B l o w i n g

S n o w

MeasurementsofCharge-to-MassRatiosonIndividualBlowingSnowParticles
D.S.Schmidt¹,J.D.Dent¹,R.A.Schmidt²

1 _{Department of CivilEngineering,Montana State University,Bozeman, Montana 59717}

2 _{US Forest Service Rocky Mountain Forest and Range ExperimentStation}

222 South 22nd Street, Laramie, Wyoming 82070

ABSTRACT

Determining the electrostaticforce actingonsaltating snow
particlesrequires knowledgeoftheelectricfieldinthe
saltation region and the charge on the particle.To date,
measurements of average charge-to-mass ratios forblow-
ingsnowsampleshavebeenmade,butSchmidtand
Schmidt (1992) indicate these may underestimate charge-
to-mass ratios forindividual particles, due to a mixture of
positive and negative charge in the samples.This paper
reports on measurements of charge-to-mass ratio forindi-
vidual blowing snow particles during a moderate blowing
snowstorminS.W. Wyoming.We measuredcharge-to-
mass ratios as largeas -208 mC/kg, significantlyhigher then
the -10 mC/kg measured by Latham and Montagne (1977)
and -50 mC/kg measured by Whishart (1969).Thislarge
charge-to-massratios was accompanied by a distribution
of positive andnegativecharge on theparticles, supporting
the conclusions of Schmidt and Schmidt (1992).

INTRODUCTION

Wind isan incredibly powerful force capable ofmoving
vastquantitiesofsnowduringblizzards.Depositionof
windblownsnowformslarge driftsthat hamperwinter
travel.In mountainous regions, cornices and wind depos-
ited snow inthe leeof ridgestrigger avalanchesthatdamage
structuresand transport systems.Effective control of these
natural phenomenonrequires a better understandingof the
physical processes in blowing snow.
Wind transports snow by three mechanisms.Particles
rollalongthesurfaceinaprocesstermedcreep.This
mechanismoccursin lightwindsand accounts forvery
little transport.Strong winds eject particles from the sur-
face in a mode of transport called saltation.Particles in
saltation bounce along the surface, rebounding to heights
typically within 10cm of the surface.Much of transport
during blowingsnowstormsmovesinsaltation.Inthe
thirdtransport mode, knownas suspension, particles travel
without impact, at heightthat may extend severalhundred
meters in polarstorms.Saltating particles are the source
for suspendedtransport, thereforeunderstandingsaltation
is essential to controlling blowing snow.
This paper reexamines themagnitudeof the electrostatic
charges thatdeveloponsaltatingsnowparticles.Most
laboratoryandfieldstudiesofsaltationhaveignored
electrostaticforces.Inthefewstudiesdirected toward
measuring particle charge, we believe the methods under-
estimate magnitudes.The remainder of this introduction
presentsa brief reviewof research on theelectrostatic force
and why previous experiments might underestimate par-
ticlecharge-to-massratios.Followingsectionsdescribe
the equations,apparatus,field procedures,and results of
ourexperiment.Ourthoughts on the implications of the
results on the saltation process are in the discussion.

THE SALTATION PROCESS

Saltating snow particles rebound from elastic impact with
the surface,following long, low trajectories in response to
forces of fluid drag and gravitation.Whether it be snow
particles in the atmosphere, orglass beads in a wind tun-
nel,theequationsthatdescribethemotionofsaltating
particles are the same.White and Schultz (1977) used a
high speed camera to photograph trajectories of saltating
glassspheresina wind tunnel.They foundtrajectories
higherandlongerthen thosepredicted fromtheoretical
equations involving only fluid drag and the gravitational
force.This indicatedthepresenceof an additionallift force.
Lift develops if the particle spins and White and Schultz
showed good agreement with the observed trajectories by
addingthe Magnus lifttothetheoreticalequations.To
produceconsistenttrajectoriesfrom the additionof Magnus
effect alone, White and Schultz assumed spin rates in the
range100 - 300 rev/s.Another force thatcould explain the
additional lift on saltatingparticles is the electrostaticforce
that results from friction between the moving particles and
the surface.

ELECTRIFICATION OF BLOWING SNOW

Acharged particle,inan electricfield,issubjecttoan
electrostatic force.The magnitude of the force is equal to
the product of the electric field and the chargeon the par-
ticle.The force acts along the electric field vector,in the
directiondeterminedbythesignoftheparticlecharge.
Schmidtand Schmidt (1992)review theresearch onthe
mechanismsthatproduce chargeseparationinsaltating
snow.These mechanisms usually produce moving parti-
clesthatare negativelycharged,whilesurface particles
become positive.Blizzardmeasurements (Schmidt, 1994)
confirmedpredictions bySchmidtandDent(1993)that
electric fields nearthe surface in saltating snow are sev-
eral orders of magnitude larger than the fair- weather field
(100V/m).
Whishart (1968)and Latham and Mongtagne(1970)re-
ported measurements of average charge-to-mass ratiosin
whichsamples of driftingparticleswere collected. Schmidt
andSchmidt(1992)alsoreportedchargeand massfor
blizzardparticles collectedin aportableFaradaycage,with
simultaneous wind speedmeasurements.Reductions, and
actualreversal,insign ofmeasured charge during wind
gustsstronglysuggestedthat eroded surface particlesof
oppositesignweremixingwithmovingparticles.This
wouldindicateaverageparticlecharge-to-massratio
determinedbymeasuringthechargeonblowingsnow
samplesanddividing bysamplemasswouldunderesti-
matecharge-to-massratiosoftheindividual snowparti-
cles.

129