Perth Today

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Perth Today

Postby Dylfish » Wed Jul 25, 2012 3:14 pm

Hey Guys,

A quick question about todays weather, I've been looking at the clouds outside and all I can see in the air is Stratocumulus which are starting to look a little bit menacing. While I was on lunch I had a look over charts and bits of information to see if I could see any precusor to the cloud buildup and the accompaning darkness.

I checked out the MSLP from earlier today and I can see were about to be engulfed by a high pressure system with the old low passing close to Adelaide now. I took from this that in theory there should be somewhat clear weather. Next I checked out this mornings SKEW-T from Perth intl and from what i've learned the atmosphere looked very stable and dry and with a rather low temperture there seemed as nothing would change in the way of the surface rapidly heating up during the afternoon to cause convective rising.

I'm wondering since looking at all the charts im familiar with at the monent there seems to be no reason that jumps out at me to form this cloud. What am I missing? Were these clouds formed by orographic lift over the hills or some type of convergance that I can't see on a chart yet?

Cheers :D

(p.s. Sorry about all the questions lately, I'm trying hard to learn on my own but I need to ask time to time)

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Re: Perth Today

Postby Pete » Wed Jul 25, 2012 9:10 pm

Yeah, I'm pretty sure you're right there, although I'm unfamiliar with the topography around Perth. What you had was a superadiabatic layer near the surface. Basically daytime heating causes an area of air to become extremely warm and thus rise. Because we're under a high pressure zone the air rapidly cools and condenses, forming cloud. Since the layer of air above the super-adiabatic layer is quite is stable, the top of the cloud becomes flat. Some helpful links.
http://books.google.com.au/books?id=IJ5 ... er&f=false
http://books.google.com.au/books?id=IJ5 ... er&f=false
http://www.theweatherprediction.com/habyhints/31/
http://www.met.tamu.edu/class/wflm/tut/conv/conv8.html
I believe the correct term for these clouds is stratocumulus cumulogenitus.
http://www.seewetter-kiel.de/fotos/cl/cl4_030.jpg
Image

Is that what you saw?

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Re: Perth Today

Postby Dylfish » Wed Jul 25, 2012 9:49 pm

More or less that's what I saw, Maybe a tad flatter?

I was out driving when the cloud cover started and I first noticed it out towards the hills area so I was thinking that might have something to do with it. I'll go through those links tomorrow morning when my brain isn't such a mush.

One question though

Basically daytime heating causes an area of air to become extremely warm and thus rise. Because we're under a high pressure zone the air rapidly cools and condenses, forming cloud


is this convective lifting or am I off the 8 ball? I've been reading about DALR and SALR and the LCL, but I keep thinking if cloud was able to form on a cold day like this due to heating of the ground (and today was cold), why don't we have cloudy days every day?

Thanks Pete

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Re: Perth Today

Postby MetRep » Thu Jul 26, 2012 4:10 pm

Pete is quite right.
The cloud yesterday was stratocumulus and is cloud underneath a strong subsidence inversion which exists due to the high pressure system. The cloud is formed by convection, even though the ground temperature is relatively low. It is all dependant on the instability below the inversion.

A cloudy day like yesterday is often referred to as anticyclonic gloom. It blocks the sun and makes for a rather cold day, especially if the cloud cleared overnight and there was a low minimum. Melbourne in winter gets a lot of these days.

As well there needs to be some surface moisture. On Tuesday the 3pm dewpoint at Perth Airport was -0.5. Yesterday it was 5.8.

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Re: Perth Today

Postby Dylfish » Thu Jul 26, 2012 6:45 pm

Thank you very much, I have still so much to learn.

So the high pressure pushes the dry air downwards and as it does it warms adiabaticlly. this gets sandwiched between instable air below and above it? I am assuming that the inversion was at about 900mb on todays 00z perth intl. sounding. Apart from the moist air below this inversion, how would i know that the air is unstable? the TAPP is to the left of the ELR throughout the whole sounding. Is there somthing else I should be looking out for instability?

Also would the Subsidence inversion always be noticeable by the fact it warms up and the dew points drops off so significantly?

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