Forecast Center

November/December 2005


This article is a courtesy copy placed on the author's website for educational purposes as permitted by written agreement with Taylor & Francis. It may not be distributed or reproduced without express written permission of Taylor & Francis. More recent installments of this article may be found at the link which follows. Publisher's Notice: This is a preprint of an article submitted for consideration in Weatherwise © 2005 Copyright Taylor & Francis. Weatherwise magazine is available online at:

PART ONE: The Puzzle

Hurricanes are often visualized in bright colors and distinctive shapes in today's era of television and computer graphics. In fact, it's easy to forget about the actual meteorological fields within the storm. In this issue's puzzle, the National Hurricane Center reports a 904 mb hurricane at coordinates 27.2N, 89.1W. Plot the hurricane location and attempt to draw all the isobars, using the storm's central pressure and the surrounding weather reports. There will be a lot of isobars near the storm, so pencil them in as best as you can!

Draw isobars every eight millibars (1008, 1000, 992, etc.) using the plot model example at the lower right as a guide. Draw a dashed intermediate isobar at 1012 and 1004 mb. As the plot model indicates, the actual millibar value for plotted pressure (xxx) is 10xx.x mb when the number shown is below 500, and 9xx.x when it is more than 500. For instance, 027 represents 1002.7 mb and 892 represents 989.2 mb. Therefore, when one station reports 074 and a nearby one shows 086, the 1008 mb isobar will be found halfway between the stations.

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Scroll down for the solution

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PART TWO: The Solution

The 2005 hurricane season was one of the most active in recorded history. Hurricane Katrina was 11th of what would be at least 17 named storms for the season. As Katrina approached New Orleans on the evening of August 28, it had already disrupted shipping traffic, and its wind field and wind-driven storm surge was beginning to damage oil rigs off the Louisiana coast. With isobaric analysis, we can get an accurate idea of just how extensive the wind field was.

You may have found drawing the concentric isobars near the storm to be a daunting task. Just when an isobar is drawn near the storm, the next one inside is drawn even more tightly. This illustrates an ever-increasing pressure gradient, or difference in pressure per unit distance, in the center of the storm. It yields an enlightening introduction to the pressure gradient force -- the force that is directly responsible for wind. Though it is influenced by the Coriolis effect and friction, we can always assume that the tighter the isobars (thus the stronger the pressure gradient), the stronger the wind.

Also, this exercise reveals just how compact the hurricane's circulation is. Television presentations often show large spiral bands and outflow cirrus extending up to a thousand miles from well-developed storms, giving a false impression of the scale of destruction. In reality, the strongest winds are actually concentrated near the center of the storm. At the time of this example, the National Hurricane Center reported that hurricane-force winds extended less than 100 miles from the storm center. This does not mean that distant regions are safe, though, as torrential rains, isolated wind gusts, and tornadoes can occur on the hurricane's outer fringes.

It is noteworthy that no readings have come from near the center of the storm. This is actually quite common. Ships, which transmit weather reports, alter their courses to avoid the storm, and buoys disappear from the reports as their sensors and communications equipment become damaged. To fill in the gaps, forecasters rely on satellite imagery techniques and hurricane reconnaissance missions, such as those of the U.S. Air Force Hurricane Hunters . All of this data combines to provide us with the best possible picture of an approaching storm.

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