The Carnegie Curve
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- Harrison, R.G. Surv Geophys (2013) 34: 209. doi:10.1007/s10712-012-9210-2
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The Earth’s fair weather atmospheric electric field shows, in clean air, an average daily variation which follows universal time, globally independent of the measurement position. This single diurnal cycle variation (maximum around 19UT and minimum around 03UT) is widely known as the Carnegie curve, after the geophysical survey vessel of the Carnegie Institution of Washington on which the original measurement campaigns demonstrating the universal time variation were undertaken. The Carnegie curve’s enduring importance is in providing a reference variation against which atmospheric electricity measurements are still compared; it is believed to originate from regular daily variations in atmospheric electrification associated with the different global disturbed weather regions. Details of the instrumentation, measurement principles and data obtained on the Carnegie’s seventh and final cruise are reviewed here, also deriving new harmonic coefficients allowing calculation of the Carnegie curve for different seasons. The additional harmonic analysis now identifies changes in the phasing of the maximum and minimum in the Carnegie curve, which shows a systematic seasonal variation, linked to the solstices and equinoxes, respectively.
KeywordsAtmospheric electricityGlobal circuitPotential gradientUniversal diurnal variation
During the first quarter of the twentieth century, the Carnegie Institution of Washington operated the Galilee and Carnegie as geophysical survey ships. Both were sailing vessels constructed of wood, allowing geomagnetic measurements to be undertaken. Overall, seven major cruises were undertaken by the Carnegie, between 1909 and 1929 which, together with atmospheric electricity observations, returned magnetic and oceanographic surveys3 and studies of the marine biology, physics and chemistry.
Preliminary atmospheric electricity measurements were made on the final cruise of the Galilee (August 1907) which continued on Cruises I, II and III of the Carnegie (1909–1914). Cruise IV (1915–1917) of the Carnegie was the first voyage well-equipped for atmospheric electricity work. For the atmospheric electricity measurements, the Carnegie’s scientific team had to pioneer new methods and instruments. At the first trials in 1907, obtaining any reliable measurements of the atmosphere’s electric field4 over the oceans seemed insurmountable due to ship motion, sail changes and the need for high-quality insulation despite spray and damp, but, by the seventh cruise in 1928/1929, the techniques had improved so substantially that their goal of continuous recording5 of the atmosphere’s electric field over the sea was reached. Analysis of the results from Cruise VII confirmed the important result that the daily electric field variation in universal time was independent of the ship’s position. A similar variation had been seen previously using the more intermittent manually recorded data from Cruises IV, V and VI (Mauchly 1921, 1923). This diurnal variation is a key part of the contemporary understanding of global atmospheric electricity (Aplin et al. 2008) and is generally known as the Carnegie curve, against which modern studies are still compared (e.g., Mach et al. 2011; Rodger et al. 2006).
Although further measurement campaigns had been planned for the Carnegie, Cruise VII sadly became its last, as the ship and all her equipment were destroyed in a petrol explosion at Apia (Samoa), on 28 November 1929. Both the ship’s master, Captain J. P. Ault, and a cabin boy died in the disaster, with five others injured. Only a few months earlier (26 August 1929), in addressing a commemorative event for the Carnegie in San Francisco, Captain Ault had remarked6 that “The story of individual endeavour and enterprise, of invention and accomplishment, cannot be told.” Whilst his own demise with the Carnegie of course brought a tragic end to these endeavours, some of the great accomplishments of the ship’s staff are nevertheless immortalised through the many research publications of the Carnegie Institution of Washington (e.g., Bauer et al. 1917; Torreson et al. 1946).7
The purpose of this paper is to provide, almost a century later, a modern summary of the methods with which the Carnegie curve was obtained, and is accordingly confined to the Carnegie’s potential gradient (PG) data, reproduced digitally in the Electronic Supplementary Material (ESM). The many other ocean air atmospheric electricity measurements made on the Carnegie, for example, of air conductivity, have themselves proved useful in studies of long-term pollution changes (Cobb and Wells 1970), as may yet the ocean air ion production rates measured during solar cycles 15 and 16, before the cosmic ray reduction associated with the grand solar maximum of the second half of the twentieth century (Owens et al. 2012). But, as no similarly extensive and thorough global surveys have been conducted—in part, perhaps, due to the loss of the research ship which would have undertaken at least some of them—this early PG data in particular retain its importance to the study of fair weather atmospheric electricity and the modern synthesis provided by the global circuit concept (Rycroft et al. 2000). In digital form, the Carnegie data source is now amenable to modern analysis, such as in considering the statistical variability not evaluated in the original study, and for seasonal analysis of the data.
2 Historical Background
2.1 Early Atmospheric Electricity Measurement Technology
Early measurements of the atmospheric potential over land used an insulated electrode exposed to the atmosphere above the surface, connected to a mechanical electrometer to allow the extent of the electrification to be found (Harrison 2004a; Nicoll 2012). The sensing electrode or “collector” used to acquire the air’s local electric potential was initially typically a flame or elevated point (Harrison 2004a). Electrometers of the eighteenth and nineteenth centuries employed electrostatic repulsion, such as with pith balls (Read 1792) or the deflection of straws or fibres (Hackmann 1994).
In 1860, Lord Kelvin described a “water dropper” collector system (Everett 1868), which consisted of an insulated tank continuously generating a spray of water. At the point where the water broke into drops, the spray transferred charge into the air until its potential became equal with the local potential of air. This was found by measuring the potential of the tank that was measured with respect to Earth, and a continuous chart record could be obtained photographically. Although in principle this provided an absolute method for determining the air potential at the spray generation point (and, therefore, the vertical potential gradient between this point and Earth), Kelvin was aware that the effect of an electrode could be to distort the local field being measured (Thomson 1859). To determine the geometrical correction, measurements without field distortion can be obtained by using a plate electrode, mounted flush with, but insulated from, level ground. Such an implementation was ultimately used at the Kew Observatory site near London (Harrison 2003; Harrison and Ingram 2005) with a procedure devised by Wilson8 (1906), near to the Kelvin water dropper recording system.
Balloon-carried measurements of atmospheric electricity parameters began in the late nineteenth century and also employed water dropper technology (Harrison and Bennett 2007; Nicoll 2012). For the measurement of air conductivity,9 an aspirated tubular sensor was developed (Gerdien 1905). This consisted of a well-insulated central electrode, mounted concentrically with an outer tubular electrode (see Nicoll 2012, Figure 5). After charging the central electrode to a known potential, air was drawn through the tube by a fan, and the rate of decay of the central electrode voltage measured. This allowed the air conductivity to be found (e.g., Aplin 2005). An additional feature of the “Gerdien tube” was that its outer electrode provided some electrostatic and physical shielding of its sensitive inner electrode.
The first PG measurements aboard a ship at sea seem likely to have been those of Exner, made between Aden and Bombay in 1888 (Simpson and Wright 1911). Clearly, the approach of using a flush-mounted plate electrode over undistorted ground was unsuitable for use on board ship. However, the need to correct for the field distortion was, if anything, even more acute than over land, as, for sailing ships, almost every location on board was strongly influenced by the presence of yards, sails and ropes, whose positions constantly changed. Realising this, in the pioneering marine PG observations of Simpson and Wright (Simpson and Wright 1911) on the Terra Nova during 1910, measurements made on board the ship were calibrated to additional measurements determined over a level beach. This site was 1,000 yards from the ship, where a long (18 m) fine horizontal wire, well insulated at each end, had been arranged. Because of its very small cross-section, and the substantial distance between the mounting posts, the long fine wire “antenna” provided minimal field distortion and, therefore, reference PG values.
The ratio of the ship-measured PG to the beach-measured PG provided the geometrical correction for field distortion, known as the reduction factor, which, once determined, could be applied to the ship PG measurements if the sail and rigging geometry remained unchanged. (The reduction factor remains a fundamental consideration for all PG measurements, in which placement of a sensor distorts the local field; see, for example, Bennett and Harrison 2006.)
One further aspect of the Terra Nova and the earlier measurements of Simpson10 in Lapland (Simpson 1906) was that the collector used was coated with radioactive material. This enhanced the local ionisation and, therefore, the air’s local conductivity, increasing the rate at which the collector acquired the air potential, giving better time resolution.
2.2 Origin of the Carnegie Oceanic Atmospheric Electricity Measurements
…we beg leave to suggest that it would be in full harmony with the proposed plan to combine with the organization of international magnetic work also the inauguration of observations pertaining to the electric condition of the Earth and of the atmosphere…
…seemed quite impracticable…the rolling of the ship, the flapping of the sails, and the varying position of the yards and boom under various sailing conditions all contributed to make the problem of reducing observations of potential-gradient to a uniform basis too complicated…
Of several atmospheric electricity instruments originally considered for evaluation on the Galilee, only air conductivity measurements, using the Gerdien aspirated device (Gerdien 1905), apparently showed any promise for ocean measurements. Air ion measurements continued to be made on Cruise I of the Carnegie (1909–1910), alongside observations of “specific conductivity” and “radioactive content”.
On the Carnegie’s second cruise (1910–1913), as well as continuing with conductivity and radioactivity measurements, PG measurements were attempted again. Radioactive collectors were used with a mechanical electrometer, with the collectors suspended on a bamboo pole extending back from the ship’s stern rail. A similar method was used on Carnegie Cruise III (8 June 1914 to 21 October 1914, between Brooklyn, Hammerfest, Reykjavik and Brooklyn), supporting an ionium15 collector from the rear of the ship. By making simultaneous ship and shore observations, at Reykjavik and Gardiners Bay, a correction for the electrostatic distortion due to the ship’s structure, known as the reduction factor, was obtained. This allowed the PG measurements for the whole of Cruise III to be calibrated, giving the average for the cruise as 93 Vm−1.
2.3 Cruises IV, V and VI
Standardisation of the measurements on the ship to open surface measurements was achieved by using simultaneous observations at sea and on land. These comparison sites were chosen to be close to level with the sea and free from trees, with a horizontal passive wire antenna of 15–20 m long employed as the reference measurement. Satisfactory sites were hard to find; however, as there were difficulties in maintaining the ship’s mooring within half a mile, the reduction factor was originally measured only for short periods (minutes) and averaged. Later, these standardisation experiments were extended to durations of a few hours, with the linearity between the two sets of measurements demonstrated before deriving the reduction factor. A further complication was the distortion of the electric field by changes in the arrangement of the ship’s sails. The PG measurements were only made when the mainsail was up with the boom to port or starboard, or when mainsail down and the boom positioned “some 2 feet over the port crutch”. These differences were quantitatively important, as apparent from the range of final values16 of the reduction factor used for 1915–1921, which varied from 2.85 (mainsail up and boom to port or starboard) to 3.77 (mainsail down and boom 2 feet over port crutch). Because changes in reduction factor affected the final values derived by the same proportional amount, they were allowed for in the results by recording the state of the sails and boom.
By the end of Cruise V, the parasol collector had become badly corroded, so the apparatus was entirely rebuilt, also implementing a stronger support rod. This arrangement (“PG2”) was made to similar dimensions to keep the reduction factor the same. Reduction factors for PG2 were first made in Colon Harbor (at the entry to the Panama Canal) on 2 April 1915, and applied retrospectively to the data from the first year of Cruise IV. Further tests after Cruise VI on nonlinearities in the reduction factor found no variation, and it was said to be “practically constant”, for PG from 120 to 480 Vm−1.
…the 24 hour Fourier wave was at the great majority of land stations in practical phase agreement on universal time with the prime daily wave over the oceans without regard to location.18
3 Potential Gradient Measurements on Cruise VII
Dates and destinations of the Carnegie during cruise VII
10 May to 18 June 1928
Newport News (Virginia, USA) to Plymouth (England)
7 to 20 July 1928
Hamburg (Germany) to Reykjavik (Iceland)
27 July to 16 September 1928
Reykjavik (Iceland) to Barbados (West Indies)
1 to 11 October 1928
Barbados (West Indies) to Balboa (Panama)
25 October 1928 to 14 January 1929
Balboa (Panama) to Callao (Peru)
5 February to 1 April 1929
Callao (Peru) to Pago Pago (American Samoa)
20 April to 6 June 1929
Apia (Samoa) to Yokohama (Japan)
3 to 23 September 1929
San Francisco (California, USA) to Honolulu (Hawaii, USA)
2 October to 18 November 1929
Honolulu (Hawaii, USA), to Pago Pago (American Samoa)
3.1 Potential Gradient Instrumentation
The photographic recording electrometer system was manufactured by Günther and Tegetmeyer, consisting of a Wulf bifilar fibre electrometer, a projection microscope, and recording box containing a motorised photographic paper roll and marking clock. The deflections of the electrometer measuring fibres were illuminated by a 110 V lamp, with their image projected onto a section of bromide photographic paper, as it was transported from an unexposed roll to a storage roll. The image on the photographic paper provided a chart record of the electrometer’s variations. Paper rolls of 150 cm length were found sufficient for a week’s record, allowing 5 cm between daily records.
which also interfered with the measurements. All these factors conspired to limit the number of fair weather days’ data eventually obtained, quite apart from the weather conditions themselves. One of the longest recognised inconveniences in fair weather atmospheric electricity measurements, spiders (Read 1792), added their unique contribution to lost hours of data by spinning webs in the cap of the shore electrometer and across the supporting insulators of the stretched wire during the calibration experiments.
In order to shelter the helmsman from the fierce tropical sun, frequently it has been necessary to stretch an awning over the stern.
Standardisation of the ship-borne PG measurements against shore measurements21 was essential to the absolute calibration of the results, and so the reduction factor required for this conversion was measured whenever possible. A stretched horizontal wire system on a land site was used to determine the undistorted “reference” PG. These determinations were made throughout Cruise VII, at Kitts Point, Maryland (5 May 1928), Engey Island, Reykavik (25 July 1928), Bridgetown, Barbados (28–29 September 1928), Easter Island (9–10 December 1928) and Apia, Samoa (10–13 April 1929). The first two standardisations yielded reduction factors which were applied to the eye-reading instrument (PG2), and the others were used with the continuous recorder system.
3.3 Summary of Measurements Obtained
Total number and proportion of fair weather days selected on Cruise VII, by month
Total number and proportion of fair weather days selected on Cruise VII, by season
Season (northern hemisphere)
Original season selection
Conventional season selection
Number of fair weather days
Proportion of data (%)
Number of fair weather days
Proportion of data (%)
4 Data Analysis
4.1 Mean Diurnal Variations
Both the original24 annual fit using the Carnegie Institution harmonic coefficients and the harmonic fit recalculated from the data in ESM are plotted in Fig. 6a. The recalculated and original harmonic fits are virtually indistinguishable. As a further check, calculations of the hourly values from the original harmonic fit are compared with hourly means from re-averaging the transcribed data in Fig. 6b. The close agreement shows that original harmonic fitting represents the data extremely well, as determined by fitting a least squares line; this line has unit gradient and zero offset to within its standard errors. This consistency gives confidence in both the reliability of the transcription of the data, and, since the original and recalculated annual Fourier coefficients give almost identical results, the harmonic analysis.
As remarked earlier, the data can be grouped seasonally, and the original data analysis divided the data into northern hemisphere winter (November–December–January, NDJ), spring (February–March–April, FMA), summer (May–June–July, MJJ) and autumn (August–September–October, ASO). Harmonic coefficients were derived for the data selected according to these groups of months by Parkinson and Torreson (1931), which were also tabulated in Israël’s text book on Atmospheric Electricity (Israël 1973). However, the coefficients were then updated in Carnegie Institution publication 568, published in 1946, and consequently, some of these coefficients (primarily the annual ones) differ from those of Table XVIII in Israël (1973). The close agreement of the 1946 CIW coefficients with those recalculated here using the transcribed original data (Fig. 6a) and the verification of their representation of the data (Fig. 6b) show that the later values represent the raw data correctly; using the CIW 568 coefficients for the annual average diurnal cycle gives a mean value of 6 Vm−1 less25 than from the coefficients of Israël Table XVIII. The difference in phase from the two sets of coefficients is negligible (<0.1 h).
Coefficients obtained from harmonic analysis of the Cruise VII data, by season
Amplitude coefficients (Vm−1)
Phase angle (degrees)
4.2 Seasonal Variations
The duration of Cruise VII of only about a year and its poor sampling of fair weather days during the northern hemisphere summer has led to difficulties in drawing conclusions from this data about seasonal variations in the PG. One approach to deriving the seasonal PG variation has been to include data from other cruises to provide further northern hemisphere summer information, although this brings with it questions about consistency of absolute calibration and selection of fair weather circumstances. Rycroft et al. (2008) combined the data from Cruises IV, V, VI and VII to provide more data in determining the change in the Carnegie curve in different seasons, but still found the maximum and minimum of the diurnal cycle barely statistically distinguishable during the summer. Adlerman and Williams (1996) combined the Carnegie data with measurements of another survey ship, the Maud, and concluded that the seasonal variation in PG showed a maximum in the northern hemisphere summer, coinciding with the known maximum in global lightning at that time. The harmonic analysis of Table 4 provides an alternative method for investigating characteristics of the annual variation, by considering seasonal variations in the Carnegie curve’s phase.
Figure 8a clearly illustrates a different seasonal phasing for the variations in the maximum and minimum hours. Comparing the left-hand side of the plot (northern hemisphere spring) with the middle part of the plot (northern hemisphere summer), the interval from minimum to maximum is greatest for northern hemisphere summer, somewhat suggestive of a contribution from the longer day lengths of the northern hemisphere summer. In surface PG measurements made at Vostok, Antarctica, Burns et al. (2005) found diurnal maxima of 2050UT for July–August and 1830UT for November–December, consistent with both the timings and seasonal change apparent in Table 4. The derived maximum and minimum times of the seasonal Carnegie curves are plotted against each other in Fig. 8b, together with the single point showing the maximum and minimum times of the annually averaged Carnegie curve. It is clear from this presentation of the data that variations of these times approximately describe an ellipse, due to a larger variation in the time of the maximum than the minimum. The spread clearly varies with the sampling (this makes the “May” and “June” values indistinguishable, for example), but, because some symmetry is still apparent, and no appreciable hysteresis is evident, it seems likely that these parameters derived from the Carnegie curve represent seasonal variations in the global circuit.
4.3 Latitudinal Variation
5 Historical Importance
As mentioned in the introduction, a major early motivation for studying atmospheric electricity was explaining the sustained electric field apparent in fair weather. An important challenge to the prevailing perspective occurred around 1900, when C. T. R. Wilson and Elster and Geitel established that atmospheric air was continuously ionised, pointing to the importance of current flow, rather than a solely electrostatic situation (Simpson 1906). Subsequently, in 1920, Wilson proposed (Wilson 1921) that currents in the atmosphere could be maintained by thunderstorms and shower clouds. In support of this hypothesis, good agreement was later demonstrated between the Carnegie curve obtained in the early cruises and the diurnal variation thunderstorm area by Whipple in 1929 (Whipple 1929), also recognised by Wilson (Wilson 1929). These developments provide some important context for the Carnegie measurements.
The Carnegie diurnal variation as a confirming idea is now briefly considered further, firstly in terms of the original thunderstorm area relations, and secondly in terms of further confirmatory ideas.
5.1 Persistence and the Thunderstorm Area Correlation
Carnegie curve PG and land areas of thunderstorms from Whipple and Scrase (1936)
Area (104 km2)
A less arbitrary alternative approach to evaluating the significance in time series with persistence is to use a Monte-Carlo method, randomising the data whilst maintaining statistical properties of the persistence in Fourier space, to evaluate how often the correlation observed actually does occur by chance (e.g., Ebisuzaki 1997). Using this approach, p < 0.01, and hence, the correlation apparent in Fig. 11 so central to the global circuit concept is unlikely to have arisen by chance.
5.2 Subsequent Diurnal Variation Determinations
The universal diurnal variation in atmospheric electricity represented by the Carnegie curve, now apparent in many of the variables of atmospheric electricity, is clearly one of the fundamental oscillations of the atmospheric system. Its early twentieth-century discovery provides thoroughly investigated reference information against which subsequent changes can be assessed, particularly for the annually averaged and northern hemisphere winter diurnal variations. Analysis of the shape of the curve, in particular the phasing of its maximum and minimum, yields a little more information, as both the times of the curve’s minimum and maximum vary seasonally, lying around an ellipse (Fig. 8). The time of the minimum of the Carnegie curve varies approximately between the equinoxes, whereas the variation in the time of the maximum, which is greater, occurs approximately between the solstices. Comparison with the classical thunderstorm area data in Fig. 11 suggests that changes in the time of the maximum occur during changes in the relative contributions of American and African thunderstorms between the southern hemisphere and northern hemisphere summers, but this simple interpretation ignores the additional contributions of electrified shower clouds (Liu et al. 2009).
Finally, the ocean PG measurements which underpin the Carnegie curve were self-evidently obtained as a result of great experimental ingenuity and diligence, alongside the development of pioneering instrumentation in the pre-electronic era. That the analysis undertaken in the early 1930s can be replicated almost exactly is a further tribute to the care with which the Carnegie Institution scientists reported, tabulated and computed their results.
Experiments clearly demonstrating fair weather electrification occurred during the summer of 1753 in England and France (Harrison 2011a), supporting the view of Lemonnier (1752) that the electrification arose from the air rather than the Earth.
Fair weather conditions are those in which no local electrification processes are occurring, without appreciable convective cloud extent. (In early literature, such weather conditions were also described as serene.)
Oceanographic studies of the Carnegie contributed particularly to understanding the deep water of the Pacific and flow in the north Pacific, notably through the work of Roger Revelle and Harald Sverdrup.
Throughout, the atmospheric electric field will be described as the Potential Gradient (PG). For a vertical electric field Ez, the PG is given by −Ez; the PG is positive in fair weather.
Continuous photographic recording was long established at land observatories. Lord Kelvin argued in September 1859 for “incessant” recording of the atmospheric electric field using such methods (Thomson 1859). [Lord Kelvin (William Thomson, 1824–1907) was President of the Royal Society and an outstanding classical physicist. In 1860, he suggested that the potential gradient’s sensitivity to environmental conditions might prove useful “…for prognosticating the weather.”].
Carnegie Institution of Washington Publication 568, page iii.
These will be referred to using the abbreviated form “CIW”, adding volume numbers and pagination as appropriate. They are now available in digitised form at http://archive.org/details/carnegieinstitution.
Charles Thomas Rees Wilson (1869–1959) was a Scottish atmospheric physicist and winner of the 1927 Nobel Prize for physics, for the invention of the cloud chamber (see also Harrison 2011b).
Air has a finite electrical conductivity because of the constant generation of molecular cluster ions, from natural radioactivity and cosmic rays. The conductivity of air is proportional to the number concentration of molecular cluster ions present and their mobility. Air’s electrical conductivity is a very variable quantity as cloud droplets or aerosol particles, which show strongly fluctuating concentrations in atmospheric air, modulate the conductivity by removing ions.
Sir George Simpson (1878–1965) travelled on the Terra Nova as meteorologist on the tragic British Antarctic Expedition led by Captain Robert Falcon Scott, and was later director of the Met Office (1920–1938).
Julius Elster (1854–1920) and Hans Geitel (1855–1923) worked together at a secondary school in Wolfenbüttel, which allowed them time for research, notably in atmospheric electricity and radioactivity. Simpson (1906) acknowledges Elster and Geitel’s work in evaluating rods coated with radioactivity as a suitable collector for Antarctic use.
Louis Bauer (1865–1932; http://library.gl.ciw.edu/ocean/background/bauer.html) championed international cooperation in geomagnetism and founded the journal Terrestrial Magnetism in 1896, subsequently becoming Terrestrial Magnetism and Atmospheric Electricity, and later the Journal of Geophysical Research.
Sebastian John Mauchly (1878–1928; Obituary: Nature 123, 215–215, doi: 10.1038/123215a0), Carnegie Institution Scientist from 1914.
CIW568, preface, iii.
A short period of the September 1928 measurements in the Atlantic has since also been found to correlate with land measurements at the UK Observatories of Eskdalemuir (Scotland) and Lerwick (Shetland), (Harrison 2004b).
CIW 568, p113 adds, in the preamble to Table 3 “Values have been interpolated for periods of 1–3 h only on days regarded as particularly quiet and undisturbed. No interpolation has been done over periods of bad weather.”
Subsequently, the need to identify fair weather conditions meteorologically became more widely recognised. For example, from 1 May 1964, the UK Met Office defined ‘fair weather’ conditions as those without hydrometeors, with no low stratus cloud, less than three-eighths cumuliform cloud and a mean hourly wind speed of less than 8 ms−1.
This is apparent in the A0 coefficient, which is alternatively 138 Vm−1 (Israël 1973) and 132 Vm−1 (CIW 568).
Subsequent work has indicated that cosmic ray ionisation varies spatially with geomagnetic latitude, with least ionisation occurring at the magnetic equator (e.g., Bazilevskaya et al. 2008). The geomagnetic effect on cosmic ray ionisation also influences the PG at the surface, and hence, a spatial variation is caused.
Reconstructions of ocean temperature anomalies do not indicate appreciable ENSO effects in 1928–1929, during Cruise VII (e.g., http://www.cgd.ucar.edu/cas/catalog/climind/TNI_N34/index.html#Sec5).
The modern study of atmospheric electricity owes a great debt to the Carnegie scientists who undertook the pioneering cruises described, and I am grateful to the Carnegie Institution of Washington for permission to use material. Keri Nicoll and Denise Hargreaves helped considerably with the data transcription and checking, and Andrew Lomas kindly generated Fig. 2a. David Marshall (University of Oxford) and Walter Munk (Scripps Institution of Oceanography) provided helpful discussion of the Carnegie’s oceanographic importance.