Howdy everyone and welcome to module 10 where we will explore wild weather with a wide range of topics. Starting with our first of severe thunderstorms need three ingredients required for them to develop. They must have moisture, instability and lift. remember air rising. Without all three of these ingredients, a thunderstorm will never develop. So, it's important to remember that we need moisture, instability, and lift. Now, the National Weather Service defines severe weather as um a system that can include any of the following. either hail that is one inch or larger in diameter, surface wind gusts of 50 knots, which is about 58 miles per hour or greater, or a tornado. So that's their definition of se severe weather has to include one of those three um hazards. But there's some other important hazards that are not considered in the definition of a se severe thunderstorm that are still important like lightning and especially flash flooding. Shockingly, flash flooding is not included in their definition of severe weather, but it's still important to remember that flash flooding can occur in severe thunderstorms. Just some review when we talked about our weather basics um a few lectures ago. Remember we assume that air parcels can heat and cool but they don't necessarily need to mix and they don't really mix in air from the outside. Remember these a diabetic processes where we can have changes in temperature by just changing the pressure. We're not actually mixing or adding or removing energy. Now the pressure of a parcel of air is assumed to adjust instantaneously to its environment. Remember this is why rising parcels of air expand and cool. Remember that's necessary for cloud formation and precipitation and ultimately severe weather. While sinking parcels of air are compressed and warmed and they don't promote cloud formation or precipitation or severe weather. We really need rising air because that rising air expands because it's experiencing different pressure. When it expands, it allows it to cool. And if it cools far enough, it will condense and form clouds. And if those clouds get heavy enough, they form precipitation. And here in this lecture, we're going to talk more about how those thunderstorms um really begin to to shape. If a parcel of air is forced to rise or lift and it's warmer than its environment, it will continue to rise. Meaning this is an unstable environment. This is favorable for thunderstorm formation. Now, if a parcel of air is forced to rise, but it is cooler than its environment, it will sink back down to its original position. This is considered to be a stable environment and does not promote thunderstorm formation. So in order to have severe weather, a parcel of air needs to be forced up. It needs to lift and that parcel of air must be warmer than its environment so it can be allowed to continue to rise. Remember warm air rising expands, cools, condenses, precipitates. That's necessary. We need an unstable environment. When parcels of air rise and cool, again, they will eventually become saturated. And as they continue to rise, the water vapor will condense into liquid water, forming a cloud. And again, if that cloud becomes heavy enough, precipitation can form. Again, all review, but all necessary to remember when we start talking about our severe thunderstorms and tornadoes. trips and rising moist air is what leads to a thunderstorm. And once we create precipitation, we often have to care about how the air is falling also because as precipitation falls, it often evaporates which changes temperature. And the falling air, this is what we call a downdraft. And the downdraft is really important because it can organize the storm, it can kill the storm, and it can create some severe hazards. So, it's really important to not just consider the updraft or the air that's rising, but we also have to care about how that air is falling as well. Now there three stages of an ordinary cell thunderstorm and we call this the cumulus stage, the mature stage and the dissipating stage. During the cumulus stage, this is when we have moist air rising and condensing often creating those puffy popcorn like clouds. The mature stage is when the dry air is entrained and it leads to a downdraft and gust front and often produces heavy precipitation. And then finally the dissipating stage where the downdraft actually cuts off moist updraft and the cloud and raindrops begin to evaporate. Now ordinary thunderstorms usually form when winds are uniform with height. So we don't really have any sheer wind happening here. All the wind is uniform as you move up in height. And these ordinary thunderstorms typically only last about 15 to 30 minutes but could be up to an hour in some cases. Here's a wonderful figure that shows the three stages of an ordinary thunderstorm. There in a we have the cumulus stage where we have warm moist air rising, expanding, cooling, and condensing forming our cumulus clouds. There in the center at B, we have the mature stage. And you can see here in this mature stage we have the the updraft but we also have the downdraft where we have precipitation occurring and that cold air is moving down creating a gust front. And then C is a dissipating stage where you can see the water um within the precipitation and the water suspended in the clouds is beginning to evaporate. Here's just some real photos of some ordinary thunderstorms that you might see um pop up here in Texas over the summer. Sometimes we can get a group of these ordinary cells or these ordinary thunderstorms at different stages of their life cycle and we call that a multi-ell thunderstorm. Almost all the convection gets organized into multi-ells where we have convergence along the gust front of a decaying cell that's lifting the air which actually initiates a new cell to form. The key to understanding all thunderstorms beyond our ordinary thunderstorms is that there is vertical shear. The updraft may be tilted and the inflow may not be cut off. This is necessary for thunderstorms that are beyond ordinary. If convection is to be sustained, the gust front must be able to generate new convection in a nearby and organized way. Vertical shear, the change of wind speed and direction with height is the key to this. This is a picture when we don't have our vertical shear. We don't have tilt. When a thunderstorm's downdraft reaches the ground, the air spreads out in all directions forming a gust front. Note that in this case any new convection will be quite separate from the cell. Now here's a simplified model describing air motions and other features associated with an intense multi-ell thunderstorm that has a tilted updraft. The severity depends on the intensity of the storm's circulation pattern. But the important part to note here is that the wind is not uniform with height. This is called our shear. And the shear actually allows that storm to tilt in such a way that the updraft rides up and over the downdraft, allowing it to form new cells and last longer. The gust front from one cell will provide the lifting for a new cell to form. And each cell lasts about 15 to 30 minutes, but the whole system can last many hours. And here's a photo of what a typical multi-ell thunderstorm looks like. When multi-ell storms grow into even larger groups, they are then called mezoscale convective systems or MCS. MCS's provide about half of the summer rainfall in the Midwest. Believe it or not, within this group of MCS's, we actually can have different types. We can have squall lines, which are long lines of convection. We can have bow echoes, which I already kind of showed you a few lectures ago, which are curved line of convection that bows out. And then we have meoscale convective complexes or MCC's, which are large circular clusters of storms. And I'll show you pictures of what these look like in radar here in a bit. So, here's those examples of those MCS's. There on the left, we have a squall line. In the center we have a bow echo and on the right we have a MCC which is observed in infrared image here. Now let's talk about how they move. Now individual cells are carried by the winds aloft the winds above. However, new cells form and old ones dissipate and the systems as a whole ends up continuing to move to the right of the wind. Now, generally, if the relative humidity in a storm environment is fairly low, we could have strong downdrafts that form and the system will move very quickly and also producing strong winds. But if the relative humidity is high, the downdrafts will be weaker and that will cause the system to move more slowly. And in a very moist environment like this, very heavy rain can fall and cause flash floods. Precipitation associated with a squall line that has a trailing stratapform cloud layer. Here's a Doppler radar composite showing a prefrontal squall line extending from Indiana southwestward into Arkansas. Severe thunderstorms are shown in this image in the red and orange colors and they're associated with the squall line produced um they produced large hail and high winds. And this this actual radar images is from October 2001 where they did indeed see large hail and high winds. Supercell thunderstorms are nearly steady rotating updrafts and a single supercell can last for many hours and these are probably the most dangerous of thunderstorms. Now just a reminder wind shear is produced when speed and direction are changing with height and because of this we get rotation as shown here. Now if we lift our rotating tube that's when we can produce cyclonic rotation and anti-cyclonic rotation within our supercell. When there is a strong wind shear, thunderstorm updrafts can rotate which forms a supercell. And the rotation keeps the precipitation in the cold downdrafts away from the updraft so that the storm can last for hours. And the wind shear created horizontal rotation which is tilted into the vertical and stretched by the updraft to make a rotating form as seen here in these two figures. Now, supercell thunderstorms, if you haven't guessed, are responsible for nearly all of the significant tornadoes produced in the US. About 80% of tornadoes come from supercell thunderstorms. Supercell thunderstorms are also responsible for most of the hailstones that are larger than golf ball size and they're also frequently known to produce extreme winds and flash flooding. Here's a side view looking down at a supercell thunderstorm. You can see we have the warm inflow with our wind shear producing a messocyone and a rotating updraft. But we also have our forward flank downdraft and our rear flank downdraft. And this allows the system to continually operating for hours and hours at a time. And if that rotation is intense enough, can even form a tornado, which we'll talk about here in in a moment. But overall, the system is moving to the northeast. Features associated with a classic tornado bearing supercell thunderstorm is viewed here from the southeast. And in this figure, the storm is moving towards the northeast. Here's what supercells often look like on radar. You can see on the left the supercell is starting to rotate. you're starting to see a lot of rotation. And here we actually have a tornado located. And that's usually where we see tornadoes being located on supercell thunderstorms. Usually gets this hook echo effect. And you can even see it over here in this radar image as well. Here, that hook echo effect. This is where we're more likely to see a tornado during our supercells. But you can see we have lots of hail and heavy rain associated with these supercells. And you can see that on both of these radar images. Here's another radar image of a supercell thunderstorm that passed through the Brian College Station area earlier this year and produced some tornadoes um south of here. And you can see those tornadoes pop up in the radar image and you can see where the heavy precipitation was located within the supercell thunderstorm. But it kind of formed a little hook echo where we saw those tornadoes form. Let's take a look at that again. Let's see if it's going to start moving again. See how we start to have a hook echo being formed here? The supercell. Well, this hook echo effect is what ultimately produced enough rotation where we saw some of those tornadoes touch down. And in this radar image, this is showing um wind direction. So we have green that indicates in one direction, red is the opposite direction. So what in the areas where we have red and green close together, that indicates, you know, wind that are coming from different directions and can indicate a tornado right in here. And that's exactly where those tornadoes were observed. Very interesting stuff. And we can tell a lot by radar. Now here we leading to the formation of severe thunderstorms, especially supercells. The area in yellow is where supercell thunderstorms are likely to form. So this is what we see at the surface. We usually have cold air being brought in, warm, humid air being brought out. And right in between the two, this is where we have our supercell thunderstorms forming. But as you move higher in the atmosphere, you see that there are different characteristics. A little bit higher, we have warm, moist air. A little bit higher, we have dry air. And you could see all the way into our jetream that we usually have a trough on our jetream that promotes the instability of air and ultimately thunderstorm formation as well. Now, here's a typical supercell scenario for Texas. This is showing surface conditions that can produce a dry line with intense thunderstorms. So, again, we have cold air being brought in. We have warm, moist air being brought out. And here we have our dry line that produces a lot of the most favorable conditions for these supercell thunderstorms to occur. And here's just some photos of some of those supercells because they are truly spectacular to to observe. different types of supercells. We have our classic examples which are essentially the diagrams that I just showed. But we also can have LP and highp which stands for low precipitation supercells and high precipitation supercells. And let me show you what each of those kind of look like. Here's our classic supercell. You can see that it is clearly a rotating storm. Then here's what our high precipitation supercell looks like. You can see a lot of precipitation associated with this rotating storm precipitation supercell. You don't see as much precipitation associated with this. So you can easily see tornadoes that get um generated with these things. Now with our high precipitation supercell, sometimes the precipitation can can hide the tornado which makes it even more um dangerous and hard to spot. Things to always remember about supercells. They are rotating storms. They always rotate. I mean, always. And they're typically much longer lived than ordinary cells. And they can usually move in a different direction than regular cells because they are rotating and they often can split off. And really, they can rotate in either way. All right. Well, supercells and other severe thunderstorms can also produce other hazards other than torrential rain, high winds, tornadoes, but also hail, which tends to be very costly and is the main reason why um homeowners insurance is so expensive here in Texas. Now, large hail only occurs where we have very strong updrafts, usually in those supercell thunderstorms we just explored. And these strong updrafts, they help keep the heavy stones aloft. So, the longer that they're able to stay afloat, they can continue to grow. And they must remain in the cloud for about 5 to 10 minutes to become golf ball sized. So, this is how we get larger hail. the longer it's up there, the bigger it's going to be. And if hail is severe, um, meaning it's greater than one inch in diameter, that's whenever it gets the thunderstorm that they're produced in that severe status. So again, an inch in diameter is about quarter size, and this can cause pretty significant damage. Now, this is a map showing the average number of days each year in which hail is observed throughout the United States. And this data is just coming from hail observed days between the years of 1950 to 1975. This is coming from Noah. And you can see here, yes, Texas is indeed on this map. We get here probably we're probably right in this yellow range. We get on average, I think yellow is two or three hail days a year, but you can see some areas of our country that are getting up to six or even eight hail days a year. And hail is is hazardous and it can cause severe damage to crops, vegetation. Um you guys probably know this well with cars. I mean, we had a pretty um damaging hail stom not that long ago here in College Station in Brian. can also damage aircraft roofs and and my my roof on my house is actually brand new because of a hail storm and windows, but they can also be deadly to people if you're caught out in one of these storms and they tend to kill livestock as well. Now, wind is also a severe weather hazard and they can cause significant damage. Now, along the gust front of a squall line, winds can be very strong, especially in those bows. Remember, we looked at the bow echo. Those bows usually contain some of the strongest winds. and a strong localized downdraft where the wind is moving down. And then remember when that downdraft hits the ground, the the gust front travels in all directions out. That can produce severe winds and we call that a micro burst. And micro bursts can be especially hazardous to aircraft. And thankfully, airports have these systems to detect them now. But in spite of their small size, an intense micro burst can induce damaging straight line winds well over 115 miles per hour. Oftentimes, the damage from these things gets mistaken for a tornado because they can be just as destructive. And in the United States, hundreds of air passengers died in micro burst related accidents in the 1970s and 80s, which is what led to intensive research on these things that ultimately led to the warning systems that are installed at airports now and have been in airports since the 1990s. Now, this system virtually has eliminated microbururst accidents for United States commercial flights. So, this is great science and engineering in action. Now, here's a figure of a micro burst where you can see the downdraft is pretty intense. It's moving downwards. When it reaches the surface, it moves outwards. And there's a picture there on the right. This is of dust clouds rising in response to outburst winds of a micro burst. This is north of Denver, Colorado. And micro burst can be associated with severe thunderstorms producing strong, damaging winds. But studies show that they can also occur with just our regular ordinary cell thunderstorms and with clouds that produce only isolated showers that may or may not even contain thunder and lightning. Now day ratios, the Houston area knows day ratios very well. Uh the span d ratio is a Spanish word for straight ahead. So, these are straight ahead, straight line, powerful winds. And here's a Doppler radar showing an intense squall line in the shape of a bow. Remember, called a bow echo. And this one's moving eastward across Missouri on the morning of May 8th, 2009. And the strong thunderstorms, which again are the red and orange color in the image, are producing damaging straight line winds over a wide area. Damaging straight line winds that extend for a large distance like this along a squall line is what we call a day ratio. And we see on average 20 day ratios in the United States every year. Here's Doppler radar imaging showing that the bow echo um has developed into a meoscale convective vortex and this happened by the afternoon on that same day. And strong straight line winds are still occurring with this severe thunderstorm as um Carbondale, Illinois here reported an unofficial wind gust of 106 miles hour. Now, these messoscale convective complexes can produce a wide variety of severe weather, including hail, high winds, flash floods, and tornadoes. And we'll spend more time talking about tornadoes in the next video. Now, here is a building near Hudson Oaks, Texas that is experiencing the damaging effects of a day ratio on June 1st, 2004. And the windstorm produced extensive damage over a wide swath across North Texas and Louisiana. Now, I've experienced something similar to this. I actually lived in North Texas in the Dallas Fort Worth area. And in high school, I told my dad I was going to church. I wanted to borrow his truck to go take myself to church. But I didn't actually go to church. This is a bad story, but I didn't actually go to church. I actually went stormchasing because the weather reports were showing a good chance for tornadoes not far from where we lived. And I didn't actually get to see a tornado, but I did see um a day ratio um very similar to this where I saw straight line winds. I thought it was a tornado at the time, but it wasn't. I saw straight line winds completely um demolish the roof of this hotel and it was crazy. Um I also do not recommend stormchasing by yourself. It's really hard to keep track of the road and the sky at the same time. So, please do not follow my example. Do not go stormchasing by yourself. Well, don't go stormchasing unless you know what you're doing, which you probably don't. So, just don't go stormchasing. And then that brings us to tornadoes. Now some supercells about 20% of them create enough rotation that the circulation will contract and reach the ground forming our tornado. Now tornadoes are defined as violently rotating columns of air in contact with the ground and below a cumulo form cloud. And here on the left is a map showing tornadoes day tornado days per year. Um so you can see here very clearly where our tornado alley is located but also pay attention to Florida's lit up over here too. Florida gets um some of the most tornadoes in our country. And then these are significant tornado days per century. So you can see um that this over the last hundred years has been a hot spot for a lot of our tornadoes. All right. So here's a map showing the distribution of thunderstorms. And you can see a lot of the south is very common to be getting thunderstorms. And worldwide, it's usually the tropics that have this high frequency of thunderstorms. So that makes sense. And thunderstorms are usually happening over land areas where we have the intertropical convergence zone. Remember, we have that warmth and moisture. Um, but thunderstorms are also important mechanisms to help us move heat forward. Now in the US, u most frequent places for thunderstorms is of course Florida, the Gulf Coast, and the central plains. Um the Pacific coast and the interior valleys probably get the fewest amount of storms, which you can see here on this map. And that occurs um these storms that they do get occur mostly in their cold seasons. And most frequent large hail actually happens in the central plains. So this map is is very nicely kind of correlating with all of those things that we talked about. Now, if anyone is interested in learning more about the processes of how all these kinds of weather events happen, please, please, please sign up for ATMO 2011. Uh, usually taught by Dr. Neielen. Uh, he's an amazing professor and he's my neighbor, so I know he's a good guy outside of campus. So, highly recommend at 2011. He's the one that helped me put together a lot of these slides that go over these weather processes. But in our next video, we will continue talking about um some weather hazards, but just focusing on tornadoes. I'll see you there.