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Giovanni Operation Technical Summary: Zonal Mean

Zonal Mean Calculation

The Zonal Mean over a given area is the sequence of data generated by taking the average over a range of longitudes for each latitude. The Zonal Mean operation in Giovanni works to generate both plots of these data, and the data itself in HDF format.

The Zonal Mean calculation only works with data in a single atmospheric layer (defined by pressure in hPA), such as 300 hPA as an example. The zonal mean cannot be calculated for a pressure range, i.e., for the range 300-400 hPA.

The zonal mean operation works by exporting the data from the dataset and then averaging along the longitude axis for each latitude point. The extent of the range of longitudes averaged, and the latitudes considered, is limited by the edges of the bounding box selected by the user.

 The zonal mean data is then visualized to create plots. These plots are characterized by degrees latitude on the x-axis and the data's mean (in the data's units) on the y-axis. This plot is saved as a PNG image, while the data is stored in an HDF file.

For queries in Giovanni that request the zonal mean for data over a time interval, the data will be time-averaged first, prior to undertaking the Zonal Mean calculation. For this reason, all averaging done for a zonal mean query follows the order of time, then space.


The inputs to the Zonal Mean service are:

  • atmospheric chemical species concentrations from a given pressure layer; (derived from HDF data Scientific Data Sets);
  • user-selected geographic coordinates
  • user-selected temporal range Begin and End values


  • PNG image of Zonal Mean values for the user-selected area
  • Corresponding HDF data file array with Zonal Mean values

Calculation Details

 Zonal mean equations


Example image

Zonal mean of CO concentrations over central Africa 

Zonal mean of CO over central Africa, April 2001.   Higher CO values near the Equator are probably due to spring biomass burning in the tropical rainforest.


Algorithm Diagram


Steps in zonal mean operation

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Last updated: Sep 09, 2009 02:28 PM ET