Chapter 8 - Cumulus Clouds and Thunderstorms

It's a warm summer's Saturday with a bright and sunny morning, and a hazy blue sky. Winds are light and variable. As you go out to get the morning paper, the air feels soft, humid, to be sure, but it carries a feeling of a nice day. Sipping your morning coffee, you and your spouse plan the day. The job jar seems full. There is the dead electrical outlet to be replaced in the garage. The lawn is calling to be cut. A dead limb needs to be pruned from the maple. There is airwork to be practiced as that new rating will not appear on its own. The groceries need replenishing and so on. You decide the airwork comes first as the air will be smoothest early in the morning.

As the morning wears on, the temperature rises and the humidity, perhaps feeling it is in a race, keeps pace. Airwork done and the plane securely tied down and buttoned up, it is time to tackle the job jar. Replacing the outlet produces perspiration. That done, the lawn is next on the list. A few cumuli start to appear. Their occasional shade and the breeze associated with their shadows really helps when you are mowing the back lawn. Stopping for a moment, you recall the last time you were out sailing. The little cumuli seemed to leave their imprint on the lake called "black water." As the cumuli came over, the gust beneath them seemed to be coming along for the ride. Lunch provides a welcome interlude in the early afternoon. The lawn is only half done.

As you finish putting the last grass clippings on the mulch pile, you notice that the cumuli grow taller and somewhat larger. Your spouse brings you a well-deserved glass of ice tea and you two sit on the back porch to relax. Watching the clouds you notice that there are fewer cumuli, but the ones which remain are getting larger, looking like a field of cauliflower heads in a hazy blue field. One of these starts to grow larger and a wavy white streamer appears over its top, looking like the pileated crest of a woodpecker. The top of the growing cumulus barges through the crest as it reaches for the stratosphere and there's a far-off rumble of things to come.

The temperature changes. There's a sudden chill in the air. The wind shifts direction. It's time to put the lawn mower away.

Just about the time you get the mower in the garage and the bag put on the shelf, a nearby flash and the accompanying thunder announce the arrival of the first giant drops of rain. Watching the huge drops splatter on the pavement, you decide to go through the breezeway into the house. The gusty winds send spray over you on your way in, and try to wrest the screen door from your hand. Time to close the windows.

As lightning and thunder are beginning to be closer, you unplug the computers, the high-fi, and the TV. Sheets of rain come down, all big drops. The willow tree in the front yard is whipping back and forth as one side of it gets wet in a gust of rain. The other side is spared only to be doused a few seconds later in another gust. Out on the back porch, sheltered from the gusts by the curtains and glass on the end, you watch the bursts of wind move the grass in the field next door in serpentine patterns reminiscent of ocean whitecaps.

Then the rain gusts stops, tailing off into a gentle patter singing on the pools and rivulets caused by the earlier inundation. The rain stops, almost as suddenly as it started. Looking out of the back porch the clouds once again appear to have defined edges. The breezes are cool and the sun is starting to peek out from behind the cloud.

The dead limb on the big old maple you had been going to trim is on the back lawn, so you go over to pick up the pieces. As you put the last one on the wood pile behind the garage, you the departing thunderstorm glides onward with cotton sails.

Let's go over this same scene from a meteorological point of view, starting with the morning sounding. During the nighttime hours, the surface cools due to radiant heat going outward into space. In the absence of cloud cover or fog, the temperature of the surface can drop dramatically. The general rule in meteorology is that the Earth's skin temperature is five degrees Fahrenheit cooler than the shelter height which is only five feet above the ground. Figure 8-1 shows a typical summer lapse rate for a day when thundershowers are likely. The sounding at dawn shows a strong nocturnal inversion. Notice also that the dew point is low above the ground reflecting the dew which has formed on the ground, cars and such. Notice the air above the nocturnal inversion is conditionally stable, that is, the air is stable as long as clouds aren't forming. If clouds do form, the air is unstable and the clouds will grow. Above the conditionally stable is the subsidence inversion, a layer dried out as the air gradually descends as it makes its way around the wave. Above the subsidence inversion, the air is once again conditionally stable.

As the sun comes out and starts warming the ground, the heat starts to bubble upward burning off the nocturnal inversion, as in Figure 8-1 b. The dew evaporates, moistening the air in the boundary layer, the layer of the nocturnal inversion. This means the relative humidity is high, but that's not news. In the early afternoon, both the nocturnal inversion and the morning dew are almost gone. The air through the lower layer is bumpy as parcels move up and down redistributing themselves according to their density. The temperature at the ground is still rising and the heating of the tarmac and parking lots encourages those who would consider cooking eggs on the pavement.

This is a good time to look for clouds because Altocumulus Castellanus some of which are shown in Figure 8-2. They are often precursors of severe convective activity as they indicate conditional instability at their level which can be set off by a very little lifting. I begin thinking no-go if I see them and I'm planning to be in the area in the late afternoon.

The sun is still warming the pavement, which has some thermal inertia, so the pavement doesn't reach maximum temperature until later in the afternoon. The sounding depicted in Figure 8-1c shows the nocturnal inversion eroded. At this point the bubbles of air are jostling up and down depending on their densities and the air is pretty bumpy to the occupants of a small single. As one of the more moist parcels ascends, the relative humidity may be high enough that condensation will start. I've tried to indicate a small bubble of cloud in Figure 8-1 c as the small round circle at 1,000 feet. The dotted line indicates the path that the temperature of the parcel will take as it starts to rise.

As convection starts to swell the nocturnal inversion has disappeared. Convection then mixes the area between the subsidence inversion and the ground. If the parcels reach the LCL, the water vapor starts to condense, and the parcels become cloudy. Once cloud has formed, the latent heat of condensation reduces the cooling of the parcels. Although it's only approximate, the parcels only change temperature -3 degrees Fahrenheit per 1,000 feet. Thus the cumuli start to grow slowly, mushrooming out as they erode the subsidence inversion.

Thus far, the processes are well-known; however, the next stage hasn't been measured and proven yet. It is based on current thinking and experiments from the laboratory. And, as you might imagine, there are at least two schools of thought as to how these inversions break up.

One school of thought, perhaps held by those who have been fortunate to watch the building cumuli downwind of tropical islands, is that the inversion is simply eroded by thermals. These thermals arrive at the base of the inversion with some upward momentum. As they run into the cap the momentum dissipates as they are slowed to a stop. During the process of hitting the inversion, they entrain some of the cooler air in themselves, resulting in a cooling of the thermal and a warming of the lower parts of the inversion.

There are some places where the thermals are more vigorous than others. Surface heating of plowed ground, an undulation in the ground surface, the edge of a sea or river breeze, an extended Taylorcell the edge of the wet ground from the previous day’s shower or the remnants of the downdraft of cooler air from yesterday's thunderstorm may provide the lifting for thermals to enhance the erosion of the inversion. There the thermals break through and a cloud is born. The first thermal through probably doesn't make it far, but the heat of condensation is enough to establish a beach head. The next one coming up entrains the cloud material and makes it further. Each successive thermal builds on the cloud material there before, and before long the cumulus has a good start on becoming a thunderhead.

The other school of thought, perhaps held by those who have watched summer thunderstorms pop up almost at random over the flat lands of the country, says the subsidence inversion also acts like a drumhead on a kettledrum. As the subsidence inversion gets thinner and thinner, because of the thermals eroding the boundary layer, the resonant frequency of the waves change. Some waves which were moving through it at reasonable amplitude are now damped while other wavelength waves are amplified.

Occasionally, a short wave the size of a small cumulus breaks and a gust of warm moist air from the boundary layer makes it to the top of the inversion. Once above the inversion, the moisture condenses and a cumulus forms; however, the inversion has reformed below it stopping the upward moving air. At the same time the wave in the inversion broke, some cool dry air from above, some of it cooled by evaporation of the cloud material in eddies along the side of the rising cloud, came downward. Since it is denser than the moist air surrounding it, it descends to the surface giving a gust of air near the base of the newly formed cloud.

As the day progresses, the capping layer becomes thinner and thinner and the shear and the thinner cap combine to make the longer waves more unstable. Fewer and fewer little cumuli break through, but the larger waves break producing the cumulus congestus. If one of these breaks in the capping inversion is large enough, the inversion doesn't reform, the seal is broken, and air from the entire boundary layer air can move up through the break. The thermals in the boundary layer move upward through the break, each building on the cloud material from the previous thermal.

Here both schools of thought converge as the cumulus congestus is simply thought to grow, thermal after thermal, into the great cotton citadels of the sky. Probably both schools of thought are correct; this is usual in science. My favorite maxim is that if there are a number of possibilities - they all contribute, the only question is which is the dominant one in any given situation.

When the cumulus congestus reach to altitudes where the temperature is -6 Celsius or so, ice starts to form adding energy to the already growing cloud. Before the cloud gets to this level, the cloud consists of droplets of liquid water. Indeed, the new parts of the cloud you see building near the top will consist of liquid droplets at temperatures well below zero. But when ice starts to form, the added heat released can kick off a growth which will only end at the stratosphere.

Water doesn't necessarily freeze when it is cooled to zero degrees Celsius. When water exists in the liquid phase at temperatures below 0 degrees Celsius, we call it supercooled water. The term freezing temperature is, in some cases, a misnomer. Ice, at normal atmospheric pressure always melts when warmed to temperatures above 0 degrees C, but water doesn't always freeze when cooled to temperatures below 0 degrees C. Big puddles, ice cubes in the freezer, and ponds rarely become supercooled; however, I've had students in the lab able to cool an inch of water in a test tube to -4 degrees C (21 degrees F) as long as it isn't disturbed. Water in very small drops may take some tens of minutes to freeze when the temperature drops well below -50 degrees C (-58 F) or so.

Water droplets, even rain-sized drops, can exist in liquid form well below "freezing" if they do not happen to have a microscopic site (called a freezing nucleus.) The ice crystals can't get started. If you take tap water and start cooling it in the laboratory, measuring the temperature every few seconds, you will get a curve like the one in figure 8-3. After the water temperature goes below zero Celsius, the temperature decline is slow but significant.

Looking into the sample, you can verify that it is still water. At some temperature below the "freezing point," the water actually freezes and the temperature rises rapidly to zero Celsius. This means that heat is released from the freezing of water. If you carry out the calculation of the amount of heat as we did earlier, you find there is around 80 calories of heat released for every gram of water which freezes.

A drop of water which is made up of water molecules, see Figure 8-4 a, all moving around the liquid. They are essentially moving independently but held together by the electrical attraction known as Van Der Wall's force. All of a sudden, one set of about 30 molecules find they are arranged in a six-sided pattern and bond. Or, as the water cools, they find that a piece of clay dust in the water attracts them to the dust's six sided crystal shape. They bond as in figure 8.4 b. Very shortly, other molecules of water will be attracted to the newly formed ice. But the molecules attracted are, for the most part, moving too fast to get stuck. The fastest ones bounce off. Only the slowest ones will stick. This leaves the faster ones remaining in the liquid. They evaporate and bump the nearby air molecules, warming the air. This warming slows the -5.5 degrees F rate of cooling to only about -3 degrees Fahrenheit, causing the cloudy air to be more buoyant than the clear cold air outside the cloud.

In a cloud the upward rushing air of the thermals pushes on air above it forcing it up also. Each one pushes the air above it starting a wave which travels upward. If there is a layer of air above the cloud which is humid enough, an ice cloud will form, called a pileation, which adds to the likelihood of ice formation in the cumulus. The waves produced were predicted by the theory but really only first noticed in the early atomic bomb tests.

The layer where snowflake formation is most efficient is where the temperature is -12 degrees Celsius or +10 Fahrenheit. This is the temperature at which the water from droplets evaporates very rapidly, flowing over to snowflakes. The growth of bigger snowflakes at the expense of droplets and smaller snowflakes is sometimes called scavenging. It is an apt term. Snowflakes grown in the lab at -12 degrees with lots of vapor and droplets around them take only a few minutes to go from microscopic sizes to a half a centimeter across. They can and do take up a lot of water vapor from the air in doing so. This causes the relative humidity to drop drastically, causing in turn, the cloud droplets to evaporate. While evaporation causes cooling, the freezing causes heating. The net effect is that the rapid growth of snowflakes causes an additional boost of heat to drive the thermals ever higher and higher, much like an afterburner in a jet.

With the formation of snow flakes in the thunderstorm, the thermals drive on to the stratosphere where the inversion slows and stops their upward progress. The lower stratosphere is like a ceiling to their growth. The thermals flatten out and move outward from the center of the storm and form the familiar anvil-shaped top of the cumulonimbus. The air may blow hail out here so flying under an anvil-shaped top can be detrimental to your progress. Occasionally a particularly vigorous thermal or group of thermals will penetrate the stratosphere, only to find the potentially warmer air is less dense and the thermals will fall back into the troposphere. Severe storm forecasters call these "overshooting tops" which warn of especially vigorous activity below.

When the garden variety storm reaches the air near the troposphere, it has probably exhausted the warm moist air in the boundary layer that fueled the storm. It stops at the tropopause but may overshoot it a little into the very stable stratosphere. The column of air driving upward forces the air already there into moving aside. Some the air which was around the cloud or in a rain shaft which was cooled by evaporation moves downward around the upward moving boundary layer air, increasing its speed as it descends towards the ground spreading out as it hits. The leading edge of this air forms a gust front, which you may occasionally hear about in a briefing. Gust fronts fan outward giving the sometimes not so gentle hint that the rain is about to start as it moves over the countryside. When you see the big drops hitting the ground, you can be almost positive that the rain was snow not long before it hit the ground. Big drop rain is a giveaway of ice formation in the clouds.

Water gives off heat when it freezes; ice absorbs heat when it melts. When snow melts the heat is removed from the air around it causing vigorous downdrafts. When they hit the ground and are vigorous enough, these downdrafts may be microbursts something pilots of big jets would rather avoid when landing.

Up in the cloud, activity is furious. Turbulence driven by the buoyant thermals abounds. Updrafts capable of giving aircraft a thousand feet per minute rates of climb are present along with downdrafts capable of doing the opposite. However, these don't usually last long for the thunderstorm is usually over in an hour or so primarily because the descending cool air usually chokes off the warm humid boundary layer air, the source of the energy.

Thunderstorms, while they produce copious rains, do not wring all the water vapor out of the air. The rain produced is actually only small fraction of the water which entered the storm. Most of the water vapor, and associated energy, moves upward in the cloud as water droplets only to evaporate as the environmental air mixes along the cloud top and edges. Thus, one major result of a thunderstorm is to move water vapor and heat energy from the surface into the middle atmosphere. A second result is that the electrical energy separation is thought to maintain the electrical charge of the ionosphere. The tremendous amount of electrical energy released during lightning is only a fraction of the total electrical energy generated.

Ice formation is thought to be a major source of energy for the electric charge generation in thunderstorms. Although these process are not completely understood, what is clear is that once lightning strikes, you can be sure that ice is being formed in these giant clouds. If you want to start an argument among cloud physicists, just state that the development of lightning has to be simple - that when clouds build to a certain height, lightning is bound to follow. So far, cloud electrification experts have identified at least eight mechanisms which cause charge separation in clouds and thunderstorms. All or any combination of these may be operating at any given time. And you can have electricity flowing from clouds which aren't even thunderstorms.

I was in the back of the science lab one bright spring day and heard a snapping noise from the amateur radio club area. Curious, I went over to that area and poked around. The noise got louder. We had been given a nice Collins amateur radio setup, a transmitter and receiver, by a widow of a ham. We had rigged an 80-meter long wire antenna over the building, being careful about lightning protection. We had also, knowing there were some good strong thunderstorms possible, rigged a knife switch between the center wire of the coaxial antenna lead in to ground. The ground wire was firmly attached to a 10-foot copper-coated ground rod just outside the lab.

Apparently the operators at the last ham club meeting had left the knife switch open and not grounded, because, after I turned the lights out to see better, I saw arcs jumping the two inches from the knife switch parts. I quickly closed the switch to the grounded position using a length of wood molding which was lying there and went to the windows to see the unpredicted thunderstorms. There weren't any. I went down the hall and out the door to look around. All that I could see were cumulus humilis moving along with a brisk, dry, cool wind. You don't need thunderstorms to produce electrical activity.

The thunderstorm or Cumulonimbus is the ultimate in cumulus clouds. Nature's summertime refreshers, these giants of the skies extend their influence from the ground up to, and occasionally above, the troposphere and for miles around their visible boundaries. They shed copious quantities of rain, occasional hail, and, somewhat less frequently, tornadoes which can plow up a furrow through the land - whether or not there are houses there.

Thunderstorms come in three basic flavors, the garden variety isolated thundershower which refreshes a summer's afternoon, the more violent squall line, and the supercell version which sometimes throws hail at the ground and then tries to sweep it up with a tornado.

Thunderstorms can be organized or isolated, forced by the topography or seemingly ignorant of the ground below. Occasionally, thunderstorms are found in areas where they seem to sprout and decay in an organized group. Figure 8-5 shows the various scales of the identified organizations of thunderstorms. Although some people feel that hurricanes are a grouping of thunderstorms, I haven't included them.

The basic garden variety thunderstorm, the single cell storm has an average radar lifetime of half an hour. Since radar can only see rain, hail, snow, and other very large objects such as airframes, rain and snow are visible on the radar for about thirty minutes. Obviously, the cloud must have started growing before the rain and snow are produced. Yes, you read right, snow. Even in the hottest summer weather in Florida or Texas, there is snow growing in the upper reaches of a thunderstorm.

Above the freezing level in thunderheads, snow contributes to two things, additional heat for buoyancy of the air and, when the snow falls into the lower reaches, the snow melts to form those huge drops that hit the ground with a resounding splat. The reason that we think that the ice phase is important in thunderstorm is that they only last a few hours. Only the ice phase processes can develop those big drops in that short time. Of course, the area can produce interesting turbulence before the particles grow big enough to have substantial radar return. Indeed, the turbulence may be just as bad just before the radar reacts as when it is painting substantial echo.

Squall lines have been seen to exist well over 12 hours and are almost always found in the warm air preceding a cold frontal passage. Squall lines may have pre-frontal waves to assist them in getting organized and staying that way. Many of the thunderstorms "imbedded" in a warm front are really the northern extensions of squall lines in the warm sector of the large-scale storm system.

The term mesoscale convective system - has been developed to discuss large-scale groupings of thunderstorms. MCSs are often simply summertime low pressure systems of the classic type, but there are enough systems which are not associated with a "storm" in the classic low pressure area sense that the acronym MCS was coined to cover these other systems which appear to have the same results. But, after all is said and done, the rest of the systems are made up of multiples of the single cell storm. The single simple thunderstorm is the building block of the other types of storms mentioned here.

In recent years, we have noticed that there is what at first appeared to be a variant of the single cell thunderstorm, called the supercell thunderstorm. These seem to spawn the biggest tornados. Since warning of tornadic activity is a main function of the NWS, these have generated a great deal of excitement. While they are most prevalent in the Great Plains (Dorothy and Toto's country), they can occur anywhere, anytime the weather conditions are right.

In these cells, the sub-cloud air has some horizontal rotational motion and at cloud base, the condensation and freezing in the upward motion provides some torque to turn the rotating air upward. Once the subcloud rotating air has made the corner upward, the friction is reduced and the spiraling roll is no longer confined between the inversion and the ground. It becomes more circular.

When freezing, be it drops freezing or snow forming or both, heat is released and the system becomes less stable, accelerating the rotating air upward. Within the cloud, the turbulence, the release of latent heat, the sharp sonic disturbances of thunder which can initiate freezing of supercooled droplets, all conspire to turn the updraft into a place of extreme turbulence.

The stretching in the vertical causes the sides to come in. The stretching increases the rotation rate of the air from the roll. If there is enough stretching and increase of rotation rate, a tornado is formed in the cloud.

Using the new Doppler radar, meteorologists have found that these vortices start forming up in the cloud and then descend towards the ground where they then become tornadoes. Nomenclature aside, they're just as dangerous while they are up in the cloud.

Expect loss of control, if not of control surfaces, near one of these behemoths.

These storms do not choke off their flow of moisture and hot air. As a cell decays, a new one forms along the upwind (southeastern) edge. So, a well-developed supercell thunderstorm may have four or five cells in a row. Usually, the newest and most vigorous one will be on the southwest end.

These ideas can lead to a graphical model of a cloud called the Lifted Index (LI) using the stability and the heating from the conversion of vapor to ice. The Lifting Condensation Level idea worked to some degree for small cumuli; however, thunderstorms are probably where it works best. The big thunderbumpers derive their inflow of air from the entire boundary layer and the air is usually quite turbulent under one of these things. Given that the air is probably neutrally stable under the cloud, the LCL idea is probably a good assumption.

The first step to find the LI is to plot the sounding. Start at the bottom and graphically find the height of cloud base but use the average temperature and dew point for the lowest 50 millibar (lowest 1,500 feet), then proceed the same way we did before. From that temperature, interpolate and follow a moist adiabatic line to 500 millibars, the middle of the atmosphere by weight. Subtract that temperature (in degrees C) from the sounding temperature (in degrees C) at 500 millibars and you have the classic LI.

If you come out with a positive number, any thunderstorms you see will probably not amount to much unless the thunderstorm is imbedded in a warm front. If the number is negative nasty ones could form. The more negative the LI gets, the worse the thunderstorms, if indeed they do get started. The idea is that, if the cloud forms, the LI indicates the readiness of the cold air at 500 millibars to switch places with that less dense cloudy air at the ground. As long as no clouds form, the density still decreases with height and the air is stable.

I hasten to add that the simulations are now being used to calculate a Model Lifted Index (MLI). If the briefer gives you a MLI, add -2 to it to get the old-fashioned graphical lifted index. I've rarely seen any MLI get worse than -6, but I suppose it could happen if the air at 500 millibars was that much colder or the model is that far wrong. While there is no theoretical limit to how negative the MLI can be, if you hear of any Lifted Indices in the -4 or smaller range, proceed with caution. Maps of these are available on the Internet.

There is a relative newcomer to the lifted index family, the Surface Lifted Index. These are calculated from the standard hourly data and the model forecasts. To calculate the Surface Lifted Index (SLI), simply determine the height of convective cloud bases the same way you have in the past, then calculate the temperature the parcel would have when it gets to 500 mb. Subtract the parcel temperature from the best guess of the 500 mb temperature you can get from the last radiosonde sounding or the numerical models and the result is the SLI. Since the SLI is taken from data right at the ground, expect it to be unrealistically buoyant when the sun is pounding the ground and probably pretty accurate just after sundown. I have heard values of -10 and -12 for the SLI in the afternoon and the thunderstorms which were around after dark were impressive.

These are empirical results, yet they have been useful to the forecaster and the briefer. Table 8-1 describes the range of values and their meanings with the various Lifted Indices currently being used.

Table 8-1 Critical Values of Various Lifted Indices


Lifted Index

Model LI

Surface LI

Weak Thunder

G.T. -1

G.T. -3

G.T. -1

Moderate Thunder

-2 to -3

-4 to -6

-2 to -10

Strong Thunder

L.T. -4

L.T. -6

L.T. -11

These Lifted Indices are one "model" of a cloud, especially the thunderstorms which forecasters have developed to help them in their job. More sophisticated models exist. One computer cloud model I know of takes two hours of computer time to simulate one cloud which supposedly lasts a half hour. It's not very useful for forecasting, but the internal numbers are very interesting and useful to research personnel. A relatively easy one to use is the Severe WEAther Threat (SWEAT - ain't it awful) index. It incorporates the wind shear but it’s tougher to calculate and has a probabilistic interpretation. As forecasters get more computer support, expect the models to be more sophisticated.

Another index which has been discussed for the last decade is to integrate the energy on the sounding. The index is called Convective Available Potential Energy or CAPE. You can often find it on skew-t log-p diagrams available on the Internet. It will be in the numbers along with the graph. This seems to give better results with fewer false alarms. The capping inversion, whether it is eroded from the bottom (one school of thought) or has internal waves (another school of thought), seems to be an important feature for triggering deep thunderstorms. However the theory plays out, expect improvements in the forecasts as forecasters get their first real looks at the data from the air surrounding these behemoths.

From the models, you might expect that tornados occur only when conditions are ripe in the Midwest. Unfortunately, this is not always the case. True, the frequency of tornadoes maxes out in the Midwest springtime. The big tornadoes are most frequent in April, with the medium-sized ones in May. The littler ones are most frequent in June. After the middle of June, the frequency of tornadoes drops drastically; however, tornadoes have occurred in every month of the year. And they have occurred in every state in the Union (including Hawaii and Alaska). Alberta has some dandy ones, with less frequent occurrences in the other Provinces.

And, just when you thought you were safely tucked away at night, I have to tell you that tornadoes also occur at night. In fact more tornadoes occur at night than during daylight in the south-eastern states during January and February. True, the days are shortest this time of year, but this doesn't help visibility much.

The supercell thunderstorms are organized along the subcloud rolls, especially in the warm sector of a cyclone. Figure 8-6 gives a schematic diagram of one of these storms. Winds near the surface are generally out of the south or southeast, bringing moisture into the storm from the Gulf of Mexico.

In a supercell thunderstorm, it appears that the roll in the subcloud layer turns aloft where a break in the cap allows it, making an almost right angle bend upward and aims toward the stratosphere. Aloft, the winds are generally from the west and are considerably drier. These winds turn the updraft from its original motion from the southeast to make it move towards the northeast.

This change in direction often causes the top of the precipitation to overhang to the east of the storm below it. When this happens, there appears to be a relatively empty volume to the radar in the thunderstorm with a cap of precipitation above. This area is often called a weak echo region. If it is capped by precipitation as in Figure 8-7, it is a bounded weak echo region or BWER.

The BWERs can be dangerous. As viewed from the cockpit, the area may be clear or may appear to have relatively calm cloudy air. The radar echo is weak and the lightning detector will have no response there. But, there may be considerable precipitation, even hail in the overhang. If there is hail directly above the aircraft, with the updraft holding it up, sooner or later that hail has to come down. And come down it will - whether there is an aircraft in the way or not. This is one reason of many why it is not a good idea to fly in or under a thunderstorm.

Figure 8-7 shows a composite of three cells in one supercell storm which were tracked for a number of hours. The large oval indicates the visible outline of the storm, the smaller ovals inside the storm are the individual cells. The three ovals pictured here are a representation of the storm at three different times.

The newest cell in each storm is where it should be, on the southeast corner. As the cell matures, a new one will form to the southeast of it. So, a storm which would otherwise be expected to be over in half an hour, is in fact, a series of cells going through their lifetimes while the total storm continues for hours. The nominal direction of movement of the storm and the average wind direction at the storm level are given by the arrows. Notice the storms don't move exactly with the winds aloft but the cells do move with the wind. Most of the supercells move to the right of the wind. Less frequently, the cells will grow on the northwest edge making the storm move to the left.

This difference in movement of the cells is not surprising to anyone who has watched ripples from a stone thrown into a pond. Individual ripples move faster than the group of ripples in a ring, with new little ripples forming on the inside of the ring and moving through the group to become the leading ripple in the ring. Just as it has made it through, it begins to disappear only to be replaced by a younger ring. Philosophers may see some parallel to life.

You will often find two or more of these lines of storms parallel to each other rolling across the plains like hedge rows.

The squall line linear shape is probably a result of ripples in the atmosphere much like the ripples in our pond. It is complicated by the fact that air in the warm sector of a low pressure area storm system is generally flowing parallel to the cold front and towards the low. Rolls often form which force the subsidence inversion into a wave which is parallel to the cold front. These waves often have relatively even spacing similar to that shown in Figure 8-8. The whole squall line will move out from the front at perhaps 20 knots while individual storms may move along the line. The actual speed of the thunderstorm will then be the sum of the speed of the line relative to the ground and the speed of the thunderstorm along the line, all in all a complicated vector equation.

I've seen some thunderstorms move rapidly along the line from the southwest to the northeast, but I've also seen other storms move the other way, from the northeast to the southwest along the line. Since the lines generally move from northwest to southeast, you can get thunderbumpers which come out of almost any direction. I've never seen a thunderstorm moving directly to the west in midlatitudes although they are common in the tropics.

As they draw their energy from the subcloud rolls, which are parallel and their upward moving parts are quite a distance apart, you will often find two or more of these lines of storms parallel to each other rolling across the plains like hedgerows. The best places to see these storms are from the door of a storm shelter with the plane securely tied down (preferably in a hanger), or from well above them. These cells will create a sizable disturbance in the lower stratosphere, so get well above them.

If you are aloft at normal single-engine altitudes, don't try to penetrate them; these hedgerows have thorns. Either head southwest staying halfway between the hedgerows or, if your flight plan is to the east, you may be able to go northeast between the storms and visit friends. Or have an early and relaxing dinner at a nearby airport. It's a much nicer feeling having the waiter put the remains of a steak in a doggie bag on terra firma than bouncing around wishing for an empty doggie bag.

Mesoscale convective systems are an area of thunderstorms and may be the culprits that, in their most severe cases produce tornado outbreaks. The term "mesoscale" is relatively new one which means the middle scale of weather phenomena. This scale is the one which incorporates phenomena which are smaller than the radiosonde network could observe and larger than the garden variety thunderstorm. Until very recently, meteorologists have not had much data on phenomena of this scale.

The new WSR-88D radar units, the Profiler network, the lightning detection network, and the advanced satellite systems are starting to provide information necessary to allow us to apply theory to these small but important scale storms. Mesoscale convective systems often contain thunderstorms which are relatively widely spaced out. In some they are quite regularly spaced, looking almost like a checkerboard. In others, the spacing is irregular.

The basic component of any of these MCSs is the single cell thunderstorm, the normal garden variety cell. These cells grow within the general area and may move in different speeds and directions than the overall group of storms. A MCS which is moving due east may contain cells that move from the southwest towards the northeast.

If you've gotten the idea that thunderstorms are bad news for a pilot, I've succeeded. Even the area nearby can have thorns. A fellow meteorologist-pilot, Dr. Bill Cotton, was analyzing some data taken nearby his home base at Colorado State University and found that the biggest vertical velocities on that particular day were in the clear air well outside the thunderhead. So, even if you're in the clear, you may not be safe. Give those beasties a wide berth. The best places to see these storms is from the ground with the plane securely tied down (preferably in a hanger).

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