Tuesday, August 13, 2013
4:25 P.M.
Last Friday night will rank up, at least in my mind, with some of the most memorable weather events in Seattle’s history. That means I’ll put it right up there with the Inaugural Day Storm, the Hanukkah Eve Storm (and associated torrential downpour), the December 2008 snow events, the 2007 Great Coastal Gale… you get the idea. Photographer Anthony May took the above picture of lightning behind the Space Needle last Friday night, and it has since gone viral. It will surely withstand the test of time and go down as one of the most incredible pictures of the Seattle skyline ever taken. Anthony’s Facebook page is here, and his personal website is anthonymayphotography.com. I HIGHLY recommend that you check them out… he takes some stunning pictures.
I have never seen such a spectacular lightning show in Seattle. Storms like this with heavy lightning and relatively light precipitation are not that uncommon for other parts of the Intermountain West. Events like this had already happened in Eastern Washington this summer. But for Seattle, they are unusual. I do remember a lightning show similar to this that must have happened in my elementary school days, but I cannot remember a show this spectacular, this prolonged, and this dangerous.
Since this event was so awe-inspiring, I am going to blog about it in four parts. The first part will be an introduction to the formation and characteristics of thunderstorms with a focus on explaining lightning. The second part will feature a thorough meteorological analysis of what happened not only Friday night in Seattle but all over the weekend throughout the state. The third part will take a look at the fires, floods, and other damage caused by these storms. The fourth and final part will tie everything together with comparison of how our most recent event compares with others in the state and others around the world.
Without further ado, let’s get started.
Most of us are familiar, whether we know it or not, with many of the meteorological phenomena we experience here in the Pacific Northwest. Warm fronts and cold fronts, high pressure and low pressure, and heat index and wind chill are just a few of the aspects of weather we experience throughout the year. Most of the stuff I talk about is readily explained. Rain forms when cloud droplets coalesce onto an airborne particle until it gets big enough to fall through the cloud. Snowflakes are the frozen analog to rain droplets and are more complicated due to their crystalline structure, but even they make intuitive sense. The sun is responsible for heating the Earth, and greenhouse gasses are responsible for preventing some of the infrared radiation reflected from Earth from escaping out to space. Lightning forms when an electrical field is created within a cloud by scientifically-debated processes and creates plasma which leads to the creation of electrical step leaders while positive streamers are forming via attraction of the positive charge of Earth’s surface to the leaders and are eventually met by the leaders to form a closed circuit that sends a return stroke that flows from positive to negative charge that heats up the air around it to five times the temperature of the surface of the sun and creates a shockwave that can be heard as thunder.
What?
Yeah, lightning is not easy to describe. I might as well split part one into separate parts. Let’s start out with some basics and go over how a cumulonimbus cloud is created.
1.1 – Creation and Characteristics of a Cumulonimbus Cloud
Cumulonimbus is a conglomerate of the Latin words cumulus and nimbus. Cumulus means a ‘heap or pile’ and is used to describe clouds of various shapes and sizes that look like somebody got a whole bunch of water droplets and decided to pile them up into one cloud. Nimbus simply means that rain falls from the cloud. Smack these two words together, and BAM!; you got a heapish cloud that rains. Some people will tell you that cumulonimbus and thunderhead are synonymous, but this is not true. Just because a cloud is a cumulonimbus does not mean that it is producing lightning.
There are four types of cumulus clouds: Humilis, mediocris, congestus, and fractus. Humulis are small and harmless, mediocris are slightly larger, congestus (also known as ‘towering cumulus’) reach high into the sky, and fractus are cloud fragments that have broken off from other clouds. I think that four different species of cumulus clouds is overkill, so let’s just talk about humilis and congestus.
I like to refer to these clouds as cotton-ball and cauliflower clouds, respectively. Cotton-ball cumulus are an indicator of fair weather. See how the tops do not rise very high? This is because the air is fairly stable and does not allow cloud-producing updrafts to rise very high into the atmosphere. With cauliflower clouds, the atmosphere is unstable and has a high environmental adiabatic lapse rate (it cools sharply with elevation), so parcels of air near the ground are quite buoyant and tend to rise. It’s these cauliflower clouds that lead to cumulonimbus clouds.
Below are the three stages of cumulonimbus development: a rising cumulus congestus cloud, a mature cumulonimbus with updrafts and downdrafts, and a dissipating storm with the updraft choked off. We’ve gone over the towering cumulus/cumulus congestus/cauliflower cumulus clouds, so let’s go over the mature cumulonimbus cloud.
If a cauliflower cumulus keeps getting bigger and bigger, it will turn into a cumulonimbus cloud. There are two types of cumulonimbus clouds: Cumulonimbus calvus and Cumulonimbus capillatus. Calvus have a puffy top and look similar to a towering cumulus cloud, but they are even larger. A mega-cauliflower cloud, if you will. Capillatus have wispy, cirrus-like tops.
Cumulonimbus capillatus have a subtype: Cumulonimbus capillatus incus. The incus means that the wispy tops of the clouds are spread out and bear resemblance to the classic ‘anvil’ shape that we associate with big thunderstorms. Compare this…
… to this…
This anvil marks the beginning of the tropopause – the region in which the troposphere is transitioning to the stratosphere. Whereas the temperature of air decreases in height in the troposphere, it increases with height in the stratosphere due to heat given off in ozone-forming chemical reactions. Because air becomes colder and thus denser when it rises, it cannot rise very far through the stratosphere as gravity simply pulls it down back to a place where it is just as dense as the atmosphere around it. Capillatus with especially strong updrafts often have what is called an “overshooting top,” which is a well-defined ‘dome’ cloud created by the momentum of the air parcel within the updraft thrusting the cloud into the stratosphere until it can go no further.
Overshooting tops are rare here in the Northwest, but I have seen them. The last one I can remember seeing was associated with a thunderstorm by Mt. Rainier on July 29, 2009 – the warmest day on record in Seattle. That storm spawned a flash flood warning from the NWS.
Here’s a picture with both cumulonimbus calvus and cumulonimbus capillatus. Can you make out the differences?
Anyway, let’s get back to the characteristics of a mature cumulonimbus cloud.
The towering cumulus cloud was formed solely by updrafts. A mature cumulonimbus has not only an updraft to continue supplying fresh air to the storm but a downdraft through which precipitation in the form of rain, hail, and even snow falls. One would think that the temperature under a downdraft would be warm due to adiabatic warming associated with sinking air, but the temperatures under downdrafts are quite cool, especially when the storm is strong. Remember, it takes an unstable atmosphere with a sharp decrease in temperature with height to form a cumulonimbus cloud, so the tops of these clouds are often exceptionally cold. To make matters even more frigid, the air can be cooled further by hail falling through the downdraft and the sublimation of ice crystals into water vapor.
Because this air is cool, it is more dense than the surrounding atmosphere. In some cases, this characteristic can be deadly. Downbursts are rapidly falling parcels of air that can cause massive destruction over an area they impact. They are very dangerous to airplanes because of the extreme turbulence, and since airplane pilots generally encounter them in the lower atmosphere where they are taking off or preparing to land, the stakes are even higher. After they hit the ground, they spread horizontally in all directions. Often times, the damage in tornadic storms is higher from these straight-line winds than the tornado itself.
Updrafts can also cause extreme winds. A massive thunderstorm producing prodigious amounts of rain and hail needs a lot of energy to support it, so air nearby won’t waste any time flowing into the updraft. These intense updrafts can cause tornadoes by shoving rotating air within the storm upright.
In addition to the overshooting top, thunderstorms have other characteristic features. Some have mammatus clouds, which form when ice crystals under the anvil sublimate and cool the air (the transition from ice to gas takes energy). This cold air sinks in little pockets and finally stops when all the ice crystals sublimate. These clouds are often indicative of a strong thunderstorm, as they can only be supported by a very moist updraft.
Two arcus clouds, shelf clouds and roll clouds, are also associated with thunderstorms. Arcus clouds are long, low-lying, and horizontal clouds which often form on the leading edge of thunderstorms, although roll clouds can be found detached from any sort of separate cloud or weather system.
Shelf clouds are ominous, wedge-shaped clouds that form on the leading edge of a thunderstorm. These form because cold air from the downdraft of a storm spreads horizontally when it hits the surface. This outflow of air originating from within the downdraft undercuts and lifts the warm, moist air rising into the storm upwards. Shelf clouds form ahead of the “gust front” of a storm, which is essentially a mini-cold front on the leading edge of strong thunderstorms.
If there is enough wind shear due to differences in wind speed with elevation, the opposite directions of the air going out of and into the thunderstorm will “spin up” a roll cloud just behind the gust front.
Put these all together, and you’ve got a typical thunderstorm! Not all thunderstorms have overshooting tops, mammatus clouds, and arcus clouds… those only occur in strong storms or supercells. Here’s an average thunderstorm below.
Cumulonimbus clouds dissipate when the updraft is choked off. If no new air is coming into the storm, the storm cannot support itself. This is usually because the downdraft has overtaken the updraft. The cloud will take on a fuzzy appearance, the wind and rain will lighten, and the skies will clear. Most rain events associated with cumulonimbus clouds, thunderstorm or not, last less than an hour.
We’ve talked about the different types of clouds. Well, I talked about the different types of clouds. You’ve tried to follow my unnecessarily long explanations. Let’s talk about the different types of thunderstorms.
1.2 – Four Types of Thunderstorms
There are four different types of thunderstorms: single-cell, multi-cell, squall line, and supercell. Supercell thunderstorms are every weather geek’s favorite thunderstorm because they are the ones that can spin up tornadoes.
Single Cell:
Single cell thunderstorms are actually quite rare. For a thunderstorm to be classified as a single cell storm, it must have no other cells in the vicinity. These storms are generally harmless and last around a half hour. Some of these storms can be stronger and produce hail, torrential rain, and microbursts, but these are the exception, not the rule, and are still short-lived. Since single-cell storms are so isolated and seem to occur at random times and locations, they are hard to predict, so cut the weatherman a little slack if you get smacked by one of these storms.
Multicell:
Multicell clusters are much more common than single cell storms. Instead of one isolated thunderstorm just hanging out in the middle of nowhere, multicell clusters consist of a group of cells moving along. Most are short lived, but since these cells have formed in a cluster, the region in which they formed must have been conducive to thunderstorm formation. As such, individual cells will constantly form up and dissipate, but the cluster as a group will retain the same general characteristics. Multicell storms are usually more potent than single cell storms, but they are just harmless punks compared to supercells.
Multi-cell Line:
Multi-cell lines, also called squall lines, consist of a more-or-less continuous long line of storms that form at or ahead of a cold front with a gust front at the leading edge. These storms often have breathtaking shelf clouds and have the heaviest rain and hail just to the west of (behind) the updraft. Squall lines can produce golfball-sized hail, weak tornadoes, and, of course, torrential rain, but they are best known for their powerful downbursts and resulting straight-line winds.
Some downbursts are so strong that they can accelerate parts of the squall line ahead of others, forming what is called a bow echo, and I was lucky enough to observe one on radar tonight (August 14). In cases with sustained winds over 58 mph, storms that take the form of a bow echo are called derechos and can cause severe damage. I checked the county alerts on the NWS homepage and did not observe any severe thunderstorm warnings associated with this echo, so it is not a derecho, just a squall line taking the form of a bow echo.
Supercell:
Supercells garner the most attention from everybody. They are extremely prominent and beautiful thunderstorms, but underneath that mask of beauty lies a ferocious beast that can cause massive hail, powerful downbursts, and most of all, tornadoes.
The anatomy of a supercell is similar to a typical strong thunderstorm. I’ve reposted the thunderstorm anatomy picture from earlier in the blog above for convenience, and I’ve posted a similar picture of a supercell thunderstorm below. The difference is that the regular thunderstorm does not rotate, while the supercell does. This is evidenced by a wall cloud and a tornado in the supercell diagram while nothing of that sort exists in a typical thunderstorm.
How does the thunderstorm get rotating? Well, first off, you need wind shear, and lots of it. Wind shear is defined as different winds blowing at different elevations in the atmosphere, so in order to have high wind shear, you need to have a large change in wind speed with height. If you choose your inertial frame of reference to be the air moving along the ground, the velocity of the air above it must be much higher for there to be any sort of wind shear. When you have high wind shear, air tends to rotate within the storm.
The high wind shear is very conducive to storm development and these storms can reach astounding heights. Supercells over 40,000 feet high are not all that uncommon. I remember I was flying on a plane at a cruising altitude of ~37,000 feet and there was a massive thunderstorm that extended well above our elevation (it was hard to judge how much higher the thunderstorm was than the airplane, but I feel like a mile would be a conservative estimate. Wind shear can also act to separate the updrafts and downdrafts in a storm. Because the downdraft cannot drown out the updraft, supercell storms can last for hours. Thunderstorms in the tropics can rise to an unbelievable 80,000 feet, but this is due to intense convection from strong solar heating. There is very little wind shear in the tropics, and these storms are usually pretty short lived.
I just… as in one minute ago… thought of an useful analogy. Think of an air parcel like a treadmill that is turned off. When you get on that treadmill and push the conveyor belt forward under your own power, you are acting as the strong wind on the top of the air parcel causing it to rotate. Since this same forward force is not being applied to the conveyor belt on the bottom of the treadmill, the conveyor belt goes down, around, and back up, where you continue to provide the horizontal force necessary to keep the belt rotating.
Second, you need very strong updrafts. According to the NWS, updrafts into a supercell are EXTREMELY strong and can reach speeds of 150-175 miles per hour. Here’s a hypothetical mathematical scenario: if you think of the storm as being on a grid with the ground being represented by the x and y axes and elevation being the z-axis, the rotating air parcels are centered on the x axis (and are thus parallel with the ground) and rotate in the direction of the y axis. If the updrafts are parallel to the z-axis, they can flip this rotating air parcel so that it is now centered along the z axis (and thus perpendicular to the ground) and rotates in the direction of the x-axis. You have a rotating updraft, or mesocyclone, and therefore a supercell storm.
Tornadic storms with a powerful mesocyclone often have what is called a “hook echo” on radar. The hook echo provides a good estimate of where the mesocyclone (and the tornado it may spawn) is.
I’d love to provide detailed explanations for how each specific raindrop in a specimen of a typical thunderstorm, but that would be of no use to you and would actually probably be deeply disturbing. I mean, you guys know I like weather and all, but describing every raindrop? They tell you that following your passions will lead you down the road to success, but as I’ve learned from past experiences, you should always do so with a seatbelt on. So let’s talk about lightning.
1.3 – Lightning
Remember that run on sentence in the first paragraph of this blog? Lightning is far and away the hardest meteorological aspect to explain. I searched for hours looking to find explanations for the formation of lightning, but for each explanation there was, there was another explanation that disagreed with it, often in an arrogant fashion. Finally, I came upon a source that seemed unbiased and actually recognized that scientists do not agree on lightning. I owe much of the following information to HowStuffWorks.com
Let’s go over one possible explanation of how lightning forms according to the folks at HowStuffWorks. I feel like they’d have the best idea of how it works.
The Causes of Lightning:
When we have any cloud, we have water droplets in the air. These are not rain droplets; they are much, much smaller, and it takes thousands of cloud droplets to make up one typical-sized rain droplet. These droplets are neutrally charged and do not electromagnetically interact with each other. They interact with each other by colliding with other cloud droplets or precipitation falling from the cloud itself. When these collisions occur, electrons are scraped off the droplets. Because electrons have been knocked off some of the neutral molecules, the molecules are now ions with a net positive charge and rise toward the top of the cloud. The electrons tend to gather toward the base of the cloud
It’s not only these collisions that are responsible for creating a giant capacitor of sorts within the cloud. As cloud droplets rise into the upper atmosphere, they cool. But just because they cool below the freezing point does not mean they immediately turn into ice. Some droplets are supercooled, meaning they are liquid at temperatures below freezing. The ice crystals that form are negatively charged, and the supercooled water droplets are positively charged. Updrafts separate the frozen and unfrozen droplets and take the supercooled, positively-charged droplets up to the top of the cloud, while the frozen, negatively-charged droplets travel to the lower portions of the cloud (they may melt on the way down, and I do not know how this melting would affect the charge).
With this charge separation comes an electric field which, like the cloud, is generally negative at the base and positive at the top. This field becomes stronger and stronger as the charges in the cloud become stronger and stronger. In fact, the field can be so strong that the electrons at the base of the cloud can repel the electrons on the surface of Earth into the ground while attracting positive ions upward. Now, you have an “electron sandwich,” with positive charges enclosing a region of negative charge. At the very base of the cloud, there is a weak net positive charge, but for the purposes of simplicity, don’t give it too much thought. Bottom = negative, top = positive.
But what good is this separation if the charges cannot travel to each other? Say, for example, we held the above charge configuration in a rubber medium. Rubber is an insulator and doesn’t allow electrons to move. The atmosphere, however, is not. It’s not an ideal conductor like copper or silver, but it’s not a complete insulator either. By not being an insulator, charges can move and align themselves in the above position. By not being an extremely effective conductor, charges can build up without being constantly transferred from one place to each other. For lightning to form, all we need is for the charge separation to become so great that a conductive path is formed through the air and provides a line through which the charges can freely interact and release excess charge. But how do we form that electric path?
Air Ionization and Plasma:
If the electric field becomes exceptionally strong (tens of thousands of volts per inch), the air itself is separated into electrons and positive ions. When this happens, the air becomes a much better conductor of electricity and is called plasma. We’ve all learned about three states of matter: solid, liquid, and gaseous, but plasma is a state of matter as well, bumping up the number of states of matter to four.* This process of separation lays out a path for electricity to travel from higher potential to lower potential in the same fashion that a mole makes a tunnel from the neighbor’s yard to your yard. It’s hard to compare plasma to mammals, but you get the idea.
*Technically, there are many other states of matter, both observed and theorized, but these only occur under very specific or extreme situations. I’m a big fan of quark-gluon plasma and Bose–Einstein condensate, but my favorite has to be strongly symmetric matter, a state in which the four fundamental forces of the universe were unified into one grand force. It is theorized that it lasted for all of 10−36 seconds after the Big Bang. Don’t you just love physics?
Step leaders:
Even though plasma is an excellent conductor of electricity, we don’t see a constant exchange of electrons and positive ions from one place to another. This is because there is not a homogeneous plasma field that would allow this. Instead, we have step leaders, which are independent paths of ionized air that stem from the negatively-charged region of the cloud. This happens with all types of lightning, but I think it is best visualized using cloud-to-ground lightning, so let’s explain it that way.
Just like people, these leaders come in all different shapes and sizes and the atmosphere is filled with particulate matter that can make the leader more likely to go in one direction than the other. If the base of the cloud and the ground are parallel, the electric flux, which is a scalar quantity that represents the rate of flow of the electric field through an area (and therefore the strength of the field), between the cloud and the ground will be maximized when the area through which the flux is being measured is parallel to the ground. By approximating the direction and magnitude of the flux by using arbitrary ‘flux lines,’ we can see this since flux lines always radiate perpendicularly from their charge surface and then move in the direction of opposite charge.
Of course, no cloud can be perfectly parallel to the ground. The cloud and ground would have to be to identical and parallel planes, and such an idealized situation is not found in nature. And then there’s the whole “the Earth isn’t flat” thing, not to mention all the particles in and characteristics of the atmosphere that would interfere with the paths of the flux lines. As such,the flux lines will not follow a path straight from the cloud into the ground. Instead, these flux lines intersect and diverge, creating a non-uniform field. It is this non-uniform field that causes the step leaders to take a path not perpendicular to the surface of the intended target.
As you can see below, there is a weak amount of positive charge at the very base of the cloud. As you can see above, I told you to not worry about it to make things easier on yourself. This charge is not sufficient to neutralize the large buildup of static electricity and the electrons continue to flow to the ground.
These leaders occur in stages. It may not look like there is a pause between them, but that is just because they are occurring so fast. They develop downward on either side of 200,000 mph, but they would be going closer to 186,000 miles per second (the speed of light in a vacuum) if they went downward in an unfragmented fashion.
Now, you’ve got leaders going toward the ground via the path of least resistance. These leaders are slightly purple. New leaders may form, but every one stays illuminated until the current has reached the ground. These leaders are NOT the big lightning strikes we are familiar with. In order for those to occur, the circuit needs to be completed. And that’s where positive streamers come in.
Positive Streamers:
I like to think of the whole leaders/streamers thing as a handshake. As public image consultant Álvaro Gordoa shows, somebody always initiates a handshake, and the person who picks up this signal immediately knows to reach their hand out and complete the gesture. The hand of the initiatior doesn’t just keep on truckin’ until it smashes into the closed fist of the now unsettled would-be recipient of the handshake. And that’s what happens with lightning. The stepped leader doesn’t just smash into the ground. It meets with what is called a positive streamer to complete the transfer of electrons from cloud to ground.
Since these leaders are negatively charged and the ground is positively charged, the charges attract each other. The ground manifests this attraction by sending these streamers, also purplish in color, into the atmosphere. Once they have been produced, they do not travel up to meet the leaders; they let the leaders come to them. In other words, the hand receiving the handshake from the initiator lets the initiating hand to all the work.
Once these two acquaintances have met, a path for the current to flow between the ground and the cloud is created, and a huge discharge follows. Positive charge from the ground races up this path toward the thundercloud and is visible as the “return stroke” of lightning that is most visible to us. That’s right: the strike we see with our naked eye actually starts on the ground and races back to the cloud. Negative charge does all the hard work trying to find a way to get to the ground, and once a connection is established, positive charge surges back up through the circuit.
Multiple Strikes:
There can be as many as 30-40 additional strikes after the initial return stroke. Remember how the step leaders coming to the ground from the base of the cloud were pointed in all these different directions? Well, once the circuit is completed, the electrons in those leaders flow through the leaders into the path of the initial strike. The leaders are only providing a path for the electrons to flow through, not neutralizing the charges themselves. These secondary strikes can be seen as branches off of the initial strike if they follow different paths or just make the initial strike look longer by taking the same pathway (and anywhere in between). Sometimes, the flash from the main strike will end while secondary strikes are occurring, and this makes the initial lightning strike flicker like a star or blind you like a strobe depending on your proximity to it.
Thunder:
The return stroke, which can be thought of as a plasma channel for charge to travel through, discharges a tremendous amount of static electricity in a very short period of time and therefore heats the air around it to extremely high temperatures – as high as 50,000 degrees C. This heating causes air radiating from the return stroke to expand in the form of a shock wave that we hear as thunder. Thunder is LOUD… my house just barely missed getting struck by lightning during ‘thundersnow’ storm (a thunderstorm where it is raining instead of snowing) on December 18, 2008. The family cat has never been the same since.
Paths of Lightning:
The example we used above was of cloud-to-ground lightning, which is the most dangerous. People, trees, and animals don’t live in clouds now, do they? There are two other paths that lightning can take: intra-cloud and cloud to cloud. Remember, it’s not just the base of the cloud and the ground that are charged. There’s a whole bunch of positive charge at the tops of the clouds, and lightning can form either in the same cloud or spread from different clouds due to the interactions between these charges. The process through which the lightning takes these paths has all the plasma and streamers and associated phenomena that we went over in cloud-to-ground lightning. Cloud-to-ground lightning is just easier to visualize.
To leave you with something to ponder, do yourself a favor and look up “ball lightning.” It’s a truly fascinating phenomenon. Now, onto part 2!
Charlie
