Atm Ocn 100 Edition 1nd Lecture 25 Outline of Last Lecture I Lightning a Charge Separation b Why are pointy objects good for creating large potentials c Positive polarity stroke d Why does lightning make a sound II Downbursts a High Lapse rates b Dry air c Evolution of Downdraf d Threat to aviation e Doppler radars Outline of Current Lecture II Review Sheet Questions and Answers ones we went over in class Current Lecture Test on Friday Review The test has 40 multiple choice questions covering chapters 18 22 Here is a synopsis of what is on the test 1 What is necessary to form thunderstorms To form thunderstorms what is needed is conditional instability and instability that is deep enough to have equilibrium level that reaches above the level to get ice If there is no ice there is no thunderstorms 2 Why does stratosphere stop thunderstorm updraf Stratosphere can stop thunderstorm updraf if it is stable Air parcels finds themselves colder then its surroundings Stratosphere is quite hot in potential temperature not in actual temperature 3 Definition of severe A severe storm is actually restrictive Severe has to have winds of at least 55 knots And hail of at least 3 quarters now it is 1 inch They made it even more restrictive to be a severe storm And then if there is a tornado which is a funnel cloud that reaches the surface that is also considered severe Lightning does not make a severe storm It is the number one killer of people in the United States or the world but it doesn t count as a severe storm Either does flash flooding There has to be strong winds large hail or a tornado 4 Causes of downdraf Why do you get a downdraf To get up updraf you need positive buoyant warm air relative to the environment which cause air to rise To get a downdraf it is a push downward that is similar to an updraf A downdraf is a strong downward push What causes it Downdraf is negative buoyancy air sinks like updraf rises What makes it cold Evaporation makes it cold And water loading Rain falling through an air parcel will make it heavier and denser in terms of putting more water molecules in an air parcel without the rain falling through it When rain is falling it reaches an equilibrium fall velocity so the force of gravity is balanced by the force of friction the acceleration stops it still falls but the increase in fall speed stops It falls at a set rate terminal velocity the friction of the air balances the weight and friction is a force upward If air is forcing upward on the droplets than the air is experiencing a force which causes it to be heavier 5 Where and whys of mammatus Mammatus clouds are typical of very dry layers with steep lapse rates at mid levels They are conducive to downdrafs as well as for mammatus They look upside down When you look at underside of anvil you see utter looking like clouds Mammatus have colorful sunsets and this they appear at the west side of the storm The texture of the mammatus can be seen well They are indicative of a situation where you have a dry high lapse rate and a mixed layer coming in from the west which is conducive to severe storms Why is the dry high lapse rate layer conducive to severe storms Dry is conducive because it evaporates water efficiently if evaporation produces downdrafs High lapse rates produce instability If the environmental lapse rate is at a dry adiabatic rate than an air parcel just needs to be a bit cooled and it would be colder than surrounding This will make the air parcel fall because it will remain colder than the surroundings and keep falling If there is a high lapse rate a little cooling of an air parcel goes a long way Lapse rates produced when the surface heats in the afernoon Air mixes is overturned and then there is more heat so there is deeper mixing Because air freely goes up and down it maintains 10 degrees per kilometer going up and down it is will mixed Unwell mixed air would be cooler at the ground but since it is mixed it keep heating and so it keeps mixing and be a uniform potential temperature This is what we have over continental America is Rocky Mountains Denver is at 5 000 feet above Sea Level and Madison is at 1 000 f above sea level Air can be heated in Denver and produce a mixed layer Let s say Madison has a mixed layer too If wind is blowing from the west the mixed layer over Denver is affected over the top of Madison as well The mixed layer that was on the ground over Denver is over Madison but up higher It is dry air because it went over the mountains but also it is dry because of the mixing that brought dry air from above to the surface If there is then a thunderstorm in Madison which is lower to the surface it will come up and drop into the mixed layer The mixed layer is unstable which can then produce a big downdraf to form in the thunderstorm in Madison and this is all because of the mixed layer that came from Denver That is called EML or elevated mixed layer it is very typical of a downstream off of a mountain The EML can form thunderstorms that can have strong even violent downdrafs This happens in Oklahoma because they are near the Rocky Mountains They are closer to Gulf of Mexico though too so they get more moisture which means more of a mixed layer Moisture and cold air from the Rocky Mountains makes this area just right or otherwise known as a Goldilocks area for thunderstorms 6 What causes cold pools A cold pool is important for thunderstorms but also in general for convective systems It is the focus of the organization It is like having a cold front that prevents air from mixing into it It is very stable A flow of air that goes through the cold pool will rise over it because the pool is cold and stable The air is forced up over it which may be forced up to the level of free convection which then could cause a thunderstorm to initiate The cold pool is important for initiating the thunderstorms but it also is important for maintenance of thunderstorms The cold air in the cold pool is cold air from evaporation and cooling from precipitation of storms Each storm has a little cold pool but they can merge together if there is a group of thunderstorms such as in a mesoscale convective thunderstorm One big cold pool may produce a big shear of air 7 Air mass thunderstorm why when Where Individual cumulous cells go through a lifecycle The lifecycle consists of three stages 1 Cumulous stage which is the initial cloud with only cloud droplets 2 Mature stage which is where precipitation downdraf and