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MET Clarification

  • Darcy
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Darcy created the topic: MET Clarification

G'Day Bob and Richard,

Near the end of my CPL exams and have found MET to be one of the more challenging due to the depth of knowledge required. I have a couple of areas which I understand the basics but further explanation would be great please.

1. Vertical Stability- ELR/ DALR/ SALR: I understand the principle of this where if a parcel of air stop rising of its own accord (DALR = ELR) the parcel is stable and if it continues rising of its own accord it is unstable (ELR ≥ DALR) or conditionally stable for stable dry air/ unstable saturated air. Applying this to a question seems to be causing difficulty however.

- The value of SALR in cloud? A: Increases as altitude increases. I thought the SALR/ DALR were constants and the ELR wasn't.
- The value of SALR is lowest when? A: Air is warm and moist.
- The more the ELR increases, the atmosphere tends towards instability; a little confused on this one.
- Meteorological conditions indicating a stable atmosphere? A: Clear skies, poor vis, turbulent and cumulus clouds- I would have thought it would be stratus clouds instead of cumulus due to little convection occurring? I'm assuming the poor vis would be due to an inversion layer?

2. Diurnal Variation: I haven't been able to find anywhere regarding which season has the greatest variation. A mate elected 'Summer' in the CASA exam and got it wrong unfortunately. I'm guessing this is due to the increased heating during the day and so the land mass will remain warmer for a longer period during the night decreasing the variation? My guess is either Spring or Autumn; leaning towards Spring though.

Cheers,

Darcy
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Richard replied the topic: MET Clarification

Hi Darcy,

Glad to see you finally made it on to the forums! There are some good questions here so let's take a look:

1) Stability and Lapse Rates
Be careful with your definitions of stability there. If the ELR = DALR then the atmosphere is neutrally stable. In other words, the air will stay where it is unless forced upwards by some force. As long as the force exists the air will continue to rise. As soon as the force stops, the air stops rising and stays where it ended up. It will not sink back to its original position as with absolute stability. This means strong updrafts can develop even though the DALR = ELR.

The Bureau of Met breaks down the relationships of ELR, DALSR and SALR as follows:
- if the ELR is less than the SALR, the air is absolutely stable, since even saturated air will cool more rapidly than the environment.
- if the ELR is between the SALR and the DALR, the air is conditionally unstable, since dry air will cool more rapidly than the environment but saturated air will not.
- if the ELR is equal to the DALR, the air is neutrally stable - but only so long as the air doesn't saturate. (You could say it is conditionally unstable too)
- if the ELR is greater than the DALR ( and therefore also the SALR) the air is absolutely unstable.

As for the value of the lapse rates, the DALR remains fairly constant at 3o / 1000ft. The SALR results from the release of latent heat when water vapoiur condenses and the rate of condensation depends on the amount of water vapour, the ambient temperature and the ambient pressure. Therefore the SALR is also dependant on these too. In fact, at sea level it is about 1.2oC/1000ft but at 18000ft, it is about 2.4oC/1000ft.

In other words, the SALR increases with height.

Since the more condensation you have the more latent heat is reduced, the SALR will be lowest when there is heaps of water vapour. The warmer the air is, the more water vapour can be present. Therefore, the SALR will be lowest when the air is warm and moist.

If ELR is increasing, the atmosphere around the air parcels is cooling more and more rapidly with height. With a high ELR (e.g. > 3oC/1000ft) even dry air rising and cooling adiabatically will be able to cool as fast as the air around it and therefore will remain buoyant and continue to rise. Therefore as ELR increases, the atmosphere will tend more and more towards instability.

A: Clear skies, poor vis, turbulent and cumulus clouds

This would indicate the presence of convetive turbulence and instability in the atmosphere. Stratiform clouds would indicate a more stable atmosphere since these are clouds forming "under protest". Given the opportunity, the air would sink back down to lower levels and this is an indicator of atmospheric stability.

2) Diurnal Variation
A good way to summarise diurnal variation in temperature is to think "How much water is there?" If there is water present as vapour, or nearby (such as over an ocean) or trapped in plants and vegetation, then the diurnal variation in temperature will reduce. This is because the specific heat of water is higher than air. A high specific heat means you need a lot of change of thermal energy before you get a change in temperature: water warms slowly and cools slowly when compared to air.

In Summer, there is also more heat in the warm summer air which can be used to keep water in a vapour state. Warm air is more likely to be moist compared to cold air. This water vapour absorbs solar heating and stops the air heating quite so much and also retains the heat preventing the air cooling quite so much during the night. That's why the diurnal variation in summer is actually less than you would expect and in the tropics there is uncomfortably little drop in temperature at night during the Summer (yes, Darwin, I'm looking at you!).

Of course, diurnal variation varies with other local factors such as windspeed, land/sea breezes, elevation etc, but if I was to place a bet, I would say the diurnal variation is going to be greatest in Winter.

Hope that helps a little and good luck with your studies!

Cheers,

Rich

Here's a good reference to check up some of these facts:
www.bom.gov.au/aviation/knowledge-centre/
www.recreationalflying.com/tutorials/met...#adiabatic_processes
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  • Darcy
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Darcy replied the topic: MET Clarification

Hi Richard,

Thanks for taking the time to reply. Your explanation on the ELR/DALR/SALR has been very helpful.

Interesting about the Diurnal variation to as a couple of mates also thought Winter.

Cheers,

Darcy
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