BUT WHY

[facthunter]

answer Question 3.

 

Well I've seen the snow alongside the road in Europe get blackened by the Diesel soot. and the headers are diesel powered aren't they?. (I'm kidding if you hadn't noticed). Spiders know when it is going to rain.. nev..

 

 

[Guest Andys@coffs]

Question 3 is for someone as omniscent as a moderator .....

Omniscent, having a non directional nose? Able to smell BS from all directions equally well?

 

I can see how having an omniscent moderator for this thread is absolutely e-scent -ual

 

that missing i is important... just ask Steve Jobs how valuable an i is.... (i-pod, i-phone,i-ripoff etc)

 

Andy

 

slinks back to his cave

 

 

[Guest ROM]

Fairly easy answer to Question 3.

 

That's when the crop is ripe but it is also about the same time that the intertropical convergence zone which is making it's way south for the southern summer starts to affect the northern Australian areas and the eastern coast.

 

This then starts to feed tropical origin moisture into the southern weather systems and across the continent from the north west and down from the north east of the continent.

 

This tropical moisture infeed combined with the increased heating of the waters to the south of the Equator, with increased subsequent evaporation creates increased instability down the east coast leading to the formation of East coast lows and troughs, the effects of which often move well inland to the Wagga region and across as far as Kerang on the Murray.

 

The area around Kerang gets about half it's annual rainfall from the summer rains whereas west of that N.S line, winter rainfall predominates.

 

You can see how the rainfall west of Wagga tapers off as you get into summer but rainfall rates continue east of Wagga and even increases ie' more clouds, during the summer by looking at the rainfall annual and monthly charts in the Bureau of Met's's web site.

 

As to the wind dying down at night, the geographical location near hills and mountains can affect ground level flows a great deal but for the most part the dying away of convection and therefore of the mixing of the lower layers allows the "drag" of the ground surface to slow wind speeds down near ground level with a change in near ground air temperatures and a consequent stratification of the air layers.

 

The most impressive example of this effect that I have personally experienced happened many years ago when this old dog was still young and fit.

 

I was sowing crop very late in mid winter on a cool night.

 

It was still the days of 180 lb wheat and fertilizer bags which we carried across our shoulders the few metres from truck to the seeder.

 

The bags were dumped onto a platform about 3 feet above the ground on the back of the seeder, from where we emptied the bags into the box on the seeder.

 

The open paddocks of the Wimmera Plains are almost dead flat for 20 kilometres around and the night was dead still with no wind at all.

 

I climbed onto the seeder platform and nearly got blown off by wind.

 

To say I was astonished was an understatement!

 

I did some experimenting while standing on the platform and discovered at about 7 or 8 feet up above ground level there was a howling gale blowing.

 

I could actually stand on that platform and by raising or lowering my hand or head less than a foot could be in a howling gale or in almost dead still air.

 

There was absolutely no indication at ground level by sound or anything else of this wind shear effect.

 

There is far more astonishing things happening in our atmosphere than most ordinary citizens ever realise.

 

Pilots are amongst the few who actually are aware of and have experienced some of these very interesting weather phenomena.

 

Count yourselves as the lucky ones and use this knowledge to try to get a working understanding of how air and the atmosphere behave as that knowledge will invariably come in very handy somewhere at sometime in your future.

 

On fly-ins, just go to your local AWS in the BOM site and have a look at the wind rose and the rainfall weekly and monthly maps for the time of the probable best month for a fly-in.

 

It ain't always when you think is the best time and for some reason no Fly-in organiser will look at this but instead will look at what Joe Blow down the flight path is doing and whethere it ties in with some other happening.

 

The actual weather patterns recorded by any surrounding AWS's over many years is never consulted.

 

 

[Captain]

As to the wind dying down at night, the geographical location near hills and mountains can affect ground level flows a great deal but for the most part the dying away of convection and therefore of the mixing of the lower layers allows the "drag" of the ground surface to slow wind speeds down near ground level with a change in near ground air temperatures and a consequent stratification of the air layers.

Good post ROM. Thanks & very interesting.

 

 

 

Re your above para .... why then does the breeze also die a little before and after dawn?

 

 

 

And I notice that it also often gets colder a little after dawn. Do you have an answer to that too, or are these issues just all about boundary layers moving up and down?

 

 

 

You are right about pilots having more knowledge of what goes on in the atmosphere. I also fly sailplanes and I was fascinated to learn that nobody really knew about the mechanism that causes thermals & Cu's until the 1930's.

 

 

 

Regards

 

 

[Guest ROM]

Well it seems us sailplane pilots are the ones who usually ask the hard weather questions or try to answer them.

 

PPL in Tigers by 1960 and sailplanes since 1963.

 

Not much flying these days as being a semi retired farmer who still depends on the generosity of his son and DIL [with a huge debt] for his income means that there is not much money for flying after the last 11 years of dry and droughts.

 

The lowest temps just after sunrise are the result of the ground cooling through the night and continuing to do so until Old Sol gets his fire into gear and starts pumping solar radiation back into the atmosphere and ground at that particular location.

 

That only happens after the sun angle has got high enough so that the solar radiation is only penetrating through a much reduced atmospheric thickness.

 

On the horizion and just above it, the solar radiation has a very long path through the lower atmosphere so little or no heat for the first 15 minutes or so and the night cooling continues.

 

Got to think about the wind dying just before and just after sun up but the answer may be in the RAA's excellent weather module in the RAA site.

 

 

[youngmic]

I believe the answer lies in the diurnal pressure variation.

 

As the illuminator (the line on the planet of light and dark, illuminator light side and terminator dark side) as it is called travels around the planet it creates its own pressure change. This in part may cause some localised wind change effect during this time. Those who fly in the arid deserts will know these dawn and dusk breezes intimately, as they can be quite pronounced in the absence of strong surface friction (trees hills and such).

 

As the planet releases long wave radiation at night the air in contact with the ground cools thus forming an inversion, an inverted adiabatic lapse rate. This creates high stability in the lower layers effectively killing vertical movement, at the top of the temperature inversion the temperature lapse rate is still at the ELR (environmental lapse rate) ~2degrees/1000'. Here you will find the pressure winds are as they were during the day.

 

Least that's what me mum reckons.

 

 

[Captain]

Here is an example of the way the wind dies around here sometimes/most times near dawn. This is today and is courtesy of Weatherzone, so thanks to them.

 

 

 

[Ross]

Hi Captain

 

I think I might have seen your dog sitting on a box North of Gundagai waiting for the diurnal wind change when his sail plane driver master usually turns up very late in the afternoon.

 

Regards

 

 

[Ross]

I was very interested to see a continuous plot of temperature and wind speed during a gliding competition recorded at Brobenah (Leeton) some years ago.

 

About all I recall was seeing the temperature plot varying up and down by about one or two degrees about every five or six minutes or so as did the wind speed over many hours once the thermal activity had started. These wind speed and temperature changes were coinciding with the passage of numerous small thermals across the airfield.

 

The average airspeed at the time was perhaps in the order of about 4 knots.

 

 

[Captain]

Hi CaptainI think I might have seen your dog sitting on a box North of Gundagai waiting for the diurnal wind change when his sail plane driver master usually turns up very late in the afternoon.

 

Regards

G'day Ross

 

They have changed the words to the song to:

 

"And the dog sits by the Cobra Trailer.

 

While his boss waits for a retrieve.

 

200 miles from To-cum-wal"

 

Best regards Geoff

 

 

[Guest ROM]

The following article from "Professional Pilots" might go some way towards answering a lot of questions about near ground weather conditions.

 

http://www.stlouishelo.org/Weather%20Near%20The%20Ground%20-%20Karsten%20Shein%20-%20Professional%20Pilot%20-%20February,%202005.pdf

 

 

[Guest Cloudsuck]

The winds at height do not vary in velocity significantly during the day/night as they do near the ground. This is most to do with the MIXING of the airmass in the lower levels caused by the surface being heated by the sun during the day, resulting in changes to the adiabatic lapse rate, and generally more instability, (bumpiness).At night, when this heating is not present, the air is calmer, and doesn't mix with the "freestream" wind so much.. Not all winds drop off at night. near high peaks you can get downflowing cold( and denser ) air, that can be quite strong, and will build up after sunset. I'm not going into the complex names for these phenomena, just the principles.. Nev.

I love a guy who knows his met. That reply is spot on and spoken like a true glider pilot.

 

 

[farri]

Yes .... but why does it usually drop just while the sun rises and falls .... then resume again an hour or 2 later?

G`Day Captain,

 

Sorry I don`t have any better answers than have already been given but I can confirm that this is the case most of the year, here at Deeral 45k south of Cairns and just 2 NM west of the ocean.

 

Here is the opposite situation.

 

When the Northerlies are blowing ,and this occurs randomly from September through to December,the wind is strongest late afternoon through to mid morning then eases in the middle of the day.

 

I`m not sure what effect the sun has in this situation.

 

Cheers,

 

Frank.

 

 

[Tracktop]

The following article from "Professional Pilots" might go some way towards answering a lot of questions about near ground weather conditions.http://www.stlouishelo.org/Weather%20Near%20The%20Ground%20-%20Karsten%20Shein%20-%20Professional%20Pilot%20-%20February,%202005.pdf

This seems to be a very good basic training / info doco

 

Could it be stored somewhere so it is easy to locate in the future?

 

like the down loads section or something. I guess the link is all we need.

 

Ray

 

 

[farri]

 

 

And I notice that it also often gets colder a little after dawn. Do you have an answer to that too, or are these issues just all about boundary layers moving up and down?

 

 

 

To keep it as simple as possible,as I understand it,this is what occurs.

 

Just as the sun is rising,the heating effect is causing the dew on the ground to evaporate,this evaporation causes a temporary decrease in temperature.

 

Frank.