A commom theme in the Yasi event was the same or very similar questions being asked by new members almost on the same page so to help I have put together this article on how cyclones form.

It is taken from various sources (Notated at the start of each detailed explantion with a link) so that people can start here and further their own study eleswhere if they want.

This isn't the complete picture by a long shot but should give people a basic understanding of how they form. Yes they do form outside these parameters and have done so in the past and will probably do so again in the future.

CYCLOGENSIS
The following factors are what are required, generally, for a cyclone to form. It does sometimes happens that even with all 6 factors in place a cyclone doesn't form.

There are six main requirements for tropical cyclogenesis:
1) sufficiently warm sea surface temperatures,
2) atmospheric instability,
3) high humidity in the lower to middle levels of the troposphere,
4) enough Coriolis force to sustain a low pressure centre,
5) a pre-existing low level focus or disturbance,
6) and low vertical wind shear.


Detailed explanation: Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core with well-defined circulations. Certain criteria need to be met for their formation. In most situations, water temperatures of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft); waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone. High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop. Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.

1) SST's: http://en.wikipedia.org/wiki/Sea_surface_temperature
Normally, an ocean temperature of 26.5°C (79.7°F) spanning through at least a 50-metre depth is considered the minimum to maintain the special mesocyclone that is the tropical cyclone.[3] These warm waters are needed to maintain the warm core that fuels tropical systems.

2) Atmospheric instability: http://en.wikipedia.org/wiki/Atmospheric_instability
is a condition where the atmosphere is generally considered to be unstable and as a result the weather is subject to a high degree of variability through distance and time [1]. Atmospheric stability is a measure of the atmosphere's tendency to encourage or deter vertical motion, and vertical motion is directly correlated to different types of weather systems and their severities. In unstable conditions, a lifted parcel of air will be warmer than the surrounding air at altitude. Because it is warmer, it is less dense and is prone to further ascent. In meteorology, instability is described by a lifted index and a K-index.

3) High humidity: http://en.wikipedia.org/wiki/Low-pressure_area
is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.

4) Coriolis force: http://en.wikipedia.org/wiki/Tropical_cyclogenesis
A minimum distance of 500 km from the equator is normally needed for tropical cyclogenesis. The Coriolis force imparts rotation on the flow and arises as winds begin to flow in toward the lower pressure created by the pre-existing disturbance. In areas with a very small or non-existent Coriolis force (e.g. near the Equator), the only significant atmospheric forces in play are the pressure gradient force (the pressure difference that causes winds to blow from high to low pressure ) and a smaller friction force; these two alone would not cause the large-scale rotation required for tropical cyclogenesis. The existence of a significant Coriolis force allows the developing vortex to achieve gradient wind balance. This is a balance condition found in mature tropical cyclones that allows latent heat to concentrate near the storm core; this results in the maintenance or intensification of the vortex if other development factors are neutral.

5) A low pressure area: (http://en.wikipedia.org/wiki/Low-pressure_area)
, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low pressure systems form under areas of wind divergence which occur in upper levels of the troposphere. The formation process of a low pressure area is known as cyclogenesis.
6) Wind Shear: Effects on tropical cyclones Wikipedia http://en.wikipedia.org/wiki/Wind_shear
Strong wind shear in the high troposphere forms the anvil-shaped top of this mature cumulonimbus cloud, or thunderstorm. Tropical cyclones are basically heat engines that are fueled by the temperature gradient between the warm tropical ocean surface and the colder upper atmosphere. Tropical cyclone development requires relatively low values of vertical wind shear so that their warm core can remain above their surface circulation center, thereby promoting strengthening. Vertical wind shear tears up the "machinery" of the heat engine causing it to break down. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low level center.

Summary: Cyclone season in Australia is between November and April, but cyclones can still occur in the month of May.
According to the Bureau of Meteorology, Australia has, on average, 13 cyclones a year. Half of these occur in the western regions.

Thanks for compiling all this SBT!

Now the issue is, getting them to read this thread first

We both know you can point people in the right direction but some refuse to follow even the most mundane help.

At least it is there for those who want to read it.

Hopefully some people will find it helpful in the future.

Thank you for this SBT. Just starting to understand the Coriolis force in physical science so this will be a big help.
So weak low-level shear is good and strong high-level shear is good? Is that correct, or does shear have to be weak throughout the atmosphere?
Also what direction does the winds have to be throughout the atmosphere. Is it similar to supercell development?

Surface: NE @ 15 knots
925mb: NNE @ 20 knots
850mb: NNW @ 30 knots
700mb: NW @ 45 knots
500mb: W @ 60 knots
300mb: WSW @ 100 knots

Like that?

Big Pete, tropical cyclones and supercells are completely different beasts. The wind profile you describe above is excellent for supercells. From what I understand, for tropical cyclones you want weak shear - "Vertical wind shear tears up the "machinery" of the heat engine causing it to break down. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low level center." - Quoted from above.

Interestingly though, tropical cyclones themselves produce strongly-sheared enviroments. Tornadoes are not uncommon from tropical cyclones - the Karratha tornado I believe was one such example. Therefore, I think you have to seperate the broader scale vertical shear, which destroys tropical cyclones, from the "tropical cyclone-induced vertical shear" which effects thunderstorms within the tropical cyclone itself.

Confusing myself now....haha

Tornados are very common within tropical cyclones and low pressure systems. The rain bands that come ashore as a cyclone approaches the coast commonly have weak rain wrapped tornado"s. By weak I mean EF1-2, however kakadu during tropical cyclone George had a EF3 move through the area. TC George was a low at that point in time as well. So even lows can produce damaging tornados. Tropical cyclone yasi would have defiantly had tornados imbedded in it's rain bands which is a scary thought !!! In the united states a hurricane like yasi would have had tornado watches all along the coast where the hurricane was going to make landfall. So really from port Douglas to Mackay in yasi should have been under a tornado watch! If only we had such advanced weather Warning systems like the US.

So in the end, the wind profile above a cyclone will have backing at height ( winds rotating at different heights ) but the wind shear is not as violent as you would see on a sounding from a supercell outbreak.

Here a few clip I found well could be very good clip to watch.

http://www.hurricanezone.net/articles/tropicalcycloneformation.html

http://www.tropicalweather.net/what_causes_typhoons.htm

http://www.bom.gov.au/cyclone/index.shtml

Big Pete - what they said above but just to expand on what Trav said water spouts are also a common feature of cyclones. It is just that the last couple have decided to cross at night so have not been observed but during daylight crossings they have been observed and photos taken. Sorry i don't have the links anymore but Trav or Nitso may have more information.

I believe even though we may not get as many tornadoes as America we should have a more specialised Warning for tornadoes because they still happen from time to time and i believe some storm in Australia has at least 1 Tornado per year if they any one know if we get more i would love to know the stats. BTW Thanks Matthew for the vids watch all of them and enjoyed 'em.

Since 90% of people live on the coast, we don't usually have anyone around in a rural storm to accually identify and then report a tornado warning.

I beleive a small study (maybe for trav and nitso???) to see when these small cyclonic tornados occur, would explain a little more why a suburb can only have 1 or 2 houses completely destroyed and leave the rest standing

True maybe a few people could get together to run a study on how frequent Tornadoes are within Tropical Cyclones and maybe how frequent tornadoes are in normals storms to as America has a known stat for the amount of tornadoes they get by personal experience i can say that the myth that Australia does not get tornadoes is busted i believe i have seen actual tornadoes 2 times and without having to leave my yard. but if people were to do this study maybe more stats could bust the myth of Australia not getting tornadoes.

Nothing to get excited about Sub Tropical. Plenty of proof that we have Tornadoes in Australia. Example

http://www.bsch.au.com/severewx/index.shtml

I don't know where the myth part comes from but there are plenty of actual tornadoes reported, photographed and offically investigated that occur from normal weather not just Cyclones. Click on the link above.

Video of Karatha Tornado this year. http://video.perthnow.com.au/1808544602/Moment-of-Impact-Karratha-Tornado (one of several actually) from TC Carlos

Pert Now photo Gallery of the Karatha Tornadoes that where spawned from TC Carlos http://www.perthnow.com.au/gallery-e6frg1vc-1226010146684?page=1

So yes cyclones can and do spawn tornados in Australia. No need to do a study.

I believe there was one reported in Makay (spelling???) earlier this year too

Yes it happened not far from me at Christmas, I didn't hear a thing and yet there was a fair bit of damage from it.

yeah there was. It destroyed a family friend's boat it did.

Here a another very good Pictures How a
Hurricanes form.

That okay any time.

I know that Mackay had at least 2 this year. One through Cremone across the Pioneer River to Canelands shopping centre and another out Homebush/Rosella area.

In 1990 Cyclone Joy that cross at Townsville produced two tornados in Mackay one went through Slade Point 10 houses were unroofed and 30 houses and caravans damaged, the other I was 13 and remember hearing a roaring noise and seeing sheets of tin flying through the air, a machinery shed near Andergrove school was ripped up and dumped at Pioneer High School.

Thank you Sir Boab Tree and Mathew, for the information that you have provided for newbies like myself. I have also found that reading previous cyclone threads, has helped me get a better understanding as well.

Your most welcome Delly D,as this is exactly why I started this thread.

there was a tornado in Bowen in 1876 and a tornado in Mackay during the "small" cyclone that smashed up Bowen during the 1950s

Currently the Coral Sea has a couple of factors in place for a cyclone to form.

SST's are well above the tempratures required, shear is non existant and the upper winds have swung into favour, just need something to kick it off like a tropical depression and a bit of storm activity and who knows?

i totally agree that when TCs come ashore they should issue tornado watches it seems with many tcs when they cross they have embedded tornadoes in them

I've asked this question in the Cyclone Heidi thread, but it has gone unanswered.

Heidi was still a Cat 1 cyclone as she neared Wittenoom, some 210 km inland. Going by the radar it looks like at 12:50 WST she formed a new eye, which persisted and grew ever larger until it tore apart at 19:00 WST.

I thought the conditions that far inland over a desert would be all wrong for an eye to form? Or am I missing something?



See : 512km Radar Loop for Port Headland, 23:00 10/01/2012 to 13:00 12/01/2012 UTC

Hey Jax,

I have read and understood that cyclone form a true visible eye on sat images at Cat 3 level and above where it opens out and there is a cloud free region with in the centre of the system.

What that radar depicts I guess is still the rotating LLCC Low Level Circulation Centre which I gues on a radar looks to be an eye but its just a point of circulation. Also as the distance from the radar to the LLCC increases the radar may struggle to receive return signals due to heavier rain between the two as well as pure distance. This may have increased that appearance aswell. Pretty remarkable though that Heidi has stayed so intact for such a long distance inland. I think Yasi last year actually made it to Central Australia intact too. Amazing things hey.

Hi Jax I was in the middle of answering that question and lost power so Interwebs fell over.

OK The eye is still visible and in a Cat 1 that is a rarety but not unheard of. Cyclones can tarvel a great distance over land as a cyclone.

Have a look at http://www.bom.gov.au/cgi-bin/silo/cyclones.cgi hover your mouse over the blue track (Cyclone George) and you will see how far this one went before it faded into a depression (the X) There are a great many that have travelled at lot further distance than George did. Just cycle through teh trackmaps in the link above.

Hi all & thanks for starting this thread.
One thing I was confused about during Yasi (and at the time, a number of people on the forum noticed the same thing) was that at ground level in a number of places, there was no wind at all, but it was howling at roof level. This was particularly noticable in various parts of Cairns, going by the posts. The resulting lack of damage below roof level in various suburbs also seemed to confer with this observation. Could anyone give me an insight as to what was happening there?
The only reason I have been given so far was that the cyclone was moving very fast & creating its own 'updraft', therefore creating a type of vacuum at ground level. Thoughts anyone?

I could have a stab at it. Maybe it was due to a similiar concept to that of a hovercraft which is explained here.

"How does the air cushion beneath the hovercraft allow the vehicle to glide so freely? The key to the ease of movement is reducing friction. Vents of slowly moving, low-pressure air are ejected downwards against the surface close below it creating the air cushion. The trapped air cushion greatly reduces the friction of the vehicle, allowing it to glide freely upon the smooth surface below. "

Bit like Yasi with the winds flowing down off the surrounding hills forcing lower pressure into the coastal areas/valleys and enabling Yasi to slide over the top????

Out there concept but could be achievable on a big cyclonic scale.

Thanks for the replies Popeye and SBT. But I'm still a bit lost, please bear with me.

You've both mentioned Yasi, as well as your example with George SBT, both category fives when they crossed. Given Heidi was a category two, I'm guessing she's done well to travel so far intact. I'll check the link you provided SBT and see if I can find any others (cat 2s) that have done the same thing. From memory, have either of you seen a Cat 2 cyclone travel so far inland intact before?

Popeye, your explanation makes a lot of sense regarding the heavy rain making returns difficult the further it gets from the radar. But on thinking about it, I'm wondering why the radar would be able to read returns on the other side of that dead spot (or eye, or blank bit, or whatever) if the heavy rain on the radar side of that spot is the reason the spot is showing in the first place? Lol, hope that makes sense.

I've noticed these blank spots above the radar before, but not away from it like this one and its got me intrigued.

It isn't unusual for this eye to continue to be seen on radar for quite a long way inland as the tight circulation is maintained. As in this case it was a cat 2 when it crossed the coast and remained pretty well a tight system for some time, gradually losing shape and form until it died from a lack of 'fuel'.
Cat 1's rarely have a decent eye that you can see and you are right, the stronger teh system the longer it will have an eye that you can see clearly.


Winds:
Yasi etc can cause local ground effects similar to what you are talking about Popeye.
To eleborate. The topography (trees, hills, buildings etc) can have the effect of lifting the winds over structures and causing a lifting of the winds a couple of meters above ground level (Drag or friction). I must point out though that flat areas such as coastal plains with little vegetation means this has no effect on lifting the winds and they will be a simialr speed 10cm abover the ground as they will be at 10 metres until the hit an obstruction like a forrest or a ridge, moutain range etc.

In Townsville during Yasi you could hear the winds screaming overhead at much greater speeds that they where at ground level. I recorded 147kph winds (before nextdoors mahogany tree fixed ir for me) from my anonmeter which is 9 meters above ground level but at ground level the speed was down to under 80 kph due to the surrounding homes and trees.

But, more often than not with a Cat 1 even over the ocean the eye cannot be easily seen.

Thanks Popeye & SBT- that makes sense to me.

So SBT, are you saying that what is showing on the radar is actually an eye? If so, that's what's got me scratching my head. If you watch the loop I linked above, that eye seems to form on the radar several hours after crossing, not before, and then after several more hours gradually loses shape way out near Wittenoom. It's difficult to make out an actual eye on the radar prior to or during the crossing, but appears to open up beautifully later at about Yandeyarra

I haven't discounted Popeye's suggestion, but am still unsure as to why the radar wouldn't see the area in the centre of the system as it moves away, but would still see the far side of it.

Ha, so much for checking it out on the satellite images. Seems the satellite was having quite the dummy spit throughout almost the entire event.

http://realtime2.bsch.au.com/vis_sat2.ht...01201131233#nav