This diagram shows the complete US Observation Record package. Most of the classes illustrated here are part of the additive data or remarks portion of the Traditional Alphanumeric Code (TAC).<br/><br/>It is important to note that the ICAO model includes only surface visibilities.  The class VariationsInObservedProperties includes an attribute for tower visibility.  The notion of "prevailing visibility" which is so important in the TAC is not modeled here, per se.  Those who decode US METARs into formats based on this model must take care to handle visibility correctly.<p/>