April 9, 1947 – The Woodward Tornado

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The morning of April 9, 1947 dawned cool, breezy and decidedly gloomy across the Southern Plains. Thick fog descended like a blanket, reducing visibility to near zero in some locations. Patchy drizzle broke out from low, sullen clouds and fell as a fine mist over the expansive fields of sorghum and winter wheat. Farmers and ranchers rose before first light, thankful for every drop of rain that could be coaxed from the sky after several months of drought. The animals, like the weather, seemed to be unsettled. Cattle huddled together as if for protection from some unknown menace. Horses whimpered and fidgeted uneasily. Milk cows protested and balked at the prospect of their morning milking. To old timers, these behaviors seemed to portend a storm. Still, April was always a fickle month, bringing radiant warmth, biting cold and booming storms to the Southern Plains in seemingly equal measure. With temperatures struggling to reach 50 degrees and a dense stratus deck choking out the morning sun, the conditions hardly brought to mind the “tornado weather” that a lifetime of living in Tornado Alley had taught residents to fear and respect. However, unknown to the people below, the vast, chaotic machinery of the atmosphere had already set into motion a series of events that would culminate in perhaps the greatest storm in the region’s long and bitter history.

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June 7-9, 1953 — The Flint – Worcester Outbreak

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Note: I’m trying something a bit different with this article. I’ve added endnotes for references and comments that don’t fit in the article itself, clicking any of them will take you to the bottom of the page. Clicking on the circumflex (^) next to each number should bring you back to the appropriate spot. If you encounter any problems, please let me know.

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It was a time of buoyant optimism in the United States. Less than a decade after the American industrial machine first roared to life in a bid to help vanquish the Axis forces in World War II, the baby boom was well underway. Though the spectre of an apocalyptic nuclear war with the Soviet Union loomed ever-present and the Korean War continued to drag on, the country was thriving. The middle class had expanded rapidly, thanks in part to the strength of domineering unions, and consumerism and conservatism came to dominate much of American life. Scientists Francis Crick and James D. Watson prepared to unveil the double-helix structure of DNA. The first production Corvette – a sleek white convertible with red interior and a black canvas top – rolled off the production line at the iconic General Motors plant in Flint, Michigan. The manufacturer’s 50 millionth car, a golden-hued 1955 Chevy Bel Aire Sport Coupe, would be produced at the same plant in November of the following year. In December, Marilyn Monroe’s nude figure would be immortalized in the first issue of Hugh Hefner’s Playboy magazine.

The bomb test codenamed "Climax" resulted in a tremendously powerful explosion in the Nevada desert.

The “Climax” bomb test resulted in a tremendously powerful explosion.

The Cold War was never far from the surface, however. On the arid, sandy Yucca Flat in the eastern portion of the Nevada Test Site, the U.S. Military conducted a frenzied series of atomic bomb tests in a never-ending effort to keep a leg up on the Soviets in the developing nuclear arms race. In the predawn hours of June 4, a Convair B-36 Peacemaker rumbled down the runway at Kirtland Air Force Base in Albuquerque, New Mexico and took to the dark skies 40,000 feet above the test range. As the first Air Force plane capable of delivering any nuclear device in the military’s arsenal without modifying its bomb bays, the B-36 quickly became the go-to choice for delivering a nuclear payload. As the doors to one of the plane’s bomb bays eased open, a relatively small, 30-inch fission bomb released and plummeted toward earth. At approximately 1,500 feet, a brilliant white flash and a gigantic, churning fireball burst into the sky. The bomb, codenamed “Climax” and a part of the curiously named “Operation Upshot-Knothole” series, was the most powerful ever tested on United States soil at the time, releasing a staggering 61 kilotons of explosive energy. The blast could be seen from Los Angeles and felt more than 500 miles away in Oregon, where it reportedly rattled windows and knocked items off shelves.

And despite a relatively quiet start to the year, the weather had been making the news as well. Just before 4:30pm on May 11, one of the deadliest tornadoes in the nation’s history tore a path through the heart of Waco, Texas. The six-story R.T. Dennis furniture store collapsed under the violent force of the wind, crushing 32 people inside. The same story repeated as pedestrians scurried inside several other large downtown buildings to escape the pounding rain, only to be killed when the structures gave way. Still others were crushed while trying to escape in their vehicles. When the dust settled and the victims had finally been extracted from the massive piles of rubble, 114 people had lost their lives. Another 13 were killed further to the west when more than a dozen blocks on the north side of San Angelo were obliterated by an extremely intense tornado. Though no one could have known at the time, it was about to get much, much worse.

This drawing, originally sketched by Truman Caldwell and completed by Waco Tribune-Herald artist Dick Boone, illustrates the tornado as Caldwell witnessed it while driving home toward the city.

This drawing, sketched by Truman Caldwell and completed by Waco Tribune-Herald artist Dick Boone, illustrates the tornado as Caldwell witnessed it while driving home toward the city.

 

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March 18, 1925 — The Tri-State Tornado

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Deep in the Ozark Mountains, in places scarcely changed through nine decades, there are legends of a monster. Though few, if any, still live to tell the tale first-hand, the tradition persists, straddling the line between fact and myth. In the Shawnee Hills of Southern Illinois, too, old-timers pass on the legend. Indeed, across three states and more than 200 miles, folks of a certain generation recall harrowing accounts by those who witnessed death drop from the sapphire sky one balmy pre-spring afternoon in 1925. Over three and a half hours, the Great Tri-State Tornado roared through the southern portions of Missouri, Illinois and Indiana, wiping town after town off the map as it ripped through forests and farmlands, over peaks and hollows, and across the mighty Mississippi River at speeds sometimes exceeding 70 mph. When the greatest tornado disaster in recorded history finally came to an end some 219 miles later, 695 people laid dead and more than a dozen towns and hundreds of farmsteads were left in splinters.

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July 6, 1893 — Pomeroy, Iowa

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The air was oppressive, clinging like a hot, damp blanket draped across the heartland of the United States. Despite a cooling rain the previous night and a thick blanket of clouds through the morning, midday sunshine pushed temperatures past 90 degrees by early afternoon in northwest Iowa. After a warm and dry beginning to the summer, rain was a welcome sight for the many farmers who wrested a living from the fertile prairie soil. As towering thunderheads began to burst into the muggy afternoon skies, however, concern began to grow. A stiff breeze picked up, blowing from the south and east with enough force to rustle trees and hold flags at attention. This, long-time residents knew, was cyclone weather.

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May 22, 2011 — The Joplin Tornado

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“Get the sirens going, get the sirens going, I’m telling you!”

The sense of urgency, bordering on panic, was palpable. The brilliant blue springtime skies had become a malevolent, roiling mass of charcoal grays and diffuse blue-greens. The murky clouds heaved curtains of rain and a spattering of hail as storm chaser Jeff Piotrowski pulled alongside a police cruiser near Seventh Street and Black Cat Road on the south side of Joplin, Missouri. The doppler radar inside Jeff’s vehicle indicated an extremely intense circulation just minutes from town, but one look to the southwest left no doubt. Lowering from the sky to engulf the horizon was a massive, billowing cone of clouds. A scattering of debris soon followed, prompting Piotrowski to issue his desperate plea. By the time the sirens wailed to life, the single deadliest tornado in more than six decades had already begun its path of devastation.

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April 11, 1965 — Palm Sunday Outbreak

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Across the upper Midwestern United States, March of 1965 was cold, snowy and miserable. The month began with blizzard conditions across the region on March 2, bringing heavy snow and a biting 50mph wind. Another, more significant blizzard would follow on St. Patrick’s Day, March 17. Several feet of snow buried the region, while 60mph winds whipped the landscape and blew the powdery snow into vast drifts tall enough to bury cars along the streets. In many locations, 1965 ranked among the top-20 coldest and snowiest Marches on record. A respite would not come until the first week of April, when a surge of warm air brought unseasonable warmth and temperatures into the low 70s. The warmth would not last long, however, as another arctic air mass settled over the region on the 8th with temperatures slipping back to the 30s and 40s.

St. Patrick's Day Blizzard. March 17, 1965.

St. Patrick’s Day Blizzard. March 17, 1965.

 

By Palm Sunday weekend a weak low pressure system was edging toward the area, drawing in warm, humid air from the Gulf Coast and pulling a mass of cold and extremely dry air behind it. A 25-knot southerly low-level jet combined with stretches of clear skies and sunshine to rocket temperatures into the mid-70s across the warm sector. With widespread dewpoints in the 60s, residents throughout the region headed outside to enjoy the first beautiful spring day of the year. In some areas, the heat grew to become oppressive. In the words of retired police chief Warren Hale of Milan, Michigan; “The day was so warm and wonderful. The family and I decided go on a picnic in the Irish Hills, because it was too stifling in the house. The heat and humidity drove us crazy so we had to just get away from it all.”

Unknown to all outside the meteorological community, a nearly unprecedented atmospheric setup was approaching from the west-southwest.

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February 19, 1884 — The Enigma Outbreak

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The first two months of 1884 had brought nearly ceaseless rain to much of the United States. Slate gray rain clouds had cast a pall over the land, blotting out what little late-winter sun there was to be had and inundating the already soggy ground. Grass and dirt turned to sloppy mud. Rivers and streams, swollen from the relentless precipitation, bulged and spilled over their banks. Meandering creeks transformed into raging rivers, major rivers into vast inland seas. For the first time in many days, the morning of February 19 brought a welcome respite. Morning broke to fair weather and mostly cloudless skies, and many engorged rivers began to slowly recede.

A broad, deep low pressure system was tracking across the upper Mississippi Valley just south of the Great Lakes, pulling in air from the south ahead of it. A strong southerly wind drawn from the Gulf of Mexico overspread the south with warm, soupy air. In Alabama, temperatures surged by ten degrees in the span of a few hours. In Mississippi, temperatures jumped nearly 20 degrees. Dewpoints rocketed to the mid- and upper-60s as winter-weary residents headed out to enjoy the springlike weather. Several hundred miles to the northwest, the story was much different. Trailing behind the low pressure system was a bitterly cold air mass, dragged down from high in the Arctic. Temperatures dropped by as much as 30 degrees in 24 hours with the passage of the cold front, bottoming out below zero in many places.

Surface chart from 7am on the morning of February 19. Strong southerly flow is evident ahead and south of the low, setting up a broad warm sector.

Surface chart from 7am. Strong southerly flow was evident ahead and south of the low, setting up a broad warm sector. Red “++++” icons indicate tornado tracks recorded in the following eight hours.

Surface chart from 3pm. Surface temperatures ranging from 60 to 75 degrees overspread much of the eastern United States ahead of the deepening low .

Surface chart from 3pm. Surface temperatures ranging from 60 to 75 degrees overspread much of the eastern United States ahead of the deepening low. Ground observations indicate temperatures in excess of 75 degrees actually reached as far inland as Kentucky and western Virginia.

Surface chart from 11pm. Enhanced low-level winds south and east of the rapidly deepening low, along with ample, deep moisture, allowed for continued tornado production through the evening and overnight.

Surface chart from 11pm. Enhanced low-level winds south and east of the rapidly deepening low, along with ample moisture, allowed for continued tornado production through the evening and overnight.

 

The negatively-tilted trough can be clearly seen on the 500mb map from noon on February 19.

The negatively-tilted trough can be clearly seen on the 500mb map from noon on February 19.

A negatively-tilted trough dug southeast across the North-Central United States, while the core of an intense jet streak rapidly rounded its base. Adding to the already dangerous situation, a bulge of extremely dry air from the west began to nose in on the layer of deep moisture over the Southeast. While winds at the surface blew stiffly from the south and southeast, winds at mid-levels howled from the southwest and upper-level winds blew west to west-northwest. Additionally, wind speeds increased substantially with height throughout the atmosphere. The resulting directional and speed shear, combined with the high instability of the warm, oppressive airmass below and the dry air from the west, created ideal conditions for supercells and tornadoes over a broad area.

By mid-morning, cauliflower-shaped thunderheads began to pop up just ahead of the cold front near the Mississippi River. In the east-central Mississippi town of Winston, the southerly winds intensified and began veering toward the west-northwest. Rain came in fits, accompanied shortly thereafter by quarter-sized hail. As the precipitation abated, sharply shifting winds and a dull roar announced the arrival of the day’s first tornado. The twister destroyed two homes and a cotton gin before lifting near Louisville. Just before noon the first killer tornado barreled through the town of Columbus. Several plantations were destroyed and one woman was killed. The tornado continued its 25-mile track across the state line into Pickens County, Alabama.

At 12:20pm CST, the first violent tornado of the day touched down near Oxmoor, Alabama. The tornado tracked just southeast of Birmingham before tearing through the southern and eastern sections of the industrial town of Leeds. At least thirteen lives were lost, including four members of the Pool family, as dozens of homes were scattered in the howling wind. Newspaper reports suggest the tornado may have had a multi-vortex structure, with eyewitnesses stating that “several black shafts darted in quick succession from the cloud toward the earth.” Survivors of the tornado recounted that much of the town was left unrecognizable. Well-constructed brick homes in the area were leveled and several foundations were damaged or swept away, indicating probable F5 damage. Bodies were found impaled by various objects, and cattle and horses were reportedly skinned and dismembered. Eleven fatalities were recorded in the Leeds area. Thirty one were reportedly “gravely injured,” though the lack of subsequent reports makes it unclear how many ultimately succumbed to their injuries.

Damage from Leeds, Alabama

Drawing of an obliterated farmhouse near Leeds, Alabama. Artist unknown.

 

 By 1:00pm, the outbreak was nearing its peak as a broken line of supercells sprawled across much of the southeast. A small funnel descended into a field near Cartersville, Georgia. Racing to the northeast, the tornado rapidly intensified as it neared the community of Waleska. Three children were killed after they had been released from school early and taken shelter in their home. At the towns of Cagle and Tate in Pickens County, many homes were completely swept away. The majority of the affected homes belonged to prominent families and were large and well-built, indicating that this tornado, too, was perhaps of F5 intensity. The tornado and associated downbursts cut a swath nearly three miles wide through the forest, leveling homes as far apart as two miles. The bodies of several of the 22 total fatalities were reportedly carried more than half a mile. In total, the tornado and associated downbursts flattened at least 50 square miles of forest.

In East Alabama, what would prove to be perhaps the deadliest tornado of the outbreak descended from the skies just north of Jacksonville. The tornado tore through the Germania tanning yard before killing ten just north of Piedmont, then known as Cross Plains. As many as 20 lives were lost at Goshen, where a schoolmaster and six children were killed in a schoolhouse said to have been “blown to atoms.” All 19 of the remaining children were seriously injured. Large cotton bales were tossed at least half a mile, and many poorly-built homes vanished entirely. At Cave Spring, Georgia, four lives were lost and a number of well-constructed homes were scoured from the Earth. By the time the tornado had come to the end of its 35 mile path of destruction in western Georgia, it had taken more than 35 lives. Newspapers of the day made little effort to document the carnage in these rural communities, and the true death toll was likely much higher.

Shortly after 3:00pm, large hail — some three inches or more in diameter — began falling across central Georgia. The wind veered from south to northwest and the temperature began to drop. Moments later, a large multi-vortex tornado began shredding and snapping trees in Monroe County. Just northwest of Haddock, many large homes were swept away and at least eleven people were killed. Unconfirmed reports indicated at least an additional 12 fatalities. Among those killed were a mother and her four children, who had taken shelter inside their home as the storm approached. Witnesses north of Macon recounted that the tornado contained “multiple dark columns rotating about a common center.” Another 50 were injured along the 30-mile path.

At 8:30pm the final violent tornado of the outbreak began a path of destruction in southeastern Anson County, North Carolina. Two were killed and several homes were destroyed in Pee Dee. The tornado barreled through the southeast edge of the city of Rockingham. Several large homes were swept from their foundations, and large hardwood trees were reportedly debarked, denuded and snapped off near ground level. At least one victim in the area was reportedly thrown nearly a mile. The tornado intensified and widened to nearly one mile as it roared to the northeast. The small railroad community of Philadelphia, about three miles northeast of downtown Rockingham, was completely obliterated, with one observer noting that only “fragments remained of the houses and cabins.” At least 25 homes were razed to their foundations, their debris scattered over large areas. At least 15 residents of Philadelphia were killed in the storm.

In total, multiple waves of tornadoes scoured at least ten states across the Midwest and Southeast. Mississippi, Alabama, Georgia, North Carolina, South Carolina, Illinois, Indiana, Kentucky, Tennessee and Virginia were all impacted by tornadoes and destructive winds. The enigma, however, begins with the final number of tornadoes — although the official number ranges from 47 to 60, the Enigma Outbreak may actually have been among the largest in history. The extent of tornado-related damage is uncertain, but is suspected to have been several million dollars, the equivalent of perhaps $100 million today. More than 10,000 structures were destroyed. The final death toll, too, is a complete mystery. While the conservative estimate is 178 — a terrible toll in its own right — many estimates have placed the final number as high as 1,200. A further 2,500 may have been injured during the outbreak. The system produced more than 37 significant tornadoes (F2 – F5) during its 15-hour duration, ranking it still among the most violent outbreaks on record.

Map of all reported tornadoes during the Enigma Outbreak. Note that the northernmost tornado tracks may actually have been downburst damage. It is likely that there were many more tornadoes that simply went unreported, and some tracks may have been tornado families.

Map of all reported tornadoes during the Enigma Outbreak. Note that the northernmost tornado tracks may actually have been downburst damage. It is likely that there were many more tornadoes that simply went unreported, and some tracks may have been tornado families.

 

In the days leading up to the event, despite a recently enacted ban on the word “tornado” and a halt to nearly all research and forecasting, an intrepid scientist named John P. Finley worked diligently to record surface and atmospheric data and attempt to piece together the puzzle of tornado outbreaks. Finley developed meticulous tornado charts to track what he believed to be the most significant factors in tornado formation, including temperatures, dewpoints, dewpoint depression, wind direction and observed conditions. In his chart for the morning of February 19, seen below, he noted what he believed to be one of the most important indications of a tornado outbreak. A bulge of dry air, now referred to as a dryline bulge, was being pulled from Texas and Louisiana and forced into an area of warm, moist air overlaying Alabama and Georgia by strong easterly mid-level winds. This phenomenon is still used today to diagnose an enhanced tornado threat.

Enigma Outbreak Surface Map

“Tornado chart” created by John P. Finley on the morning of February 19. Dewpoint isolines indicate a bulge of dry air from Louisiana cutting into the existing tongue of moist air over Alabama and Georgia, boosted by a strong southerly low-level jet. Added arrows represent dry air intrusion (orange) and moist air surge (green).

 

Because of the extremely dynamic nature of the storm system in place, a range of severe weather effects were experienced over a vast swath of the eastern United States. The upper Midwest was gripped by an intense blizzard, with gale-force winds and snowfall rates exceeding one to two inches per hour in some areas. Much of the Ohio Valley experienced the violent winds and torrential rains of a derecho, with widespread and severe non-tornadic wind damage. Thousands of trees were “prostrated by the extreme wind,” and telegraph communications were cut off for several days. Across the Ohio Valley, the Ohio River and its many tributaries had already spilled over their banks. The additional rainfall from the storms of the 19th and 20th led to flooding which exceeded all records. Homes were swept away by deadly floods in Louisville, Kentucky; Jeffersonville, Indiana; and many other towns along the Ohio and its tributaries. At Union Point in Greene County, Georgia, hail fell in such volume that the ground was covered to a depth of three inches.

Though the true extent of the Enigma Outbreak have been lost to the ravages of time, it still stands among the most widespread and violent tornado outbreaks in history. The tornado which struck Philadelphia and Rockingham remains the deadliest tornado in the history of North Carolina. Limited newspaper resources and shoddy reporting also makes it difficult to place this outbreak historically. Damages and deaths in rural areas were very poorly reported, and the deaths of African-Americans were not counted nor recorded. Given the large African-American population in the affected areas, it’s likely the death toll was much larger than the officially accepted number. The nickname is certainly well-deserved, and the many mysteries of the Enigma Outbreak will continue to draw intrigue well into the future.

May 27, 1997 — The Jarrell, Texas Tornado

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Nestled in the Blackland Prairies at the edge of Central Texas’ Hill Country lies the small community of Jarrell. It’s a land of contrasts, with a small-but-developing town center carved into the low, rolling hills and patchy farmland. Clumps of Indian paintbrush and Texas bluebonnet provide blotches of color against the muted greens and browns of shortgrass and crop fields. A sharp, black, six-lane ribbon of asphalt bisects the town, carrying drivers between the metropolitan areas of Dallas, Waco and Austin.

If the town of Jarrell is divided by that flat, sun-baked stretch of Interstate 35, so too are its people. To the east of the interstate, the march of progress has taken hold full-force. A brand new water tower glints in the midday sun, rising more than 200 feet above its surroundings. Newly elected mayor Dewey Hulme talks excitedly about his plans for a reinvigorated Jarrell, one in which the new water tower and a sprawling 46-acre town center — planned out on a vast 153-acre parcel of land near the interstate — will begin to attract the kind of commercial and residential growth that could transform the town.

To the west of I-35, there is no water tower. There are no freshly built developments and new businesses. There is no talk of transformation. Instead, there is the stark land surrounding County Road 305. There is a park with a small community center, flanked by two emerald-green baseball fields. There is a simple memorial plaque, white lettering etched into sepia-toned granite, and a semicircular entrance ringed by twenty-seven trees. Most of all, there is the ever-present memory of the day nature conspired to wipe 50 homes and 27 of Jarrell’s mothers, fathers, children, friends and neighbors from the face of the Earth.

Jarrell Memorial Park

The Jarrell Memorial Park, with several of the 27 trees planted to honor tornado victims.

 

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Meteorology 101 — The Basics

 

As anyone who has ever watched a local television meteorologist can tell you, the weather isn’t always easy to predict. The atmosphere is a maddeningly complex, chaotic system, and even the best forecasts involve some level of uncertainty. Fortunately, the basic mechanisms of the weather are relatively well understood, even if the results aren’t always predictable. The goal of this Meteorology 101 series is to provide a basic understanding of those processes, which will aid in the ability to forecast the weather and diagnose it as it happens.

The Atmospheric Engine

Although we generally don’t recognize it, we live at the bottom of a vast ocean. Unlike the liquid water that makes up the oceans we’re familiar with, however, the Earth’s atmosphere is composed of many onion-like layers of air. This air, despite being a gas rather than a liquid, still behaves in much the same way as any other fluid. This invisible ocean of nitrogen, oxygen, argon and other trace gases flows, compresses, pools and deforms as it travels around the Earth, and the science of meteorology is an attempt to understand and predict these motions and their results.

As with any fluid, the atmosphere constantly seeks stability. In an ideal world, the air would be the same temperature and humidity throughout the entire atmosphere. Of course, we don’t live in an ideal world. Solar radiation, the rotation and tilt of the Earth and other factors ensure that stasis is never reached. The weather can be thought of as a great engine which seeks to rid the atmosphere of this imbalance and return it to a state of equilibrium, and throughout this series we’ll look at various ways in which it does this. But first, let’s examine how this imbalance originates.

Although the atmosphere is a continuous body of air, it is composed of many smaller air masses. An air mass is a volume of air that has roughly the same temperature and moisture content throughout. There are four primary types of air masses, named by where they originate. “Continental” air masses are so-named because they originate over large land masses, and they tend to be drier. “Maritime” air masses originate over large bodies of water and tend to hold more moisture. These air masses are further defined by their temperature, with “tropical” air masses having a higher temperature than “polar” air masses. Taken together, the four air mass types are continental tropical (cT), continental polar (cP), maritime tropical (mT) and maritime polar (mP). A fifth air mass, continental arctic (cA) contains dry, bitterly cold air and is seen much less frequently in the middle latitudes.

Air Masses

Common positions of the various air mass types across North America. Arrows represent the most common directions of flow for each air mass.

 

These air masses move like large tectonic plates across the globe and, like the earthquakes that result from the collision of two plates, most large-scale weather occurs when two air masses collide. The meeting of two air masses is called a front, and the type of front depends on how the air masses are interacting. Because the majority of widespread weather occurs near fronts, they are a key factor in understanding and forecasting storms. Below are the types of fronts as well as the weather that is normally expected along them. Below each header is the symbol used for each front on surface analysis maps.

Cold Front

Cold fronts occur when a mass of cold air moves into and collides with an existing mass of warm air. Because cold air is denser than warm air, the cold air mass drives underneath the warm air mass like a wedge, lifting it higher into the atmosphere. The warm air mass eventually rises high enough to begin condensing, forming clouds and rain. The cold air strongly forces the warm air to lift upward, often leading to thunderstorm development or heavy snow in the winter. For this reason, the majority of severe weather occurs near cold fronts.

Warm Front

A warm front occurs when a warm air mass moves into an existing mass of cool air. Because the warm air is already in motion and the cold air is stationary, the warm air lifts above the cold air with much less force. This leads to much less energy for storm development and often causes steady, prolonged rain and occasionally weak storms instead. Because warm fronts move much more slowly than cold fronts, rainy conditions generally persist for a longer period of time.

Stationary Front

When warm and cold air masses meet and neither has enough force to move the other, a stationary front is formed. Along the front, the lifting is very weak and storms are rare as a result. Because stationary fronts move very little, they can cause steady rain or snow over the same general area for many days. Many flooding and heavy snow events are caused by stationary fronts, which are sometimes referred to as being “stalled” over an area.

Occluded Front

An occluded front is the most complex of the various fronts. When a swiftly moving cold air mass overruns a slowly moving warm air mass, it can sometimes wedge completely underneath the warm air mass and begin mixing with the cool air on the other side. When this happens, the warm air mass is cut off from the ground, or occluded, and is is lifted further into the atmosphere. Weak lifting may cause widespread clouds, as well as rain or snow and occasionally storms.

Under Pressure

As we mentioned briefly earlier, one of the primary causes of the imbalance that drives the weather is solar radiation. Because of the tilt and rotation of the Earth, its surface isn’t heated evenly. This, in turn, causes the air masses above these cooler and warmer areas to also become cooler or warmer because the temperature of an air mass is determined largely by the temperature of the surface beneath it. In addition to contributing to the different thermal properties of the air masses, this uneven heating also influences the surrounding air pressure.

Air pressure is essentially a measure of the weight or thickness of the atmosphere above any given point. A thicker volume of atmosphere is heavier because it contains more air molecules, and vice versa for a thinner volume of atmosphere. Temperature affects the pressure of a mass of air by expanding or contracting it. As a mass of air becomes warmer, it becomes less dense and begins to rise and expand. The air high in the atmosphere begins to flow away from the rest of the air mass as it warms and expands, leaving less air beneath and creating a lower air pressure. Conversely, a cooling air mass contracts and becomes more dense, allowing additional air high in the atmosphere to flow inward, adding additional weight to the air mass and creating higher pressure.

As air rises in a low pressure system and flows outward high in the atmosphere, more air near the surface rushes inward to fill the void. This air is also ultimately lifted upward and dispersed, acting as an engine or pump that continues to run until the surrounding atmosphere is no longer imbalanced. The same mechanism works in reverse in a high pressure system, where air flows outward near the surface and more air rushes inward in the upper atmosphere. The air in the upper atmosphere then flows downward toward the surface, called subsidence, until it too eventually reaches the surface and flows outward. However, this inward and outward flowing air does not flow in a straight line. Combined with friction from the surface, the Earth’s rotation imparts rotational energy to the air, called the Coriolis force.

Coriolis Force

The Coriolis force causes wind to be deflected, imparting rotational energy. The diagram shows the direction of rotation for each hemisphere.

 

Since low pressure areas cause air to be lifted higher into the atmosphere and high pressure areas cause air to sink, low pressure is generally associated with clouds and stormy weather while high pressure normally brings fairly calm, clear weather. Although high pressure areas play an important role in the weather, low pressure systems are of most interest for our purposes. A typical low pressure system consists of a cool air mass to the north, a warm, moist air mass to the south and east and a cold, dry air mass to the west and northwest. In an effort to maintain stability, the warm, southeastern air flows toward the north and the cold, northwestern air flows to the south. This setup creates a warm front to the east of the high pressure and a trailing cold front to the south.

Low Pressure System

A typical low pressure system with its associated air masses and fronts. The air from each air mass flows inward toward the area of lowest pressure, indicated by the “L” on the map.

 

One final piece that often plays a role in severe weather, particularly in the Great Plains, is a dry line. A dry line is a semi-permanent boundary that often separates the warm, moist air mass across the southeastern and central United States and the hot, dry air mass originating from the desert Southwest and northern Mexico. The boundary between very moist air and very dry air often serves as a focal point for severe weather when low pressure systems pass through.

Low Pressure System With Dry Line

A low pressure system with a typical dry line, seen here in light orange.

 

Now that we have an understanding of the large-scale  forces (called “synoptic scale”) that factor into the weather, we’ll look at the smaller-scale forces (called “mesoscale”) that factor into storm development in the next installation.