Hi,

I have two questions with regards to diurnal range of temp. and wind and ELR.

I know factors like cloud cover, atmospheric stability, surface type, wind speed, humidity, effect of latitude etc would alter the diurnal range of temperature but what about wind? I'd like to know more about the diurnal variaton of wind and the factors involved. Would it be things like, land/sea breeze, katabatic/anabatic winds?

With reference to the ELR, why do Density Height calculations use the standard ELR if the actual ELR is not 2 deg. per 1000 ft and varies from day to day and throughout different layers of the atmosphere on any one day. Does this mean that for most part the ELR is 2 degrees and not 3 degrees (or any other figure) lapse rate. Also, is the SALR constant like the DALR?

Thanks,

Frederik

Land v's sea breeze and katabatic v's anabatic are examples of common diurnal variations in wind. There is another common diurnal variation referred to as atmospheric decoupling. It is common under high pressure influences where there is not a lot of convective mixing occurring in the lower levels of the atmosphere. At night, particularly when it is cloudless, the outgoing long wave radiation cools the earth's surface and the adjacent air. Because this lower air is then more dense, it tends not to mix at all with the air above. The synoptic scale gradient wind at around 3000 ft AGL continues, but the air close to the earth's surface tends to become calm (or close to) under these conditions. This is why the wind strength at the surface often drops after sunset under the influence of high pressure systems. This is also one reason that low level jets can become so strong overnight - the boundary with the still air below is virtually frictionless.

As for the ELR, just like you never get a perfect ISA atmosphere, you will never get a 2 degree per 1000' lapse rate in a real atmosphere. It is an average. If you want to know what the actual atmosphere looks like in your location on any given day, you can consult the atmospheric temperature soundings that the Bureau conducts daily in many locations. They are accessible on the web at www.bom.gov.au/aviation/observations/aerological-diagrams/

Neither the SALR or DALR are constant, but for practical purposes they are close enough to the CASA figures for use in the lower atmosphere (below about 10-15,000'). One thing worth noting that CASA does not factor in to the syllabus is that the dewpoint of a given parcel of air does not remain constant as it rises (it falls by around half a degree per thousand feet). This means in the real world, if you want to calculate a theoretical convective cloud base you should divide the dew point depression by 2.5 degrees to find the number of thousands of feet at which saturation will occur, rather than dividing by 3 degrees, which is the DALR and the number to use in CASA exams (but not the real world).

That was a good explanation. Thanks Ray.