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Calculating Cloud Base with ELR/DALR/SALR

  • Aaron
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Aaron created the topic: Calculating Cloud Base with ELR/DALR/SALR

G'day Guys,

I was working through a couple of questions in regards to working out potential cloud bases with the ELR, DALR and SALR. I came across this question:

On a hot summer's day, the temperature at Canberra Airport is 35C The dewpoint at the time is 29C. Assuming that a suitable environmental lapse rate exists, at what height above the aerodrome would cumulus clouds be likely to form?

The answer I have been given is 2500ft, and I have no idea how you get that answer!

Any help would be appreciated.

Cheers,

Aaron
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  • Bungee
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Bungee replied the topic: Calculating Cloud Base with ELR/DALR/SALR

Basically cloud will form when a warm 'humid' air mass rises until it reaches dew point (condensation). This hot air will only rise if it is warmer than the environment and will only form cloud if it is humid, otherwise it will just create turbulence. Cumulus cloud is formed when the cloud is warmer than the environment, hence it keeps on rising, this is due to heat being released its SALR (about 1.5Ëšc per 1000') is lower than the ELR (about 2Ëš per 1000'). I think this is called 'Conditional Stability' so if you find this in Bob Tait text book you should find how it all works.

Is this question in the Bob Tait book? if so maybe you can give the excersize/page number as a reference and it will be easier to help you. I am not sure how the answer is 2500' without knowing more than 'a suitable ELR exists'.

cheers
Ben
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  • Aaron
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Aaron replied the topic: Calculating Cloud Base with ELR/DALR/SALR

G'day Ben,

Thanks for that, I have figured out how to do the question with the help of my Uni lecturer. (I fly with the University of SA out at Parafield)

Happy Flights,

Aaron
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  • Bungee
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Bungee replied the topic: Calculating Cloud Base with ELR/DALR/SALR

Thanks Aaron
I wouldn't mind knowing how the answer is 2500' if you wouldn't mind sharing the solution.
Cheers
Ben
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bobtait replied the topic: Calculating Cloud Base with ELR/DALR/SALR

The key to this question is to understand that dew point temperature itself lapses with increased height [or more correctly with dropping pressure]. As an unsaturated parcel of air rises, it cools at the dry adiabatic lapse rate of 3°/1000 feet. However the dew point temperature withing the rising parcel also drops at about .6°C/1000 feet. Given your figures, the temperature within the parcel at 2000 feet would be 29°C but the dew point would also drop to 27.8°C.

At 2500 feet, the temperature within the parcel would have dropped at the dry adiabatic lapse rate to 27.5 °and dew point temperature would have dropped to 27.5°C. So the cloud base would be at 2500 feet.

I'm sure you would not get a question like that in a CASA exam.

Bob
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  • Aaron
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Aaron replied the topic: Calculating Cloud Base with ELR/DALR/SALR

G'day Ben,

As Bob said above, that is the solution to the question!

And Thank you Bob, I was talking to me Uni lecturer about it today and he said that you wouldn't get it in a CASA exam.

Cheers,

Aaron
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