The Ups and Downs are Graupel Shocking
In his day, Ben Franklin was all charged up about lightning. He was the international expert on the subject. He became the toast of the town in Europe. That's Ben for you.
What about CED? What is our source material for this lightning blog?
Here it is: Houze, R. A. (1993). Cloud Dynamics, Academic Press and Atmospheric Science. John M. Wallace and Peter V. Hobbs (2006) Academic Press.
Franklin's dabbling in all things electrical! Indeed, his use of kites for electrical observations is well known. Less well known are his 1750 planned experiments on the more frightful side of his work. He sought to prove that thunderstorms were electrified. In his experiment on the subject, a man would be placed in an insulated sentry box with a 30-foot rod out the top of the sentry box and then wait for the sparks! Faith in Franklin was great. The experiments were conducted in 1752 and Franklin suggested that indeed, there were positive and negative layers in the thunderstorm clouds.
If you are ever lucky enough to make a balloon ascent through a thunderstorm with an electric charge meter on board, you will find that the atmosphere has three layers defined by temperature and electrical charge: Bottom, middle, and top.
The ground and the air up to the bottom of the clouds, or up to where the temperature is around -10 C, is positively charged.
The middle layer of the atmosphere where temperatures range from -10 C to -20 C is the layer of negative charges.
Above this layer (at still colder temperatures), positive charges are the rule. The middle layer is called the "charge-reversal temperature".
Cloud micro-physicists still argue about how it gets to be this way. We can just say that that is the way it is. Two theories are offered. One model involves graupel, a.k.a. soft hail or snow pellet precipitation. The other is called the "convection model&".
Who begat the graupel? Well, we start with cloud droplets not yet ready to fall out of the clouds. It is made of supercooled water with temperatures colder than 0 C. and could be as cold as -40 C, at which temperature freezing is automatic. This freezing is called homogenous.
Between 0 C and -40 C, freezing is called heterogeneous and requires an ice nucleating material to get the icing started. That is the subject of other CED blogs. When the cloud droplet spheres of supercooled water heterogeneously freeze you have graupel. The graupel now begins to grow in size at the expense of liquid water droplets. As it grows it begins falling and participation in electrical charge separation. The bigger the voltage gradient, the more likely lightning will follow.
One theory to explain all this is called the precipitation hypothesis. In this theory, falling graupel, the precipitation, takes on a negative charge on its way down and leaves behind, at the top of the cloud, little non-falling positively charged ice crystals. The big, falling crystals take on the negative charge, and the small ice particles left behind take on what is left of the positive charges. All this makes for charge separation and large potential gradients, the stuff of lightning. At warmer temperatures (warmer than -15 C), the graupel takes on positive charges, thus the warm bottom layer of the cloud gets to be positive.
Another hypothesis called the "convection hypothesis" involves updrafts and downdrafts, picking up positive or negative charges. Think of a downdraft down to the surface. The cloud top air, rich in positive, charges the bottom layer. Updrafts carry the positive charges to the cloud top layer.
Biogenic compounds now take the stage. Biogenic hydrocarbons (terpines and hemiterpines) agglomerate to form petroleum spheres. When 14,000 terpine molecules or so accumulate in a spherical drop (a haze particle) it has a positive charge. In a thunderstorm, these biogenic particles are lifted into clouds and play in the electrical life of the thunderstorm. Storms without a source of the charged biogenic nuclei from the surface, e.g. marine areas, tend to have little lightning compared to the terrestrial world.