Previously, the API relied on the ISD network of weather stations for historical weather observations, while using the MADIS network of weather stations for real-time observations.

As of today, we now use the MADIS network for historical observations going back to 2002. (ISD is still used beyond this, as well as to fill in gaps in MADIS coverage.)

This primarily means better coverage, as MADIS has about twice as many active weather stations as ISD does. It also corrects some of the occasional data glitches found in the ISD data (in particular, some locations where precipitation was not defined or always reported as zero).