At 1800 UTC, a trace of precipitation has fallen during the past 6-hours. What should be reported in remarks?

In this context, the trace of precipitation that has fallen during the past 6 hours indicates a very light amount, specifically not measurable in standard reporting terms. When reporting weather observations, a trace is commonly represented by a specific code that denotes it has fallen but is less than 0.01 inches.

In aviation weather reports, when a trace of precipitation occurs, the appropriate code to include is often a numeric representation. The code “60000” signifies that a trace of precipitation is present, with “60” indicating the recorded amount (in terms of hundredths or tenths) of liquid precipitation and the last "00" relating to the fact that the total precipitation accumulation is still zero.

This coding system helps pilots and meteorologists understand precipitation conditions at a glance, as it standardizes the manner in which such data is expressed. This is why the trace of precipitation is indeed required to be reported, ensuring that all relevant weather conditions are documented comprehensively.