WEATHER WISE

Met codes for reporting GEOFF SMITH & LOUIS BOKOR The changes: Gone are the eighths (OKTAS)
which measure cloud amount in
and forecasting cloud INCE J ULY OF THIS YEAR , SOME METAR/SPECI, TTF and TAF. They are now
have changed, bringing
Australia into line with
international practice.
S critical changes to the way of reporting
and forecasting cloud have occurred
which affect cloud codes.
replaced with cloud amount descriptors.(The
cloud amount descriptors used in the
abbreviated, plain language, low-level area
The changes implement the international forecast conform to the same standard.)
standard adopted by the International Civil The descriptive terms now used for cloud
Aviation Organization (ICAO) and the World amounts are:
Meteorological Organization (WMO) in 1993. • SKC (sky clear) = no cloud.
Use of the standard in Australia has been • FEW (few) = 1 to 2 OKTAS.
endorsed by CASA after an analysis of the • SCT (scattered) = 3 to 4 OKTAS.
potential impact showed there would be no • BKN (broken) = 5 to 7 OKTAS.
resulting penalty to operators. • OVC (overcast) = 8 OKTAS.
The result for international pilots is close to (If appropriate CAVOK is used.)
world-wide uniformity in coded versions of Minimum conditions: To find out below
actual and forecast weather. The format also minimum conditions, the amounts of fore-
removes unnecessary detail on cloud types.
cast cloud layers below the minima must be
looked at cumulatively as follows:
• FEW plus FEW is equivalent to SCT (4/8ths or
less)
• FEW plus SCT is equivalent to BKN (more
than 4/8ths)
• SCT plus SCT is equivalent to BKN or OVC
(more than 6/8ths)
Observations: In METAR/SPECI:
• Cloud amounts are in ICAO standard
descriptors.
• Only towering Cumulus (TCU) and Cumu-
lonimbus (CB) cloud types are identified.
For TTF METAR/SPECI:
• Cloud amounts are in ICAO standard
descriptors.
RAINFALL INTENSITY: WHAT THE REPORTED RATES MEAN • Of the observed cloud reported, only
INTENSITY RATE OF FALL PAST 10 MINUTES VISUAL APPROXIMATION towering Cumulus (TCU) and Cumu-
lonimbus (CB) cloud types are identified.
SLIGHT up to 2mm/hr 00.2mm Individual drops easily identifiable, • The statement of trend appended to the
wets dry surface slowly, puddles form observation uses ICAO descriptors for cloud.
very slowly, small streams may flow in
Only forecast CB cloud type is identified.
gutters and down pipes.
In TAF, cloud is forecast using ICAO descrip-
MODERATE 2.2 – 6mm/hr 00.4 – 1mm Individual drops not clearly identifiable, tors for cloud amount. Again, the only fore-
puddles form rapidly, some spray visible cast cloud type identified is CB.
over hard surfaces. Rainfall in METAR/SPECI: At aerodromes
HEAVY 6.2 – 50mm/hr 1 – 8.2mm Rain seemingly falls in sheets, misty spray where there is an automatic weather station
visible over hard surfaces, down pipes and (AWS), METARs and SPECIs will usually include
gutters flow strongly, may make a roaring a rainfall group. This will be included in the
noise on roofs. message in the form “RMK RF00.0/000.0”
For example:
VIOLENT >50mm/hr >8.4mm Gutters and down pipes overflowing,
spray to a height of several centimetres TTF SPECI YSSY 1200Z 18010KT 9999 FEW006
over hard surfaces, may cause a loud BKN010 OVC100 21/21 Q1006 RMK
roaring noise on roofs. RF00.0/002.8
TEMPO 1200/1500 3000 DZ SCT004 BKN008
Note: Rainfall intensity is defined by the World Meteorological Organization. It may not reflect local Explanation: The TTF SPECI YSSY 1200Z
subjective assessment
contains the group “RMK RF00.0/002.8”

24 flig ht safet y aust r alia, november 1998

 WEATHER WISE
The first number RF00.0 is the amount of rain broader geographical area. By way of “NOSIG” means “no significant change” and
recorded in the last 10 minutes before the example, a forecaster might expect thunder- does not mean “no significant weather”.
observation (in millimetres and tenths).In this storms to develop well off the coast to the east
case NIL rainfall is reported in the last 10 of Sydney aerodrome and then track north- Louis Bokor is the assistant national manager (avia-
minutes before 1200Z. east. In such a situation the forecaster may be tion and defence weather services) for the Bureau of
The second number /002.8 is the amount confident that the storms will not affect the Meteorology. Geoff Smith is the regional aviation and
of rainfall recorded since 9am local time and aerodrome. defence manager (NSW) for the Bureau.
accumulates for up to 24 hours until 9am There may be no mention of thunder-
local time on the following day. storms in either the TAF or TTF forecasts. HOW TO GET A
Rainfall in AWIB broadcasts: The Bureau of However in such a situation the METAR
Meteorology aerodrome weather information observation should, if possible, reflect the WEATHER FORECAST
broadcast system (AWIB) provides the last 10 observed storms in remarks and also, if To receive a fax forecast, register as an
minute rainfall amount from the AWS updated possible, comment on the observed move- AVFAX user by calling 07 3866 3573.
every minute. ment. Dial 1 800 804 166 to order a fax forecast
This information can be useful when taken The information provided in the METAR if you have a tone dial telephone and a fax
into account with other weather information remarks section indicates that although the
available for the site. However, rainfall infor- machine. Remember to have your location
aerodrome is clear, some flight paths may be codes handy, as you will need them.
mation should be used with caution. The affected.
limitations of the automated rain gauge may If you have a rotary dial telephone, with
The following could reflect the situation:
result in no record of rain when only very no access to a fax machine, you can get a
TTF METAR YSSY 0500Z 04015KT 9999
slight drizzle is falling. FEW045 FEW045CB 30/21 Q1008 RMK
forecast by calling 1 800 805 150. The
A report of no rain in the last 10 minutes briefing officer will read out your requested
RF00.0/000.0 THUNDERSTORMS 15 NM TO
does not indicate that there is no precipita- weather forecast.
EAST MOVING TOWARDS THE NORTH EAST
tion or any associated low cloud in the area. To get further information after having
NOSIG
METAR/SPECI: METAR/SPECI aerodrome
A similar situation could apply with other obtained the forecast, call the numbers
weather reports are expected to be represen-
types of weather, with fog being a particularly published in ERSA under pre-flight
tative of conditions at the aerodrome and its
immediate vicinity. The cloud reported in common example. A fog could form over a briefing services. Technical elaboration on
aerodrome weather observations is cloud nearby river but the fog might only be forecasts is available by calling:
observed over the whole of the celestial dome mentioned in the METAR/SPECI remarks • Darwin 08 8982 2833
(horizon to horizon) from the observing section as “RMK FOG IN THE DISTANCE”with • Brisbane 07 3229 1854
location. no mention of reduced visibility or fog at the
• Townsville 07 4779 5999
The weather reported as aerodrome aerodrome.
• Sydney 02 9296 1527
weather is weather occurring at and/or near If, with good conditions reported at the
aerodrome itself, the statement of trend (in • Canberra 02 6247 0411
the aerodrome. Weather reported as being in
the TTF) is added to indicate “NOSIG”, the (available 0300-1900 local daily)
the vicinity (coded using the term VC) is
weather not at the aerodrome but within 8km good conditions at the aerodrome are not • Adelaide 08 8366 2617
(5nm) of the aerodrome perimeter. expected to change significantly within the or 08 8362 1928
For example, if there are showers or thun- 3-hour period following the time of the • Perth 08 9263 2253 or 08 9263 2254
derstorms situated at some distance from the report. The internet
observing site but within 8km of the aero- In this case the TAF also might indicate that Reports and forecasts are now also
drome, they would be reported as showers or the formation of fog at the aerodrome is not available on Airservices Australia
thunderstorms in the vicinity (VCSH or VCTS). expected over the longer term even though web site: http://www.airservices.gov.au
TAF and TTF: TAF and TTF are forecasts for the patch of fog over a nearby river area is and the Bureau of Meteorology
the aerodrome and may not be representa- expected to persist.
web site: http://www.bom.gov.au.
tive of the weather conditions over the Remember that in the trend the statement

Test yourself on the a) Only reported below minima at 0100Z Conditions at Canberra are forecast to be
and forecast to immediately improve to above below alternate minima:
new cloud codes minium conditions. a) Between 0800Z and 2000Z.
(Lowest alternate minimum at YMAY 2011ft, b) Reported to be below alternate minima b) Between 0800Z and 1100Z and for periods
YSCB 1912FT) and forecast to continue for 3 hours from not exceeding 30 minutes from 1800Z TO
1) SPECI YMAY 2100Z VRB03KT 8000 0100Z. 2000Z.
2000E NOSIG WX FEW002 SCT008 08/07 c) Not reported or forecast to be below c) Only between 0800Z and 1100Z.
Q1028 RMK FOG TO EAST minima. d) Only between 0800Z and 1800Z.
Conditions at Albury at 2100Z are: 3) TAF YSCB 270648 Z 0820 02010 KT e) Between 0800Z and 1800Z and for periods
a) Below alternate minima. 5000 LIGHT RAIN BKN 008 OVC 100 not exceeding 30 minutes from 1800Z TO
b) Not below alternate minima. FM 11 16015 8000 RAIN SHOWERS 2000Z.
2) TTF SPECI YSCB 0100Z 36015KT 9999 FEW008 SCT015 BKN030 FM18 22010KT
FEW005 SCT015 10/08 Q1008 NOSIG 9999 FEW 010 SCT 030 INTER 1820 8000 Answers:
Conditions at Canberra at 0100Z are: RAIN SHOWERS FEW 008 SCT 015 1) a 2) b 3) e

flig ht safet y aust r alia, november 1998 25