Frequently Asked Questions - Imagery
Q: What is satellite imagery?
A: In its most simple form, satellite imagery is a picture of the earth from space. These images are generated using sensors that perceive the various light or temperature wavelengths to create an image of clouds, water vapor or land.
Q: What is the difference between polar and geostationary satellite imagery?
A: There are two main differences between these types of satellites; the orbit height and orbit direction.
Geostationary satellites orbit the earth along the equator about 35,800 km (22,300 miles) above the Earth, high enough to allow the satellites a full-disc view of the Earth. They orbit the earth at the same rate as the earth so that it appears as though the satellite stays above a fixed spot on the surface. This allows the satellite to provide a constant vigil for atmospheric "triggers" for severe weather conditions such as tornadoes, flash floods, hail storms, and hurricanes. When these conditions develop the GOES satellites are able to monitor storm development and track their movements.
Polar satellites orbit the earth from pole to pole about 870 km (540 miles) above the earth and orbit roughly 14.1 times daily. Data from the POES series supports a broad range of environmental monitoring applications including weather analysis and forecasting, climate research and prediction, global sea surface temperature measurements, atmospheric soundings of temperature and humidity, ocean dynamics research, tropical cyclone monitoring, volcanic eruption monitoring, forest fire detection, global vegetation analysis, search and rescue, and many other applications.
Q: What is a satellite footprint?
A: The geostationary satellite footprint is the area of the earth visible to the satellite. The ability to accurately view portions of the earth deteriorate as the edge of the footprint is reached. Multiple geostationary satellites are used to provide overlap, and improve the coverage for these areas. The best way to visualize a satellite footprint may be to view the full disk image from the different geostationary satellites. See this page for full disk image examples.
Q: Do you archive the imagery you put online?
A: Other than our 21 day archive, the Operational Significant Events Imagery (OSEI) Archive, and a few additional Tropical systems, we do not archive imagery at our site. We are hoping to expand the 21 day archive to include some of the more popular loop imagery, but at this time none of the sector or floater imagery is kept online past about 24 hours.
Q: Where can I get older imagery?
A: The primary archive for all NOAA data including imagery is the Comprehensive Large Array-data Stewardship System (CLASS) - http://www.class.noaa.gov/. You may also check out other archives from our archive page (http://www.osdpd.noaa.gov/ml/imagery/archive.html).
Q: Can you make the image loops longer?
A: In theory the loops could be made longer. Past experience has shown that longer loops have caused issues by taking longer to load and requiring more system resources on the part of our users. As a result, we have tried to maintain a useful balance, by creating both standard and enhanced loops. If even longer loops are desired, the images have unique file names and can be downloaded and saved to make loops as long as desired with any software of your choice. Imagery is deleted on a daily basis, usually around 03Z (10PM EST or 11PM EDT). Some sectors are deleted more frequently, with floater imagery deleted when moved. In the future, we hope to provide a few days worth of data archived from our sectors to make it easier for users to generate longer loops on their own systems.
Q: Why can't I watch your image loops on my phone?
A: We have not yet been able to set up our imagery to work with specific phone technology. Some of our loops will work with some phones or tablets that work with flash. Updating the technology to HTML5 or similar is on our "to do" list. In the mean time, the University of Wisconsin does have a specific page for "PDA Animated Weather" which includes satellite imagery and radar loops in animated gif format. It can be found at http://www.ssec.wisc.edu/data/paw/
Q: Can I view my house, my block or my town on satellite?
A: Satellite imagery is not very good for seeing small detail from space. In most cases, the resolution is too coarse to be of any value. Geostationary satellite resolution is at best 1 km, with polar a little better at 250m. What that means is that one pixel in the satellite image at best represents 250 square meters or one square kilometer. Only large objects would be visible in more than one or two pixels.
Q: What is "Floater" imagery?
A: Floater imagery is a name given to a set of loops that has no preset location. As originally set up, the image set was established so that the resulting loop could easily be moved or be allowed to "float" over the ocean to follow a tropical system as it developed. Floaters are also used to quickly set up a loop over any given location, fires, heavy precipitation events, earthquakes (for relief efforts) and any other areas of interest. Tropical floaters are set up automatically and should appear shortly after the Tropical Analysts begin classifications. Tropical Floaters are turned off within 12 hours of the last classification.
Q: Why are you getting rid of java loops?
A: Well we were going to remove them after converting to flash, but we've had many requests to keep them. However we will no longer be supporting the java applet when it comes to resolving problems or dealing with updates. The applet developer made his last update in June, 2007, and replaced it with the flash application. They both use the same images, overlays and text files.
Q: On the "Fronts" overlay, what does the dashed yellow line mean?
A: The dashed yellow line indicates a low pressure trough. For more information on the weather symbols used as part of the "Fronts" overlay, the Weather Service has put together an informative web page to explain all of the weather symbols - http://www.hpc.ncep.noaa.gov/html/fntcodes2.shtml.
