AIP Australia GEN 3.5 and ENR 1.1
Understanding weather reports is a fundamental knowledge criterion for flight operations. AIP GEN 3.5 and ENR 1.5 details the definitions and terminology for the pilot. The page will summarise the AIP section.
TEMPO and INTER
When included in a terminal area forecast, a TEMPO and INTER indicate significant weather changes from previous conditions, temporarily, or intermediate in nature. The TAF format is:
ddhh/ddhh, for example: 0209/0215 means from 0900 until 15 UTC on the 2nd
TEMPO is used for periods of 30 minutes or more but less than 60 minutes in duration.
INTER is used for periods less than 30 minutes.
FM and BECMG
FM and BECMG are known as change groups. They indicate a significant change in the weather condition from a time, or over a period of time.
FM (from) is used when rapid changes occur at an expected time. FM is presented in the TAF as:
FMddhhmm, for example: FM271100 means from 1100 UTC on the 27th
BECMG (becoming) is used when the changes are expected to develop at a regular or irregular rate during the specified time period. It is presented in the TAF as:
BECMG ddhh/ddhh, for example: BECMG 2711/2712 means between 1100 and 1200 UTC on the 27th.
In both cases (FM and BECMG), the new conditions will continue until the end of the validity period of the TAF/TAF3, or until replaced by another FM or BECMG.
Cloud Height Datum
Aerodrome forecasts (TAF and TAF3) indicate cloud height above aerodrome elevations. In other forecasts cloud heights are expressed as a flight level or with reference to mean sea level (AMSL or MSL)
Amendments (AMD) to forecasts are issued when necessary and when changes are expected during the period of validity of a given forecast.
See the TAF brochure from the Bureau of Meteorology: