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I see maps like this one all the time online and in the news. A moving map that shows clouds and stuff moving around with time. Where do they get the data from? Is it from a single source location or multiple locations? Also, is it free? Is it open for public access?

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Images showing clouds mostly come from geostationary satellites. Geostationary satellites are at an altitude of about 36000 km, which allows them to monitor the same area of the Earth continuously. They stay in place relative to the Earth's surface above the equator thanks to balance between gravitational and centrifugal force. There are a few such satellites designated to operational weather forecasting, notably:

These satellites contain passive remote sensing instruments measuring incoming radiation in a number of channels, mostly visible and infrared. Visible is only available during the day, while infrared is available all the time. Radiation is captured on lines of sensors by scanning the view, which generally takes a few minutes to complete. Geostationary satellites are unable to capture polar regions above a certain latitude, because of the angle.

In addition to geostationary satellites, there are polar orbiting weather satellites:

Polar satellites fly much lower (~800 km) and cannot provide continuous global coverage, only swathes along their orbits.

All of this data can be used in the assimilation (initialization) process of weather forecasting models. In turn, one can produce images of cloud cover from the output of the models, see e.g. wettercentrale.de.

NASA operates mostly research satellites, not so much operational weather satellites, although they do cooperate with NOAA on some of the missions. In Europe, the counterpart is ESA, who run some satellites on their own (e.g. Sentinel and in the past Envisat and ERS-1,-2) and cooperate with EUMETSAT.

The data from satellites is downlinked to communication centers (sometimes routed via other communication satellites), where it is processed by various algorithms into products (data files). Product files can be downloaded over the Internet by national weather services or are made available publicly to anyone (all data from NASA satellites is public - see e.g. NASA Global Change Master Directory).

Weather radars are generally located on the ground. They operate by sending short pulses of microwave radiation and measuring the reflected (backscattered) radiation from water/ice droplets. As the microwave radiation has relatively large wavelength (on the order of one millimeter), it is not very sensitive to cloud droplets, and you can see mostly bands of precipitation. Weather radars are operated locally by national weather services. You need roughly one per 250 km to get a good coverage. As with data from weather satellites, they are very important in assimilation of numerical weather prediction models. There are also spaceborne radars, e.g. DPR on GPM Core Observatory, TRMM, or CloudSat.

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The clouds that are shown on a radar map are a satellite picture that has been superimposed onto the map.

The radar data is from various National Weather Service weather radar stations across the country. The satellite cloud data is provided NASA.

The radar and cloud pictures for the USA are free and available to the public through Intellicast and the NWS (National Weather Service) websites. Both of these sites will be helpful to you in learning more about both of these features/services.

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    $\begingroup$ The NWS site is fairly easy to navigate; here's a quick tip though: click on the region where you want to see the weather. Somewhere on that screen there will be a place you can click to view the radar! :) $\endgroup$ – L.B. Dec 5 '14 at 18:30
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    $\begingroup$ how about other countries? Is there a similar international agency like that? Is anyone monitoring the ocean areas? $\endgroup$ – KingsInnerSoul Dec 5 '14 at 18:31
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    $\begingroup$ @KingsInnerSoul you'll have to ask a question about a specific country to get answers about that. Each country is different and not all of them make their data publicly available (and some have private sector companies with private observing networks to fill this gap, and even then, some charge for access). $\endgroup$ – casey Dec 5 '14 at 18:35
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    $\begingroup$ I think that this Q&A has to either internationalize itself or explictly mention that it is only relevant for USA. $\endgroup$ – Gimelist Dec 6 '14 at 8:34
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    $\begingroup$ @michael this site is not only for the US, and there are many non-American contributors here. People sometimes forget that what happens in their own country is not universal, and have to be reminded ;-) $\endgroup$ – Semidiurnal Simon Dec 7 '14 at 8:15

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