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NASA Looks Into the 'Heart' of Severe Weather
An unexpected EF2 tornado brushes the Huntsville, Ala., skyline on Jan. 21, 2010.

An EF-2 tornado forms over the University of Alabama campus in Huntsville, Ala., on Jan. 21, 2010. Image Credit: UAH/Christopher Schultz
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The evening sky above Huntsville, Ala., held an eerie look on Thursday, Jan. 21, but few knew looming overhead was an EF-2 tornado waiting to descend on a downtown neighborhood. The Huntsville storm system didn't produce an abnormally large amount of lightning, typically a key indicator of severe weather, and the weather community was focused on larger hail-producing thunderstorms moving through southern Tennessee that looked more threatening. Scientists at NASA's Marshall Space Flight Center in Huntsville are studying these recent storms by looking at data from three unique weather monitoring tools to gain a better picture of how storms evolve to produce both heavy rain or large hail, and subsequent strong winds or tornadoes.

On Monday, Feb. 22, 2010 at 3 p.m. EST, physical scientist Walt Petersen of NASA's Marshall Space Flight Center in Huntsville, Ala., answered your questions about severe weather and how NASA is using cutting-edge technology to improve forecasts.

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(Moderator) Brooke: Welcome to today’s Web chat with NASA physical scientist Walt Petersen. Our topic is severe weather and how NASA is using cutting edge technologies to help improve forecasts. Please remember to stay on topic! This is a moderated chat. It may take a few moments for the queue to catch up to your question, so please don’t leave if you don’t see your question right away.

Walt: Hi everyone, Walt here. We had an early question. Why are NASA scientists interested in studying storms and how they form? What do you hope to learn from the recent tornado that hit Huntsville? That’s a good question. Here at NASA’s Marshall Space Flight Center in Huntsville, Ala. we’re working to understand how a storm works and that knowledge will lead to better and more timely predictions and warnings. You have to know how a storm forms to understand them, so we need to make measurements and observations. We’re combining data from three unique weather monitoring tools to gain a better picture of how storms evolve to produce both heavy rain or large hail, and subsequent strong winds or tornadoes. We’re using the Advanced Radar for Meteorological and Operational Research, or ARMOR, radar and NASA Lightning-Mapping Array System and disdrometer data to understand storm precipitation types – rain, snow or hail — and how those amounts relate to the amount of lightning produced. By studying all these tools together we're able to connect the dots between precipitation formation, properties, and movement, and the development of dangerous weather phenomena.

Sundar:  Hi Walt. Do you foresee that this hard weather is all due to the global warming?

Walt:  No, extreme weather is common in our current climate.

(Moderator) Brooke: To ask a questions, please type out your question below and hit the "Ask" button on the right. This is a moderated chat. We're answering questions as soon as possible. Many thanks for your patience.

Steve: How is the ARMOR radar different than weather radar (Doppler?) already in common use?

Walt:  ARMOR transmits in two polarizations. That enables us to see particle sizes and shapes.

WxDan:  I'm all for improving severe weather forecasts, but I'm curious as to why NASA is taking the lead in this field instead of NOAA/NWS?

Walt:  It's not that we're taking the lead. We're working with NOAA and the National Weather Service to improve severe weather forecasting. NASA's niche is satellite remote sensing.

Erin:  How do the two polarizations of the ARMOR radar allow you to see particle size and shape?

Walt:  Picture a football. Now picture hitting that football with one wave that's horizontally oriented and then one that's vertically oriented. The horizontally oriented return will be larger because the football is larger in the horizontal. If we compare the horizontal to vertical returns, the difference between those tells us something about the shape.

Hawkofva:  So is the dual-polarization something we can expect to see in NEXRAD in the near future?

Walt: Yes, in the next 2-3 years.

Steve_C:  So weather radar now can't see particle sizes and shapes? By that do you mean whether the particles are cloud droplets, raindrops, or snow?

Walt:  It's very sensitive to size, but not necessarily the shape. Weather radar like NEXRADs don’t  typically see cloud droplets very well.

Sundar:  Only recently from about 5 years we have seen this extreme weather as common in our current climate. Is this a concern?

Walt:  Well, it's not clear to me that the last five years have been any worse, but severe weather is always a concern.

Erin:  How does the amount of lightning tell you about the amount of precipitation?

Walt:  Lightning is the result of the generation of electric charge. And the electric charge is the result of collisions between large ice particles and small ice particles. In thunderstorms, the large ice particles contribute a large fraction of the precipitation, so the larger the ice and the more lightning, the more precipitation.

(Moderator) Brooke:  This is a moderated chat. We're answering questions as soon as possible. Many thanks for your patience.

Rambler:  Would you explain a little about how satellite remote sensing works?

Walt:  Imagine looking from space down at the Earth with your eyes. Your eyes collect light at visible frequencies, and your brain makes an image. With a satellite, we look the same way, but we look at many different frequencies. And, that information is also processed to create information that is specific to a certain physical process. For example, cloud top temperatures are inferred from infrared wavelengths.

fire1968:  Hello Walt, my questions is…today we have supercomputers for calculating. Why is it so difficult to predict the weather?

Walt:  Even if we had the best computer imaginable, the equations that we use for weather prediction require extremely accurate initial conditions at very high resolution. So, if our initial conditions aren't accurate, the forecast will eventually also not be accurate.

Hink:  Do you see NWS offices offering training on Dual Polar or will it be vendor driven or other options?

Walt:  The National Weather Service currently offers its offices Dual Polar training.

Steve_C:  And how does knowing the SHAPE of the particle help in forecasting severe weather events?

Walt:  The shape of the particles tell us a couple of things. First, it can tell us whether we're dealing with large hail or heavy rain. The shape and type of particle are also related to the resulting evaporational cooling that occurs in thunderstorms, that helps drive strong winds and circulations.

Lhgross:  Will this technology allow you to increase lead time on severe weather events locally?

Walt:  Yes, it will.

WxDan:  The NASA article on the Huntsville tornado and the ARMOR radar ( talks about using disdrometers to augment the radar data from ARMOR. Can you talk more about disdrometers and how they work?

Walt:  Distrometers are typically the size of a rain gauge, and they measure the particle sizes, fall speeds, and sometimes actually image the particles. Distrometers use either images or pressure sensors to detect the size of the particles and count their numbers. From that information, we can deduce the particle number concentrations and precipitation rates.

Erin:  Why do you think that the storm in Huntsville didn't produce the signs that normally alert you that it was becoming a large storm?

Walt:  It didn't have as much lightning, and it was relatively shallow compared to Great Plains super cell thunderstorms.

Hawkofva:  Was there intent to relate lightning frequency and dual-pole radar scans to the tendency of a storm to produce a tornado, or was the observation of the Huntsville EF-2 merely a lucky coincidence? Are either of those tools any more useful than traditional Doppler radar for early warning systems?

Walt:  We weren't originally scanning the storm for tornadic activity. We were scanning the storm for precipitation studies. However, the storm evolved to produce a tornado. Yes, in a sense, we were lucky.

Rambler:  Is it possible that cloud top temperatures be altered by infrared wavelength devices?

Walt:  Yes, it is. It depends on what frequency you're measuring. Some frequencies are more sensitive to different altitudes in the troposphere.

Sundar:  What’s the diff between tornado and hurricanes?

Walt:  Tornadoes are very locally confined, intense vortices. Typical size is less than half a mile (and that's extreme). Hurricanes are LARGE storm systems, covering areas of 1,000 kms or more, and they rely on a heat source resident in the upper ocean.

Erin:  How did you become interested in researching severe weather?

Walt:  When I was a junior in high school I lived in Nebraska. There was LOTS of severe weather. :)

Sundar:  It’s really gr8 that NASA organizes such events as this chat.

Walt:  Thank you -- we enjoy it. Thanks for your questions.

(Moderator) Brooke:  Thanks for all the good questions so far. We're answering questions in the queue as soon as possible. Many thanks for your patience and keep the good questions coming.

Erin:  What new technologies are being worked on to help in predicting severe weather?

Walt:  There are new satellite instrument technologies. For example, geostationary lightning  mappers. There are also new rapid-scanning ground-based radar systems being developed. And of course, the ability to numerically forecast and analyze these events is highly dependent on our computational abilities, which are constantly improving.

Rambler: That's interesting about infrared wavelength devices. That's therefore an alternate method to the more common chemical seeding of hail/storm clouds?

Walt:  Actually, when we talk about infrared devices we're talking about seeing how tall or intense a storm is, as opposed to trying to modify one.

Erin:  How do you know the altitude that you are measuring with a specific infrared frequency?

Walt:  For example, we know that infrared radiation coming out of the top of the Earth's atmosphere at a wavelength of 11 microns originates at the Earth's surface or from the top of any object in the way of the Earth's surface, like a cloud. So, by measuring the amount of radiance detected at that frequency, physics tells us that we can infer the temperature of that entity, and the temperature is a good measure of the altitude.

WxDan:  Besides the technologies used in the Huntsville storm, what else is NASA using to advance the study of severe weather? I hear GOES-R mentioned a lot as the "next big thing." Are there other new technologies being developed that maybe aren't getting as much press?

Walt:  Other technologies include space born weather radar and passive microwave remote sensing. For example, NASA has a current mission called the Tropical Rainfall Measurement Mission, data from which is often used for hurricane “nowcasting” and prediction. The Precipitation Science in Space- NASA TRMM Web site has a lot of information: The same thing can be said for the future NASA Global Precipitation Measurement Mission. Check out the Precipitation Science in Space- NASA GPM Homepage:

Sundar:  Just one technical question: as I understand it’s possible to get early alarm of upcoming storms / hurricanes -- is it possible to manipulate the severity  so that they don’t peak  in populated areas, possibly get it peaked in oceans ?

Walt:  That's still a topic of discussion and also some scientific controversy.

Haulward:  Initially I want to thank NASA for providing all of the scientific materials and knowledge, here and other places every time.

Walt:  Thank you very much!

Haulward:  How do chemical elements and aerosols effect the intensity of severe weather?

Walt:  There's a theory that suggests that the more aerosols you have, the more intense a thunderstorm can become because the aerosols affect the precipitation process.

Erin:  In what types of results do computer models appear to fail the most in attempting to predict weather?

Walt:  Summertime precipitation.

(Moderator) Brooke:  To submit a question, please type it out at the bottom in the yellow bar area at the bottom of the chat window and submit it by clicking on the 'Ask' button. We'll answer the question as soon as possible. Thanks for the good questions so far. Keep them coming!

Walt:  Before I forget, an earlier visitor mentioned GOES-R. This is a great link for information on that:

Eric_EV94:  Has anyone considered the effect on air masses that form tornados may intensify over large bodies of water like the lakes in the Tennessee River?

Walt:  Yes. Ultimately warm, water vapor-laden air is typically important to creating severe storms. So, if you have large, warm water bodies adjacent to or embedded within land masses, then you have a situation that is helpful to formation of storms in certain situations. In fact, large rivers like the Tennessee or the Amazon often create their own breezes that enhance the likelihood of cloud and storm intensification.

tina86:  So how bad was that [Huntsville] tornado?

Walt:  The Huntsville tornado was an EF-2 with winds at approximately 100/mph.

Gvillamil:  Why do you think that the Huntsville storm system didn't produce as much lightning as other events?

Walt:  The storm was shallower and typically storms that produce lots of lightning are much deeper. And, the fact that this storm didn't produce as much lightning is actually a current topic of research within our group.

Hawkofva:  If you're able to infer the size and shape of particles, is there any research being done to classify non-weather echoes such as birds?

Walt:  Absolutely. As an example, ornithologists can use polar metric radar to study bird migrations and other scientists have used the technology to examine insect types. We typically use the information to eliminate these pesky biological flyers from our data. :)

Erin:  How much do you use sunspot cycles in analyzing general weather patterns?

Walt:  That's an active area of research in the space physics community, and it’s also a big question for climate variability.

Eric_EV94:  Do we historically have more tornados in the Tennessee Valley during El Nino or La Nina years?

Walt:  Excellent question. Typically in El Nino years, the Southeastern U.S. experiences cooler and wetter weather than usual. However, I don't know if there is as strong a correlation between severe weather and El Nino for our area.

Erin:  How often are low-lightning but powerful storms produced?

Walt:  Quite frequently. For example, individual storms in hurricanes over the ocean don't typically produce much if any lightning. Occasionally we also see storms in land-falling hurricanes that don't produce much lightning, but do produce weaker tornadoes.

Sundar:  Is there any way that we can generate electricity and energy by these storms. if yes has it been done?

Walt:  That question has been examined over the last 200 years. :) The problem is one of being in the right place at the right time with a battery that can actually hold the massive amounts of charge and energy transferred in a lighting flash. Not currently feasible!

Kdschult:  Has anyone looked into the frequency of quickly forming tornadoes around Madkin and Weeden Mountains on Redstone [Arsenal] and the microclimate they might form? Over the years there seems to be more here than other places. I am thinking of the 1974 that hit Parkway City Mall, the Airport Road [tornado], etc.? Just curious if the land formation seems to funnel ground in some way that might be significant. Does ARMOR help with air patterns like that?

Walt:  The formation of tornadoes due to interactions with hilly terrain like Weeden and Madkin Mountains is actually a current topic of research in the UAH/Marshall collaborative severe weather group.

Gvillamil:  To follow my last question...I did a research where I found that temperatures at 700mb needed to be around 7 to 11 degrees C to produce considerable lightning...Do you think that would have been a factor in why there was fewer lightning?

Walt:  Lightning production in clouds is sensitive to the temperature at which the large ice particles exist at. The deeper the large ice particles penetrate into the storm, the more lightning you typically get. We often look to see how deep a radar echo extends past the minus 10 degrees C level to estimate how much lightning a storm will produce.

Eric_EV94:  Do you measure the radar signature to eliminate birds from Doppler r like military radars?

Walt:  Yes, once we identify an echo as being a bird or flock of birds, those echoes can be removed from the data.

WxDan:  Are you doing any research into the effects on dual-pole radar returns by alternative energy sources like wind farms?

Walt:  In the Midwest, wind farms typically act as a block to weather radars. Right now, those issues are being worked out.

Rambler:  NASA issues weather data directly to various country's National Weather offices, and only the respective national weather office determines if warnings should be issued accordingly. Is my understanding correct?

Walt:  Yes, NASA provides data and even training for use of NASA products. The National Weather Service is tasked with warning the public.

Eric_EV94:  Were there any reported micro-bursts from the tornado on Jan 21st?

Walt:  Not that I'm aware of.

Haulward:  Why does stormy weather or some showers effect satellite communications?

Walt:  Satellites transmit their information at microwave frequencies. Sometimes if the path between the satellite and receiver is blocked by a very intense storm, the transmission is attenuated.

Erin:  When radar that could discern particle shape was first being used, was it considered that it would be picking up bugs, or was that realized later?

Walt:  It was probably realized shortly thereafter.

Erin:  Is identifying and removing critter radar echoes done manually, by a computer program, or some combination?

Walt:  We can do it both ways.

(Moderator) Brooke:  Thanks for all the good questions so far. We're answering questions in the queue as soon as possible. Many thanks for your patience and keep the good questions coming.

Walt:  As a general point, these are also some great lightning safety links: ,

Hawkofva:  In addition to the tornado, Huntsville also recently experienced some wintry weather as well. Was there any opportunity to observe a thundersnow event with ARMOR and the lightning detection systems?

Walt:  We haven't had a thundersnow event...yet. However, ARMOR is extremely useful for showing us where it is snowing vs. where it's raining.

(Moderator) Brooke:  We've got time for a couple more questions before the end of our chat at 4 p.m. EST. Submit them below using the yellow bar to type them out and then click on the 'Ask' button to send them in.

Walt:  Also, as more general information…if you want to see where lightning is in real-time from our sensor networks, here are some links:
The NASA MSFC Lightning Mapping Array data/images for North Alabama is a good resource:

NASA MSFC Lightning Mapping Array data/images for Washington D.C.:

Another great link is NASA's MSFC Lightning Mapping Array data/images for Cape Canaveral:

Erin:  Why do computer predictions of weather have difficulty with summertime precipitation?

Walt:  The initial conditions used in the models are typically not well-resolved.

Rambler:  Hypothetically, is it chemically possible to increase the potential effects of a hail or tornado storm cloud?

Walt:  There are some theories that suggest this should be possible, but it's difficult or even impossible to prove.

Eric_EV94:  Do you have any recommendations for personal weather stations <$1K?

Walt:  Yes, Google "weather stations" for lots of options.

Erin:  Thank you for being here and answering all of our questions! =D

Walt:  My pleasure. I appreciate the questions.

Tina86:  Have you been my a tornado before?

Walt:  I've observed them before in the Great Plains and in Colorado. And, one time I even observed one at sea.

(Moderator) Brooke:  Thanks to all of you for all the great questions, and thanks to our guest scientist, Walt Petersen! Check back in the next day or so for a posted transcript of today’s chat.

(Moderator) Brooke:  Learn more about some of the technologies discussed today at:


Kim Newton, 256-544-0034
Marshall Space Flight Center, Huntsville, Ala.