B l o w i n g

S n o w

The multi-strobe system(Bird and Jairell,1989) provides
continuouslight from ahalogenlamp, andaccuratelytimed
flashesfromeightelectronicstrobes.Theresultisa
photographic image showing a sequence of dots, defining
particle location,along lowintensitywhite streakon ablack
background.Anadjustableslit(setto1cm),on aclear
plastic window sealed tothe bottomofthe electric field
chamber,confines illumination to a region perpendicular
to the plates and centered in the chamber.A small fan on
themulti-strobehousingremovesheatproducedbythe
halogen lamp.
The timing circuit performs two functions.It opens the
camera shutterwhen aparticleisdetected, andtriggers
the strobe sequence after a delaythat allows the particle to
movefromthedetectorintotheelectricfieldchamber.
Particles are detectedby a snow particle counter (SPC) that
senses theparticlesshadowin alightbeam (Schmidt,1977).
Weused a35-mm film camera with motor drive, databack,
and55-mm,f1.4lenstorecordparticleimages.A
microprocessorcontrolsthetimingcircuit,allowing
programmabletimedelays.Forourexperiment,a time
delay of 121.6 ms was set between particle detection and
first strobe flash, with20.0 ms intervals betweeneachflash.

Field Procedure

The experiment was conducted on 8 January 1996 at the
Chimney Park trailhead, 96 km west of Laramie, Wyoming
on Highway 130.Snowplowoperators forthe Wyoming
Highway Department assisted us in forming a 2.1 m high
snow bank,at the west end of the trailhead parking lot.
Suitable snow cover existed upwind of the site, though no
new snow had fallen in several days.A mobile laboratory
provided electricity and shelter forcomputers (as well as
investigators).Supportingmeteorologicaldata included
wind speed anddirection, air temperature, and humidity,
allmeasured1mabovethesurfacenearthe topofthe
snowbank.
The apparatus was set up in the parking lot just down-
wind of the snowbank.Aroof,level with the top of the
snowbank,prevented the apparatus from being drifted in
(Fig.2a).A20-cmlayerofsnow,placedontheroof,
smoothedtheapproachtotheinletofthedevice.The
electric fieldchamberwasleveled and theinletaligned
with the wind.
A length scale was defined forimage analysis by sus-
pending a section of metric ruler at the center of the field
of view forthe first twopictures ofeach film roll.Four
rollsof36exposure(ASA1600)wereexposed between
1400and 1700-h,in low-level drifting (no noticeable sus-
pension).

Analysis Procedure

Figure3:Image showing trajectories of the first four parti-
cles in Table1.Strobe number five failed to flash through-
outtheexperiment.Theimagewasenhancedand
converted to black and white forthis figure.Continuous
traces were converted to black in the process.
Of the 136 images we exposed, 50 showed particle im-
ages.We transferred these tocompactdiskforanalysis
using computersoftware.Figure 3 showsanexampleimage
fromwhichparticlelocationwasdigitized.Wealso

132

digitizedtheendpoints
of the same100-mm seg-
ment in all images of the
ruler,todeterminea
length scale.
Verticaldistancebe-
tweendots,dividedby
the20-msstrobeinter-
val,determined particle
fall velocity.If the par-
ticle was at terminal ve-
locity,an average value
wascomputedforv
Horizontaldistancebe-^{T .}

tweenthe firstandlast
dotonthepathdeter-
mined horizontal deflec-
tion,x.The time,t,for
thisdeflectionwasthe
productofthe20-ms
strobeintervalandthe
numberofintervalsbe-
tweenthe firstandlast
dot.Thesemeasured
values of vT, x, and t, to-
gether with E= 147.65-
kV/m, and g = 9.81-m/s ²
providethearguments
required to evaluate the
charge-to-massratioof
the particle using equa-
tion (13)

Table 1.Strobe number five failed
to flash throughout theexperiment.
The image was enhancedand con-
verted to black and white forthis
figure.Continuoustraceswere
converted to black in the process.

In selecting particle traces foranalysis,two criteria were
used to ensure particles were traveling at terminal veloc-
ity,and that measured deflections resulted only from the
electrostatic force:(a) At least three strobe dots had to be
visible to check for terminal velocity.(b) A trace couldnot
approach orcross othertrajectories,to be certain the de-
flection was not influencedby otherparticles.

EXPERIMENTAL RESULTS

The largestvariationin measureddistanceon theeightruler
images was less then 1%, therefore we used the averageof
419 du/mm to convert coordinates of the strobe dots from
digitizer units(du) to actual distance in mm.11 traces met
thecriteriaforanalysis.Table 1listmeasurementsand
computed charge-to-mass ratios forthese particles.Data
forthe four particles in figure 3 is listed first.Windspeed
decreased from 10-m/s at the beginning of sampling to 5-
m/s nearthe end.Temperature decreased from +1^o C to -
2^o C, withrelativehumiditydroppingfrom 93%duringpeak
drifting, to 70% as winds decreased.

Error Analysis

We estimated the errors forcomputed charge-to-mass ra-
tios in Table 1 from the errors in eachargumentof the com-
putation.Table2 lists these values.A worst-case analysis
foraparticle with500-mm/sfallvelocity,deflected 15-
mmin 100-msbyan electric field of148-kV/mmgives a
maximum error of 4%in q/m, equal to 1.5- mC/kg.Percent
error increased as deflections decreased.