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Electric space charges over melting snow on the ground
PressLtd. Printedin Northern Ireland Journalof Atmospheric andTerrestrialPhysics. 1965,Vol.27,pp. 91to 99. Pergamon
Electric space charges over melting snow on the ground R. B. BENT and W. C. A. HUTCHINSON Department of Physics, university of Durham (Received 3 July 1964) Al&&--Records of space charge concentration and potential gradient over a melting snow cover indicate an average concentration of 5500 negative elementary charges/cm3 in the first metre above the ground, with a wind speed of 10 m/set, and an average of 770 positive/cm3 between 1 and 21 m. The upper positive space charge is accounted for by the blowing of snow on neighbourmg high land. The lower negative charge in the air near the ground, persisting in spite of the wind, can be explained if a separation of charge occurs at the surface, the positive charge remaining on the melting snow. 1. INTRODUCTION DURING the winter of 1963-64
some records were made of the space charge concentration at two heights, 1 and 2 m respectively, over ground covered or partially covered with snow. During one period with wind blowing at some 10 m set-1 over melting snow the record indicates a separation of electric charge, and provides results of interest because of the fundamental role which ice and water play in some theories of cloud electrification. There seems to be general agreement that when snow is blown by the wind the potential gradient near the ground is raised considerably above its normal fine weather positive value, and SIMPSON and SCRASE (1937) based their theory of thunderstorm charge separation partly on such observations made in the Antarctic between 1910 and 1913. A more detailed distribution of space charge accompanying blowing snow has been deduced by MAGONO and SAKURAI (1963) from observations of potential of the air at different levels up to 3 m. They found negative charge lying above positive in the first metre, and positive space charge above that. LATHAM (1964) has made an analysis of observations of snowstorm electrification. He concludes that all well authenticated observations on the electrification of blowing or sliding snow can be interpreted in terms of the temperature gradient theory of LATHAM and MASON (1961), according to which there will be a separation of charge along a temperature gradient in a piece of ice, the colder end becoming positive. There are, however, reports of electrification observed when snow or ice is melting. MAGONO and KIKUCHI (1963) measured the charges on snowflakes which melted
as they
ing snow applied
fell through
consisted the
melting
were obtained
a heated entirely
snowflakes
with artificial
that on one occasion sign of charge
almost
cylinder.
Periods
of negatively
flakes. positive.
when When
the failheat
was
became predominantly Similar results snowflakes. RAMSAY and CHALMERS (1960) found
when there was a quite sudden
on the precipitation
DINGER and GUNN (1946)
were chosen
charged
reported
changed that
within they
91
change
from
sleet to snow the
t min from positive
found
positive
charging
to negative. of melting
-’ -.
Arl’:ir{A’I’r~s
The observations were made at, the Uurham .ITniversity Observatjory #it<%,whcrc the instruments were mounted on a steel Iat,t,icc mast 22 m high. ‘l’ht: space charge was measured with collectors which haw alwatly been described bv HEW (19ti4). ‘L’hey incorporate glass woot filters which t,rap virtually all the large and small ions, the net, charge per unit vol [Ime of air being nwasured c(~ntin~~ousl~~x\.ith a Faraday cage connected to an elrctrometer. ‘I’he cotlectors have proved extremely r&able over a period of several months. For these ohswvations the,y were monnt~ed at I and 2 m respectively. A field mill was set. Hush with t,hc surface of tlw ~rountf at. 40 m from the mast, and Mr. H. I,. (.‘ollin l)rovided some data front his t) 20 Nar. 1964, lOOO---121.5hours. It will be seen lat,er that period C is of outstan~~ing interest,. Throughout period A the potential gradient at the ground was positive and the space charge values at both 1 and 2 m kept close together and were positive. For the whole of period 13 the potential gradient at the ground was positiw ‘l’&le 1. Range ofnlxmvut~ions
A I9 Ike* 13w 1900 hours
Period .~~~..
.~~~~ .~..
.--.
-----
Space charge, e crnm3 at 2 m tvt 1 III I’okntial gratlicnt . \‘m-’ 2tt0 III \l’ind Speed, m see-l Direction, dog ‘f’emp. at 1.2 m, “F Relative humidity, at 1.2 m at 8 cm Clondition \I-e&her
of
snow
in periods A. B, C: and T). Bkmentary IS ‘0 I>(‘l* 1330 1800 hortrs
chargcl e
C! 17 Ma 154Om 1840 hours
1.6 1: IO‘-“” (’ I) 20 Mar loot)I 2 t 5 t~oLws
.
--
-! (10~.~00)
(10~.-600)
-1(‘LOO-600)
( IO0 4300)
--(loo--ALTO)
( I oo-400) ( 100-400) ( fOC)- 800)
-1( 10% 500)
.-:wi
‘~(ZOO-600)
4-11 300 so- 33
3- 9 288 30. -35
7-14 120 35~38
3 180 37 -39
“/, 60 -53 52- 55
Kot meking Fine
47 47 Not melting Fin,
50-60 54.-64 Meking Fine, hazy
90 95 Melting Fine, wry mkty
Electric Table
space charges over melting
2. Space charge and potential 1542 hours
At 21 m 2m lm Om
snow on the ground gradient
C
1638 hours + 220 Vm-l
-t 330 Vm-l + 640 Vm-l
in period
93
+ 100 e cme3 -150 e cmP3
+500 Vm-l + 400 Vm-l
no observations were made at 2 m. the space charge at 1 m was positive; Period C was quite different. At the ground the potential gradient was continuously positive; on the two occasions when data from Mr. Collin’s mills were available it was more positive at 1 m than at 0 m, but much less positive at 21 m. The space charge was always negative at 1 m and always positive at 2 m from the start of the period until 1800 hours, after which the records gradually approached one another and coincided at 1830 hours. For the whole of period D the potential gradient at the ground was negative, and the space charge values at 1 and 2 m kept close together and remained negative. A summary of the recorded values is given in Table 1, together with the following additional information: wind direction and speed at 10 m, air temperature at 1.2 m, relative humidity at 1.2 m and 8 cm, and condition of the surface. Where the values changed appreciably during the period the range is shown. Table 2 shows values of space charges and potential gradients on two occasions in period C. In all cases the space charge density is expressed in the units e cm-3 where the magnitude of the electronic charge e = + 1.6 x lo-l9 C. Various portions of the space charge and potential gradient records are shown in Figs. l-4. In periods A and B the ground was completely covered with snow, with no In the first half of period B the 8 cm relative humidity was apparent melting. and
Fig.
1. Typical
record in period A.
I-4 Zmin
Fig.3. Find part, nt‘ pwiotl (‘. higher than that at l-2 m; later the l-2m ,value was the higher. During period C the snow cover was only partial, it was wet and certainly not blowing, and was rapidly disappearing. fn fact. at t,he Durham Science Laboratories site about 1 km away the routine observations on t*hc state of the ground included a reference to snow at 1500 hours but none at 1800 hours. Before period I$ there was a new fall of snow; this was
Electric space charges over melting snow on the ground
95
2min.
Fig. 4. Typical record in period D. melting rapidly during that period, and was noted in the routine observations at 1200 hours but not at 1500 hours. Here it is convenient to include other relevant results from obse~ations between December 1963 and May 1964. It was not uncommon for space charge densities to be as much as 5500 e cmm3, with corresponding potential gradients at the surface. The records were made almost invariably during fine or fair weather, when an excess of positive space charge appeared to be normal, so for convenience here we may regard negative densities exceeding 300 e cm-3 and positive exceeding 400 e cm-s as being unusual. There were ten such periods with negative space charge, and in eight cases there was mist or fog locally or humidity was in any case high. In one of these ten periods however the weather was fine with no report of mist within 100 km. Thus there seems to be a close connexion between these negative space charges and mist, and we may safely regard this as the phenomenon investigated by ~HAL~IERS (1952), who showed that the negative potential gradient occurs do~~n~lind of electric power overhead transmission lines where presumably the corona discharge in misty conditions gives an excess of negative ions to the atmosphere. Durham Observatory is practically surrounded by these transmission lines at distances between 1 and 6 km. Of the nine high positive space charge periods with densities exceeding 400 e cm-3 there were four when dry snow which might blow in the wind was lying either at Durham or on high laud rising above 300 m to W, S or SE. On other occasions when snow lay on the high land the space charge density was lower but never highly negative. For the periods A and B the wind was between W and NW, and the weather reported from high moorland 25 km or so in that direction was fine, frosty, and with snow lying. In period C the wind was from SE, and from the high land in the Yorkshire Moors 40 or 50 km in that direction there were reports of snow cover
ivhich was not! melting. I)uring period D the wind vvas from S. and all reports fr01r1 bha’t a,nd other directions. at, all levels. referred to ally’ remaming 51101~ CO\'f'l ;iy meltSing. There lt’as also tvidesprend mist. or fog. 4. l)IS(‘I~SSION The most striking feature in these observations is the persistenctl for nearly 3 hr, in period (‘. of negat,ive space charge at, 1 m and positive at 2 III, in spite of st,rong turbulent mixing due t,o the wind blowiug at some IO m see I. Almost as remarkable is the way this difference falls to zero, apparently as the last traces of snow melt and disappear. These features may be seen in Figs. 2 and 3. The space charge patt’ern is different, in periods A and II where there was a cornparable wind. with presumably as much mixing, but little or no melting. There was yet anot her pattern in period 11 when there was melting \vith much lighter wind. The obvious first step is to consider whether the electrical state of the air during period (’ can be accounted for I)y inst,rumental fauts or spurious effects from the surroundings. The spaw charge apparatus leas run reliably and consist,ently for some 161 hr up t,o t’hc: cud of May 1964. Tlw t,wo collectors have indicated practically equal space charges at the two levels. I anti 2 m. wit)h litt,le dependence 011 wind speed, and often t her-c has been a close wrrclation betwwn spaw charge auti There is every reason to have confidence in this apparat~us. potential gradient. especially as during period (’ thr space charge records finally come t’ogether for the two levels. as seen in Fig. 3. Moroovcr. imniediat~ely preceding this period there was a zero check for 3 hr. We might perhaps expect to find excess of space charge of om sign to be associated nith certain wind directions. No such connexiort is apparent from t,he thirty-nine recorded periods covering bhe I fi t hr. Let us now examine the four periods mow closely. From what, has been said. periods A and K show no unusual features, and the posit,ive space charges on those days are in keeping with the wind speeds and cover of sno\\’ not melting. The humidity. it is t,o be noted, was comparatively lolv. As already mentioned in Section 3, t,he wind was I)lowing from W and NJ\‘, from high land vvliere blowing During period I) another snow might- well accomlt for the positive space charge. snow cover was melting rapidly and disappearing. HurnidiCes wtre high. \vit$h upward transfer of water vapour. The wind speed was only 3 m sw I, and yet t,here was no differenct~ in space charge concentSration at the t,wo levels. Hew w;t’ must cxaminc two 1)ossibilitics. A charge separation might, have occurred clear the surface. witjh bhc positive charge lyjng close to t’he ground wc~ll below I m. The field rnill at the surface would be affected to t,he same extent numerically hi, the space charges of both signs, unless t,hc: positive charge remairwti wholly or The other possibility is that on tShis day of widepartSly attached to the surface. spread mist or fog t,he negative space charge at. both levels had been produced at the electric power transmission lines referred to in Section 3. t3y Poisson’s equation the pot,ential gradient tr’T’/dZ or E’, its gradient dP/dZ and the space charge densit,y p are related as follon3: Il i* Cl’
Electric space charges over melting snow on the ground
where E,, is the permittivity
of empty space.
97
Then
where p is the mean value of p in the interval 6Z. Let us consider typical values of p and the potential gradient at the ground for this period, e.g. -200 e cm-3 and -350 Vm-1. If the fine weather potential gradient is 100 Vm-l, then 6P is -450 Vm-1 and by equation (1) we calculate 6Z to be 120 m. This is the effective height to which the negative space charge reaches, and is not unreasonable since the transmission line tower height is 40 m. In period C melting was obvious, and the humidity records indicate an upward transfer of water vapour. As shown in Table 2, at 1638 hours there are values of space charge density for two heights and of potential gradients for three. The mill at 1 m is fixed so as to work in the inverted position, an exposure factor being applied. Normally the exposure factor would be different when there was a high space charge density, but in this case the observed density at 1 m was too small to introduce serious error. Using values of potential gradient at 0 m and 1 m we find from (1) for that interval that p is -5500 e cm-3. Likewise between 1 and 21 m j is +770 e cm-3. The corresponding value at 1542 hours was +860 e cm-3. Let us now consider a vertical column of air of cross section 1 cm2 stretching up to the limit of the space charge. If we assume a fine weather value of 100 Vm-l, then at ground level the departure of the potential gradient from that value is a measure of the space charge contained in the column, namely QSZ. From (1) this is ---E~~F, and the value is +1,600,000 e cm- 2. Using p for the interval O-l m we get a total of -550,000 e cm-2 for that layer, and likewise -2,000 e cm-2 between 1 and 2 m, and +1,460,000 e cm-2 between 2 and 21 m. Differences yield 692,000 e cm-2 lying above 21 m. These values, with other relevant data, are shown in Fig. 5, which gives an impression of the electrical state of the air during period C. From this figure it appears that the negative space charge layer is concentrated near the ground, especially since the measured density at 1 m was only - 130 e cm-3, yet below this level the average was -5500 e cm-3. There is in fact more support for this view. On one occasion in this period the 1 m mill output was being recorded continuously. A numerical fall in negative space charge density at 1 m was accompanied by a fall in the 1 m mill indication of positive potential gradient, showing that most of the negative space charge is not close enough to this mill to have much effect on its output. It will be seen from Fig. 5 that there is a large overall excess of positive space charge. Now the concentrations, negative at 1 m and positive at 2 m, persist in spite of the mixing produced by winds between 7 and 14 m see-l. It would appear then that there is a charge separation which occurs locally so that the space charges are detected before the wind disperses them, and that the separation occurs close to the ground. Periods C and D both involve melting snow. During C there is a much stronger wind, during D there is mist. At the same time, in C, the wind was from SE, and could perhaps bring in the overall positive space charge from snow blowing on the Yorkshire Moors. These observations in period C are consistent with a process in which meltmg snow acquires a positive charge and the air the corresponding negative charge. In
HElGHl IN METRE hot to SCU!Qi
i
-i
I._-.I
-..._
--
--. & t
-130ecm+
-
-2p000cm‘*
I _._
-
~__. I -
I -.
-
--
_-
/ ‘. 550#000‘2cm-”
-----
6. CONCLGSION
The exceptional conditions in period C, persisting for some 2 hr and giving place to a more usual state as the melting snow finally disappeared, suggest that the charge is being separated while the snow is melting rapidly. Negative charge is transfe~ed It is possible but unlikely that this process provides much of the to the atmosphere. negative charge in period D, since the records show no difference in the concentrations at 1 m and at 2 m. Now in period D the wind was very much lighter, only about
Electric space charges over melting snow on the ground
99
m set-l. We may conclude that the charge separation occurs when a wind of some 10 m set-l blows over rapidly melting snow.
3
~c~~Z~e~e~~-We are indebted to numerous observers for isolation about weather conditions in their localities. One of us (R. B. B.) is in receipt of a maintenance grant from the Department of Scientific and Industrial Research.
REFERENCES BENT R. B. &ALMERS J. A. DINC+ER J. E. DINQER J. E. and GUNN R. LATIIAM 5. LATHAM J. and MASON B. J. MA~ONO C. andKIKUCHI K. MAUONO c. andSAJX’RAI K. MASON B. 5. and MATTHEWS J. B. MATTHEWS J. B. and MASON B. J. RAMSAY M. W. and CHALMERS J. A. SIMPSON G. C. and SCRASE F. J.
1964 1952 1964 1946 1964 1961 1963 1963 1964 1963 1960 1937
J. Atmosph. !Pew. Phys. 26, 313. J. Atmosph. Xerr. Phyzx 2, 155.
Quart. J. R. Met. Sot. 90, 208. Terr.Magn. Atmos. Elect. 51, 477. Quart. J. R. Met. Sot. 90, 91. Proe. Roy. Sot. A. 260,523. J. Met. Sot. Japan. 41,270. 6. Met. Sot. Japan. 41,211. Quart. J. R. Met. Sot. 90, 208. Quart. J. R. Met. Sot. 99, 376. Quart. J. R. .iiet. Sot. 86, 530. Proc. Roy. Sot. A.161,309.
Electric space charges over melting snow on the ground R. B. BENT and W. C. A. HUTCHINSON Department of Physics, university of Durham (Received 3 July 1964) Al&&--Records of space charge concentration and potential gradient over a melting snow cover indicate an average concentration of 5500 negative elementary charges/cm3 in the first metre above the ground, with a wind speed of 10 m/set, and an average of 770 positive/cm3 between 1 and 21 m. The upper positive space charge is accounted for by the blowing of snow on neighbourmg high land. The lower negative charge in the air near the ground, persisting in spite of the wind, can be explained if a separation of charge occurs at the surface, the positive charge remaining on the melting snow. 1. INTRODUCTION DURING the winter of 1963-64
some records were made of the space charge concentration at two heights, 1 and 2 m respectively, over ground covered or partially covered with snow. During one period with wind blowing at some 10 m set-1 over melting snow the record indicates a separation of electric charge, and provides results of interest because of the fundamental role which ice and water play in some theories of cloud electrification. There seems to be general agreement that when snow is blown by the wind the potential gradient near the ground is raised considerably above its normal fine weather positive value, and SIMPSON and SCRASE (1937) based their theory of thunderstorm charge separation partly on such observations made in the Antarctic between 1910 and 1913. A more detailed distribution of space charge accompanying blowing snow has been deduced by MAGONO and SAKURAI (1963) from observations of potential of the air at different levels up to 3 m. They found negative charge lying above positive in the first metre, and positive space charge above that. LATHAM (1964) has made an analysis of observations of snowstorm electrification. He concludes that all well authenticated observations on the electrification of blowing or sliding snow can be interpreted in terms of the temperature gradient theory of LATHAM and MASON (1961), according to which there will be a separation of charge along a temperature gradient in a piece of ice, the colder end becoming positive. There are, however, reports of electrification observed when snow or ice is melting. MAGONO and KIKUCHI (1963) measured the charges on snowflakes which melted
as they
ing snow applied
fell through
consisted the
melting
were obtained
a heated entirely
snowflakes
with artificial
that on one occasion sign of charge
almost
cylinder.
Periods
of negatively
flakes. positive.
when When
the failheat
was
became predominantly Similar results snowflakes. RAMSAY and CHALMERS (1960) found
when there was a quite sudden
on the precipitation
DINGER and GUNN (1946)
were chosen
charged
reported
changed that
within they
91
change
from
sleet to snow the
t min from positive
found
positive
charging
to negative. of melting
-’ -.
Arl’:ir{A’I’r~s
The observations were made at, the Uurham .ITniversity Observatjory #it<%,whcrc the instruments were mounted on a steel Iat,t,icc mast 22 m high. ‘l’ht: space charge was measured with collectors which haw alwatly been described bv HEW (19ti4). ‘L’hey incorporate glass woot filters which t,rap virtually all the large and small ions, the net, charge per unit vol [Ime of air being nwasured c(~ntin~~ousl~~x\.ith a Faraday cage connected to an elrctrometer. ‘I’he cotlectors have proved extremely r&able over a period of several months. For these ohswvations the,y were monnt~ed at I and 2 m respectively. A field mill was set. Hush with t,hc surface of tlw ~rountf at. 40 m from the mast, and Mr. H. I,. (.‘ollin l)rovided some data front his t
A I9 Ike* 13w 1900 hours
Period .~~~..
.~~~~ .~..
.--.
-----
Space charge, e crnm3 at 2 m tvt 1 III I’okntial gratlicnt . \‘m-’ 2tt0 III \l’ind Speed, m see-l Direction, dog ‘f’emp. at 1.2 m, “F Relative humidity, at 1.2 m at 8 cm Clondition \I-e&her
of
snow
in periods A. B, C: and T). Bkmentary IS ‘0 I>(‘l* 1330 1800 hortrs
chargcl e
C! 17 Ma 154Om 1840 hours
1.6 1: IO‘-“” (’ I) 20 Mar loot)I 2 t 5 t~oLws
.
--
-! (10~.~00)
(10~.-600)
-1(‘LOO-600)
( IO0 4300)
--(loo--ALTO)
( I oo-400) ( 100-400) ( fOC)- 800)
-1( 10% 500)
.-:wi
‘~(ZOO-600)
4-11 300 so- 33
3- 9 288 30. -35
7-14 120 35~38
3 180 37 -39
“/, 60 -53 52- 55
Kot meking Fine
47 47 Not melting Fin,
50-60 54.-64 Meking Fine, hazy
90 95 Melting Fine, wry mkty
Electric Table
space charges over melting
2. Space charge and potential 1542 hours
At 21 m 2m lm Om
snow on the ground gradient
C
1638 hours + 220 Vm-l
-t 330 Vm-l + 640 Vm-l
in period
93
+ 100 e cme3 -150 e cmP3
+500 Vm-l + 400 Vm-l
no observations were made at 2 m. the space charge at 1 m was positive; Period C was quite different. At the ground the potential gradient was continuously positive; on the two occasions when data from Mr. Collin’s mills were available it was more positive at 1 m than at 0 m, but much less positive at 21 m. The space charge was always negative at 1 m and always positive at 2 m from the start of the period until 1800 hours, after which the records gradually approached one another and coincided at 1830 hours. For the whole of period D the potential gradient at the ground was negative, and the space charge values at 1 and 2 m kept close together and remained negative. A summary of the recorded values is given in Table 1, together with the following additional information: wind direction and speed at 10 m, air temperature at 1.2 m, relative humidity at 1.2 m and 8 cm, and condition of the surface. Where the values changed appreciably during the period the range is shown. Table 2 shows values of space charges and potential gradients on two occasions in period C. In all cases the space charge density is expressed in the units e cm-3 where the magnitude of the electronic charge e = + 1.6 x lo-l9 C. Various portions of the space charge and potential gradient records are shown in Figs. l-4. In periods A and B the ground was completely covered with snow, with no In the first half of period B the 8 cm relative humidity was apparent melting. and
Fig.
1. Typical
record in period A.
I-4 Zmin
Fig.3. Find part, nt‘ pwiotl (‘. higher than that at l-2 m; later the l-2m ,value was the higher. During period C the snow cover was only partial, it was wet and certainly not blowing, and was rapidly disappearing. fn fact. at t,he Durham Science Laboratories site about 1 km away the routine observations on t*hc state of the ground included a reference to snow at 1500 hours but none at 1800 hours. Before period I$ there was a new fall of snow; this was
Electric space charges over melting snow on the ground
95
2min.
Fig. 4. Typical record in period D. melting rapidly during that period, and was noted in the routine observations at 1200 hours but not at 1500 hours. Here it is convenient to include other relevant results from obse~ations between December 1963 and May 1964. It was not uncommon for space charge densities to be as much as 5500 e cmm3, with corresponding potential gradients at the surface. The records were made almost invariably during fine or fair weather, when an excess of positive space charge appeared to be normal, so for convenience here we may regard negative densities exceeding 300 e cm-3 and positive exceeding 400 e cm-s as being unusual. There were ten such periods with negative space charge, and in eight cases there was mist or fog locally or humidity was in any case high. In one of these ten periods however the weather was fine with no report of mist within 100 km. Thus there seems to be a close connexion between these negative space charges and mist, and we may safely regard this as the phenomenon investigated by ~HAL~IERS (1952), who showed that the negative potential gradient occurs do~~n~lind of electric power overhead transmission lines where presumably the corona discharge in misty conditions gives an excess of negative ions to the atmosphere. Durham Observatory is practically surrounded by these transmission lines at distances between 1 and 6 km. Of the nine high positive space charge periods with densities exceeding 400 e cm-3 there were four when dry snow which might blow in the wind was lying either at Durham or on high laud rising above 300 m to W, S or SE. On other occasions when snow lay on the high land the space charge density was lower but never highly negative. For the periods A and B the wind was between W and NW, and the weather reported from high moorland 25 km or so in that direction was fine, frosty, and with snow lying. In period C the wind was from SE, and from the high land in the Yorkshire Moors 40 or 50 km in that direction there were reports of snow cover
ivhich was not! melting. I)uring period D the wind vvas from S. and all reports fr01r1 bha’t a,nd other directions. at, all levels. referred to ally’ remaming 51101~ CO\'f'l ;iy meltSing. There lt’as also tvidesprend mist. or fog. 4. l)IS(‘I~SSION The most striking feature in these observations is the persistenctl for nearly 3 hr, in period (‘. of negat,ive space charge at, 1 m and positive at 2 III, in spite of st,rong turbulent mixing due t,o the wind blowiug at some IO m see I. Almost as remarkable is the way this difference falls to zero, apparently as the last traces of snow melt and disappear. These features may be seen in Figs. 2 and 3. The space charge patt’ern is different, in periods A and II where there was a cornparable wind. with presumably as much mixing, but little or no melting. There was yet anot her pattern in period 11 when there was melting \vith much lighter wind. The obvious first step is to consider whether the electrical state of the air during period (’ can be accounted for I)y inst,rumental fauts or spurious effects from the surroundings. The spaw charge apparatus leas run reliably and consist,ently for some 161 hr up t,o t’hc: cud of May 1964. Tlw t,wo collectors have indicated practically equal space charges at the two levels. I anti 2 m. wit)h litt,le dependence 011 wind speed, and often t her-c has been a close wrrclation betwwn spaw charge auti There is every reason to have confidence in this apparat~us. potential gradient. especially as during period (’ thr space charge records finally come t’ogether for the two levels. as seen in Fig. 3. Moroovcr. imniediat~ely preceding this period there was a zero check for 3 hr. We might perhaps expect to find excess of space charge of om sign to be associated nith certain wind directions. No such connexiort is apparent from t,he thirty-nine recorded periods covering bhe I fi t hr. Let us now examine the four periods mow closely. From what, has been said. periods A and K show no unusual features, and the posit,ive space charges on those days are in keeping with the wind speeds and cover of sno\\’ not melting. The humidity. it is t,o be noted, was comparatively lolv. As already mentioned in Section 3, t,he wind was I)lowing from W and NJ\‘, from high land vvliere blowing During period I) another snow might- well accomlt for the positive space charge. snow cover was melting rapidly and disappearing. HurnidiCes wtre high. \vit$h upward transfer of water vapour. The wind speed was only 3 m sw I, and yet t,here was no differenct~ in space charge concentSration at the t,wo levels. Hew w;t’ must cxaminc two 1)ossibilitics. A charge separation might, have occurred clear the surface. witjh bhc positive charge lyjng close to t’he ground wc~ll below I m. The field rnill at the surface would be affected to t,he same extent numerically hi, the space charges of both signs, unless t,hc: positive charge remairwti wholly or The other possibility is that on tShis day of widepartSly attached to the surface. spread mist or fog t,he negative space charge at. both levels had been produced at the electric power transmission lines referred to in Section 3. t3y Poisson’s equation the pot,ential gradient tr’T’/dZ or E’, its gradient dP/dZ and the space charge densit,y p are related as follon3: Il i* Cl’
Electric space charges over melting snow on the ground
where E,, is the permittivity
of empty space.
97
Then
where p is the mean value of p in the interval 6Z. Let us consider typical values of p and the potential gradient at the ground for this period, e.g. -200 e cm-3 and -350 Vm-1. If the fine weather potential gradient is 100 Vm-l, then 6P is -450 Vm-1 and by equation (1) we calculate 6Z to be 120 m. This is the effective height to which the negative space charge reaches, and is not unreasonable since the transmission line tower height is 40 m. In period C melting was obvious, and the humidity records indicate an upward transfer of water vapour. As shown in Table 2, at 1638 hours there are values of space charge density for two heights and of potential gradients for three. The mill at 1 m is fixed so as to work in the inverted position, an exposure factor being applied. Normally the exposure factor would be different when there was a high space charge density, but in this case the observed density at 1 m was too small to introduce serious error. Using values of potential gradient at 0 m and 1 m we find from (1) for that interval that p is -5500 e cm-3. Likewise between 1 and 21 m j is +770 e cm-3. The corresponding value at 1542 hours was +860 e cm-3. Let us now consider a vertical column of air of cross section 1 cm2 stretching up to the limit of the space charge. If we assume a fine weather value of 100 Vm-l, then at ground level the departure of the potential gradient from that value is a measure of the space charge contained in the column, namely QSZ. From (1) this is ---E~~F, and the value is +1,600,000 e cm- 2. Using p for the interval O-l m we get a total of -550,000 e cm-2 for that layer, and likewise -2,000 e cm-2 between 1 and 2 m, and +1,460,000 e cm-2 between 2 and 21 m. Differences yield 692,000 e cm-2 lying above 21 m. These values, with other relevant data, are shown in Fig. 5, which gives an impression of the electrical state of the air during period C. From this figure it appears that the negative space charge layer is concentrated near the ground, especially since the measured density at 1 m was only - 130 e cm-3, yet below this level the average was -5500 e cm-3. There is in fact more support for this view. On one occasion in this period the 1 m mill output was being recorded continuously. A numerical fall in negative space charge density at 1 m was accompanied by a fall in the 1 m mill indication of positive potential gradient, showing that most of the negative space charge is not close enough to this mill to have much effect on its output. It will be seen from Fig. 5 that there is a large overall excess of positive space charge. Now the concentrations, negative at 1 m and positive at 2 m, persist in spite of the mixing produced by winds between 7 and 14 m see-l. It would appear then that there is a charge separation which occurs locally so that the space charges are detected before the wind disperses them, and that the separation occurs close to the ground. Periods C and D both involve melting snow. During C there is a much stronger wind, during D there is mist. At the same time, in C, the wind was from SE, and could perhaps bring in the overall positive space charge from snow blowing on the Yorkshire Moors. These observations in period C are consistent with a process in which meltmg snow acquires a positive charge and the air the corresponding negative charge. In
HElGHl IN METRE hot to SCU!Qi
i
-i
I._-.I
-..._
--
--. & t
-130ecm+
-
-2p000cm‘*
I _._
-
~__. I -
I -.
-
--
_-
/ ‘. 550#000‘2cm-”
-----
6. CONCLGSION
The exceptional conditions in period C, persisting for some 2 hr and giving place to a more usual state as the melting snow finally disappeared, suggest that the charge is being separated while the snow is melting rapidly. Negative charge is transfe~ed It is possible but unlikely that this process provides much of the to the atmosphere. negative charge in period D, since the records show no difference in the concentrations at 1 m and at 2 m. Now in period D the wind was very much lighter, only about
Electric space charges over melting snow on the ground
99
m set-l. We may conclude that the charge separation occurs when a wind of some 10 m set-l blows over rapidly melting snow.
3
~c~~Z~e~e~~-We are indebted to numerous observers for isolation about weather conditions in their localities. One of us (R. B. B.) is in receipt of a maintenance grant from the Department of Scientific and Industrial Research.
REFERENCES BENT R. B. &ALMERS J. A. DINC+ER J. E. DINQER J. E. and GUNN R. LATIIAM 5. LATHAM J. and MASON B. J. MA~ONO C. andKIKUCHI K. MAUONO c. andSAJX’RAI K. MASON B. 5. and MATTHEWS J. B. MATTHEWS J. B. and MASON B. J. RAMSAY M. W. and CHALMERS J. A. SIMPSON G. C. and SCRASE F. J.
1964 1952 1964 1946 1964 1961 1963 1963 1964 1963 1960 1937
J. Atmosph. !Pew. Phys. 26, 313. J. Atmosph. Xerr. Phyzx 2, 155.
Quart. J. R. Met. Sot. 90, 208. Terr.Magn. Atmos. Elect. 51, 477. Quart. J. R. Met. Sot. 90, 91. Proe. Roy. Sot. A. 260,523. J. Met. Sot. Japan. 41,270. 6. Met. Sot. Japan. 41,211. Quart. J. R. Met. Sot. 90, 208. Quart. J. R. Met. Sot. 99, 376. Quart. J. R. .iiet. Sot. 86, 530. Proc. Roy. Sot. A.161,309.