For many moons, the IEM has been processing an archive of one minute interval data from Iowa ASOS (primary airports) sites provided by the National Center for Environmental Information (NCEI, formally NCDC). This news item details an expansion of data processed by the IEM to provide data from all available sites provided by NCEI.

Twice a day, NCEI connects to many of the ASOS weather stations in this country to download its one minute interval data tables. NCEI then collects up this information and provides it in monthly files on their website here. For the lay person and even me, the weather programming nerd, the dataset provided by NCEI is difficult to work with and doesn't really have documentation on what exactly the data format is.

The IEM has made a best attempt effort to ingest this information and provide a simple download of the data found in these files. For some stations, data dates back to the year 2000.

While NCEI updates their available files twice per day, the IEM is just syncing them once every morning (around 8 AM). There is some delay with the most recent observation available for a given site, but generally a days worth of one minute data arrives within the next two days. Rewording, if some big event happens at your local ASOS site and you want to see the one minute data, this service won't have it for a day or two in the future.