TEMPO, INTER, FM and BECMG AIP GEN 3.5
TEMPO and INTER indicate significant variations, from the previous given mean conditions, of a temporary or intermittent nature, expected during the period which is given in:
- TAF format: ddhh/ddhh, for example: 0108/0114 means from 08 until 14 UTC on the 1st or
- TTF format: hhmm/hhmm, for example: 0630/0900 means from 0630 until 0900 hours UTC.
TEMPO is used when variations from the forecast mean conditions are expected to last for 30 minutes or more but less than 60 minutes in each instance, and which in the aggregate are not expected to cover more than half the given period. For instance, the variations take place sufficiently infrequently such that the mean conditions remain those of the preceding part of the forecast.
INTER is used when variations from the forecast mean conditions are expected to last for periods less than 30 minutes in each instance and which, in the aggregate, are not expected to cover more than half the given period. For instance, the variations take place sufficiently infrequently such that the mean conditions remain those of the preceding part of the forecast (AIP GEN 3.5).
The change groups FM (from) and BECMG (becoming) are used when significant changes (both deteriorations and improvements) from the preceding information that are more lasting in nature.
FM is used when rapid changes are expected at the specified time, and is given in:
- TAF format: FMddhhmm, for example: FM301000 means from 1000 UTC on the 30th or
- TTF format: FMhhmm, for example: FM1815 means from 1815 UTC.
BECMG is used (in TAF only) when the changes are expected to develop at a regular or irregular rate during the specified time period, and is given in:
- TAF format: BECMG ddhh/ddhh, for example: BECMG 3010/3011 means between 10 and 11 UTC on the 30th.
In both cases (FM and BECMG), the new conditions will continue until the end of the validity period of the TAF/TTF, or until replaced by another FM or BECMG.
Sun and rain illustrating a FM period
Cloud height datum
In aerodrome and trend forecasts, cloud heights are given above aerodrome elevations. In other forecasts, heights are expressed:
- as a flight level or
- with reference to mean sea level.
Amendments (AMD) to forecasts are issued as necessary when changes are expected during the period of validity of a given forecast.