AIP GEN 3.5 Section 3 gives general information regarding aviation weather reports and forecasts.
AIP GEN 3.5 Section 18 gives specific information regarding area forecasts for use below FL200.
Some important points to note include :
- TEMPO and INTER on an area forecast indicate significant variations in weather conditions from the previously given mean conditions. The conditions are of a temporary or intermittent nature, expected during the period identified in the forecast. The period of time during which the variation is expected is given in the format ddhh/ddhh eg. 0108/0114 means from 0800 UTC until 1400 UTC on the 1st.
- TEMPO is used to indicate a change in the prevailing weather conditions that is expected to last for periods of between 30 and 60 minutes in each instance;
- INTER is used to indicate changes expected to occur frequently for periods of leass than 30 minutes duration, the conditions fluctuating almost constantly, between times specified in the forecast.
- The amount of Cumulonimbus (CB) cloud is indicated on non-aerodrome forecasts as:
- ISOL = isolated – for individual CBs;
- OCNL = occasional – for well sepereated CBs;
- FRQ = frequent – for CBs with little or no spacing.
- GOOD refers to visibility of 10 km or greater over the entire forecast area;
- FM = from and BECMG = becoming: are used when significant changes (both improvements and deteriorations) are expected to occur;
- Cloud Height Datum: in aerodrome and trend forecast, cloud heights are expressed above the aerodrome elevation. In other forecasts heights are expressed as a flight level or with reference to mean sea level;
- PROB% is used to indicate an expected 30 or 40% probability of occurence of a weather phenomena eg. PROB30 Fog 1822 – there is a 30% probability of fog occuring between 1800 and 2200 UTC.
We have an aviation weather decode card available for download which can be used as a quick reference guide for interpreting aviation weather reports including ARFOR, TAF, METAR, SPECI and SIGMET.
Here is an additional resource including example severe weather area forecasts.
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Decoding Aviation Weather Reports - ARFOR - Technical data content credited to Mr Steve Griffin