Hello Everybody! It’s been a long, hot few days here in the Midwest and across the country for that matter. In fact, currently 28 states encompassing 995 thousand sq miles and roughly 150 milion people are covered with a heat advisory of one type or another… So I have figured the last thing everyone wanted to hear about is some more hot air.
In this post, I’ll be covering the basics of the area of the central United States known as Tornado Alley, starting with the big picture. Nearly 80% of all tornadoes every year tear across this swath of land. Why do more tornadoes touch down here than anywhere else on earth? Lets begin by talking about some basic climatology and how the bigger picture really influences weather on smaller scales.
The Global Circulation Model is a basic depiction of high and low pressure regions in addition to general wind directions, whether it be easterly or westerly. (See Diagram Below.)
As you can see, not all weather patterns move from west to east globally like they do here in the “mid-latitude” United States. For example, in Brazil, most of the weather systems move in from the Atlantic Ocean in the east. Why do the winds change at different latitudes? To answer it simply, it has to do with the uneven heating of the Earth’s surface. The Earth is also in a constant struggle to stay in balance. Since our planet is tilted at a 23.5° angle, some parts of the Earth will get more direct sunlight and in turn get heated more efficiently depending on the time of the year/orbit. For the most part however, the tropics (areas between 23.5° N – The Tropic of Capricorn and 23.5° S – The Tropic of Cancer) receive the most sunlight throughout the year and that’s why they are much better for warm beach vacations, rather than Northern Saskatchewan. Getting back on track… This uneven heating allows for the warmer air near the equator to rise and spread out (Low Pressure) and eventually cool and sink (High Pressure) near the 30° area of latitude. This causes air to want to constantly circulate to try to attain a healthy balance, which it will never achieve, but you can’t blame it for trying. Of course, this is just a simplification of this model. Nearing the 60° area of latitude, you have the Arctic Circle, with more Low Pressure and converging, rising air.
So what does all of this Global Climatology jibber-jabber have to do with Tornado Alley? The answer is Everything. Come spring time in the Northern Hemisphere, the Earth begins to tilt back towards the sun. This obviously welcomes more sunlight hours, and the temperatures also tend to get warmer with the extra sunlight. As the warmer and more moist air masses begin to creep northward from the Gulf of Mexico, they clash with the remaining cold dry air from the previous winter. This sets up an epic battle between the two differing air masses across the Great Plains, aka Tornado Alley. The actual area of combat stretches from Texas to North Dakota and even into the Canadian Prairies later in the season. This is all based on how far the warm air advances as summer moves in.
In a smaller scale, the Great Plains are a breeding ground for tornadoes, similar to a swamp for mosquitoes. The main causes for this include the clashing air masses as well as the topography of the United States. The two main mountain ranges in the USA are the Rockies and the Appalachian Mountains to the east. These mountain chains are North-South orientated ranges and they actually help to funnel in the differing air, the warm, moist air from the Gulf towards the cold, dry air from our Canadian neighbors to the north, just like the scene out of the movie 300.
These two differing air masses do not like to mix, in fact they hate each other. The cold, dry air from the north will sweep under and push up the warmer, more buoyant air from the south. This follows the simple principle that cold air sinks whilst warm air rises. As the cold air presses into the warm air, also known as a cold front, the warmer air is forced upwards where it condenses, forms clouds and eventually storms.
Now that you understand the big picture to an extent, lets talk about the smaller scale. There are 3 main ingredients necessary for general storm formation. These ingredients are as follows:
The first ingredient, moisture, is supplied by the main bodies of water that are on the outskirts of the United States, including the Pacific Ocean, Atlantic Ocean and especially the Gulf of Mexico. The water temperature is directly related to the amount of moisture that gets transferred to the air/atmosphere. The warmer the water, the more easily moisture can evaporate and get taken with the wind so to say. That is why the warm gulf waters are a huge player in the spring thunderstorms that roll into the Great Plains, but moisture is not the only ingredient needed.
Instability comes in to play when you have warmer air near the surface and colder air aloft. Elementary Physics Lesson: Heat is transferred via 3 methods, Convection, Conduction and Radiation. Radiation is how energy from the Sun heats our planet through the vacuum of space. Conduction is the transfer of energy from particle to particle, so when two particles come into contact, the energy is transferred. Conduction is most effective in solids. The last method of heat transfer is Convection. This heat energy is transferred by the movement of the particles, and this takes place mostly in the air. Phew, ok, you made it through that alright. Let me tell you what on Earth that has to do with the air. Since the most effective form of heat transfer is Conduction, this means that solids will be the most efficient “warmers.” Ex: The Ground. When the Sun’s Radiation heats the Earth, it is predominantly from the ground up. The air closest to the ground will get warmer than the air above it most of the time, and when you have a significantly cooler body of air floating above the warmer air, then voila! You have instability. The warmer air will naturally want to rise above the colder air.
Finally, you could have all the moisture and instability in the world and still not have a storm! You need the final ingredient, Lift. A lifting mechanism, or trigger, is the last ingredient needed to really get the storms going. You can get this in a variety of fashions, and I’ll try not to get too deep into detail. Types of lift include Fronts, where the cold and warm air masses at the surface collide, a Dryline, which is another type of front, where the difference between the air masses is the amount of moisture. Another trigger for storm genesis is an outflow boundary from a already formed thunderstorm. The rush of cool rain and air downward acts as a mini-cold front and can help to develop more thunderstorms. The last form of lift is terrain or Mountains. These are a great lifting mechanism as they force air upward where it then condenses and causes localized rain showers.
When all these ingredients come together, you should in theory end up with a storm. These ingredients tend to come together quite well in the central plains of the United States. However, not every storm is tornadic. There is one more ingredient I left out. The ingredient can turn any normal thunderstorm into a special, violently rotating storm known as a Supercell. The ingredient is Wind Sheer, or the turning of winds with height. As the warm air rises and forms into a thunderstorm, the different directions of wind will begin to turn the storm on it’s axis. Another type of sheer, Speed Sheer, will help the storm to tilt a bit on its vertical axis so it can continue to feed on warm moist air and not choke itself on the cold air and rain it produces. See the diagrams that I borrowed from the NWS depicting the two types of sheer.
When the sheer is in place and a thunderstorm morphs into a rotating Supercell, tornadoes are much more prone to form. The strong updrafts in the supercells actually tilt the horizontally rotating columns of air to a vertical position, creating a rotating updraft.
This final bit of rotation can be the last key to tornado genesis as it forms a broad area of low level rotation in a supercell. We meteorologists call this a mesocyclone, which is usually near the hook echo of said supercell. Another diagram below has been added so I don’t lose you completely.
If the conditions are right, the rotation within a mesocyclone will tighten up, and a lowering cloud will appear. This is called a rotating Wall Cloud, named because of it’s rather square appearance. This is the final precursor to a funnel cloud and or tornado if the funnel reaches the ground. This is what a wall cloud looks like at ground level.
If the rotation is tightened enough, and the conditions are right, a funnel will most likely appear at the base of the wall cloud. If the funnel reaches the ground, it is then classified as a tornado.
Tornadoes can form anywhere, given the right conditions, but as I have explained they are more prone to form in the Great Plains of the United States because of all the aforementioned reasons.
If anybody has any questions, please e-mail me! Brad@SWATChasers.com. I know I have covered about a years worth of studies and crammed it into one blog posting. Hopefully everyone has learned something about the weather, and in particular Tornado Alley. If you would like to suggest a topic for the next blog, either send me a suggestion via e-mail or make the request on our Facebook page. Also, don’t forget to visit our websites, www.SWATChasers.com and www.SWATTours.com. Some people are Weather Wise, some people are otherwise. Don’t be the latter! As always, thank you for your support and thanks for reading what I have to say.