ON METEOROLOGICAL TEPHIGRAMS

©  Charles Warner   24 Nov 2015 edition

The meteorological tephigram is explained briefly, with emphasis on the aesthetic appeal it offers to an observer informed via cloud photography. This tephigram refers to clouds shown on the main page.

e30tef6
Recall Answer 31: Static instability yielded the narrow castellanus towers. A stable layer very close to the surface allowed patterning by wave propagation . Winds turned with height because friction slowed the wind at the surface, while a pressure gradient acted in another direction. These clouds were recorded at 8.8°N, 112.9°E, altitude 6500 m, 1118 local time on 30 Dec 1978, looking N. ( back for a look at the corresponding photo; Answer 31 is here being enhanced).

The trace of the Temperature of the atmosphere, on the right, shows an approximate match with that for the moisture content of the air, represented by the Dew Point Temperature, on the left. Increase in energy is signalled if Temperature rises; also if the Dew Point Temperature rises. On tephigrams the two traces commonly move in opposed directions. If Temperature is increased then usually Dew Point Temperature is decreased: a balance is indicated: Warmth often accompanies dryness; coldness often accompanies increased moisture content. [An account of the distincive clouds shown in the Frontispiece of my main page was published in Weather, 58, pp 84-9, Feb 2003, along with an appropriate discussion of a tephigram representative of its circumstances.]

The tephigram e30tef6 shown above is pleasing to the eye of an enthusiast: Note how the layer near the surface - up to ~870 hPa - is well mixed, and that the lowest 20 hPa, the lowest 200 m in altitude, is slightly different in character. It features a dry adiabatic lapse rate of Temperature. The Temperature trace leans to the left at the dry adiabatic Temperature lapse rate, 9.8°/km. From about 970 hPa up to the 880 hPa level the lapse rate is ~6.5°/km; this is the usual, recognized, moist adiabatic lapse rate for our tropical oceans. Convective mixing leads to this lapse rate: there were many clouds fulfilling their role of getting the received solar heat off the surface.

There is a disparity between the examination for Answer 31 in which a stable layer capable of yielding waves has been inferred, and the paragraph above in which no such singularity has been noticed. However there are minor wiggles, and I infer that these do represent real disturbances.

e29tef1
Tephigram e29tef1 is to represent clouds seen early in the WMONEX mission flown on 29 Dec 1978. I find it not very pleasing. Temperature and Dew Point did not always vary in harmony as anticipated by theory. [J.V.Iribarne and W.L.Godson wrote the classic textbook of the time Atmospheric Thermodynamics, D.Reidel, 1973]. Errors of measurement with the WMONEX dropwindsondes did occur, so one has to be circumspect. Users like me of the dropwindsonde data received magnetic tapes of WMONEX data, whence we made print outs of processed sonde data; The last line of data composed of Pressure, Temperature, Dew Point Temperature, Wind direction, Wind speed, ... did not always indicate that the surface had been represented accurately. The traces are taken down to a surface drawn by hatch lines, labeled with the recorded surface pressure to the nearest hPa.

A marked temperature inversion at pressure 750 hPa is obvious. This is usual during the Winter monsoon. It results from subsidence of the whole upper air mass (at very roughly the speed a human baby could crawl). The tephigram features an inversion at 925 hPa, and it features air masses of apparently different character superimposed, worthy of close attention should one want to infer what had been happening. .

e29tef18
Tephigram e29tef18 is to represent these clouds. The surface pressure, ~990 hPa, is too low probably by ~10 hPa; note also the paucity of wind data. Data were produced at very great vertical resolution, as may be seen from the traces of Temperature and Dew Point, and the winds were averages over ~100 hPa of descent. Modern data are better than these - state of the art for 1978. Delights of coupling exquisite photos with exquisite tephigrams remain to be explored.