EarthLabs > Hurricanes > Lab 6: Why Keep an Eye on the Barometer? > 6A: Katrina Pressure vs Wind

Why Keep an Eye on the Barometer?

Part A: Air Pressure and Wind Speed in Hurricane Katrina

In August of 2005, Hurricane Katrina hit the Louisiana, Mississippi, and Alabama coasts. Katrina had the third lowest air pressure reading ever made for hurricanes up to that point, and it became the deadliest and most destructive hurricane to hit the U.S. in 80 years.

You may have already examined the daily data for Katrina, or plotted or explored its storm track. Now, you'll use Katrina as a case study to examine the relationship between air pressure and wind speed.

  1. Download and open the Hurricane Katrina Data (Excel 22kB Aug9 07) file into Microsoft Excel or another spreadsheet application.
  2. The data are from the HURDAT database, but they are presented in a slightly different format than you've seen before: the location, air pressure, and wind speed data are on separate lines for each of the four daily reports.
  3. Stop and Think

    1: Look over the columns of Katrina's air pressure and wind speed. Do you see a pattern between the two values? If so, describe it.
  4. You won't need the location information for this activity: select the columns that show Latitude and Longitude then choose Edit > Delete.
  5. Generate a double line graph. Show air pressure and wind speed in a stacked line with markers, both plotted against date/time. Label your axes and include a title and legend for the graph.
    Hurricane Katrina observation data with time, air pressure, and wind speed selected for graphing.

    Select the Date/Time, Air Pressure, and Wind Speed headers and columns in your spreadsheet.
    Excel chart wizard step 1

    Click the Chart Wizard button or Insert on Excel's menu bar and select Chart.

    Select Line under Chart type then choose the Stacked line with markers button under Chart Sub-type.

    Click the Next > button.

    Step 2 of importing Katrina Data

    Step 2 of 4: Because the data are organized in vertical columns, the Columns button should be selected.

    Click the Next > button.

    Step 3 of importing Katrina Data

    Step 3 of 4: Give the chart a title and enter labels for the X and Y axes. Both air pressure and wind speed will be plotted along the Y-axis, but you can only enter one Y-axis label at this point. Start with pressure, measured in millibars.

    Check settings on the other tabs in the Chart Options dialog to control the format of your graph. Make your graph as easy-to-read and interpret as possible.

    Click the Next > button.

    Step 4 of 4: Decide where you want your graph and click the Finish > button.

  6. Add a secondary Y-axis label and scale for the Wind Speed data.
    Select the pressure series of data

    Click any data point along the line of wind speed values to select the entire series.

    Right-click and choose Format series...

    Right-click (ctrl-click with a one-button mouse) the line to access a pull down menu and select Format Data Series...

    Adding the secondary title

    Click the Axis tab and select the Secondary axis button.

    Click OK.

    Chart Options dialog

    With your chart selected, go to Excel's main menu bar and choose Chart > Chart Options...

    On the Titles tab, enter an appropriate title for the Second value (Y) axis.

    Click OK.

  7. Save your file.
  8. Examine the graph. Interpret the lines to figure out what they tell you about how conditions changed through the life of the storm. Use your graph plus the tabular data in your spreadsheet to answer the questions below.
  9. Checking In

    • At what air pressure did Katrina's winds become fast enough to classify it as a hurricane? In other words, at what pressure did the wind speed reach an average of 65 knots or higher?
      983 millibars (August 26 at 0Z)
    • What was Katrina's minimum air pressure? What was its maximum wind speed?
      Minimum air pressure was 902 millibars. Simultaneously, the maximum wind speed was 150 knots (August 28 at 18Z).

    Stop and Think

    2: Look for evidence in the data that one of the variables controls the other. In other words, try to find an instance where a change happens first in one variable and its effect is seen later in the other variable. Describe what you find and how you interpret that relationship.

    3: Focus in on the first several air pressure readings for the storm. What happened to the wind speed as the air pressure decreased? Give a specific example (a range of dates and times) when the air pressure decreased and tell what happened to the wind speed over that time. Describe another specific example of what happened to wind speed while pressure increased.

    4: Based on your analysis, write a summary statement describing the relationship between air pressure and wind speed in Hurricane Katrina.