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Lows associated with hot air: the penny dropped.


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For *years* I could not get my head around how come lows are associated with hot air and not cold air. The first problem I had was that I associated rain with cold weather. (Now, I suppose that is due to a cold front lifting warmer moist air).

 

The main problem I had was that when you heat a gas, pressure increases, rather than decreases. Like if you heat a balloon, the gas will have a higher pressure and thereby expand the balloon. *Today* I realised that because atmospheric pressure is due to the weight of the air, so the the air becomes less dense, pressure decreases. And, air becomes less dense when you heat it. 

 

As an aside, for an air mass to expand, it would still have to have an increase in pressure. Otherwise there will be no way for to do the work of pushing away the air above it. So, maybe lows have a higher pressure gradient between low and high altitudes. 

 

Correct me if I am wrong. 

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Air pressure at ground level varies considerably as highs and lows passover. I often have to crank my altimeter back a couple of hundred feet to match local elevation. Last Wednesday it was out more than ever.
Five minutes into my flight I noticed the altimeter was reading  1,000’  lower than OzRunways….Woops-  I’d cranked in the wrong direction.

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Low pressure is associated with rising air. Air is always moving from a high pressure area to one of low pressure and accumulating moisture along the way. The Coriolis effect of the earths rotation makes it move in a clockwise direction in the Southern hemisphere gaining speed as it moves towards the centre of the low pressure area. By contrast high pressure is associated with descending air and is dryer moving down and outwards in an anti clockwise direction in the Southern hemisphere towards lower pressure areas. There are lots of very good books on how weather works.

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On 22/11/2021 at 8:00 PM, APenNameAndThatA said:

For *years* I could not get my head around how come lows are associated with hot air and not cold air. The first problem I had was that I associated rain with cold weather. (Now, I suppose that is due to a cold front lifting warmer moist air).

 

The main problem I had was that when you heat a gas, pressure increases, rather than decreases. Like if you heat a balloon, the gas will have a higher pressure and thereby expand the balloon. *Today* I realised that because atmospheric pressure is due to the weight of the air, so the the air becomes less dense, pressure decreases. And, air becomes less dense when you heat it. 

 

As an aside, for an air mass to expand, it would still have to have an increase in pressure. Otherwise there will be no way for to do the work of pushing away the air above it. So, maybe lows have a higher pressure gradient between low and high altitudes. 

 

Correct me if I am wrong. 

I read this in The Age newspaper today re La Niña. I thought I’d understood it then read your post and reviewed…. I’m studying this article  now. 
 

https://www.abc.net.au/science/articles/2013/01/31/3679358.htm 
 

All helps stay clear of IMC. 

 

A semi-permanent band of low pressure stretching from Antarctica to the southwest corner of the country, known as the West Australian trough, moves westward along the continent’s southern coastline towards Tasmania. Low-pressure systems typically bring strong winds and rain, but the effect of the West Australian trough on the eastern seaboard is counterbalanced by a semipermanent cell of high pressure in the Tasman Sea near Tasmania, which guides the low-pressure system towards the Antarctica. But the normally reliable highpressure cell in the Tasman Sea has broken down. ‘‘ That means these [wet low pressure] systems are free to flow on and just bubble across the Australian continent, rather than being steered south of the continent as they go across,’’ Dr Risbey said.
 

 

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As I have eluded to in an earlier post, I am the long lost 4th brother of The Three Stooges.

Reading the recent topics on methanol and outback survival, and the above weather conundrum, I have taken note about my lack of knowledge and personal attitude and offer the following as my self inflicted flagellation.

1. Carefully select the correct coloured Garbage Bag

2. Take Glen’s advice and don’t drink the methanol 

3. Place bag over head and then inhale the methanol instead….the outback   aviator’s version of “chroming “

4. Once finished, come to the conclusion that I don’t travel far enough to leave one air mass to the next as I don’t fly RPT aircraft.

5. Get into the plane, point it at the next airfield, then land and have a coffee.

6. Repeat step 5 but in the opposite direction and return to the hangar.

 

Steps 5 and 6 are to be repeated as often as possible so as to ensure stress free Recreational Aviation 

 

Ken

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6 hours ago, Bosi72 said:

Recommended reading BOM Manual of Aviation Meteorology. 

There is also an online course:

https://learn.bom.gov.au/course/view.php?id=136

 

click eLearning module

This is really good. My least favourite subject. Aviation weather. Finally making more sense. I’ve combined here the ABC high and low article and the BOM info. Hopefully I have it right and will just reverse in northern hemisphere. Lows go with clock in Southern Hemisphere and ACW northern. 
 

 

13A400CE-6BE3-4154-8971-12679504415E.jpeg

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You can also visualise winds using https://www.windy.com/

 

1. click anywhere on the screen to set the windspeed and direction flag

2. turn on pressure lines

3. you can see the winds at different altitudes

4. you can see the winds in the future

 

windy.thumb.png.4ae0bff57d30c5c550cf00aaa0439acf.png

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Windy is amazing and accurate even days out. No excuses to be caught by bad weather now. Get Windy to know what is going on, NAIPS to be legal.

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Where the wind came from and the nature of the airmass is nearly everything about weather Understand Lapse rate as well.  Weather is a most interesting subject that has always been difficult to get good references to. Nev

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Windy for some reason is the most accurate Weather app available. It is strange being based on 4 different worldwide systems, the default being ECMWF. BOM is so convoluted and hard to follow and often wrong & NAIPS, well, not worth the effort really but that is what we are supposed to use. As a volunteer with SES I make my predictions mainly based on Windy data which conflict with what the highly paid hierachy say quite often. The last 2 major events that were supposed to happen according to our glorious masters failed to materialise and my (Windy) predictions which I even put in writing to our local team were spot on.

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At the last casa avsafety seminar I attended the topic was "weather to fly". Someone asked the question about using alternative weather sources, only NAIPS is legal was the official reply. I remember thinking the unfortunate pilot in the example case may have had a better understanding of the big picture and how the weather was going to unfold for the return journey by using windy.

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A lot of "operators" only care about the need for an alternate or holding fuel so they can advise their fuel requirements.  Bugsmashers should go into it more than that.  Whether (and When) to go is something they have full control over. .Nev

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