GAFOR routes

lionel wrote:

My understanding now:

Your understanding agrees with mine, but it is not compatible to what chrisn and jgmusic writes — that the reference altitude is the minimum altitude along a route.

Add to that France where it is also an area forecast and a reference altitude (see https://aviation.meteo.fr/gafor_expair.php). I am not sure how the reference altitude is calculated for France though.

Wolfgang

Airborne_Again wrote:

The cloud height given in the code is relative to the reference altitude for each different route or area.

Yes, that’s what I meant, but I didn’t make myself clear. My understanding is that the reference altitude is the highest ground level along a route — or the highest representative ground level within an area.

For some value of “representative” :)

My understanding now:

• In Germany (area forecast), the reference height is a height that is generally higher than most terrain in the area, but not 100% all the hills / peaks / …
• In Austria (route forecast), the reference height is the highest terrain in a corridor “approximately 3nmi wide” along the route. (Reference Austrocontrol description)
• In Switzerland (route forecast), the reference height is the highest point along the “linear” route, no corridor thickness. So e.g. often it will be a saddle point, and it will be the elevation of that saddle point. (Reference: Swiss documentation in German which I might have misunderstood.) I assume it is rounded (hopefully up) somewhat, since it is shown with always hundreds of feet.

Mooney_Driver wrote:

No. And I agree it can be confusing.

The cloud height given in the code is relative to the reference altitude for each different route or area.

Yes, that’s what I meant, but I didn’t make myself clear. My understanding is that the reference altitude is the highest ground level along a route — or the highest representative ground level within an area. But that doesn’t agree with

jgmusic wrote:

…the reference altitude can’t be lower than the minimum altitude

Which leaves me confused.

Airborne_Again wrote:

The GAFOR refers to cloud height above ground.

No. And I agree it can be confusing.

The cloud height given in the code is relative to the reference altitude for each different route or area.

(Picture reference DWD)

This pic shows what this means. The prognosis bases on the reference height. If in an area that one is, say, 6500 ft, then all the cloud base prognosis bases on that, regardless if there is ground down to 2500 ft in that area.

The Germans further define undercathegories where you can see if it is visibility or celiling which is limiting.

In the routes for the Swiss GAFOR, it is the same thing.

(picture by Meteo Swiss)

Note the reference heights next to each route. That is what the cloud base is based upon.

So let’s say you want to fly to Ambri from Lugano. The GAFOR route 72 is forecast with Marginal, so 1000 ft cloud over reference. 7200 ft is the minimum passing height of the Gotthard, so overflying that one is really tight, however, to fly in the valley to Amrbi (3200 ft) should be possible based on the clouds as the forecast ceiling would be around 8200 ft AMSL.

jgmusic wrote:

Makes sense.

It gets me confused. The GAFOR refers to cloud height above ground. The reference altitude can’t at the same time be the minimum flight altitude and the ground reference altitude for cloud height.

For Switzerland, the map
shows "MNM ALT ", e.g. 8200 ft for the Gotthardpass. That’s the reference altitude for the GAFOR-route. There are many peaks around that are higher.

Not in Germany, and definitely not in the alps.

This is the GAFOR area map for Germany, with the reference altitudes. The bits in read are areas above the reference altitude, to prevent single peaks or small hilly areas from having too much influence on the wider area.