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I have data taken from over 2300 ascents through the atmosphere at various locations around the world,

for every 500 meters vertically upwards I have averaged the temperature and plotted it: Lapse Rate

Its quite clear the lapse rate is dropping near ground level for every location. Near ground level is supposed to be unsaturated, meaning a lapse rate of approximately -2C/1000 feet however for the data I have the average never goes more negative than -1.5C/1000 feet and drops to below -1C/1000 feet near the surface.

Can someone explain this?

An excel file with the data can be found here if you wish to look into it yourself: https://www.dropbox.com/s/0k44i1mc4nxt8pu/Cities.xlsx

Edit: I have also updated the spreadsheet to contain differential (the gradient) of the curves.

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Edd
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  • Since the question is about lapse rate, you might want to plot lapse rate with altitude -- ie the forward difference of that plot with height, might make it clearer what you're trying to ask. – tpg2114 Dec 31 '13 at 00:00
  • @tpg2114 I have updated the spreadsheet link in the question, the spreadsheet now contains a plot of dT/dZ against altitude – Edd Dec 31 '13 at 12:56

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There are two things that I can think which will influence the lapse rate close to the ground.

The first is the heating from the ground itself. Depending on the location and the season, there may be considerable heating of the air due to stored energy in the ground or due to things like cities, roads, concrete, etc.. Since all of your data lines are labeled with city names, my guess is there may be a non-trivial heat island effect going on, preventing the air from cooling as rapidly as one might expect.

The second is that close to the ground the measurements are well within the planetary boundary layer. Within the layer, most of the classical assumptions don't hold and many of the approximations made throughout the rest of the atmosphere are invalid close to the ground. There is a lot of turbulent mixing, viscous effects from the surface of the Earth, etc. all of which may change the lapse rate from what is "expected".

tpg2114
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