Lecture 24 Outline of Last Lecture I. Weather of the dayII. Hail, some radar informationIII. Where does hail occur?IV. LightningV. Where lightning occursa. United StatesVI. More Lightning Outline of Current Lecture II. Lightninga. Charge Separationb. Why are pointy objects good for creating large potentials?c. Positive polarity stroked. Why does lightning make a sound?III. Downburstsa. High Lapse ratesb. Dry airc. Evolution of Downdrafd. Threat to aviation e. Doppler radars…Current LectureTest is FridayWednesday we will have a review sessionLightning We have been talking about the fair weather electric field. The fair weather electric field has a predominance of negative charges at the surface and positive charges in the air; this is a lefover effect of thunderstorms.These charges are called ions. If the ions are missing an electron, and they have a positive charge, or if they have excessive electrons they are considered negatively charged. Ions are mixed into the air and into the surface of the earth. Atm Ocn 100 1nd EditionWhen a cloud forms you tend to get an accumulation of positive charge at higher levels and negative charge at lower levels. If you look at what happens it is due to precipitation in the cloud. The crystals in the upper part of the cloud have pointy surfaces and make bigger ice particles. On the pointy surfaces, electrons move to the edge of the particle, and when this happens the large particles come into contact with smaller ice particles to which then the electrons from the large particles jump to the smaller particles. This then means that there is a negative charge in the larger particles, which then settle to the ground. The smaller particles have the positive charges.Because larger particles are falling and ice crystals will have the predominance of negative charges at the tips, the ice particles will have a stronger gradient between ice crystal and the larger droplets/ particles above which means there is more charge jumping across. The jumping makes ice crystals become increasingly positive charge from the larger droplets/ particles, which are switching electric charges. The reason the smallerice crystals only interact with the bottom of the large particles is that the whole thing is setting and falling. The large particles are falling faster than the ice crystals because they have smaller surface to volume ratio. The ice crystals are slower, so the large particles catch up and they intercept at the bottom side because that’s where the ice crystals are drawing toward the other particles. Ice crystals negative on top and then switched too positive. This is called a charge separation. Once you get charge separation you can form discharges if the separation reaches a critical point where you have a large potential between the negative and positive charged parts of the cloud. The cloud can begin to discharge. The charge differential within the cloud is positive above and negative below. But the ground in general is more positive charge partially because that is built in response to negative charged cloud above. You get a strong gradient between negative charge and positive charge in ground. Right below the anvil however, the ground is relatively negative because there is so much positive charge positive in the anvil. What happens is that electrons jump off; they work their way down until the ions reach the ground and then there is an ionization path which lightening bolts can form in and then have a return stroke.You can get multiple return strokes forming. This is a discharge of the cloud that was formed by the interaction of microphysics in the cloud and in particular the ice phase. They key thing is the mixture of slow movingpointy ice crystals in the cloud with the larger sometimes water coded graupel particles that are falling faster. The forced interaction between these two is kind of like electricity from feet on the ground. A charge is exchanged by the fact that the crystals have pointy structures. When you see lightning occurring it looks like it is coming down from above. When ionization comes down andreaches a point on the surface, it goes to pointy objects like tree branches or lightning rods. The pointy object is very good for creating large potentials. Why are pointy objects good for creating large potentials?The reason for this has to do with the electric field. If there is a pointy object the electric field spreads around the object. At the point you get a strong gradient between positive charge and negative charge above. The pointy surface with a broad charge distribution winds up with a concentration of difference right at the points or point. That increased potential is likely that charges will start jumping. This is less likely in a flat object. Positive polarity stroke happens because of positive anvil electrons flow underneath it on the ground and you get a large potential. Electrons cannot just flow all the way to the cloud because of the air in between may not be totally charged, they need ionization paths in order to jump. But you can create a path and once that path is formed the electrons can jump quickly to change the charge of another space. If there is no path, electrons do not pass easily through the air. There also can be interactions between charges inside and outside of clouds. The charge can go outside the cloud and land several kilometers outside assuming an ionization path is formed. Lightning has a sound associated with it. Thunder and lightning is the same phenomenon. Thunder is the sound of lightning. Why does lightning make a sound?Lightning heats the air thousands of degrees. It heats the air rapidly and strongly. The hot air has to expand in order to satisfy the equation of space. Heat expands and then it compresses and cools. And then it collapsesback again; the expansion produces compressional waves that move outwards those are sound waves so we can hear them as thunder. Thunder is air expanded and contracted by heating that goes on from lightning. Sound waves emitted by lightning may not travel in a straight line. When density of air changes rapidly that cancause sound waves to bend. They can be slowed down or sped up. When changed, sound waves bend and do not travel in a straight line.During the evening hours the ground in the lower layers of atmosphere cools so there is a high density level near the ground, sound waves that hit this would slow down and be retracted off that layer and go up in