Monday, January 18, 2010
What is Lightning?
To understand what lightning is we must first look at the water cycle which exploits 2 well-known phenomenon. Evaporation is the process of a liquid absorbing heat and changing to a vapor. Condensation is the process of vapor or gas losing heat and returning to liquid form. As vapor rises, it enters cooler and cooler areas in the atmosphere, losing heat in the process. When enough heat has dissipated, vapor becomes liquid. Snow/sleet occurs when the atmospheric temperature is below the freezing point, the vapor, in turn, freezing instead of liquefying.
During an electrical storm upper portions of clouds become positively charged and the lower portions become negatively charged, creating an electric field. The process of charge development is still unclear. As charge builds in the cloud, the lower portion of the cloud, which is negatively charged, repels all the earth's electrons deep into the earth's surface - so the ground itself acquires a positive charge.
Next, the electric field itself creates a conductive path for the negative cloud bottom to contact the positive ground. The conductive path occurs because the negative charge "wants" to travel towards a positive charge and become neutralized. Remember, right before lightning strikes, a tremendous amount of voltage has built up ... ~tens of thousands of volts per inch (compare to your house voltage which is ~230 V). Voltage is the energy per unit charge. Current is the ensuing flow of energy. Think of voltage as the cause, and current as the effect.
The electric field ionizes the surrounding air - that is, it separates the air into positive and negative charge just like the cloud. The air ionized, now the charge seeks various paths to reach the ground. Differences in levels of air ionization, dust, etc. makes some paths better then others. Also, if a cloud's lower portion is not straight, the ensuing lightning will branch out. All of the above is what creates "branch-like" lightning. Thus, it is incorrect to say lightning always follows the shortest path, as the shortest path may not be the most conductive. Note - lightning can strike between clouds as well if the charge is as polarized as it is when lightning strikes the earth. There is also lightning which occurs above clouds - that is, lightning that strikes upwards into the atmosphere!
When lightning strikes, current is passing from the clouds to the ground, eventually neutralizing the charged clouds and air. Lightning bolts, traveling at 186,000 mps, reach temperatures of ~55,000ºF vs. the sun's surface (9,800ºF). The lightning bolt's heat causes the surrounding air to expand rapidly and literally explode, resulting in a shock wave known as thunder.