NASA's Worldview website now allows you to explore IMERG global satellite estimates of rainfall and snowfall from 1998 to the present. Researchers and organizations have been using IMERG data since 2014 (Portier 2024; Portier et al. 2023), but this new integration makes it available as imagery in Worldview, a user-friendly website for visualizing many of NASA's Earth science datasets. Worldview is now being updated every hour with the most recent global IMERG estimates. 

IMERG stands for Integrated Multi-satellitE Retrievals for Global precipitation measurement (GPM). By collecting data from a constellation of satellites united by the GPM Core Observatory, IMERG provides high-quality estimates of precipitation across the entire Earth. IMERG has many uses, from helping researchers understand the water cycle and severe weather events, to aiding decision-makers  in the agricultural, health, energy, and transportation sectors. Over the years, a team of NASA scientists have improved the IMERG algorithm's accuracy over land and ocean (Huffman et al. 2023a,b,c).

A major benefit of IMERG is its ability to continuously monitor precipitation over regions of the Earth without ground-based instruments, such as oceans and remote areas. The image below shows the precipitation falling from Hurricane Helene shortly before it made landfall in Florida in Sept. 2024 (West et al. 2024). In the image you can see how IMERG estimates precipitation over the land as well as the ocean, where there are no rain gauges or ground radars.

An IMERG snapshot of the precipitation falling from Hurricane Helene as displayed in NASA's Worldview website. The estimate covers a 30-minute period near the end of September 26, 2024.

NASA runs the IMERG algorithm three times for each 30-minute period. The precipitation estimates from the first run become available just 5 hours after satellite observations are made, and that run is called IMERG Early Run. This near real-time dataset is useful for forecasting floods and monitoring large storm systems. A second run, called IMERG Late Run, occurs about 15 hours after observations are made. About four months later, the IMERG algorithm is run one last time when more observations and calibration datasets are available as inputs. This last run is called IMERG Final Run. This research-quality product is useful for studying droughts and other seasonal deviations from normal conditions. It can also be used for post-season reexamination of major events such as hurricanes. The Worldview website displays the IMERG Final product when available and IMERG Early otherwise.

Viewing IMERG in Worldview

If you haven't used NASA's Worldview website before, it is easy to learn how. Start by visiting https://worldview.earthdata.nasa.gov. There is no app to download, no user registration, and no charge.

When you first visit Worldview it loads satellite imagery of the most recent observations of the Earth's surface and clouds. Precipitation is not initially displayed.

It takes two steps to display IMERG precipitation in Worldview. The first step is to add the layer that contains 30-minute IMERG precipitation estimates. To do so, click the “Add Layer” button in the lower left corner. In the panel that pops up, type "imerg" into the search field. In the search results, check the box next to the entry for 30-minute IMERG. To dismiss the Add Layer panel, click the "X" symbol in the upper right.

The Add Layer panel in Worldview.

The second step is to move a few hours into the past to find the most recently available IMERG product.  To do so, move the time slider a little to the left in the timeline along the bottom of the Worldview window. Alternatively, you can use the arrow icons to move the clock display back a few hours. This time adjustment is necessary because IMERG precipitation estimates are created approximately 5 hours after satellite observations are made and it takes another hour or so to convert the data into Worldview imagery.

The date-time selection interface along the bottom of the Worldview window.

The 30-minute IMERG imagery should now be visible with green, yellow, and red colors indicating increasingly intense rainfall rates, and cyan, blue, and purple colors indicating increasingly intense snowfall rates, shown as the liquid-water equivalent. To adjust the map's zoom level, use the plus and minus buttons in the Worldview window or the scroll function on your mouse or trackpad. To change the map's location, drag the map with your mouse or trackpad. By default, Worldview shows coastlines. For more detailed political boundaries and place labels, turn on optional layers. These optional layers can be found at the top of the layer list on the left side of Worldview.

The IMERG algorithm estimates the precipitation rate at the middle of each 30-minute period, i.e., 15 minutes and 45 minutes after the hour. The units of precipitation rate are the millimeters of rain-equivalent depth that would accumulate in an hour if the observed precipitation rate (rainfall or snowfall) were to persist for a full hour. If the precipitation is frozen, then the value that IMERG reports is the depth of liquid water that would result if the frozen precipitation were melted before its depth is measured. The liquid-equivalent depth of snowfall is relevant to the water storage in snowpack, which influences how much water will become available to plants once the snow melts. Because fresh snow contains air (i.e., it's fluffy), about 4 to 20 inches of snowfall will melt down to just an inch of liquid water (Baxter et al. 2005).  IMERG states the liquid-equivalent depth of snowfall, but most weather reports state the depth of snowfall in its original, frozen form. 

One advantage to viewing IMERG precipitation imagery within Worldview is that one can hover over an image in Worldview to obtain a numerical range for the precipitation rate represented by that pixel of the image. The numerical range is displayed in the color bar on the left side of the browser window. This color-table-interrogation feature is available only if you are using a device with a mouse that can hover, such as a laptop or desktop but not a mobile device.
 

The Worldview color bar for rainfall. The downward-pointing white triangle indicates the color of the pixel in the IMERG image that is currently under the mouse pointer. The numerical range below the arrow is the range of precipitation rates associated with that color.

Advanced Worldview Features

To understand the evolution of a large storm system that travels across a continent or ocean over several days, it may be useful to look at daily-average precipitation rates instead of 30-minute precipitation rates. Worldview contains both durations of IMERG imagery. To view the daily imagery, click the “Add Layer” button. In the search field, search for "imerg" again, this time ensuring that both daily and 30-minute datasets are selected. This action will cause both daily and 30-minute imagery to be included in the layer list on the left side of the browser window. Now, you will be able to quickly turn these layers on and off using the Hide/Show-Layer icons that look like eyes, located along the left margin of the layer list.

The 24-hour-average IMERG precipitation rate for the same date shown in the 30-minute image near the top of this story (September 26, 2024). As is the case with the 30-minute image, the 24-hour image also shows rain associated with Hurricane Helene.

Using Worldview, you can save a sequence of 30-minute IMERG images for future reference. To do so, click the movie-camera icon near the lower left corner of the browser window. Then select a date-time range and the desired frames per second. Three frames per second is a good setting, and a 12-hour period is a good duration for a sequence of 30-minute IMERG images. To avoid flickering in the animation, avoid starting or ending the movie at exactly 0 or 30 minutes after the hour.

The pop-up panel for selecting the start and end date-times for saving a sequence of images in Worldview.

Next, click the blue Create Animated GIF icon. This action will take you to a screen where you can select a geographic region for the animation. Move the boundary of the rectangle defined by a white dotted line until the rectangle covers the region of interest. To avoid creating an unnecessarily large animation, reduce the resolution to just 5 km per pixel.

Once the geographic region is selected, click the Create GIF button. Wait a few minutes for Worldview to create the animated GIF. Once it is created, you can click the Download button to save a copy to your hard drive. Web browsers and Microsoft Word will play the animation stored in an animated GIF file. In contrast, other applications like MacOS Preview and Photoshop Elements will list the images stored in the animated GIF but not play the animation.

Click here to view a sample animated GIF. It shows IMERG precipitation rates for the southeast United States as Hurricane Helene approached landfall in Florida during the last 12 hours of Sept. 26, 2024.

Obtaining IMERG in a Scientific Data Format

If you wish to use IMERG in your research or add it to your application, most likely you will want to download it in a scientific data format. To download the IMERG HDF5 files for the day you are currently displaying in Worldview, click the Data tab at the top left of the Worldview window.  The Data tab is to the right of the Layers and Events tabs.  When you are done downloading files, click the Layers tab so that you can once again see the list of layers loaded into your Worldview session.

IMERG HDF5 files can be read, analyzed, and displayed in scientific programming languages such as Python, Matlab, and IDL. These files can also be displayed by installing the THOR data viewer on your computer. THOR is designed specifically to display GPM data products like IMERG on a map of the Earth. For information about downloading THOR, visit the Precipitation Processing System (PPS) website or read the tutorial.

People unfamiliar with scientific programming languages but comfortable with geographic information systems (GISs) may want to download the GeoTIFF translation of IMERG from PPS. These GeoTIFF files contain precipitation rates multiplied by 10 and stored as two-byte integers. A ReadMe file provides information on downloading and displaying the GeoTIFF translation of IMERG. If you encounter any trouble downloading IMERG data, contact the GPM project using the contact page.

Credits: Story and visualization by Owen Kelley (PPS/GMU). Edits by Jacob Reed (NASA GSFC / Telophase)

References

Baxter et al., 2005: A Climatology of Snow-to-Liquid Ratio for the Contiguous United States. Weather and Forestcasting, 20, 729–744, Figs. 3 and 5.

Huffman, G. J., D. T. Bolvin, D. Braithwaite, K. Hsu, R. Joyce, C. Kidd, E. J. Nelkin, S. Sorooshian, J. Tan, and P. Xie, 2023a: NASA Global Precipitation Measurement (GPM) Integrated Multi-satellitE Retrievals for GPM (IMERG) Version 07, Algorithm Theoretical Basis Document (ATBD). https://arthurhou.pps.eosdis.nasa.gov/Documents/IMERG_V07_ATBD_final.pdf.

Huffman, G. J., D. T. Bolvin, R. Joyce, O. A. Kelley, E. J. Nelkin, J. Tan, D. C. Watters, and B. J. West, 2023b: Integrated Multi-satellitE Retrievals for GPM (IMERG) Technical Documentation. white paper, https://arthurhou.pps.eosdis.nasa.gov/Documents/IMERG_TechnicalDocumentation_final.pdf.

Huffman, G. J., D. T. Bolvin, R. Joyce, O. A. Kelley, E. J. Nelkin, J. Tan, D. C. Watters, and B. J. West, 2023c: IMERG V07 Release Notes. white paper, https://arthurhou.pps.eosdis.nasa.gov/Documents/IMERG_V07_ReleaseNotes_final.pdf.

Kelley, O., 2025: Displaying Global Precipitation Measurement (GPM) Data Using the THOR Data Viewer. NASA Goddard Precipitation Processing System (PPS), 32pp. https://gpmweb2https.pps.eosdis.nasa.gov/pub/THOR/version_2.3/tutorial.pdf.

Kelley, O., 2023: The IMERG multi-satellite precipitation estimates reformatted as 2-byte GeoTIFF files for display in a Geographic Information System (GIS). NASA Goddard Precipitation Processing System (PPS), 46 pp., https://arthurhou.pps.eosdis.nasa.gov/Documents/README.GIS.pdf.

Portier, A., 2024: Using GPM Precipitation Data to Help Inform Decisions and Policies Throughout the Globe. GPM 10-in-10 Webinary Series: Application, NASA, https://www.youtube.com/watch?v=NPg_2H-XxR4.

Portier, A., D. Kirschbaum, M. Gebremichael, E. Kemp, S. Kumar, I. Llabres, E. Snodgrass, and J. Wegiel, 2023: NASA’s Global Precipitation Measurement Mission: Leveraging Stakeholder Engagement & Applications Activities to Inform Decision-making. Remote Sensing Applications: Society and Environment, https://doi.org/10.1016/j.rsase.2022.100853.

West, J., S. Lang, and J. Reed, 2024: Powerful Hurricane Helene Makes Landfall in Florida’s Big Bend. GPM website, https://gpm.nasa.gov/applications/weather/news/powerful-hurricane-helene-makes-landfall-floridas-big-bend.