Introduction to Thunderstorms

It is estimated that there are as many as 40,000 thunderstorm occurrences each day world-wide. This translates into an astounding 14.6 million occurrences annually! The United States certainly experiences its share of thunderstorm occurrences..

It is in this part of the country that warm, moist air from the Gulf of Mexico and Atlantic Ocean (which we will see later are necessary ingredients for thunderstorm development) is most readily available to fuel thunderstorm development.

All thunderstorms require three ingredients for their formation:

• Moisture,
• Instability, and
• a lifting mechanism.

Sources of moisture

Typical source of moisture for thunderstorms are the oceans. However, water temperature plays a large role in how much moisture is added to the atmosphere.

Recall from the Ocean Section that warm ocean currents occur along east coasts of continents with cool ocean currents occur along west coasts. Evaporation is higher in warm ocean currents and therefore puts more moisture into the atmosphere as compared to the cold ocean currents at the same latitude.

Therefore, in the southeastern U.S. the warm water from the two moisture sources (Atlantic Ocean and Gulf of Mexico) helps explain why there is much more precipitation in that region as compared to the same latitude in Southern California.

Instability

Air is considered unstable if it continues to rise when given a nudge upward (or continues to sink if given a nudge downward). An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft.

In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As this parcel rises it cools and some of the water vapor will condense forming the familiar tall cumulonimbus cloud that is the thunderstorm.

Sources of Lift (upward)

Typically, for a thunderstorm to develop, there needs to be a mechanism which initiates the upward motion, something that will give the air a nudge upward. This upward nudge is a direct result of air density.

Some of the sun's heating of the earth's surface is transferred to the air which, in turn, creates different air densities. The propensity for air to rise increases with decreasing density. This is difference in air density is the main source for lift and is accomplished by several methods.

Differential Heating

The sun's heating of the earth's surface is not uniform. For example, a grassy field will heat at a slower rate than a paved street. A body of water will heat slower than the nearby landmass.

This will create two adjacent areas where the air is of different densities. The cooler air sinks, pulled toward the surface by gravity, forcing up the warmer, less dense air, creating thermals.

Fronts, Drylines and Outflow Boundaries

Fronts are the boundary between two air masses of different temperatures and therefore different air densities. The colder, more dense air behind the front lift warmer, less dense air abruptly. If the air is moist thunderstorms will often form along the cold front.

Drylines are the boundary between two air masses of different moisture content and divides warm, moist air from hot, dry air. Moist air is less dense than dry air. Drylines therefore act similarly to fronts in that the moist, less dense air is lifted up and over the drier, more dense air.

 

The air temperature behind a dryline is often much higher due to the lack of moisture. That alone will make the air less dense but the moist air ahead of the dryline has an even lower density making it more buoyant. The end result is air lifted along the dryline forming thunderstorms. This is common over the plains in the spring and early summer.

 

Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled, more dense, air acts as a "mini cold front", called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form.

Terrain

As air encounters a mountain it is forced up because of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.

How much water is in that cloud?

Types of Thunderstorms

Thunderstorm Hazards - Hail

Updrafts in Action

 

Overview

SKYWARN^® is a concept developed in the early 1970s that was intended to promote a cooperative effort between the National Weather Service and communities. The emphasis of the effort is often focused on the storm spotter, an individual who takes a position near their community and reports wind gusts, hail size, rainfall, and cloud formations that could signal a developing tornado.

Focusing on hail, based upon samples picked at random the student learn to estimate the size of hail.

Procedure

Choose a student to select balls from the box. Once the ball is selected have the student tell the class the number on the ball. The students should write that number on their paper.

Pass the ball, row by row, around the class allowing the students to hold it and estimate the ball's diameter in inches. Once the entire class has written their estimates, place it aside.

Repeat the procedure with the next ball chosen from the box. (you can save time by allowing several balls to be passed through the classroom at the same time.) Once the last estimate has been made, tell the students which number ball represented which size.

Discussion

Take a poll of the class asking their results of their estimates. For example, hold up a 1" ball and ask...

• How many students had the correct estimate?
• How many estimated the ball was greater than 1"? If so, by how much?
• How many estimated the ball was less than 1"? If so, by how much?

Again holding up the 1" ball say the National Weather Service defines a severe thunderstorm as one containing hail size of 1" (2.5 cm) or larger (and/or any wind speed 58 mph (93km/h) or greater).

Live weatherwise

When the National Weather Service issues a severe thunderstorm warning it means a severe thunderstorm is occurring, is imminent, or has a high probability of occurring. The warning will contain...

• County or counties affected by the severe weather event,
• Warning expiration time,
• Location and direction of storm movement,
• Locations in the path of the storm, and
• Additional information and/or call-to-action statement(s).

Remember, due to the nature of a thunderstorm's size, there may be a severe thunderstorm warning in effect for your county but you may experience mostly blue skies. Know where the storm is in relation to your location and which direction it is moving.

Just because a thunderstorm may not be severe that does not mean cause damage. A thunderstorm can be deadly due to lightning alone. If you can hear thunder, you are close enough to be struck by lightning. Seek shelter indoors immediately and remain indoors until 30 minutes after the last thunder is heard.

Damaging wind from thunderstorms is much more common than damage from tornadoes. In fact, many confuse damage produced by "straight-line" winds and often erroneously attribute it to tornadoes.

The source for damaging winds is well understood and it begins with the downdraft. As air rises, it will cool to the point of condensation where water vapor forms tiny water droplets, comprising the cumulus cloud we see.

Near the center of the updraft, the particles begin to collide and coalescence forming larger droplets. This continues until the rising air can no longer support the ever increasing size of water drops.

Once the rain drops begin to fall friction causes the rising air to begin to fall towards the surface itself. Also, some of the falling rain will evaporate. Through evaporation heat energy is removed from the atmosphere cooling the air associated with the precipitation.

As a result of the cooling, the density of the air increases causing it to sink toward the earth. The downdraft also signifies the end of the convection with the thunderstorm and its subsequent decrease.

When this dense rained-cooled air reaches the surface it spreads out horizontally with the leading edged of the cool air forming a gust front. The gust front marks the boundary of a sharp temperature decrease and increase in wind speed. The gust front can act as a point of lift for the development of new thunderstorm cells or cut off the supply of moist unstable air for older cells.

Downbursts are defined as strong winds produced by a downdraft over a horizontal area up to 6 miles (10 kilometers). Downbursts are further subdivided into microbursts and macrobursts.

Microbursts and Macrobursts

A microburst is a small downburst with an outflow less than 2½ miles (4 kilometers) in horizontal diameter and last for only 2-5 minutes. Despite their small size, microbursts can produce destructive winds up to 168 mph (270 km/h). Also, they create hazardous conditions for pilots and have been responsible for several disasters. For example...

1. As aircraft descend (above) into the airport they follow an imagery line called the "glide slope" (solid light blue line) to the runway.
2. Upon entering the microburst, the plane encounters a "headwind", an increase in wind speed over the aircraft. The stronger wind creates additional lift causing the plane to rise above the glide slope. To return the plane to the proper position, the pilot lowers the throttle to decrease the plane's speed thereby causing the plane to descend.
3. As the plane flies through to the other side of the microburst, the wind direction shifts and is now a "tailwind" as it is from behind the aircraft. This decreases the wind over the wing reducing lift. The plane sinks below the glide slope.
4. However, the "tailwind" remains strong and even with the pilot applying full throttle trying to increase lift again, there may be little, if any, room to recover from the rapid descent causing the plane to crash short of the runway.

Since the discovery of this effect in the early to mid 1980's, pilots are now trained to recognize this event and take appropriate actions to prevent accidents. Also, many airports are now equipped with equipment to detect microbursts and warn aircraft of their occurrences.

A macroburst is larger than a microburst with a horizontal extent more than 2½ miles (4 km) in diameter. Also a macroburst is not quite a strong as a microburst but still can produce winds as high as 130 mph (210 km/h). Damaging winds generally last longer, from 5 to 20 minutes, and produce tornado-like damage up to an EF-3 scale.

In wet, humid environments, macrobursts and microbursts will be accompanied by intense rainfall at the ground. If the storm forms in a relatively dry environment, however, the rain may evaporate before it reaches the ground and these downbursts will be without precipitation, known as dry microbursts.

In the desert southwest, dust storms are a rather frequent occurrence due to downbursts. The city of Phoenix, AZ typically has 1-3 dust storms each summer due to the cooler dense air spreading out from thunderstorms.

On July 5, 2011, a massive dust storm resulted in widespread areas of zero or near zero visibility in Phoenix. The wind that produced this storm was generated by downbursts from thunderstorms with winds up to 70 mph (110 km/h).

Heat Bursts

Dry downbursts are responsible for a rare weather event called "Heat Bursts". Heat bursts usually occur at night, are associated with decaying thunderstorms, and are marked by gusty, and sometimes damaging, winds combined a sharp increase in temperature and a sharp decrease in dew point (humidity).

The process of creating a dry microburst begins higher in the atmosphere for heat bursts. A pocket of cool air aloft forms during the evaporation process as for any downburst. But as the precipitation falls it evaporates before reaching the ground. The cool dense air sinks by the pull of gravity but since there is no rain drops to absorb heat, the air then warms due to compression.

In fact, it can become quite hot and very dry. Temperatures generally rise 10 to 20 degrees in a few minutes and have been known to rise to over 120°F (49°C) and remain in place for several hours before returning to normal. One such heat burst occurred in Wichita, KS on June 9, 2011.

Derechos

If the atmospheric conditions are right, widespread and long-lived windstorms, associated with a band of rapidly moving showers or thunderstorms, can result. The word "derecho" is of Spanish origin, and means straight ahead. A derecho is made up of a "family of downburst clusters" and by definition must be at least 240 miles in length. Learn more about derechos.

A tornado is a violently rotating (usually counterclockwise in the northern hemisphere) column of air descending from a thunderstorm and in contact with the ground. Although tornadoes are usually brief, lasting only a few minutes, they can sometimes last for more than an hour and travel several miles causing considerable damage. Tornadoes are the #3 most hazardous aspect of thunderstorms (#2 is lightning).

In a typical year around 1200 tornadoes will strike the United States. The peak of the tornado season is April through June with more tornadoes striking the central United States than any other place in the world. This area of the country has been nicknamed "tornado alley."

Wind Shear

Most tornadoes are spawned from supercell thunderstorms. Supercell thunderstorms are characterized by a persistent rotating updraft and form in environments of strong vertical wind shear. Wind shear is the change in wind speed and/or direction with height.

Directional wind shear is the change in wind direction with height. In the image below, the view is looking north. The wind near the surface is blowing from the southeast to the northwest.

As the elevation increases the direction veers (changes direction in a clock-wise motion) becoming south, then southwest, and finally, west.

Speed shear is the change in wind speed with height. In the illustration below, the wind is increasing with height. This tends to create a rolling affect to the atmosphere and is believed to be a key component in the formation of mesocyclones which can lead to tornadoes.

Strong vertical shear is the combination of a veering directional shear and strong speed shear and is the condition that is most supportive of supercells.

The updraft lifts the rotating column of air created by the speed shear. This provides two different rotations to the supercell; cyclonic or counter clockwise rotation and an anti-cyclonic of clockwise rotation.

The directional shear amplifies the cyclonic rotation and diminishes the anti-cyclonic rotation (the rotation on the right side of the of the updraft in the illustration - located right).

All that remains is the cyclonic rotation called a mesocyclone. By definition a supercell is a rotating thunderstorm.

When viewed from the top (left image), the counter-clockwise rotation of the mesocyclone gives the supercell its classic "hook" appearance when seen by radar. As the air rises in the storm, it becomes stretched and more narrow with time.

The exact processes for the formation of a funnel are not known yet. Recent theories suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone.

However, mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on May 3, 1999 in Oklahoma.

The Tornado Itself

The funnel cloud of a tornado consists of moist air. As the funnel descends the water vapor within it condenses into liquid droplets. The liquid droplets are identical to cloud droplets yet are not considered part of the cloud since they form within the funnel.

The descending funnel is made visible because of the water droplets. The funnel takes on the color of the cloud droplets, which is white.

Due to the air movement, dust and debris on the ground will begin rotating, often becoming several feet high and hundreds of yards wide.

After the funnel touches the ground and becomes a tornado, the color of the funnel will change. The color often depends upon the type of dirt and debris is moves over (red dirt produces a red tornado, black dirt a black tornado, etc.).

Tornadoes can last from several seconds to more than an hour but most last less than 10 minutes. The size and/or shape of a tornado is no measure of its strength.

Occasionally, small tornadoes do major damage and some very large tornadoes, over a quarter-mile wide, have produced only light damage.

The tornado will gradually lose intensity. The condensation funnel decreases in size, the tornado becomes tilted with height, and it takes on a contorted, rope-like appearance before it completely dissipates. Learn more about tornadoes from the NWS Storm Prediction Center's FAQ.

The Enhanced F-Scale

 

EF-Scale wind speeds

EF scale	Class	Wind speed		Description
		mph	km/h
EF0	weak	65-85	105-137	Gale
EF1	weak	86-110	138-177	Moderate
EF2	strong	111-135	178-217	Significant
EF3	strong	136-165	218-266	Severe
EF4	violent	166-200	267-322	Devastating
EF5	violent	> 200	> 322	Incredible

The Fujita (F) Scale was originally developed by Dr. Tetsuya Theodore Fujita to estimate tornado wind speeds based on damage left behind by a tornado. An Enhanced Fujita (EF) Scale, developed by a forum of nationally renowned meteorologists and wind engineers, makes improvements to the original F scale. This EF Scale has replaced the original F scale, which has been used to assign tornado ratings since 1971.

The original F scale had limitations, such as a lack of damage indicators, no account for construction quality and variability, and no definitive correlation between damage and wind speed. These limitations may have led to some tornadoes being rated in an inconsistent manner and, in some cases, an overestimate of tornado wind speeds.

The EF Scale takes into account more variables than the original F Scale did when assigning a wind speed rating to a tornado. The EF Scale incorporates 28 damage indicators (DIs) such as building type, structures, and trees. For each damage indicator, there are 8 degrees of damage (DOD) ranging from the beginning of visible damage to complete destruction of the damage indicator. The original F Scale did not take these details into account.

For example, with the EF Scale, an F3 tornado will have estimated wind speeds between 136 and 165 mph (218 and 266 km/h), whereas with the original F Scale, an F3 tornado has winds estimated between 162-209 mph (254-332 km/h).

The wind speeds necessary to cause "F3" damage are not as high as once thought and this may have led to an overestimation of some tornado wind speeds.

There is still some uncertainty as to the upper limits of the strongest tornadoes so F5 ratings do not have a wind speed range. Wind speed estimations for F5 tornadoes are left open ended and assigned wind speeds greater than 200 mph (322 km/h).

Except for heat related fatalities, more deaths occur from flooding than any other hazard. Why? Most people fail to realize the power of water. For example, six inches of fast-moving flood water can knock you off your feet.

While the number of fatalities can vary dramatically with weather conditions from year to year, the national 30-year average for flood deaths is 127. That compares with a 30-year average of 73 deaths for lightning, 68 for tornadoes and 16 for hurricanes.

National Weather Service data also shows:

• Nearly half of all flash flood fatalities are vehicle-related,
• The majority of victims are males, and
• Flood deaths affect all age groups.

Most flash floods are caused by slow moving thunderstorms, thunderstorms that move repeatedly over the same area or heavy rains from tropical storms and hurricanes. These floods can develop within minutes or hours depending on the intensity and duration of the rain, the topography, soil conditions and ground cover.

Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Furthermore, flash flood-producing rains can also trigger catastrophic mud slides.

Occasionally, floating debris or ice can accumulate at a natural or man-made obstruction and restrict the flow of water. Water held back by the ice jam or debris dam can cause flooding upstream. Subsequent flash flooding can occur downstream if the obstruction should suddenly release.

TURN AROUND, DON'T DROWN^®

Each year, more deaths occur due to flooding than from any other thunderstorm related hazard. Why? The main reason is people underestimate the force and power of water. Many of the deaths occur in automobiles as they are swept downstream. Of these deaths, many are preventable, but foolish people drive around the barriers in place that warn you the road is flooded.

Whether you are driving or walking, if you come to a flooded road, Turn Around...Don't Drown!. You will not know the depth of the water nor will you know the condition of the road under the water.

Of the three deaths which occurred as a result of the Fort Worth tornado, March 28, 2000, one death was due to flooding. The man who drowned was a passenger in a car with his girlfriend, the driver. They approached a low spot with water flowing over the road due to very heavy rain. Flooding was a common occurrence at this location with heavy rains and the danger was well marked.

As the driver drove her car into the water she became frightened as the water rose higher and higher around her vehicle. She backed out to higher ground. The passenger said the water was NOT too deep and he would prove it by walking across to the other side. He never made it.

Follow these safety rules.

• Monitor the NOAA Weather Radio, or your favorite news source for vital weather related information.
• If flooding occurs, get to higher ground. Get out of areas subject to flooding. This includes dips, low spots, canyons, washes etc.
• Avoid already flooded and high velocity flow areas. Do not attempt to cross flowing streams. If you enter a flowing stream and the water gets above you knee, TURN AROUND, DON'T DROWN.
• If driving be aware that the road bed may not be intact under flood waters. Turn around and go another way. NEVER drive through flooded roadways! If your vehicle stalls, leave it immediately and seek higher ground. Rapidly rising water may engulf the vehicle and sweep you and your occupants away.
• Do not camp or park your vehicle along streams and washes, particularly during threatening conditions.
• Be especially cautious at night when it is harder to recognize flood dangers.

We say "Follow these safety rules" and most folks say "Yeah, yeah, whatever. It's not going to happen to me." A simple Internet search of flash flood victims (and some seen from inside the vehicle) will show you the reason to just TURN AROUND, DON'T DROWN.

Severe weather rarely happens without any warning. While we will never be able to pinpoint when and where severe weather will develop, we can identify broader areas with the potential for the development of severe weather. It is your responsibility to check the weather forecast, which may be often several times daily, to see if you are, or will be, under a risk of severe weather.

The weather office charged with monitoring and forecasting the potential for severe weather over the 48 continental United States is the Storm Prediction Center (SPC) located in Norman, OK. Use the information provided by SPC to give you early critical information concerning the threat of severe weather in your locale.

Convective Outlooks

Convective Outlooks consist of a narrative and a graphic depicting severe thunderstorm threats across the continental United States. The outlook narratives are written in technical language, intended for sophisticated weather users, and provide the meteorological reasoning for the risk areas.

This product also provides explicit information regarding the timing, the greatest severe weather threat and the expected severity of the event. The graphics that accompany the narratives provide vital information to help plan your day.

The convective outlook graphics display up to six different color categories to reflect the six likelihood of occurrences and/or increased severity of a severe weather event(s). The four convective outlooks issued (Day 1, Day 2, Day 3 and Days 4-8) are...

Day 1	This is the risk of severe weather today through early tomorrow morning. Day 1 forecasts are issued five times daily; 06z (around midnight), 13z (around sunrise), 1630z (mid-morning), 20z (mid-afternoon), and 01z (early evening). This is the forecast you will see on SPC's frontpage.
Day 2	Day 2 continues from the ending of Day 1 (tomorrow morning) for the next 24 hours. These are issued twice daily; 07z (around midnight) and 1730z (around noon).
Day 3	This is the forecast for the subsequent 24 hours. Day 3 forecasts are issued daily by 0830z on standard time and 0730z on daylight time (after midnight).
Days 4-8	A severe weather area depicted in the days four through eight period. It is issued at 10z daily (early morning) and indicates a 15% or 30% or higher probability for severe thunderstorms (e.g. a 15% or 30% chance that a severe thunderstorm will occur within 25 miles of any point).

Following are the meanings of the colors used in convective outlooks.

These are the official definitions. The reason for the "AND's", "OR's" and "WITH OR WITHOUT's" is the atmosphere is complicated with many different conditions that can occur. For example, there will be times when the number of severe weather events will be high but the overall intensities will not necessarily be extreme. Conversely, there may only be one or two severe events expected but the intensity of the event(s) will be extremely high.

Therefore, below is a simplified version of the official definitions.

Still Confused?!? Just know that the greater the threat (from Slight to High), the greater the risk for severe weather which could be either in number of events or intensity or both.

The following are current severe weather outlooks from the Storm Prediction Center (click to enlarge - takes you to the SPC website)

Today

Tomorrow

Day 3

Days 4-8

Public Severe Weather Outlooks

The Public Severe Weather Outlooks (PWO) are issued when a potentially significant or widespread tornado outbreak is expected. This plain-language forecast is typically issued 12-24 hours prior to the event and is used to alert National Weather Service field offices and other weather customers concerned with public safety of a rare, dangerous situation.

The Public Severe Weather Outlook is reserved for for all high risks and for moderate risks with a strong risk for tornadoes and/or widespread damaging winds. The SPC issues about 30 PWOs each year.

Mesoscale Discussions

When conditions appear favorable for severe storm development, SPC issues a Mesoscale Discussion (MCD), normally 1 to 3 hours before issuing a weather watch.

SPC also puts out MCDs for mesoscale aspects of hazardous winter weather events including heavy snow, blizzards and freezing rain. MCDs are also issued on occasion for heavy rainfall or convective trends.

The MCD basically describes what is currently happening, what is expected in the next few hours, the meteorological reasoning for the forecast, and when/where SPC plans to issue the watch (if dealing with severe thunderstorm potential). Severe thunderstorm MCDs provide you with extra lead time on the severe weather development.

Severe Weather Watches

When conditions become favorable for severe thunderstorms and tornadoes to develop, SPC usually issues a severe thunderstorm or tornado watch.

Tornadoes can occur in either type of watch, but tornado watches are issued when conditions are especially favorable for tornadoes. Severe thunderstorm watches are blue with tornado watches in red.

Watches are large areas, 20,000 to 40,000 square miles, and are issued by county. They are numbered sequentially (the count is reset at the beginning of each year). A typical watch has a duration of about four to six hours but it may be canceled, replaced, or re-issued as required. A watch is NOT a warning, and should not be interpreted as a guarantee that there will be severe weather!

When a watch is issued, stay alert for changing weather conditions and possible warnings. Any warnings will be issued by your local NWS Weather Forecast Office.

When a severe weather watch is issued close to your location but does not include your county, you should still remain alert.

The atmosphere rarely follows straight lines, and thunderstorms do not always remain within the man-made boundaries we create around them. When SPC feels confident about the possibility of severe weather in a specific area, they try to issue a watch at least one hour prior the onset of severe weather.

In some instances the phrase "THIS IS A PARTICULARLY DANGEROUS SITUATION" will headline a watch (called a PDS watch). The "particularly dangerous situation" wording is used in rare situations when long-lived, strong and violent tornadoes are possible.

PDS watches are issued when, in the opinion of the forecaster, the likelihood of significant events is boosted by very volatile atmospheric conditions.Usually this decision is based on a number of atmospheric clues and parameters, so the decision to issue a PDS watch is subjective.

There are no hard threshold or criteria. PDS watches are most often issued in association with "high risk" convective outlooks.

Introduction to Lightning

Lightning is one of the oldest observed natural phenomena on earth. At the same time, it also is one of the least understood. While lightning is simply a gigantic spark of static electricity (the same kind of electricity that sometimes shocks you when you touch a doorknob), scientists do not have a complete grasp on how it works, or how it interacts with solar flares impacting the upper atmosphere or the earth's electromagnetic field.

Lightning has been seen in volcanic eruptions, extremely intense forest fires, surface nuclear detonations, heavy snowstorms, and in large hurricanes. However, it is most often seen in thunderstorms. In fact, lightning (and the resulting thunder) is what makes a storm a thunderstorm.

At any given moment, there can be as many as 2,000 thunderstorms occurring across the globe. This translates to more than 14.5 MILLION storms each year. NASA satellite research indicated these storms produce lightning flashes about 40 times a second worldwide.

This is a change from the commonly accepted value of 100 flashes per second which was an estimate from 1925. Whether it is 40, 100, or somewhere in between, we live on an electrified planet.

The conditions needed to produce lightning have been known for some time. However, exactly how lightning forms has never been verified so there is room for debate.

Leading theories focus around separation of electric charge and generation of an electric field within a thunderstorm. Recent studies also indicate that ice, hail, and semi-frozen water drops known as graupel are essential to lightning development. Storms that fail to produce large quantities of ice usually fail to produce lightning.

Forecasting when and where lightning will strike is not yet possible and most likely never will be. But by educating yourself about lightning and learning some basic safety rules, you, your family, and your friends can avoid needless exposure to the dangers of one of the most capricious and unpredictable forces of nature.

Charge Separation

Thunderstorms have very turbulent environments. Strong updrafts and downdrafts occur with regularity and within close proximity to each other. The updrafts transport small liquid water droplets from the lower regions of the storm to heights between 35,000 and 70,000 feet, miles above the freezing level.

Meanwhile, downdrafts transport hail and ice from the frozen upper regions of the storm. When these collide, the water droplets freeze and release heat. This heat in turn keeps the surface of the hail and ice slightly warmer than their surrounding environment, and a "soft hail", or "graupel" forms.

When this graupel collides with additional water droplets and ice particles, a critical phenomenon occurs: Electrons are sheared off of the ascending particles and collect on the descending particles. Because electrons carry a negative charge, the result is a storm cloud with a negatively charged base and a positively charged top.

Field Generation

In the world of electricity, opposites attract and insulators inhibit. As positive and negative charges begin to separate within the cloud, an electric field is generated between its top and base. Further separation of these charges into pools of positive and negative regions results in a strengthening of the electric field.

However, the atmosphere is a very good insulator that inhibits electric flow, so a TREMENDOUS amount of charge has to build up before lightning can occur. When that charge threshold is reached, the strength of the electric field overpowers the atmosphere's insulating properties, and lightning results.

The electric field within the storm is not the only one that develops. Below the negatively charged storm base, positive charge begins to pool within the surface of the earth (see image right).

This positive charge will shadow the storm wherever it goes, and is responsible for cloud-to-ground lightning. However, the electric field within the storm is much stronger than the one between the storm base and the earth's surface, so most lightning (~75-80%) occurs within the storm cloud itself.

 

How Lightning Develops Between The Cloud And The Ground

A moving thunderstorm gathers another pool of positively charged particles along the ground that travel with the storm (image 1).

As the differences in charges continue to increase, positively charged particles rise up taller objects such as trees, houses, and telephone poles.

A channel of negative charge, called a "stepped leader" will descend from the bottom of the storm toward the ground (image 2).

It is invisible to the human eye, and shoots to the ground in a series of rapid steps, each occurring in less time than it takes to blink your eye. As the negative leader approaches the ground, positive charge collects in the ground and in objects on the ground.

This positive charge "reaches" out to the approaching negative charge with its own channel, called a "streamer" (image 3).

When these channels connect, the resulting electrical transfer is what we see as lightning. After the initial lightning stroke, if enough charge is leftover, additional lightning strokes will use the same channel and will give the bolt its flickering appearance.  

Tall objects such as trees and skyscrapers are commonly struck by lightning. Mountains also make good targets. The reason for this is their tops are closer to the base of the storm cloud.

Remember, the atmosphere is a good electrical insulator. The less distance the lightning has to burn through, the easier it is for it to strike.

However, this does not always mean tall objects will be struck. It all depends on where the charges accumulate. Lightning can strike the ground in an open field even if the tree line is nearby.

Lightning can be divided into two types:

• Flashes with at least one channel connecting the cloud to the ground, known as "cloud-to-ground" discharges (CG); and
• Flashes with no channel to ground, known as "in-cloud" (IC), "cloud-to-cloud" (CC), or "cloud-to-air" (CA).

The lightning process is more or less the same for both types.

Step 1

A typical CG lightning strike initiates inside the storm. Under the influences of the electric field between the cloud and the ground, a very faint, negatively charged channel called a "stepped leader" emerges from the storm base and propagates toward the ground in a series of steps about 160 feet (50 meters) in length and 1 microsecond (0.000001 seconds) in duration.

In what can be loosely described as an "avalanche of electrons", the stepped leader usually branches out in many directions as it approaches the ground, carrying an EXTREMELY strong electric potential: about 100 MILLION volts with respect to the ground and about 5 coulombs of negative charge.

Between each step there is a pause of about 50 microseconds, during which the stepped leader "looks" around for an object to strike. If none is "seen", it takes another step, and repeats the process until it "finds" a target.

It takes the stepped leader about 50 milliseconds (1/20th of a second) to reach its full length, though this number varies depending on the length of its path. Studies of individual strikes have shown that a single leader can be comprised of more than 10,000 steps!

Step 2

As the stepped leader approaches the ground, its strong, negative charge repels all negative charge within the immediate strike zone of the earth's surface, while attracting vast amounts of positive charge. The influx of positive charge into the strike zone is so strong that the stepped leader actually induces electric channels up from the ground known as "streamers".

When one of these positively charged streamers connects with a negatively charged stepped leader (anywhere from 100 to 300 feet (30 to 100 meters) above the surface of the earth), the following steps occur in less than 100 microseconds.

Step 3

The electric potential of the stepped leader is connected to the ground and the negative charge starts flowing DOWN the established channel.

Step 4

An electric current wave, called a "return stroke", then shoots UP the channel producing a brilliant pulse. It only takes the current about 1 microsecond to reach its peak value, which averages around 30,000 amperes.

This "return stroke" is more than 99% of a lightning bolt's luminosity and is what we see as lightning. The stroke actually travels FROM the ground INTO the cloud, but because the strike takes place so quickly, to the unaided eye is appears the opposite is true.

Step 5

An electric current wave, called a "return stroke", shoots UP the channel as a brilliant pulse. Behind the wave front, electric charge flows up the channel and produces a ground current. It takes the current about 1 microsecond to reach its peak value, which averages around 30,000 amperes.

The "return stroke" produces more than 99% of a lightning bolt's luminosity and is what we see as lightning. The stroke actually travels FROM the ground INTO the cloud, but because the strike takes place so quickly, to the unaided eye is appears the opposite is true.

After the return stroke ceases flowing up the channel, there is a pause of about 20 to 50 milliseconds. After that, if enough charge is still available within the cloud, another leader can propagate down to the ground. This leader is called a "dart leader" because it uses the channel already established by the stepped leader and therefore has a continuous path.

Dart leaders give lightning its flickering appearance and normally are not branched like the initial stepped leader. Not every lightning flash will produce a dart leader because a sufficient charge to initiate one must be available within about 100 milliseconds of the initial stepped leader.

The dart leader carries additional electric potential to the ground and induces a new streamer from the ground. The dart leader's peak current is usually less than the initial stepped leader and its return stroke has a shorter duration than the initial return stroke. As additional dart leaders are produced, their peak currents and return stroke durations continue to decrease.

Dart leaders and their return strokes don't necessarily have to use the same cloud-to-ground channel that was burned by initial stepped leader. If a dart leader takes a different path to the ground, the lightning will appear to dance from one spot to another. This is known as "forked lightning".

The combination of each leader (stepped and dart) and their subsequent return strokes is known collectively as a "stroke". All strokes that use the same channel constitute a single "flash". A flash can be made up of a single stroke, or tens of strokes. (The highest number of strokes ever recorded in a single cloud-to-ground flash is 47!)

Notice, in the greatly slowed down animation , the top part of a lightning flash is not connected to anything physical in the cloud, as the cloud itself is not conductive. What happens is the lightning channel branches out inside the cloud in a tree-like shape, and draws free electrons to it (the negative charge in the lower half of the thunderstorm).

Lightning Safety

Frequently Asked Questions