Decoding taf

[extricate]

Hi there,

What's your take on the visibility from 2521/2522 where there's a BECMG BKN011?

I was reading Jepps on it mentioned, "When the visibility is expected to improve and change to or pass through one or more of the following values, or when the visibility is expected to deteriorate and pass through one or more of the following values: 150, 350, 600, 800, 1500 or 3000m, the trend forecast shall indicate the change."

Does that mean visibility will be 6000 as per TAF forecast? I'm just wondering, there's also a TEMPO from 2512/2518 where visibility is 800m. Does this have precedence over the 6000m as per forecast?

[eckhard]

The vis is forecast to be 800m from 2512 to 2518.
Outside that time period, it’s forecast to be 6000m.

There is no ‘precedence’ involved for the visibility figure, although it’s true that some TAFs do invoke that principle.

This is a good example of why I prefer the North American format, which gives ‘FM’ time periods, meaning ‘From’. I think it reduces the chance of misunderstanding as it is a full forecast from that particular time until the next ‘FM’ time period. No need to try to remember which element(s) of the previous period still apply.

[Officer Kite]

I suspect that you're confusing visibility with cloud base.

BKN110 is telling us our cloud base is 1,100 feet. 6000 is telling us our visibility is 6km. Two very different things.

First things first, BECMG indicates a permanent change lasting 2 hours or more is forecasted to occur within the time period specified ... BECMG 2521/2522 simply means a permanent change in conditions between 2100 and 2200, our new cloud base is gonna be 1,100', the exact time of this change is unknown, just that it's between 2100 and 2200.

Now that's the cloud base (it also happens to be the ceiling considering its BKN) ... the visibility is something else as mentioned.

A TEMPO is a change in the weather, but it's not a permanent change. It's a change lasting a minimum of 30 mins but no greater than an hour. So basically TEMPO means there's a change in the weather for at least a half an hour to 1 hour during the time period specified.The key is realising that outside of the TEMPO time specified, the conditions are to be the same as they were previous to the TEMPO time period, they revert back (or never change if they weren't included in the list of things to change beside the TEMPO notice).

As you correctly alluded to, the 6000M visibility at the start applies unless we're told its changed by a new visibility being mentioned. The only time that occurs is during the TEMPO notice of a change to 800M, however as above, this is only a change lasting 30-60 mins at some point in time within the specified time period. Outside of that, the 6000M from the start prevails. It would only change if a new visibility was offered after a BECMG notice.

[extricate]

Thanks for the replies. But how can the visibility improves so much within 3 hours? From 800m to 6km

[Flying Wild]

Look at what is on the same line. It says 0800 SN... Which is 800m vis in moderate Snow. Once snow abates, vis returns to 6km

[Stab Bar]

Could also easily change that much with a dissipating fog, not that that's the case in that TAF, but there are a number of ways you could get a big change like that.