B l o w i n g

S n o w

Onlyparticlesthatpassthroughthedetectorenterthe
electric field chamber.The fourth sub-unit,the imaging
system,iscomprised ofthedetector,camera, and multi-
strobes system.The purpose of this last unitis to illumi-
nateandphotographtheparticlesin theelectric fieldcham-
ber.
The expansion chamber slows air flow and allow parti-
cles to settle.Constructed of sheet metal,the expansion
chamber maintains the same ratios as the smaller drift trap
reported by Mellor(1960).The expansion chambercon-
nectstothe rest oftheapparatus bymeansofa rotating
cap.This allowed the direction of the nose cone inlet to
be adjustedfor different winddirections.Oncepositioned
the cap was fixed in place and sealed by an adhesive fas-
tener (duct tape).
The extension tube allows the particles to reachtermi-
nal fall velocitybefore entering the measurement region.
Schmidt (1981) reports size distributions of saltating par-
ticles with mean equivalent diameters near 200 mm.Us-
ing a computeriteration scheme, we determined 24 cm is
requiredfor a 200 mm ice sphere to reacha terminal veloc-
ityof53cm/s.The vertical dimensionoftheextension
tubewas55cminordertoinsurethatmostparticles
attained terminalvelocity.The imagesprovideatest of
this requirement
The electric field chamber is sealed, providing a still air
region.Twoaluminumplates, (figure 2b) connectedto high
voltage power supplies, produce a horizontal electric field
across a 20 cm plate separation.Leveling mechanisms on
the bottom corners of the chamber allow leveling to set the
electric field perpendicularto gravity.Flat black paint on
all interior surfaces of the chamber reducereflections, and
black velvet on the chamber wallopposite the camera gives
highcontrasttotheparticleimages.Thehighvoltage
supplies,adjustableoverarangeof0-20kV,providea
variable electric field.Plate voltage measuredwith a high
voltage probe during the experiment were + 14.93 kV and
-14.60kV, giving an electric field strength E = 147.65 V/
mm.Highervalues interfered with the particle detection
circuit.

Separating variables and integrating both sides of expres-
sion (10) gives _{[!](1-e- b t)dt=(--)[!]b}
a^dx

or

t+(--)e¹
b^{- b t=(--)x+cb}
a²

(11)

Weevaluatethe integrationconstant c
2^{by defining thefirst}
known location of the particle as x = 0 at time t = 0.If we
apply this boundary conditionto equation (11),c ₂=1 / b,
and_x=--[
a_+(--)1
b^tb^{(e-b t -1)]}

from which

a=------------^{b2 x}
[bt=(e ^{- b t}-1)]

(12)

Re-writing equation (12) in terms of the definition of a and
b,and solving the expression forq/m gives

(--)=--------------------^q(g/V
m_E[(g/VT)t+(e^{T)2 x}
- gt/ V_{T -1)]}

(13)

This equation is the basis of ourexperimental technique.
It evaluates charge-to-mass ratio q/m, from measurements
ofdeflectionx,overtimet,andtheparticle'sterminal
velocityv _T.Electricfield strength mustbeknown,and
gravitational acceleration assumed constant.

EXPERIMENTAL METHODS

Apparatus

Figures 2 shows theexperimentalapparatus.Four sub-units
makeupthedevice.Theexpansionchamberextracts
particlesfromsaltation,allowingsometofallintothe
extension tube, where they accelerate to terminal velocity.

Figure 2a:Apparatus for measuring charge-to-mass ratios of individual

blowing snow particles.

Figure 2b:Schematic diagram of electric field chamber and photo-

imaging system.

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