Microlighters

CLU-less and I were at the runway this morning and the subject of wind direction came up. How come the weather department says the wind is northerly and it actually seems to be coming from somewhere else.

Well it's because the weather always give the wind in True degrees. All your runways are in Magnetic degrees. So what you have to do is look on a map to find your local magnetic variation (in our case it's 23.6deg W and add this to the published wind direction.

Example, from weathersa, the aviation forecast says 180deg 05kt then the wind will be 203.6deg Magnetic in our case, i.e. straight down runway 20

Today the wind is 320deg 07kt so it is 343deg Magnetic. This should be 37deg crosswind to our runway 02.

If runways are marked with thier magnetic degrees does that mean the marking will change, or have changed, over the years as the local variation changes?

I flew a while with Cluless. At Altona on ground level as well as looking down at some of the smoke emissions while flying it was clearly an easterly wind close to ground level, yet estimated by the crab angle on my return flight at a higher altitude I was sure of a northerly wind as predicted. Can it be that the surface winds are a different direction than the main stream at a higher altitude?

If there is no wind which runway do you use? This is assuming that there is no one else in the circuit...

Tobie wrote:I flew a while with Cluless. At Altona on ground level as well as looking down at some of the smoke emissions while flying it was clearly an easterly wind close to ground level, yet estimated by the crab angle on my return flight at a higher altitude I was sure of a northerly wind as predicted. Can it be that the surface winds are a different direction than the main stream at a higher altitude?

Winds closer the surface deviate due to surface friction. Can't remeber which way, but it opposite direction in N hemisphere... Higher level winds are more constant... Also depends on High and Low position in relation to yur position and the speed of the pressures movement. If isobars close together and front moving fast they will change fast, while if in middle of a pressure cell it should be quite constant....

PMarshall wrote:If there is no wind which runway do you use? This is assuming that there is no one else in the circuit...

Depends on local rules, neighbours etc. Morning Star defaults to 20 so we don't harass the Ostrich Farm to the north of us unnecessarily. I think Stellenbosch defaults to 01 because the farmer to the south tends to moan.

Tobie wrote: I flew a while with Cluless. At Altona on ground level as well as looking down at some of the smoke emissions while flying it was clearly an easterly wind close to ground level, yet estimated by the crab angle on my return flight at a higher altitude I was sure of a northerly wind as predicted. Can it be that the surface winds are a different direction than the main stream at a higher altitude?

Yes Tobie, it's called the Coriolis effect. When the wind is close to the surface the surface resitance changes/bends it's direction. Here in the southern hemisphere the surface wind could be as much as 30 Deg different (anticlockwise when viewed from above or Clockwise when viewed from the ground) to the upper air at say 5000 feet, i.e. surface wind of 180deg will be 150deg higher up at 5000ft. So in your case an easterly, at the surface would tend to be North Easterly higherup. At 10000ft it can be as mmuch as 60deg difference

Have a look at the aviation section at weather SA for the wind profiles.

See how the wind at 9800 feet deviates from the low level wind

Griffin wrote:

If runways are marked with thier magnetic degrees does that mean the marking will change, or have changed, over the years as the local variation changes?

Yes, that does happen. Years ago the runways at Durban were 05/23, now they are 06/24.

Regards,
Arnulf

I always wonder why they don't cut a big round grass circle and then you can land any way you like with any wind direction.

Thats the way it used to be many years ago. Thats why we still have the term "Airfield" It used to be a big field, and you could land any direction. The old aeries did not have brakes, were all taildraggers, and some did not even have a tailwheel, only a skid of some sorts. So your directional control was not that hot in a crosswind once you ran out of rudder control.

Another point - regarding wind, is that when a Tower gives you wind direction, the direction is MAGNETIC, as someone said earlier, the winds are given in true bearings, that is correct, but within the confines and area's of a field, magnetic.

kb wrote:Another point - regarding wind, is that when a Tower gives you wind direction, the direction is MAGNETIC, as someone said earlier, the winds are given in true bearings, that is correct, but within the confines and area's of a field, magnetic.

Interesting, I didn't know that. It does make sense so you can directly relate it to the runway hading without having to first calculate the magetic deviation.

Thanks kb

All winds below 50 ft are given in Magnetic. Above that are given in true. So if you look at the airfield TAF's and Metars they are magnetic. If you look at the upper wind charts they are true.