Monday Feb. 16, 2015
We'll devote today's class and most of Wednesday to qualitative
descriptions of cloud electrification processes.
The picture below is from the supplementary
lecture on cold cloud/ thunderstorm cloud structure.
The tripolar charge structure is what a viable electrification
process needs to explain.
The main charge centers are first an upper
positive charge center at 1(a) and smaller lower positive charge
centers (1c). There is more of a layer of negative charge at
(1a) that seems always to be found at temperatures between -10o
and -30o C.
Screening layers are found at the top and sides of the cloud
(2a and 2b in the figure). These form because of the abrupt
drop in conductivity as you move from outside the cloud into the
cloud aren't part of the electrification process.
The figure below presents some of the evidence supporting the
idea that the negative charge is found between -10o and
-30o C in
The figure shows the locations
of negative charge neutralized during cloud-to-ground and
intracloud discharges in clouds in Florida, New Mexico, and
Japan. We'll look at how this is done later in the
week. In each case the negative charge is found in the
same temperature range despite differences in cloud heights,
cloud base altitudes, and cloud thicknesses. (source:
Krehbiel, Paul R., "The Electrical Structure of
Thunderstorms," Ch. 8 in The Earth's Electrical Environment,
National Academy Press, Washington, 1986)
And just so you don't think you now have everything
figured out, here is a figure that demonstrates that the
distribution of charge in some thunderstorms can be much more
complex than the simple tripolar model discussed above. The
figure shows charge in the convective part of a mesoscale
convective system (source: Stolzenburg,
in Thunderstorm Convective Regions 3. Synthesis," J. Geophys.
Res., 103, 14097-14108, 1998).
We'll start with the convective process of cloud
The convective theory starts
with the positive charge found in the air above the ground
during fair weather (Fig. A). In Fig. B an updraft and a
cloud have started to form and positive charge is swept upward
into the cloud where the charge attaches to cloud
particles. Negative charge carriers in the surrounding
air are drawn to the positive charge in the top of the cloud
and form a screening layer. Cloud edge motions shown in
Fig. C then begin to carry this negative charge down and into
the middle center of the cloud to form the main negative
charge center. The electric field at the ground
intensifies because of the close proximity of negative charge
in the cloud. Objects on the ground go into corona
discharge and "spray" positive charge into the air (Fig.
D). This is one way of accelerating the charging
While cloud motions can clearly have an effect on the
distribution of charge inside a thunderstorm, the convective
theory is generally not considered to be a viable mechanism
for the initial electrification of thunderstorms. It is
hard to understand how cloud motions would always form the
main layer of negative charge in the -10o
and -30o C region
of the cloud, for example.
In the inductive process an existing (initially fair
weather) field induces charges in precipitation particles (in
the same way charges were induced on a conducting sphere in a
The downward pointing fair weather E field induces equal
amounts of negative and positive charge on the tops and
bottoms of different sized precipitation particles.
Because the particles are moving in different directions
or at different speeds, they collide. During the
collision some of the charge on each particle is
neutralized. Each particle is left with net charge
following the collision. Positive charge moves
upward, negative charge downward.
The direction of charge motions is consistent with the
direction and strengthening of in-cloud electric fields.
The field at the ground under a thunderstorm points
upward, the opposite of the fair weather field. The
direction of the in cloud field is the same as the fair
weather field. Thus the directions of charge
movement in the inductive process is consistent with
thunderstorm charge distribution.
There are a variety of types of particles that can
- riming ice
When two water droplets collide, they will often stick
together. If make just a glancing collision, there
won't be any charge neutralization because there isn't any
charge induced at the particle's equators. Thus a
collision between two water droplets would not seem like
it could separate significant amounts of charge.
Ice particle - ice particle collisions don't neutralize much
charge because the particles don't remain in contact for very long
and the lower electrical conductivity of ice means charge doesn't
flow as readily as it does through water.
Water - ice particle and ice - riming particle collisions might
work, but the general feeling still is that the inductive process
is not able to start with a fair weather E field and turn it into
a thunderstorm strength field.
Before we leave the inductive process behind a quick
demonstration of an apparatus that depends on induction (and
positive feedback) to produce a surprising amount of charge from
tap water. The apparatus is a Kelvin
Water Dropper or Kelvin Electrostatic Generator. A
picture of the apparatus used in class is shown below at left
Water from a reservoir (at the top of the photo) travels down
two sections of tubing and out of two plastic nozzles positioned
near the middle of the photo. The falling streams of water
fall through two metal rings (colored red and green) and into two
plastic cups at the bottom of the picture. The cup on the
left is electrically connected to the ring at right (the red
ring), the cup on the right is connected to the left (green)
ring. The two cups are also electrically connected to 5 neon
lamps wired together in series. The right photo above is a
close up of the neon lamps.
The operation (as I understand it) is explained below.
Let's assume that the water in the left cup (Pt. 1) contains
some + charge (it is equally likely that it
contains - charge). The left cup is electrically
connected to the right (red) ring, Pt. 2. The positively
charged red ring will cause (by induction) negative charge to
build up at the bottom end of the falling stream of water.
The stream then breaks up into drops which carry the negative
charge to the cup on the right (Pt. 3). This is connected to
the left ring at Pt. 4. As shown in the close up at Pt. 5
positive charge is induced at the tip of the stream of
water. As the stream breaks into drops the drops carry more
positive charge to the cup on the left. Charge builds up
until a spark flashes across the spark gap (or until the bank of
neon lamps flashes in the apparatus used in class).
There are a lot of videos on YouTube. This first video
(from Reinhard Schumacher at Carnegie Mellon University) shows an
apparatus that produces a visible spark and also contains a
sensitive electroscope. Here's a second video
(from Thomas Kim) that produces a visible and audible spark (note
how the sound of the dripping water also stops a second or two
before the spark).
The non-inductive process, also called the Reynolds, Brook,
Gourley process is generally thought to be the most viable
explanation for the rapid initial electrification of
thunderstorms. It doesn't require a existing field.
The process is shown in general terms below. We will look at
some of the details later.
Basically graupel collides with a snow crystal, in the
presence of supercooled water droplets, and then depending on
the environmental temperature, the graupel ends up with
negative or positive charge and the ice crystal ends up with
the opposite polarity. At temperatures colder than about
-15 C the snow crystal ends up with positive charge and the
graupel is negative. The different sizes and fall
velocities of the two types of particles means they will tend
to separate after the collision. The negatively charged
graupel tends to accumulate in the middle of the cloud and the
snow crystal is carried up to higher parts of the cloud.
At temperatures warmer than about -15 C the polarities
reverse. We will see that this charge reversal
temperature can vary depending on the cloud liquid water
content. The letters A, B, and C show how this process
can account for the three main charge centers in a
thunderstorm (A & C are the upper and lower positive
charge centers, respectively; B is the main negative charge
The following figure shows the experimental apparatus used
to make the initial measurements of charging. [source: S.E.
Reynolds, M. Brook, and Mary Foulks Gourley, "Thunderstorm
Charge Separation", J. Meteorology, 14, 426-436, 1957)
Two metal balls at the ends of arms mounted on a rotating
shaft simulate graupel particles in a cloud (pt. 1).
These balls become electrically charged the electrical path
from the balls to a sensitive electrometer has been
highlighted in yellow. A little bit wider view of the
apparatus is shown below.
Temperature and liquid water content (LWC) are measured near
the spinning arms (pt. 2). Liquid water content is
really just a measure of the concentration of supercooled
Warm water at pt. 3 is the source of water vapor.
Cooling coils (pt. 4) cool the air inside the chamber.
Cold air will sink and probably establish a vertical
temperature gradient. The spinning metal ball could be
raised or lower to change the temperature of the surrounding
air. Replicas of ice crystals that form can be collected
at pt. 5 for later examination.
This would not be an easy experiment to conduct. It
would be difficult to create, measure, and monitor the cloud
environment in the chamber and the charging that does occur is
probably very weak and a sensitive electrometer would be
On Wednesday we'll look at some of the details from more
recent studies of the non-inductive electrification
process. Results from some of the work done by a group
at the University of Manchester Institute of Science and
Technology (UMIST) are summarized on the graph below (source:
Influence of the Rime Accretion Rate on Charge Transfer
during Crystal/Graupel Collisions," J. Geophys. Res., 103,
The + and - polarities in
the figure refer to the charge on the metal target that
simulates a graupel particle in these laboratory
experiments. The charge depends on the
environmental temperature and on the cloud liquid water
content as shown in the figure above. EW on
the vertical axis is effective liquid water content and is the
product of liquid water content and collision efficiency (what
fraction of the supercooled water droplets colliding with the
target stick and eventually freeze to the target).