Why Keep an Eye on the Barometer?
Part B: Direct Comparison of Air Pressure and Wind Speed
You've seen that air pressure and wind speed are related: as air pressure drops, wind speed increases. Is there a particular formula for this relationship though? For instance, for every 10 millibars decrease in air pressure, does the wind speed increase by a predictable amount? Is there a certain air pressure at which the wind speed can be expected to reach 65 knots, the characteristic that classifies a storm as a hurricane?
The quantitative relationship between air pressure and wind speed can be revealed by creating a graph that compares the two variables directly. This graphing technique is called a scatter plot.
Create a Scatter Plot
- Use the same file as you worked with in Part A. Select the Air Pressure and Wind Speed columns and generate an XY Scatter plot to compare pairs of values.
In your worksheet, select the columns that contain Air Pressure and Wind Speed. Click the Chart Wizard icon, or click Insert on Excel's menu bar and choose Chart.
Under Chart type, select XY (Scatter); under Chart sub-type, select the Scatter option.
Click the Next > button.
On Step 2 of 4, the Columns button should be selected. Click the Next > button.
On Step 3 of 4, enter a name for your chart and its axes. Values from the left-most column of your spreadsheet will be plotted along the X-axis and the column on the right will be plotted on the Y-axis.
Click the Legend tab.
As each point on your chart represents one value from each of the two columns of data, a legend isn't appropriate. Uncheck the Show Legend box.
Click the Next > button.
On Step 4 of 4, decide where you want your chart and click Finish.
- Every point on the graph shows the air pressure and wind speed for a specific time during the storm. Put your cursor over several different points on the graph to read the ordered pair of data it represents.
- What was the storm like (air pressure and wind speed) at the time represented by the highest point at the far left of your graph?
150 knots and 902 millibars: the strongest winds and lowest air pressure recorded
- What was the air pressure when the wind speed reached 65 knots?
Stop and think
5: Use values from the graph to estimate how much the wind speed increased as the air pressure decreased by approximately 100 millibars.
Add a Trendline
You probably noticed that the points of your scatter plot seem to lie along a straight line. You can use Excel to define the line that is the best fit for your data.
- Select the series of data points and add a linear trendline to your scatter plot. Show the equation of the trendline on the chart as well.
- Click any of the data points in the graph to select the data series, then right-click (ctrl-click with a one-button mouse) to access a pull down menu.
- Under Trend/Regression type, choose Linear.
- Click the Options tab and check the Display equation on chart box.
- Click and drag the equation to an uncluttered part of the graph.
Select Add Trendline...
Stop and Think
6: The equation for the line is in the form y = mx + b. Plug an air pressure value (x) of 940 millibars into the equation and calculate the wind speed (y) predicted by the line. Show your work. Did the equation do a good job of predicting wind speed?
7: Do you think the trendline helps to clarify the relationship between air pressure and wind speed for this hurricane? Tell why or why not.