Introduction to Navigation – What’s the weather like at an airfield?
Before you go anywhere, let’s check the weather. There is no point in sitting down and planning a trip if the weather is not going to permit safe flying.
Let’s start with the weather at your departure and destination airports.
We can get a very good read on what is likely to be going on from the National Aeronautical Information Processing System (NAIPS). You should register for an account at Air Services, and this can then be accessed directly to read weather forecasts and submit flight plans. It can also be integrated into other apps such as OzRunways.
Once you have registered for an account you will be able to access a number of different weather forecasts. The first one we will examine is the Terminal Aerodrome Forecast (TAF).
A typical TAF might look like this
TAF AMD YMEN 252336Z 2600/2612
19008KT 9999 LIGHT RAIN BKN015
FM260200 19010KT 9999 SHOWERS OF LIGHT RAIN SCT015 BKN025
BECMG 2609/2610 18008KT 9999 LIGHT DRIZZLE BKN012
INTER 2600/2601 9999 BKN012
TEMPO 2601/2605 4000 SHOWERS OF MODERATE RAIN BKN008
INTER 2605/2609 4000 SHOWERS OF MODERATE RAIN BKN015
It might look a little intimidating at first, but we can break it down line by line, and expand on the shorthand that it is written in.
TAF AMD YMEN 252336Z 2600/2612
Terminal Aerodrome Forecast – Amended – For Essendon Airport
The ‘Amended’ is telling us that there was a previous forecast issued which has been adjusted now that further information has come to light.
TAF AMD YMEN 252336Z 2600/2612
The date and time this TAF was issued
Let’s break that time down – it’s not how we normally write time in everyday life.
252336Z
The day of the month. In this case it is the 25th.
The time in the 24hr clock – 23:56 (or 11:56 pm).
The time zone in use. Z is shorthand for the time in London based on Greenwich Mean Time (GMT). To convert to the local time here in Melbourne we need to add 10 or 11 hours depending on whether Daylight Savings Time is in force. In this particular case the difference is +10 hours, so the local time would be 09:56 – on the next day (i.e. 26th of the month).
Please note that this time is only the time at which this forecast was issued. It does not tell you the validity period of the forecast – that comes next.
When will this weather happen?
TAF AMD YMEN 252336Z 2600/2612
The start date and hour to the finish date and hour.
Here it is from 26th day at 00 hours (or midnight) until the 26th day at 1200 hours (or midday). But again we must remember that this is based on the London GMT timezone, so we have to add 10 hrs (or 11 in summer). This means the validity is from 10am local time on the 26th until 10pm local time on the 26th. If you are planning to fly to another airport you must have a TAF for that airfield that is valid from at least 30 minutes before your estimated time of arrival, to 60 minutes after your estimated time of arrival. So in this instance we could be planning to land at Essendon anytime between 1030 am local and 9 pm local.
Now we get to the weather that can be expected.
The first line of weather information is what the weather is forecast to be from the start of the validity period. In this case from 10am local time on the 26th of the month.
19008KT 9999 LIGHT RAIN BKN015
190 is the direction in ‘degrees true’ FROM which the wind is expected to be blowing. So this wind is coming approximately from the south. 08KT is the speed, in this case 8 Knots (approximately 15km/hr).
9999 – The horizontal visibility in metres. If it is 9999 then the visibility is 10,000M or greater.
Light Rain (sometimes abbreviated to -RA) is expected from Cloud which is ‘Broken’ at 1500’. The term ‘Broken’ means 5 to 7 eighths of the sky is obscured. 015 represents the height in hundreds of feet above the airfield, so 1500′ above the airfield.
The next line says:
FM260200 19010KT 9999 SHOWERS OF LIGHT RAIN SCT015 BKN025
From 26th at 0200Z (i.e. from 12 noon local time) the weather will change fairly swiftly to become wind from the south at an increased speed now of 10 kts. The visibility is still 10,000M or more, but the light rain has changed to showers of light rain. This could be abbreviated as -RASH.
The cloud is now Scattered (3 to 4 eighths of the sky covered) at 1500 feet above the aerodrome, but also has another layer that is Broken at 2500’ above the aerodrome.
And the next line:
BECMG 2609/2610 18008KT 9999 LIGHT DRIZZLE BKN012
Becoming – The weather will gradually change between 0900Z on the 26th and 1000z on the 26th to 8Kts of wind from the south, visibility 10000m or more, with light drizzle and a broken cloud base at 1200’ above the airfield.
Some weather is intermittent or temporary
INTER 2600/2601 9999 BKN012
TEMPO 2601/2605 4000 SHOWERS OF MODERATE RAIN BKN008
INTER 2605/2609 4000 SHOWERS OF MODERATE RAIN BKN015
These ‘INTER’ and ‘TEMPO’ lines indicate additional weather that will happen at the same time as the weather forecasts listed earlier in the first part of the TAF.
INTER refers to periods of weather which will last for ups to bit no more than 30 minutes. These might happen just once or multiple times throughout their validity period.
TEMPO is similar, but refers to passing weather conditions that will last for between 30 and 60 minutes.
There are many more abbreviations and you can find all the details at the Bureau of Meteorology here