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# MY NASA DATA Lesson:

## Using Hovmöller Plots to Better Understand Temperature and Salinity

Image courtesy Image courtesy MND LAS – NOAA PMEL

Purpose:
The purpose of this lesson is for students to learn what a Hovmöller plot is and several ways that they can be used to look at different data sets.
Estimated Time for Completing Activity: 50 to 60 minutes
Prerequisite
• Computer with Interent access
• Basic understanding of plotting and using a key to understand various types of plots
Vocabulary:
Background:

Naomi, Mark, and Sammy are three friends that met playing video games online. One day they were discussing the best time for them to log in to play together. They discovered that they are all in the same time zone. Not only that, they used Google Maps to find out that they also live at the same longitude, 86 W. Sammy lives in Alabama and said that the weather there is warm most of the year, and does not get too cold during the wintertime. Mark's hometown is in Kentucky, and mentioned the weather there is warm half of the year, and the other half is cold. Naomi, on the other hand is from Michigan where it is cold for a good portion of the year.

### Where Naomi, Mark, and Sammy Live

The map shows the longitudinal line that goes from 86W through 31N. All three friends live along this line.

The three friends wanted to make a visual representation of the temperature across the latitude span of their hometown to see how the temperature changed over time. So, after some research, they decided to make a Hovmöller (also known as Hovmoller) Plot, which shows data such as temperature, along a selected longitude or latitude over a period of time.

### Hovmöller plot of the Temperature over Naomi, Mark, and Sammy's Homes

Hovmöller Plot on the longitude line 86W between Alabama and Michigan, latitudes 44N to 32N from June 2006 through 2008

Naomi, Mark and Sammy created the color-coded plot and saw how different the temperatures were from Alabama to Michigan even during the same season. Reds and oranges represent temperatures between 68F (20C) to 90F (32C). The color yellow and green colors represent 42F (6C) to 66F (19C). And the temperatures for blue to purple are 41F (5C) to 17F (-8C).

### Hovmöller plot of the Temperature over Naomi's Home

Here we have the Hovmöller Plot for the latitude around Naomi's hometown. We can see how the blue and purple take up a good portion during the months of December through March. We can also notice how from the end of September to the beginning of December it has yellow and green showing cooler temperatures. Finally, May through September are red orange in color, but it is not a dark red, indicating that it does not get extremely hot there.

### Hovmöller plot of the Temperature over Mark's Home

In Mark's hometown, the plot shows how there is a good spread between the hotter colors, and colder or cooler colors. Notice that there are cold temperatures, but there is not a lot of purple, meaning it does not get below freezing very often. We can also see that there is dark red in the graph, showing that high temperatures occur, especially in late summer.

### Hovmöller plot of the Temperature over Sammy's Home

We can see that at Sammy's hometown there is a lot of dark red across the plot, and small stretches of time where there is yellow and some green. This shows that it is warm most of the year and the weather cools off during the winter. We can also see that there is no blue nor purple indicating that in Sammy's hometown does not get cold at all.

### Hovmöller Plot between Alabama and Michigan from june 2006 through June 2008

Hovmöller Plot on the longitude 86W between Alabama and Michigan, latitudes 44N to 32N from June 2006 through June 2008

Here we have the whole Hovmöller Plot and we can see the changes in temperature as you go along the latitude and over time. The higher the latitude the colder it gets, the lower the latitude the warmer it gets. We can also see the temperature change in cycles as time passes.

### Moving on to other Applications of the Hovmöller Plot

Now that they understand how to make these types of plots, the three friends decide to look for other patterns in the world. Sammy, who wants to be an oceanographer, decides to create a Hovmöller plot at the mouth of the Amazon River to look at changes in salinity there.

Salinity is important to track to be able to understand ocean circulation. It changes the density of seawater and makes it sink or float, and influences deep ocean currents. Evaporation and precipitation, ice formation and melting, and river runoffs influence salinity. Combined with other oceanographic information, they can find out how it affects climate in the short term as well as in the long term.

Procedure:
1. Go to the Live Access Server Advanced edition found in the Lesson Links above
2. On the top left, there will be a button that reads "Data Set", click it. A tab will appear with more options. Look for "Oceans" and click on the + sign, that is before it. A list will appear underneath, look for "Sea Surface Salinity (AQUARIUS)", and click it. On the top left of the pop up, underneath the "Close" button, it should read: "oceans, Sea Surface Salinity (AQUARIUS)". Then click close.
3. On the left side there is a list of information that you can manipulate to receive the data you are looking for. In that list look for "Hovmöller Plots", and underneath it click "Latitude time". Now, look for the "Start date time" and change the date to Aug 2011. On the "End date time" change it to Aug 2012.
4. Above where you changed the start and end dates, there is a compass star with 4 boxes surrounding it, this is where you put down the coordinates to find the area you want to sample. The top and the bottom box change the latitude, and the left and right box change the longitude. Since we are looking for the salinity across a latitude, on the top box write "30N" and on the bottom box write "30S". Then on the left or right box write "27W". You can see that the orange line in the map above changes to the coordinates that are written down.
5. On the top left, next to the "Data Set" button, there is a button that says "Update Plot" written in red, click it. Then you should see the map in the center of the page change into a plot. The Y axis will have the latitude variable and the X axis will have the time variable. On top of the graph is a legend with the salinity expressed in color to identify in the graph. The graph that you end up with should look like the figure below.
6. ### Example Plot from the LAS – Step 5

7. Right click the graph and click on the "Save As" option to save the graph for later use.
8. Now, go back to the "Data Set" button, and look for "Atmosphere" and click the + sign, that is before it. On the new list look for "Precipitation" and click on its + sign. Now look for "Monthly Precipitation (TRMM)" and click it. Close the "Data Set" tab.
9. At the "Start date time" put the date Aug 2011, and for "End date/time" Aug 2012.
10. For the latitude, on the top box write "30N" and for the bottom box write "30S", and for the longitude on either the left or right box write "27W".
11. Click on "Update Plot". You will see a multicolored graph, with the latitude on the Y axis and the time on the X axis. On the top of the graph is the legend for the amount of precipitation expressed in color. If everything was done correctly the graph should look like the figure below.
12. ### Example Plot from the LAS – Step 10

13. Right click on the graph and click "Save As" to use it later.
14. Now you can compare both graphs and see the relationship between amount of precipitation and the surface salinity level
15. Now that you know how to make a Hovmöller plot, try to make one yourself. Create a salinity and a precipitation Hovmöller plot, using the same coordinates as the example procedure, but for the starting date use Aug 2012 and the end date Aug 2013.
Questions:
1. Is there a noticeable trend in the graph? Explain.
2. What other ways could these friends use a Hovmöller Plot? Consider what other data (like precipitation, salinity, wind patterns) could be compared.
3. If you joined the group of friends and looked at your latitude (imagine being in the same longitude), would you have warmer or colder winters than Mark? What about a group of friends at a different longitude?
4. If you could create your own Hovmöller Plot, what would you look for and why?
Extensions:

Here you are going to create another Hovmöller that will be taken across a longitude. When looking at a Hovmöller plot at a longitude point, the Y axis becomes the time, and the X-axis becomes the longitude. Do this my clicking "Longitude-time" under the Hovmöller Plots menu. Now, create a salinity and precipitation Hovmöller Plot at the latitude "1.5N" and longitude from "60W" to "30W". For the start date use Aug 2011 and for the end date Aug 2013.

1. Where in South America are these plots being taken from?
2. Looking at the two plots you created side by side, do you see a relationship between the two datasets? Explain.
3. What do you think is the cause of the low salinity points in the salinity plot? Why is this occurring?

Lesson plan contributed by A. deCharon,C. Jean Lauter,A. Nevàrez