In our next lecture we will change directions and start covering cloud electrification. 

First we need to do a little review and have a look at the structure of a cold thunderstorm cloud.  Cold refers to the fact that much of the cloud is found at high enough altitude that it is at below freezing temperatures and contains ice crystals.  This is usually the case for thunderstorm clouds even in Tucson on the hottest day of the summer.



The cloud above is meant to be fairly typical of a summer thunderstorm in S. Arizona.  Cloud base is found above the top of Mt. Lemmon (9000 ft. altitude).  The freezing level is usually well below 20,000 feet altitude.  The top of the cloud in this figure is found between 35,000 and 40,000 feet (some stronger storms might get higher than that).

The important part of the cloud,  both for precipitation formation and electrification, is the middle, mixed-phase region.  There you find both ice crystals and supercooled water droplets (water droplets cooled to below freezing that are unable to freeze).

Before we try to understand why it is difficult for water droplets to freeze it might be worth noting that the formation of water droplets does not occur as you might have imagined.  You might have thought that once the relative humidity in the air (RH) reaches 100% that water vapor would simply condense and form little droplets.  This is not the case; we will find that small particles in the air called condensation play an essential role in cloud formation.

We'll first illustrate that when the air is saturated with water vapor (the relative humidity is 100%) the rates of evaporation and condensation above a flat surface of water will be equal. 

Small droplets however evaporate more rapidly than a flat surface of water   This is called the curvature effect and is illustrated above (6 arrows of evaporation from the smallest drop, 4 arrows from the middle drop, and 3 arrows from the largest drop above which is the same rate as you would find over a flat water surface).



In this graph (this is a plot of the Kelvin equation and is adapted from this source) we can see very small drops evaporate much more rapidly than larger drops.  These droplet radii are very small but a droplet that forms would start out very small.  A droplet that did form would quickly disappear; condensation from the moist surroundings would not be enough to overcome the high rate of evaporation.  A droplet needs somehow to reach a critical size with a lower enough rate of evaporation before it will be in equilibrium with its surroundings.


Particles in the air, cloud condensation nuclei (CCN), make it much easier for cloud droplets to form.  Water vapor could simply condense onto a particle of appropriate size as shown above.  The water droplet could effectively start at, rather than grow to, the critical size and would be in equilibrium with its surroundings.


This figure illustrates an additional scenario.  Some particles will dissolve when water condenses onto them.  The solution droplet has a lower rate of evaporation than a droplet of pure water.  This is the solute effect (Raoult's law).  Only two arrows of evaporation are shown coming from the middle solution droplet above.  Because that is less than the 3 arrows of condensation being supplied by the moist surroundings, the droplet is able to grow.  Once the droplet has gotten a little bigger,  the solution concentration will decrease and the effect on evaporation rate will diminish. 

Note that it is possible for small droplets of solution to form and be in equilibrium with their surroundings when the relative humidity is less than 100%.  These are called haze droplets.

Ice crystals in a cloud can basically form in two ways


Water vapor can turn directly to ice or supercooled water droplets can freeze.  Just as was the case with condensation and the formation of water droplets, the formation of ice crystals is much easier if an "ice crystal" nucleus is involved. 

The problem is that there aren't nearly as many materials that can act as an ice nucleus as there are condensation nuclei.  Silver iodide is used in cloud seeding.  Kaolinite is a clay material (that was used at one time in Kaopectate for the treatment of diarrhea, bismuth subsalicylate is now used).  Certain bacteria also are effective ice nuclei (bacteria are added to water in snow-making operations at ski resorts to ensure that the water freezes when sprayed onto the slopes).



Once an ice crystals forms, it is able to grow relatively quickly in the moist environment in the cloud even when the water droplets do not.  Ice crystals evaporate (actually they sublime, they change directly from solid to gas) at a slower rate than water droplets.  Because the surrounding air is moist enough to keep the water droplets in equilibrium (3 arrows of condensation balancing 3 arrows of evaporation in the figure above), and because water vapor will condense onto the ice crystals at the same rate, the ice crystals will grow and become snow crystals (just bigger ice crystals). 

Snow crystals come in lots of different shapes (called "habits", a plate is sketched above) depending on the amount of moisture in the cloud and the temperature.  A snow crystal might have a diameter of 100 μm or so, that's a tenth of a millimeter.  You really need to look through a microscope to see them.  You'll find plenty of photomicrographs of some snow crystals and a lot more information at www.snowcrystals.com.

It used to be (and maybe still is) that people would make replicas of snow crystals by allowing them to fall onto a microscope slide coated with formvar (a plastic resin material of some kind dissolved in acetone or something like that).  The crystal would melt and evaporate but would leave behind an impression in the formvar.  The acetone in the formvar would evaporate and the formvar would harden leaving a permanent record of the snow crystal that could be examined or photographed under a microscope.

A couple of more things you need to be familiar with before we start talking about electrification processes.


The snow crystal in the picture above is falling and colliding with supercooled water droplets.  Once the droplets come into contact with ice they quickly freeze.  This process is called riming or accretion.


An unrimed snow crystal.  This particular shape (habit) is known as a dendrite I believe.  This photograph and the one at right are from this source.
 A rimed snow crystal (also a dendrite it looks like).  The snow crystal is probably 100 to 200 μm across.  The supercooled water droplets that have stuck and frozen to it are usually 10 to 20 μm in diameter.



If this goes on long enough the snow crystal can get completely covered with frozen droplets, so much so that the original snow crystal is hidden inside.  The resulting particle is called graupel, soft hail, or snow pellets.  Graupel particles can grow up to maybe 1/4 inch across.  They have a frosty white appearance and resemble a miniature snow ball. 



A pretty good picture of graupel (source of this image) because it also gives you some idea of its size (I think that is a dime, not a nickel).

Graupel is really not hail (graupel is sometimes referred to as soft hail).  Hail usually starts with a graupel core and then has alternating layers of clear ice and rime ice (the frosty white ice that makes up graupel).  In Tucson hail usually has just a graupel core and a single layer of clear ice.  The appearance is quite distinctive and clearly different from graupel.  In the big severe thunderstorms in the Central Plains the hailstones can have many layers of rime ice and clear ice.



An excellent photograph of truly golf ball size hail (source of this photo, click on "Hail Rings" in the Storms category) .  Two of the hailstones appear to have been cut in half so that you can clearly see the alternating layers of rime ice and clear ice.

This is the last picture.  It shows the normal distribution of charge in a thunderstorm.  This is what a viable cloud electrification process needs to be able to explain.

Note first of all the cloud has a rough tripolar structure consisting of a main negative charge center (1a), an upper positive charge center (1b), and lower positive charge centers (1c).  All are found at temperatures colder than freezing.  The main layer of negative charge (1a) seems always to be found at temperatures between -10 C and -30 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.

E fields under the thunderstorm at the ground are typically 1000s of V/m (100 to 300 V/m is typically found during fair weather).  Enhancement of the E field at the points of sharp objects on the ground often go into corona discharge and spray positive charge into the air near the ground.  The ground under the main part of the thunderstorm is also positively charged.