The Richter Scale of Solar Flares

by Dr. Tony Phillips (

A solar flare is an explosion on the Sun that happens when energy stored in twisted magnetic fields (usually above sunspots) is suddenly released. Flares produce a burst of radiation across the electromagnetic spectrum, from radio waves to x-rays and gamma-rays.

Scientists classify solar flares according to their x-ray brightness in the wavelength range 1 to 8 Angstroms. There are 3 categories: X-class flares are big; they are major events that can trigger planet-wide radio blackouts and long-lasting radiation storms. M-class flares are medium-sized; they can cause brief radio blackouts that affect Earth’s polar regions. Minor radiation storms sometimes follow an M-class flare. Compared to X- and M-class events, C-class flares are small with few noticeable consequences here on Earth.

This figure shows a series of solar flares detected by NOAA satellites in July 2000:

Each category for x-ray flares has nine subdivisions ranging from, e.g., C1 to C9, M1 to M9, and X1 to X9. In this figure, the three indicated flares registered (from left to right) X2, M5, and X6. The X6 flare triggered a radiation storm around Earth nicknamed the Bastille Day event.

Peak (W/m2)between 1 and 8 Angstroms
 I < 10-6
 10-6 < = I < 10-5
 10-5 < = I < 10-4
 I > = 10-4

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Rads on a Plane: Hot Seats in First Class

by Dr. Tony Phillips (

July 30, 2015 — Many people think that only astronauts have to worry about cosmic radiation. Not so. Regular air travelers are exposed to cosmic rays, too. This week,’s Dr. Tony Phillips and the students of Earth to Sky Calculus flew across the United States to conduct a transcontinental launch of space weather balloons. They took radiation sensors on board the plane to find out how many cosmic rays they would absorb during the flight. Here are the data they collected flying east:

Radiation levels in the cabin of the Airbus 319 (Spirit Airlines flight 640) tripled within ten minutes after takeoff, and were nearly 30 times ground level by the time the plane reached cruising altitude at 39,300 feet. Summing over the entire flight, the sensors measured about 1 mrem of radiation–similar to a dental x-ray.

There was no solar storm in progress. The extra radiation was just a regular drizzle of cosmic rays reaching down to aviation altitudes. This radiation is ever-present and comes from supernovas, black holes, and other sources across the galaxy.

The Earth to Sky team consisted of five people who sat in three different locations: First Class, over the wings, and in the back row. Would they all absorb the same dose? No. On this particular flight, dose rates were highest in First Class and lowest near the toilets in the rear. The front-to-back ratio was as high as 13%. This gradient is not understood; presumably, it has to do with the way cosmic rays interact with the plane’s fuselage and fuel tanks.

Five days later, following a successful Transcontinental Balloon Launch, the team flew back to the west coast. Once again they flew on an Airbus 319 (Spirit Airlines flight 641), non-stop from Boston to Las Vegas. The results were similar:

As before, the First Class seats registered the highest dose of radiation–as much as 6% higher than the wings and rear of the plane. On this flight we added a second radiation sensor to First Class to confirm the effect. Both sensors agreed: ionizing radiation was slightly higher in the front of the plane.

Because cosmic rays come from space, radiation inside the airplane grows stronger as the airplane ascends. This plot shows how the dose rate changed as a function of altitude throughout the July 23rd flight:

Note how radiation levels remain low at altitudes below ~15,000 ft. Earth’s atmosphere does a good job shielding those altitudes from cosmic rays. Above 15,000 ft, however, dose rates climb rapidly as the plane ascends.

The radiation sensors are the same ones that Earth to Sky Calculus routinely flies onboard helium balloons to measure cosmic rays in the stratosphere. They detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.

Noctilucent Cloud Season Begins (May 2015)

by Dr. Tony Phillips (

May 23, 2015: NASA’s AIM spacecraft has spotted a luminous patch of electric-blue drifting across the Arctic Circle. The sighting marks the beginning of the 2015 season for noctilucent clouds (NLCs). “The first clouds appeared on May 19th–a bit earlier than usual,” reports Cora Randall, AIM science team member at the University of Colorado. They are located at longitude +90o in this polar image recorded by AIM’s CIPS instrument:

The first northern-hemisphere NLCs of 2015, recorded by AIM/CIPS on May 19th

“It is always good to see the beginning of another season,” says James Russell of Hampton University, principal investigator for the AIM mission. “What surprises will it bring? We will see. The clouds have never disappointed us.”

NLCs are Earth’s highest clouds. Seeded by meteoroids, they float at the edge of space more than 80 km above the planet’s surface. The clouds are very cold and filled with tiny ice crystals. When sunbeams hit those crystals, they glow electric-blue.

Noctilucent clouds first appeared in the 19th century after the eruption of super-volcano Krakatoa. At the time, people thought the clouds were caused by the eruption, but long after Krakatoa’s ash settled, the clouds remained. In those days, NLCs were a polar phenomenon confined mainly to the Arctic. In recent years they have intensified and spread with sightings as far south as Utah and Colorado. This could be a sign of increasing greenhouse gases in Earth’s atmosphere.

Data from AIM have shown that NLCs are like a great “geophysical light bulb.” They turn on every year in late spring, reaching almost full intensity over a period of 5 to 10 days. News flash: The bulb is glowing. Stay tuned for sightings.

Noctilucent Clouds, Behaving Strangely

by Dr. Tony Phillips (

March 2, 2015: The southern season for noctilucent clouds (NLCs) has come to an end. NASA’s AIM spacecraft observed the last wisps of electric-blue over Antarctica on Feb. 20, 2015. The end of the season was no surprise: The polar clouds always subside in late summer. Looking back over the entire season, however, reveals something unexpected. In an 8-year plot of Antarctic noctilucent cloud frequencies, the 2014-2015 season is clearly different from the rest:

These data come from the AIM spacecraft, which was launched in 2007 to monitor NLCs from Earth orbit. The curves show the abundance (“frequency”) of the clouds vs. time for 120 days around every southern summer solstice for the past 8 years.

“This past season was not like the others,” notes Cora Randall, a member of the AIM science team and the chair of the Department of Atmospheric and Oceanic Sciences at the University of Colorado. “The clouds were much more variable, and there was an enormous decrease in cloud frequency 15 to 25 days after the summer solstice. That’s when the clouds are usually most abundant.”

What does this mean? Previous research shows that NLCs are a sensitive indicator of long-range teleconnections in Earth’s atmosphere, which link weather and climate across hemispheres. The strange behavior of noctilucent clouds in 2014-2015 could be a sign of previously unknown linkages. “Preliminary indications are that it is indeed due to inter-hemispheric teleconnections,” says Randall. “We’re still analyzing the data, so stay tuned.”

Now attention turns to the northern hemisphere, where the season for NLCs typically begins in May. Will the northern season ahead be as strangely variable as the southern season, just concluded? Says Randall, “I can’t wait to find out.”

Planets in a Bottle — more about yeast

Yeast are simple, unicellular fungi. The most common forms of yeast — baker’s and brewer’s yeast — are strains of the species Saccaromyces cerevisiae. Yeast is often taken as a vitamin supplement because it is 50 percent protein and is a rich source of B vitamins, niacin, and folic acid.

Yeast microbes are probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-year-old bakeries and breweries. Only in the last 150 years, since the experiments of Louis Pasteur, have scientist begun to explore how yeast works. Pasteur first proposed the production of carbon dioxide from yeast as responsible for raising a loaf of bread in 1859.

Yeast Facts:

As little as two pounds of yeast starter can raise 500 pounds of bread dough.

Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids. These wild varieties contributed some of the earliest kinds of sourdough bread mixes which did not depend on adding starter cultures.

Yeast is also a popular organism for studying genetics. Baker’s yeast is one of only a half-dozen microbes on Earth whose unique gene script has recently been comprehensively deciphered. Notable in the yeast gene is a host of signals that trigger the microbe to protect itself against extremes in cold and heat, called thermal shock proteins. Hopes now run high in the biological community that over the next several years, more than 50 to 100 additional microbes will also provide comprehensive genetic scripts for their lifecycles, including how these organisms might survive under relentless swings in near boiling water, deep ice, or even in the core of an active volcano vent and nuclear reactors.

The yeasts, like most fungi, respire oxygen (aerobic respiration), but in the absence of air they derive energy by fermenting sugars and carbohydrates to produce ethanol and carbon dioxide. When yeast are supplied with both sugar and oxygen, the colonies grow up to 20 times faster through cell division than without oxygen.

The “Stuff of life” as reconstitutable for simple experiments in extreme environments and astrobiology, including freeze-dried nutrient mix (far left, dried skim milk powder) and fast-growing Baker’s yeast (middle, powdered Saccharomyces cerevisiae), and as shown with the key element for life, water, added (right shown in vials from left to right in last panel, with left, metabolic dye to measure growth (chemical called resazurin), middle, reconstituted skim milk with color dye added, and right, yeast.

In 1815, Guy-Lussac understood how yeasts convert the simplest sugar, glucose (C6H12O6) to ethanol:

C6H12O6 (glucose)--->2CO2 (carbon dioxide) + 2C2H5OH (ethanol)

The great medical microbiologist, Louis Pasteur, played a central role in proving this conversion to ethanol required living organisms, rather than a chemical catalyst. Pasteur showed that by bubbling oxygen into the yeast broth, the cells could be made to stop growing, but ferment vigorously–an observation later called the Pasteur Effect.

Many higher animals share this property of oxygen balance with yeasts. When given nutrient (sugar) and oxygen, they will burn fuel quickly like a stoked fire, but when deprived of oxygen, they will reproduce by cell multiplication and division (rather than metabolize). This kind of behavior–burn fuel or divide–is common to many biochemistries and these kinds of organisms are classified as facultative anaerobes; they essentially scrounge a meager living out of whatever particular circumstances are handed to them.

Unlike many kinds of fermenting bacteria (such as yogurt making or lactic acid microbes), yeasts don’t require anything but sugar and water to maintain fermentation and growth. For example, their nutrient broth can be free from other complex molecules such as amino acids, minerals or vitamins, since the yeasts’ history of austere conditions in nature has brought them to a unique state of self-sufficiency, even by microbial standards. The ingeniousness of adaptation makes yeasts one of the most studied and robust microbes.

Suggested Reading 

The Early Days of Yeast Genetics. (1993) edited by Michael N. Hall and Patrick Linder. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

The Molecular and Cellular Biology of the Yeast Saccharomyces cerevisiae: Gene Expression. (1992) edited by Elizabeth W. Jones, John R. Pringle, and James R. Broach. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York.

Mortimer, R.K. and Schold, D. 1985. Genetic map of Saccharomyces cerevisiae, Edition 9. Microbiological Reviews 49: 181-212.

Planets in a Bottle

Students demonstrate Mercury in a BottleMar. 16, 1999: NASA/Marshall’s “Life on the Edge” program is barely a month old but it’s already producing results in some grade school classrooms.

“It was wonderful,” says Mrs. Nancy Walters, whose 3rd grade class recently tried some simple microbiology experiments with yeast. “The kids felt like they were doing real science, and they couldn’t stop talking about it for days.”

Life on the Edge is an educational program that aims to expose grade school students to some of the basic principles of astrobiology and to explore the possibilities for life elsewhere in the Solar System. The program began just over a month ago when 50 lb of yeast and other microbes were delivered to a summit in California’s White Mountains. Conditions there present severe challenges for most forms of life, so it is a good place to test the response of microbes to extreme environments. Some of the microorganisms will remain there for months, and some for longer than a year before they are retreived and distributed to classrooms for experimentation.

“Eventually we’ll be sending thousands of yeast packets to schools around the country,” says Dr. John Horack, director of science communications at the NASA/Marshall Space Sciences Lab. “But even before the microbes are ready to go we have to develop some simple lab protocols that kids can use to measure how their samples were affected by exposure. That’s why we’re going into classrooms now to test some of our ideas.”

One of these ideas, called “Planets in a Bottle,” was field-tested in a 2nd/3rd grade class room in February.

Moon in a Bottle “‘Planets in a Bottle’ is a simple way to test the viability of yeast samples, and a great way to teach young students about conditions on other planets,” explained Dr. Tony Phillips, who is evaluating the concept in classrooms. “The basic ingredients for a planet in a bottle are 1 cup of warm water, 3 sugar cubes, a 1/4 oz. packet of yeast, a half liter plastic water bottle, and a nine inch party balloon. Simply mix the sugar, water, and yeast in the bottle, and cap the bottle with the balloon. A healthy sample of yeast will inflate the balloon to 12 inch circumference in less than an hour.”

What happens is this: In the nutrient broth — warm water containing both dissolved oxygen and sugar — yeast metabolizes the sugar and produces carbon dioxide. The rate of carbon dioxide production at any given instant is proportional to the number of healthy microbes in the bottle. Because the yeast are constantly reproducing through cell division the number of microbes increases exponentially. Likewise, carbon dioxide production increases. The balloon inflates slowly at first, then rapidly accelerates.

a healthy yeast sample inflates a balloon with carbon dioxide In practice the balloon inflates to maximum volume in about 45 minutes. That’s when the yeast have consumed all the available nutrient. At room temperature the cells remain viable for several hours afterward and then begin to die. The maximum volume of CO2 and the time required to produce the gas can be used to estimate the number of healthy microbes in the original sample.

Students monitor yeast growthClick here for a sample “Planets in a Bottle” lesson plan

“Two weeks ago we visited Mrs. Walter’s 3rd grade classroom in Bishop, CA” continued Dr. Phillips. “The class was divided into seven groups, each with the basic ingredients for a Planet in a Bottle. Rather than have every group do the same experiment, we added variations so that each bottle would represent a different planet. For example, the Moon has no atmosphere to protect its surface from solar UV radiation. So, one group exposed their yeast to a UV lamp before adding the microbes to the nutrient mix, creating a “Moon in a Bottle.” Another group used scalding hot orange juice as a nutrient mix for ‘Venus in a Bottle.’ Citric acid in the orange juice served as a substitute for sulfuric acid in Venus’s hot atmosphere.”

Above: Young scientists monitor yeast growth in a bottle labelled “Pluto”. In this case the yeast were frozen for weeks before being added to the nutrient mix.

“Clearly we can’t reproduce true planetary conditions in a simple water bottle, nor did we pretend to, but these excercises have powerful teaching value. Every kid in Mrs. Walter’s class now knows that Venus has acid in its atmosphere thanks to the orange juice experiment, and they also learned that weak acids are not deadly to yeast,” Phillips said.

Students discuss the results in Mrs. Walter's 3rd grade class “My students were really excited when their balloons began to inflate,” recalled Mrs. Walters, “but the best part came at the end when we measured the sizes of the balloons and held a classroom debate about the results. We argued about which planet was most congenial to yeast and what the limitations of our results were. It felt like real science.”

Left: Students in Mrs. Walter’s 3rd grade class debate the question: “Which planet is really best for yeast?”

NASA scientists have a crowded schedule of classroom visits planned in the months to come, even though the Life on the Edge yeast container won’t be retreived for some time. The goal is to develop safe and effective classroom protocols before the yeast packets are distributed nationally.

“We don’t want to spoon feed students with overly-detailed protocols,” says John Horack,” That’s not science. But, we do want to give them a good starting point for their own creative experiments with extremophiles. The only way to do that is by spending lots of time in the classroom now, while the microbes are still in the White Mountains.”

To view a prototype lesson plan for “Planet in a Bottle” yeast experiments click here. Readers are invited to try the experiments (they are lots of fun) and we welcome comments from educators and others to improve our procedures. Please send comments and suggestions to

Planets in a Bottle –Lesson Plan

This is a prototype lesson plan for “Planet in a Bottle” yeast experiments .

Objective: The student will measure the viability of yeast samples and explore environmental conditions which affect the health of yeast microbes. The yeast samples may be common store-bought Baker’s yeast, or more exotic forms which have been exposed to extreme environments as part of the Earth to Sky balloon program.

Overview: Students mix yeast with a nutrient broth consisting of warm water and table sugar in a plastic water bottle. A common 9 inch party balloon is used to cap the bottle. As yeast digest the sugar they produce carbon dioxide and inflate the balloon. A healthy 1/4 oz sample of baker’s yeast can inflate a balloon to 12 inch circumference in less than 30 minutes. Simple variations of this experiment may be used to discover environmental factors that inhibit or promote the health of the yeast colony. Students can compare these factors to conditions on other planets. ingredients for a Planet in a Bottle


  • 1 cup lukewarm water
  • 3 cubes sugar
  • 1 quarter-oz package of yeast
  • 1 empty half-liter plastic water bottle
  • 1 nine or ten inch party balloon
  • 1 cloth measuring tape
  • 1 small funnel (optional)


  1. Mix water + sugar in water bottle until the cubes are dissolved.
  2. Using the funnel add yeast, the gently swirl the mixture.
  3. Cap the bottle with a balloon.
  4. Use the cloth measuring tape to measure the circumference of the balloon every 15 minutes.

a healthy yeast sample inflates a balloon with carbon dioxide This basic recipe can be considered an “Earth in a Bottle.” It is a warm, healthy environment for yeast with plenty of nutrients. The total amount of CO2 in the balloon when it reaches its greatest volume is proportional to the number of healthy yeast microbes present in the initial sample. For the procedure outlined above, the balloon will achieve its maximum volume less than two hours after the yeast are added to the nutrient mix.

The rate at which the balloon inflates is proportional to the growth rate of the yeast colony. After the yeast are added to the nutrient broth they begin to divide and increase in number. As the colony size increases so does the rate of CO2 production, so long as there is an ample supply of nutrients. If the environment inside the bottle is conducive to yeast growth, the maximum rate of CO2 production will be high. Conversely, if the environment is hostile to yeast, the maximum rate of CO2 production will be low.

Students can begin to explore conditions on other planets with simple variations to the basic recipe. Although we cannot create truly accurate extraterrestrial conditions in a water bottle, there are many simple variations that are representative of conditions on other planets. A few examples are listed below:

Example variations:

  • Mercury — Mercury’s surface is very hot. Mercury in a Bottle: Boil the water before adding sugar and yeast.
  • Venus — Venus is very hot, and has an acidic atmosphere. Venus in a Bottle: Instead of water and sugar, use scalding hot orange juice as a nutrient mix. Citric acid in the juice represents sulfuric acid in Venus’s hot atmosphere. Lemon juice or vinegar can also be used to increase the acidity of the nutrient mix. Venus’s atmosphere also has a high pressure, so that the simulation can be made more realistic by heating the nutrient mix in a pressure cooker.
  • Students monitor yeast growthThe Moon — The moon has no atmosphere, so that yeast on its surface would be exposed to a strong vacuum and solar radiation. Moon in a Bottle: Expose the yeast to a vacuum, using a hand pump bell jar, and to radiation in a microwave oven and/or from a UV lamp.
  • Mars — Mars is cold and has a thin atmosphere which allows much solar UV radiation to penetrate to its surface. Mars in a Bottle: freeze the yeast, then expose the microbes to ultraviolet radiation from a UV lamp before adding yeast to the nutrient mix.  Note:  Flying the yeast to the stratosphere on an Earth to Sky research balloon gives the yeast a very Mars-like experience.
  • Europa — this moon of Jupiter may harbor the largest ocean in the solar system. The icy surface is a combination of pure water ice, Epsom salts, and unknown minerals. Europa in a Bottle: Freeze a briny mixture of water and Epsom salt. Break the ice into chips and mix the salty ice chips with a cold nutrient solution.
  • Callisto — this moon of Jupiter may have a salty ocean beneath its frozen crust. Callisto in a Bottle: Add common table salt or Epsom salts to the nutrient mix to simulate a salty environment.
  • Pluto — Pluto is the most distant planet from the sun and is very cold. Pluto in a Bottle: freeze the yeast in a deep freezer before adding to the nutrient mix.

Growing Peril for Astronauts?

 NASA’s successful test flight of Orion on Dec. 5th heralds a renewed capability to send astronauts into deep space. A paper just published in the journal Space Weather, however, points out a growing peril to future deep space explorers: cosmic rays. The title of the article, penned by Nathan Schwadron of the University of New Hampshire and colleagues from seven other institutions, asks the provocative question, “Does the worsening galactic cosmic ray environment preclude manned deep space exploration?” Using data from a cosmic ray telescope onboard NASA’s Lunar Reconnaissance Orbiter, they conclude that while increasing fluxes of cosmic rays “are not a show stopper for long duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic radiation remains a significant and worsening factor that limits mission durations.” This figure from their paper shows the number of days a 30 year old astronaut can spend in interplanetary space before they reach their career limit in radiation exposure:

According to the plot, in the year 2014, a 30 year old male flying in a spaceship with 10 g/cm2 of aluminum shielding could spend approximately 700 days in deep space before they reach their radiation dose limit. The same astronaut in the early 1990s could have spent 1000 days in space.

What’s going on? Cosmic rays are intensifying. Galactic cosmic rays are a mixture of high-energy photons and subatomic particles accelerated to near-light speed by violent events such as supernova explosions. Astronauts are protected from cosmic rays in part by the sun: solar magnetic fields and the solar wind combine to create a porous ‘shield’ that fends off energetic particles from outside the solar system. The problem is, as the authors note, “The sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the Space Age. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of cosmic rays in the Space Age.”

The shielding action of the sun is strongest during solar maximum and weakest during solar minimum–hence the 11-year rhythm of the mission duration plot. At the moment we are experiencing Solar Max, which should be a good time for astronauts to fly–but it’s not a good time. The solar maximum of 2011-2014 is the weakest in a century, allowing unusual numbers of cosmic rays to penetrate the solar system.

This situation could become even worse if, as some researchers suspect, the sun is entering a long-term phase of the solar cycle characterized by relatively weak maxima and deep, extended minima. In such a future, feeble solar magnetic fields would do an extra-poor job keeping cosmic rays at bay, further reducing the number of days astronauts can travel far from Earth.

To learn more about this interesting research, read the complete article in the online edition of Space Weather.

Solar Eclipse in the Stratosphere

On Oct. 23rd, 2014, just as the New Moon was about to pass in front of the sun, the students of Earth to Sky Calculus launched a helium balloon carrying a Nikon D7000 camera. Their goal: to set the record for high-altitude photography of an eclipse. During a two-hour flight to the edge of space, the camera captured 11 images of the crescent sun. The final picture, taken just a split second before the balloon exploded, was GPS-tagged with an altitude of 108,900 feet:

To put this achievement into context, consider the following: Most people who photographed the eclipse carefully mounted their cameras on a rock-solid tripod, or used the precision clock-drive of a telescope to track the sun. The students, however, managed the same trick from an un-stabilized platform, spinning, buffeted by wind, and racing upward to the heavens at 15 mph. Their photos show that DLSR astrophotography from an suborbital helium balloon is possible, and they will surely refine their techniques for even better photos in the future.

Hey thanks! The students wish to thank for sponsoring this flight. Their $500 contribution paid for the helium and other supplies necessary to get the balloon off the ground. Note the Automation Direct logo in this picture of the payload ascending over the Sierra Nevada mountains of central California:

Another notable picture shows the payload ascending over clouds, which blocked the eclipse at ground level but did not prevent photography from the balloon.

Readers, would you like to sponsor a student research flight and have your logo photographed at the edge of space? Contact Dr. Tony Phillips to get involved.

Electric Hurricanes

by Dr. Tony Phillips (this article originally appeared in Science@NASA)

January 9, 2006: The boom of thunder and crackle of lightning generally mean one thing: a storm is coming. Curiously, though, the biggest storms of all, hurricanes, are notoriously lacking in lightning. Hurricanes blow, they rain, they flood, but seldom do they crackle.

Surprise: During the record-setting hurricane season of 2005 three of the most powerful storms–Rita, Katrina, and Emily–did have lightning, lots of it. And researchers would like to know why.
An infrared GOES 11 satellite image of Hurricane Emily. Yellow + and – symbols mark lightning bolts detected by the North American Lightning Detection Network. The green line traces the path of the ER-2. Click to view electric fields measured by the aircraft during the flight.

Richard Blakeslee of the Global Hydrology and Climate Center (GHCC) in Huntsville, Alabama, was one of a team of scientists who explored Hurricane Emily using NASA’s ER-2 aircraft, a research version of the famous U-2 spy plane. Flying high above the storm, they noted frequent lightning in the cylindrical wall of clouds surrounding the hurricane’s eye. Both cloud-to-cloud and cloud-to-ground lightning were present, “a few flashes per minute,” says Blakeslee.

“Generally there’s not a lot of lightning in the eye-wall region,” he says. “So when people see lightning there, they perk up — they say, okay, something’s happening.”

Indeed, the electric fields above Emily were among the strongest ever measured by the aircraft’s sensors over any storm. “We observed steady fields in excess of 8 kilovolts per meter,” says Blakeslee. “That is huge–comparable to the strongest fields we would expect to find over a large land-based ‘mesoscale’ thunderstorm.”

see caption
The ER-2 en route to a hurricane. [More]

The flight over Emily was part of a 30-day science data-gathering campaign in July 2005 organized and sponsored by NASA headquarters to improve scientists’ understanding of hurricanes. Blakeslee and others from NASA, NOAA and 10 U.S. universities traveled to Costa Rica for the campaign, which is called “Tropical Cloud Systems and Processes.” From the international airport near San Jose, the capital of Costa Rica, they could fly the ER-2 to storms in both the Caribbean and the eastern Pacific Ocean. They combined ER-2 data with data from satellites and ground-based sensors to get a comprehensive view of each storm.

Rita and Katrina were not part of the campaign. Lightning in those storms was detected by means of long-distance sensors on the ground, not the ER-2, so less is known about their electric fields.

Nevertheless, it is possible to note some similarities: (1) all three storms were powerful: Emily was a Category 4 storm, Rita and Katrina were Category 5; (2) all three were over water when their lightning was detected; and (3) in each case, the lightning was located around the eye-wall.

What does it all mean? The answer could teach scientists something new about the inner workings of hurricanes.

Actually, says Blakeslee, the reason most hurricanes don’t have lightning is understood. “They’re missing a key ingredient: vertical winds.”

Within thunderclouds, vertical winds cause ice crystals and water droplets (called “hydrometeors”) to bump together. This “rubbing” causes the hydrometeors to become charged. Think of rubbing your socked feet across wool carpet–zap! It’s the same principle. For reasons not fully understood, positive electric charge accumulates on smaller particles while negative charge clings to the larger ones. Winds and gravity separate the charged hydrometeors, producing an enormous electric field within the storm. This is the source of lightning.

A hurricane’s winds are mostly horizontal, not vertical. So the vertical churning that leads to lightning doesn’t normally happen.

Lightning has been seen in hurricanes before. During a field campaign in 1998 called CAMEX-3, scientists detected lightning in the eye of hurricane Georges as it plowed over the Caribbean island of Hispaniola. The lightning probably was due to air forced upward — called “orographic forcing” — when the hurricane hit the mountains.

“Hurricanes are most likely to produce lightning when they’re making landfall,” says Blakeslee. But there were no mountains beneath the “electric hurricanes” of 2005—only flat water.

It’s tempting to think that, because Emily, Rita and Katrina were all exceptionally powerful, their sheer violence somehow explains their lightning. But Blakeslee says that this explanation is too simple. “Other storms have been equally intense and did not produce much lightning,” he says. “There must be something else at work.”

It’s too soon to say for certain what that missing factor is. Scientists will need months to digest reams of data gathered in this year’s campaign before they can hope to have an answer.

Says Blakeslee, “We still have a lot to learn about hurricanes.”