Are hurricanes getting more intense? Lasting longer? More frequent?
The answers may be blowing in the wind, but we'll look at hurricane data from
the National Hurricane
Center to try and elicit answers to these questions. But first, let's look
at how hurricanes form.
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.
Data Activity
Using archived hurricane
data from the National Hurricane Center, we'll look at the
hurricane seasons from 1995 - 2011 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.
The hurricane data sheets summarize the named Atlantic storms from 1995 through
2011 with:
- Maximum wind speed in knots/hour (1 knots
= 1.15 miles/hour)
- Highest hurricane category attained by the
storm (according to the Saffir-Simpson scale)
- Storm duration in days
- Total estimated damage costs given in US
dollars (includes damage estimates outside of the US)
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.
- What trends, if any, do you see in the number of total storms and the number of
severe storms over time? Are the numbers markedly increasing or decreasing?
- What trends, if any, do you see in the wind speeds and storm durations? Are these
markedly increasing or decreasing?
- What societal factors influence the total estimated damages (in US$) caused by
hurricanes, and why is it illogical to judge a hurricane’s intensity by the
cost of its damages? (Think
about the affected area’s population size, wealth of population, property values,
etc.)
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:
