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Question: There are talks about big storms in the 1930s as we are talking about hurricane Irma. What tools did weather forecasters have to know storms are approaching? — Rebecca Stahlbusch
Answer: In the later 1800s, weather stations had been established at a number of Caribbean island locations to monitor weather conditions for signs of tropical cyclones, and during the early 1900s the advancement of radio technology added substantially to this ability to detect storms and provide useful measurements, both by allowing for observations to be transmitted by ships at sea in the vicinity of storms, and also by allowing for the development of the radiosonde by the 1930s, which allowed for measuring weather variables not just at the surface, but over a deep profile of the atmosphere at several of those locations. The idea of also watching for storms and tracking their movement by way of aircraft reconnaissance flights came up and gained support in the 1930s, although it was a bit into the 1940s before this method became truly established.
Sep. 20, 2017 | Tags: general meteorology, hurricanes, instruments
Question: What was that low rumbling thunder type sound that lasted a long time - maybe 5 mins - near the beginning of the storm on Friday, September 1st? — Carol Witt
Answer: The storms that day were fairly numerous, some were arranged in long bands, and several produced a large amount of frequent lightning. In that kind of a situation, one cause for the long-duration rumbling you noticed is the combination of thunder arriving from multiple lightning discharges at differing distances from your location. When there is very frequent lightning, some cloud to ground and a good bit of in-cloud and cloud-to-cloud as well, the sound of thunder can become almost continuous. The can be enhanced even more if some of the strikes are along a lengthy lightning channel that happens to line up so that one end is a good deal closer to you than another end, which can lead to a long period of thunder from one strike, since the sound from the more distance end takes longer to reach your location. Finally, some of the storms that day produced outflows that created a temperature inversion that can sometimes bend sound waves downward in a way that allows them to travel farther than they usually would. This could have added to the ability for multiple rumbles of thunder, some from closer storms and some more distant, to blend together over a period of time in the way you observed.
Sep. 19, 2017 | Tags: lightning, thunderstorms
Question: I took pictures of an upside down rainbow on the afternoon of the eclipse last week while fishing at Cedar Island. I had never seen one. Is there an association with the eclipse? — Dick Fritz
Answer: We suspect that you saw either a partial 22-degree halo (if you saw mainly the lower portion of the halo it would look like an "upside down rainbow"), or perhaps, if the sun was fairly low in the sky at the time, something called the "circumzenithal arc," which has commonly been referred to as an upside down rainbow. These are both associated with high, fairly thin cirrus clouds composed of the right type and of ice crystals, which existed in many ares along the southeast coast on the day of the eclipse. That was just a coincidence, however, that was not related to the astronomical event itself. You can read more about the halo and arc, including diagrams, explanations and galleries of example photos, at www.atoptics.co.uk/halo/common.htm.
Sep. 18, 2017 | Tags: astronomy, atmospheric optics, clouds
Question: How is it possible that upper clouds can go in a different direction than lower clouds? — Pamela O'Connor
Answer: One of the complexities of the atmosphere is the fact that horizontal variations in temperature (for example, across a front, across land/water boundaries, from cloudy areas to sunny locations, and more) lead to variations with height in the gradient and direction of differences in pressure. Since the wind blows in a direction more or less parallel to lines of equal pressure and at speeds that depend on how large the change in pressure is over a given horizontal distance, the changes with height in the direction of and intensity of pressure variations leads to corresponding changes with height in wind direction and speed in many cases. When these common variations occur at the same time that multiple layers of clouds are visible, the clouds at each layer will move along with the wind at that altitude, making the differences in wind evident by the differing motions of the clouds. This kind of vertical shear is quite common, although there can also be situations where over a limited area the wind direction and speed doesn't vary a lot with height, and during those periods there can be clouds at multiple levels that move at roughly the same speed and direction.
Sep. 17, 2017 | Tags: clouds, general meteorology, winds
Question: What was the actual path of Hurricane Harvey compared to its daily forecast track along the way? Do you have a graph for this? — John Streit
Answer: We don't have that on our web site, but there is something you can approximate it with on the National Hurricane Center web site. If you open www.nhc.noaa.gov/archive/2017/HARVEY_graphics.php?product=current_wind in a window and stop it toward the end of the lapse, the track of the storm will be left behind on the map as a dashed black line. You can put this window on one side of your screen. Then open a new window beside the first one and go to www.nhc.noaa.gov/archive/2017/HARVEY_graphics.php?product=5day_cone_with_line_and_wind. On that page, you will see an animation of all the 5-day forecast paths that you can visually compare to the actual center track. Note that with Harvey, there was a period where the storm dissipated as a closed low, so the NHC did not issue advisories and forecast tracks for a time, until the storm regenerated over the Gulf of Mexico. The dashed line track in the first window therefore connects the two separate storm center tracks with a long straight line.
Sep. 16, 2017 | Tags: cool sites, hurricanes, past weather
Question: A recent seven day forecast showed a predicted high of 84. The hourly breakdown for the same day never exceeded 73. Where does this discrepancy come from? Which should we give the most attention to? — Rich
Answer: In order to allow for our forecast page to be set to cover any input zip code or city, the hourly forecast pulls data from a gridded National Weather Service database that covers the entire country. The same database also populates 7-day forecasts outside of the local region surrounding the Triangle. However, the 7-day forecast you see on our main weather page for the Triangle, along with the text description of that forecast, is input directly by those of us in the WRAL WeatherCenter. In most cases, the two forecasts are reasonably close to one another, but in rare cases, usually involving complex weather patterns with nearby fronts or fast-moving disturbances, there will be a larger discrepancy in the forecast.In the particular instance you asked about, we forecast a high of 84 degrees, while noting that temperatures might run a good deal cooler just to the north and west. The gridded database reflected a forecast that the sharp gradient in temperature would have the Triangle area remain on the cool side of the boundary through the end of the day. The eventual observed high for that day at RDU was 83, while the high at Greensboro was only 68. Note that it is fair to say that the temperature in the Triangle area that day could easily have swung either way based on a relatively small shift in the location of the southeast edge of a shallow cool and cloudy wedge of air trapped between the mountains and the middle of the state.
Sep. 15, 2017 | Tags: maps & codes, wral.com
Question: A recent Friday night we witnessed something I have never seen. I have the video of a lightning display in clouds just north of our Marina. The lightning never seemed to touch the ground but went around and around in the clouds. No thunder was heard. The show went on for at least 45 minutes. If you can explain I know many people that witnessed this with us that would like to know what this was. — Mike Inman
Answer: That was a really active storm, electrically! Usually, that meas the storm formed in a very unstable environment, at least through the middle layers of the atmosphere where the temperature drops from above to below freezing. This can result in very strong updrafts that cause a lot of turbulence in that layer that results in many collisions between water droplets, supercooled water droplets and small ice crystals. These collisions lead to transfers of charge between the particles, with heavier particles concentrating in some parts of the cloud and lighter ones in others. The end result is several pockets of enhanced positive and negative charge in different parts of the cloud, and sometimes a big difference in charge inside the clouds and that outside. When the potential differences grow large enough, the insulating properties of the air in between those centers breaks down and lightning occurs to balance, or partially balance, the charges. In this case, more than in many others, the updrafts were apparently strong and continuous enough to rapidly rebuild potential differences between various parts of the cloud and surroundings, or to build new ones. It's hard to say of there was also some cloud to ground lightning with the storm, as it may have been far enough away that you couldn't see the bottom of it. In general, there are about 5-10 lightning discharges that never reach the ground for every one that does. The lightning you saw would have been generating thunder as well, but at sufficient distance (usually around 10 miles) or under the right conditions of temperature profiles through the lower atmosphere (which can bend sound waves in such a way as to prevent them from reaching some areas that are closer to the source than 10 miles), the thunder may not be audible at your location.
Sep. 14, 2017 | Tags: general meteorology, lightning, thunderstorms
Question: What is the height in the atmosphere of a typical hurricane? 5K, 10K feet, more? — Helen
Answer: They can vary some, but around 40-50,000 feet isn't unusual. The strongest winds in tropical cyclones tend to be in the lowest 8-12,000 feet, especially from a few hundred feet up to around 5,000 or so, which is a distinction from extratropical storms (frontal lows like nor'easters), in which winds frequently increase with height up to around 30,000 feet or more.
Sep. 13, 2017 | Tags: hurricanes, winds
Question: I am curious as to why the forecast models have Hurricane Irma going to the west as it makes its way north instead of East like most hurricanes in the past? — Chris Ferguson
Answer: As you note, it is common for tropical systems that have reached the western end of the semi-permanent sub-tropical high pressure ridge over the Atlantic to curve north for a ways, then northeast or east as they encounter more westerly steering flow on the north side of the ridge. However, as we drew closer to Irma's arrival at the west end of the, models picked up on a developing mid-level trough digging south/southeast from the central Plains states toward the Gulf coast. This feature was expected to create a steering flow toward the northwest once Irma reached about north Florida or southern Georgia. Of course, there is always potential for such a subtle or delicately balanced steering pattern to change by the time the storm moves a ways north.
Sep. 12, 2017 | Tags: general meteorology, hurricanes
Question: System 10L never received a name although it achieved tropical storm status. Why no name? — Rodney Harvill
Answer: You're referring to a system the National Hurricane Center designated as Potential Tropical Cyclone 10. Despite some model forecasts that the system would become a fully tropical or subtropical system, it remained under the influence of significant vertical wind shear that seemed to inhibit a well-formed low-level circulation center from developing. Instead, the system remained in the form of an elongated trough of low pressure with multiple weak and transient centers of lower pressure that never really consolidated into a single defined center until it pulled away from the coast and became an extratropical low pressure center along a cold frontal boundary. Even though the system did have some pockets of sustained wind around 40 mph, that alone doesn't meet the criteria for being designated a tropical cyclone and receiving a name. It came close, but never quite got there...
Sep. 11, 2017 | Tags: hurricanes, maps & codes, past weather
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51