# 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.
- 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.

## Checking In

- 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?
- 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.

## 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.