The IEM website provides a massive archive of raw observations from METAR format reporting stations. These all are lumped together in per-state or per-country networks labelled as "ASOS". The database housing these observations has over one billion rows!

This news item denotes a few improvements made to the Download Interface.

• There are new columns available for 1, 3, and 6 hour ice accretion reports. Please note that very few sites have the sensor that reports the ice value. Computed apparent temperature (feels like) and relative humidity values are included as well.
• There is now an option to specify how missing values are represented in the download. Previously, they were always "M", but this causes some software systems issues. You can now have it be blank or by the 'null' string.
• There is also an option on how Trace precipitation reports should be represented. Previously, the value 0.0001 was used, but now values of T, blank or 'null' string can be used.

The service should be faster now with some optimizations made. Please let me know of any odd data or issues you encounter.