SALR + DALR
<!–[endif]–>Well, as we draw to the close of week 3, it’s time to hear from me again.
This week’s main culprits have been the joys of adiabatic lapse rates, and the resulting terminology which is rife with TLA’s and FLLA’s (Three-Letter and Four-Letter-Long acronyms respectively). Also in the mix is a trip to Gibraltar and musical delight.
Well, as ever, I’ll start with the work.
All subjects are progressing at quite a pace, with the Chapters flying (no pun intended) by in Principals of Flight, Engines, Radio Nav, General Nav and Instruments. It’s been a week of Altimetry and compression ratios, of Mach-numbers and Great Circles. Notice that I’ve tactically not mentioned either Systems or Met in the list of ‘fast running’ subjects.
Meteorology (or ‘weather’ for the plebeians is a wondrously weighty subject.
We’ve spent the last few classroom hours looking at a phenomenon called Adiabatic Lapse . Adiabatic Lapse from a Met perspective can essentially be said the tendency of a parcel of air to cool as it rises (air can be modelled as discrete parcels thanks to its poor heat conductivity). The rising is caused by some kind of trigger action (Turbulence, Convection, Terrain, or movements of larger air-masses). The air rises, and due to the drop in pressure with height it expands, decreases in density and cools ().
The rate at which this parcel of air cools with height is known as the Dry Adiabatic Lapse Rate (DALR) and weighs in at approximately 3°C per 1000ft. Why ‘Dry’? Well, this lapse rate holds true provided the parcel of air isn’t saturated with water vapour. Air with less than 100% Relative Humidity (even air at 99.999999% Relative Humidity) is deemed ‘dry’. The reason for this is simply that air at 100% RH is holding the maximum water vapour it can for its temperature – in order to cool some of that water vapour must condense out as droplets.
Now, to turn water into water vapour requires energy. We say this energy is stored in the water vapour as ‘Latent Heat‘. It follows that when that water condenses it must relinquish some of its latent heat. So as a parcel of saturated air cools, it is in turn heated by the release of latent heat. The release of latent heat is about 0.4°C for every 1°C the air would cool were it dry. Hence the Adiabatic lapse rate for saturated air (the Saturated Adiabatic Lapse rate or SALR no less) is:
3°C-(3×0.4°C) = 3°C-1.2°C = 1.8°C per 1000ft
But what does all this mean?
These lapse rates compare nicely against something called the Environmental Lapse Rate.
If we imagine a parcel of air at ground level at 60% Relative Humidity and +15°C. Lets also imagine that there is water vapour contained within this parcel and that it will reach 100% Humidity at 0°C (Cooler air = less capacity to carry water vapour).
Our little parcel of air hits the side of a mountain and gets pushed up (that our Trigger action). As it rises it cools at 3°C/1000ft (DALR, because it’s not saturated). This will happily continue until our little parcel gets to 5000ft. At 5000ft the air will be at 15°C – (5x3°C) = 0°C. This is the ‘Dew Point‘ at which the parcel of air becomes saturated, and conveniently for this little example it’s also the point at which the mountain stops. But before we consider what this means, we must figure what the parcel of air will do next.
For this we compare the temperature of the air in our parcel with the temperature of the air in the surrounding atmosphere. The surrounding atmosphere follows the Environmental lapse rate, and this is the -actual- real-time temperature lapse rate in the atmosphere (and the temperature can increase with height as well as fall, because the world is complicated and hates Met students).
If the air is hotter than the surrounding atmosphere at the end of the trigger action (i.e. – we’ve reached the top of the mountain) then the air will continue to rise because hotter air is less dense. If it is cooler than the surrounding air it will Fall. Surprisingly if the parcel of air is the same temperature as the surrounding atmosphere it will remain at that height. It just so happens that the atmosphere at the top of this mountain is at -5°C, so our parcel of air at 0°C will continue to rise.
Its temperature will decrease again, but this time because it’s reached 100% Relative Humidity it will cool at the SALR (1.8°C/1000ft). Loosing water vapour to condensation all the time.
Lots and lots of small droplets of liquid water condensing out of the air.
High up in the sky.
Them’s Clouds.
It follows that if you know the changes in temperature throughout the atmosphere as you climb and you know the temperature and humidity of the air at groud level, you can make a fairly well educated guess at the height of the cloud base.
Well, this lesson is over.
I’ll let you all know about the joys of Gibraltar and the arrival of a very special package in subsiquent posts throughout the week.
Here’s to week 4!















