Author Topic: Thunderstorm Features  (Read 7803 times)

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zacaroo

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Thunderstorm Features
« on: 06 January 2007, 07:05:21 AM »
Ok, I haven't been on a proper chase yet but when storms come around my local area I try and distinguish the features such as the gustfront, shelf cloud etc.

My problem is most storms around here are the high precipitation variety so it is hard to see most features. Or maybe I am looking at the wrong things?

1) I don't really know what an RFB is. My understanding it is the lowest part of the storm with no precipitation falling from it?
2) What is inflow and outflow? Inflow bands is the shelf cloud, correct? Then what does the outflow look like?
3) Where is the updraft region in the middle of the storm?

Thanks, hopefully I get some responses.

Offline Jimmy Deguara

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Re: Thunderstorm Features
« Reply #1 on: 07 January 2007, 04:17:56 AM »
Hi,

What you have asked here are very important questions.

When convection bubbles develop further, as you would know, they can form thunderstorms. When thunderstorms do develop, there still usually is more air pusching the cloud tops upwards. This is called the updraft air. Now if this updraft air is coming from the ground, what is taken away from the ground has to be replaced. This comes from anywhere on the side usually where the wind is coming from. This is the simplest way to explain inflow - the air rushing into the storm.

Now take that enormous thunderstorm growing. It cannot grow forever. Something has to give. At this point, a nice dark base has formed because there is so much material in the cloud covering the sun. This is called the rain free base (some Australian chasers nicked named this to be RFB and have caused confusion to many others).

If the updraft cannot support the weight of the mass of moisture particles in the cloud including the hailstones that may have formed, precipitation begins to fall. This massive of air and moisture or even hail descends (goes down) toward the ground and is called the downdraft. Most of the rain we get from thunderstorms come from the downdraft.

Now we have an interesting interaction here. You have the air going in and up inflow and the outflow air coming down. If the air coming down which is cooler, spreads out and pushes the inflow air out of reach, the storm begins to die. It needs to breathe. Pulse storms that go straight up and hardly move basically die after half to one hour period because the outflow air has spread out so far that the inflow is cut off.

Now with severe storms, the updraft and the downdraft are a little more organised. Depending on the winds pushing the storms and what is going in BEFORE the storm develops, that tells meteorologists and storm chasers which storms are organised enough to chase. In other words, most severe storms besides pulse storms can last much longer and produce much more severe weather. This is because:

- the updraft is stronger and can hold far more moisture and hail in the air including larger hailstones
- the outflow in some storm types does not cut off the inflow as fast so the storm can last longer
- the downdraft can fall at such a pace that they can cause more storm updrafts on the side (although this can occur with non-severe storms)
- the rain free bases are often far more impressive

So the thing to look out for the next time you watch storms develop is the cycle - how the updraft first grows, rain begins to fall and then the rain with the downdraft can begin to cut off the inflow and the storm looks weaker and mushy. The lightning activity also may decrease during and after this stage.

So to answer your question? The updraft and downdraft are within the storm. Where they are located DEPENDS on how the storm formed in the first place based on the winds flowing if there was any! For instance, if there was an easterly wind and the winds were above were from the west, the updraft cumulus tower will rise and tilt forward a little from the west. The whole cloud will begin moving based on the speeds at the different levels. How the downdraft falls depends on the tilt and where the rain free base forms and where the rain falls. This is where you can see the visible part of the downdraft. Sometimes, the air falling down may not be so visible or even invisible. If you are near the downdraft though you will feel the cooler air.

Regards,

Jimmy Deguara
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Offline Jimmy Deguara

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Re: Thunderstorm Features
« Reply #2 on: 07 January 2007, 04:32:55 AM »
I separated the question regarding inflwo bands because it is a little more complex to explain and is a separate part of the whole storm question.

Firstly, inflow is air rushing into the storm and up. However, sometimes and especially in places like Tornado Alley, the air rushing into the storm may be on the ground and above the ground as well. We call this deep inflow. You may have inflow rushing in at levels of about 1 to 2km in depth - now that is deep inflow. What you get then is formation of INFLOW BANDS. These are bands of cloud connected with the storm you see as it approaches. Because they are much rarer in Australia, we don't often get the chance to see them and often get confused what they could look like. You will see the cloud along the inflow flowing in towards the storm!

Beaver Tail is a form of inflow but forms slightly differently. They also tend to develop in storms called supercells. The "beaver tail" looks often like a beaver's tail so was named as such. This feature develops when the air rushing down and around the front in a supercell meets the inflow. Because supercells are very organised, there seems to be a balance maintained. Where these meet above the ground at base level is where the "beaver tail" forms. Most of the "beaver tails" occur in organised Tornado Alley storms. However, there have been some local Australian examples:

beaver tail examples




inflow bands





Try more than one inflow bands



Try beavery tail and also you can just notice inflow bands into the storm


I hope this helps.

More extensive reading is available here:

http://www.australiasevereweather.com/techniques/index.html

Regards,

Jimmy Deguara
« Last Edit: 07 January 2007, 05:17:25 AM by Jimmy Deguara »
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zacaroo

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Re: Thunderstorm Features
« Reply #3 on: 07 January 2007, 05:37:38 AM »
Thanks Jimmy I reckon that will help me some more. I already knew that when lightning activity decreases the storm is in it's weakening process. I will tell you how my studies of what I asked you about went.

Cheers,
Zac

Offline Jimmy Deguara

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Re: Thunderstorm Features
« Reply #4 on: 07 January 2007, 05:46:17 AM »
Hi,

I hope I never gave the wrong impression in regards to the decrease in lightning activity as always meaning weakening storms.

Sometimes, even powerful storms go through periods of active and non-active and then active lightning. However, it you look at the storm and has perhaps dropped in size, has less rainfall, become mushy in appearance, sun is shining through etc, then yes with lightning decreasing you can almost be certain that the storm is weakening.

Just be careful that one storm weakening can give rise to another storm or one is developing next to it anyway.

Regards,

Jimmy Deguara
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zacaroo

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Re: Thunderstorm Features
« Reply #5 on: 07 January 2007, 06:11:24 AM »
Yes we have had that a few times. I waited for nearly an hour for one storm to come through but it weakened and went north of us and then I saw another forming. I could see big updrafts? I will post the picture of what is pretty sure updrafts. It moved in and over the next 40mins it moved closer and closer, until lightning struck with in 5kms. Which put the WHOLE power out where I was viewing from street lights, houses, traffic lights etc, the next bolt was closer. I was videoing, and as soon as the bolt finished I turned the camera off and I missed the thunder on my cam as doing so but I was on my bike. I have some videos. The core of the storm passed to the south of Redcliffe though.

No comments required but for readers, please be careful storm chasing on a bicycle. Lightning is one of the number one killers from storms.
« Last Edit: 07 January 2007, 06:30:25 AM by Jimmy Deguara »

zacaroo

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Re: Thunderstorm Features
« Reply #6 on: 07 January 2007, 06:34:58 AM »

Offline Jimmy Deguara

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Re: Thunderstorm Features
« Reply #7 on: 07 January 2007, 07:20:12 AM »
I can see the updraft as you suggest - also I can see part of an anvil above. The anvil is the top of the storm and spreads out because the air rising at this level has reached the equilibrium point - it cannot rise much further and spreads out at that level. Fast moving updrafts punch through this level sure but the overall storm tops are at this level.

Regards,

Jimmy Deguara
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centralian

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Re: Thunderstorm Features
« Reply #8 on: 28 June 2007, 05:01:35 PM »
Hello I am new here and have just read the best explanation of thunderstorm features.
Have posted a photo of an isolated storm to the east of Alice Springs that exploded within 30mins back in 2001.


Offline Michael Bath

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Re: Thunderstorm Features
« Reply #9 on: 30 June 2007, 01:24:14 AM »
Welcome Centralian !   I encourage you to post reports in the severe storms or tropical weather section from your part of the country and please don't hesitate to start threads if you have questions or have significant weather to report.

regards, Michael

Slight edit by Jimmy
« Last Edit: 08 July 2007, 12:45:51 PM by Jimmy Deguara »
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Offline Mike

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Re: Thunderstorm Features
« Reply #10 on: 30 June 2007, 03:15:33 AM »
Howdy all.  I'd like to know with regard to low pressure systems over land and resultant things; when the term 'couplet' is used - what is this and do you have a MSLP diagram showing such a thing?

Mike
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centralian

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Re: Thunderstorm Features
« Reply #11 on: 07 July 2007, 04:46:29 PM »
Thanks M.B. for the welcome, I can't wait for summer and the storms!, we get both tropical influenced storms and frontal influenced storms, Will update with photo's when they come.

Regards Jason .