500 mb Forecast Actual

2. 500 mb Forecast Actual

3. SurfaceForecast Actual

4. Nov 17 Forecast 500 mb Surface

6. How does it work? • It is common knowledge that lightning is generated in electrically charged storm systems, but the precise method of cloud charging still remains elusive • Certain things are known…however

7. How does it work? • An electrical field exists in any region exposed to positive or negative charges • Air is a good insulator (opposite of conductor like copper) … therefore charges do not flow easily from one electrical field to another • So very strong electrical fields must exist before charges will move through air

8. How does it work? • Thunderstorms are charged due to interface charging and inductive charging • Both involve collisions between upward moving small ice particles and falling hail. • Interface charging occurs when electrons (negative charges) jump from the lighter rising particles to the heavier falling particles. • Positively charges accumulate from the upper portion of the cloud to the anvil, negative charges accumulate in the lower half of the cloud

9. How does it work? • Inductive charging occurs as a feedback mechanism to interface charging. • Once opposing electrical fields are established in a cloud, collisions between falling hail and rising ice particles are even more effective in stripping electrons. • The negative charges in the ice align themselves at the top of the ice particle and the positive charges align themselves at the bottom of the hailstone

10. How does it work? • The ground is typically negatively charged • When a strong negative charge builds in the lowest layer of a cloud, the negative charges at the surface are repelled away (charges of the same sign repel each other, opposite charges attract • In a mature thunderstorm there is a positively charged field at the surface, an negatively charged field in the lower cloud, and a positively charged field in the upper cloud (like an oreo cookie)

11. So…Here is What Charge Separation Looks Like in a Mature Storm

12. The Strike: Step Leaders and the Return Stroke • When the electric field in a local area exceeds 3 million volts per meter a cloud to ground stroke begins • Electrons surge toward the cloud base to the ground in a series of ‘steps’ called ‘step leaders’ • Each step is between 50-100 meters long and take paths of least resistance toward the ground • Just before the step leader reaches the ground the charge field becomes so great that a flow of positive charges (called the return stroke) moves from the ground as sparks to meet the step leader • See figure 20.6 in the book… the step leaders are yellow, the return stroke is white

13. The Strike: Dart Leaders and Their Return Strokes • Once the ionized channel is established between the cloud and ground, additional step leader type discharges may occur, these are called ‘dart leaders’ • Similar to step leaders, before they reach the ground/object the charge field becomes so great that a flows of positive charges (return strokes) move from the ground as sparks to meet the dart leaders • Up to about 20 dart leader/return strokes can occur with one discharge event

14. Leader and Return Strokes

15. The Strike and the Flash • After the step leader (or dart leader) and the return stroke meet, the ionized air (plasma) has completed its journey to the earth, leaving a conductive path from the cloud to the earth. With this path complete, current flows between the earth and the cloud. • This discharge of current is nature's way of trying to neutralize the charge separation. Nature’s balancing act. • Any time there is an electrical current, there is also heat associated with the current. Since there is an enormous amount of current in a lightning strike, there is also an enormous amount of heat • In fact, a bolt of lightning is hotter than the surface of the sun • This heat is the actual cause of the brilliant white-blue flash that we see.

16. Thunder • When a leader and a streamer meet and the current flows (the strike), the air around the strike becomes extremely hot. So hot that it actually explodes because the heat causes the air to expand so rapidly. The explosion is soon followed by what we all know as thunder. • Thunder is the shockwave radiating away from the strike path. When the air heats up, it expands rapidly, creating a compression wave that propagates through the surrounding air.