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Question: I saw a never-seen-by-me formation of clouds over my yard in Fayetteville recently. I have posted them at bit.ly/1MRn0K2. Please give me the name of these clouds and what caused this formation. — James A Artis
Answer: We took a look at the photo you referenced and it appears you'd already found the most likely cloud type, which is altocumulus undulatus. Depending on altitude, a similar appearance could result from clouds called stratocumulus undulatus and we can't be certain from the photo which applies here (these appears to be too low to be cirrocumulus undulatus). The "undulatus" part of the name refers to the parallel rows of cloud which are usually formed due to vertical wind shear that creates an internal gravity wave near or on a sharp vertical density gradient (often associated with a temperature inversion). The waves in this case are not unlike those that wind blowing across the top of a water surface might create. If the air at the level of the waves has just the right amount of humidity, then clouds can form where the air travels upward and cools to the point of condensation, and then as air flows through the wave and down the other side, the air can warm and dissipate the cloud droplets, leading to the gaps between one row of clouds and the next. Note that if moisture is very extensive, there may be a solid overcast that simply has a wavy pattern to the thickness, or if moisture is insufficient, the waves may exist but go unseen due to no cloud being formed. Another name for this type of cloud pattern is "billows."
Apr. 2, 2015 | Tags: clouds, cool sites, general meteorology
Question: What exactly do you mean when you talk about "energy" in the atmosphere? For instance, with a snow storm back on 2/25/15, you talked about energy being over New Mexico that would move across Texas into the Gulf developing a low pressure system which brought all the rain and snow. — Dylan
Answer: When you hear a generic reference to energy of that sort in a weathercast, it usually is a short-hand way to reference enhanced areas of cyclonic rotation aloft (known more technically as vorticity). These areas can be due to strongly curved flow, to strongly sheared flow along a horizontal gradient in wind speeds, or, in many cases some combination of both. When these areas make forward progress (usually in the form of moving with a west-east component at our latitude range) they tend to induce diverging flow and strong upward motion on their downwind side, which can lead to the development or enhancement of low pressure near the surface, along with formation of extensive cloud cover and precipitation, assuming sufficient moisture is available.
Apr. 1, 2015 | Tags: general meteorology, maps & codes
Question: What is record high barometric pressure for RDU? — Boyd Strain
Answer: Records going back to 1948 for the Raleigh-Durham airport indicate the highest barometric pressure occurred on February 13, 1981, when the pressure topped out at 30.99 inches as high pressure built into the region behind a strong cold front. The lowest pressure on record at RDU was associated with passage of the March 1993 "Storm of the Century", when the pressure dipped to 28.58 inches.
Mar. 31, 2015 | Tags: records/extremes
Question: With the recent peaks and valleys of warm and cold days, I was wondering what is the highest temperature change in a 24 period? — David
Answer: We checked daily observations of low and high temperature going back to 1887 for the Raleigh area and found that the largest span of temperatures within a single calendar day occurred on Feb 1, 1981, with a low of 8 degrees followed by a high of 59 for a 51-degree change.
We also enlisted the aid of the Southeastern Regional Climate Center (SERCC) and their database of hourly observations from the RDU airport going back to 1944. By checking every hourly temperature against all others reported in the next 24 hours, they found the following: the largest increase in temperature within that time span also began on Feb 1, 1981, as the temperature went from 10 degrees at 5AM that day to 63 at 5AM on Feb 2nd, for a 53-degree climb.
Just last year we experienced the largest decrease in 24 hours or less, when the temperature fell from 61 at 6AM on Jan 6, 2014 to 9 degrees at the same time on the 7th, a 52-degree drop.
Those are our local records. By comparison, the greatest temperature swing within a 24-hour span that has been measured and published in the record books occurred from 14-15 January, 1972 in Loma, Montana. Within that period the temperature rose from -54 degrees Fahrenheit to +49 degrees, for a tremendous warmup of 103 degrees. Of course it is possible that larger changes have occurred somewhere, and simply went unmeasured.
Mar. 30, 2015 | Tags: past weather, records/extremes
Question: Was reading the answer related to dew point and was wondering - is it possible for the temperature to get lower than the dew point? I realize the dew point can fluctuate but when I see its value I would like to think (especially during the winter) that is the lowest the temperature can get! — Tony Amitrano
Answer: As a practical matter, the temperature can't fall below the dew point at a given moment. However, as you note simply looking at a dew point value and assuming the temperature can't fall below that level may not work out. This can occur for a few reasons. First, if drier air blows into the area, the dew point will fall and this can allow the temperature to drop beyond what the initial dew point reading was. In addition, if the temperature of the ground, cars, roofs and other objects falls to that of the dew point, that can result in dew forming on those objects, which removes water vapor from the air near the ground, causing the dew point to fall. Again, once this happens then the air temperature can become colder than the earlier dew point value would have suggested. A similar process can occur with moist air flowing over colder ground or a cold body of water. The air may be forced by the cold surface underneath to become colder than the dew point was initially. However, as it cools, water in the air will condense into liquid droplets (fog or low clouds) so that the dew point decreases and remains equal to or lower than the temperature.
Mar. 29, 2015 | Tags: general meteorology, humidity/dew point
Question: I was wondering can it snow in 40 degree weather. I have seen it snow in NC before in 40-45 degree before and everyone thinks I am making it up. They say it impossible, it has to be 32 degrees. Is this true or can it snow in the 40s? — Lea Johnson
Answer: In this case, "they" are quite wrong. For snow to form, the temperature at the altitude where the snow crystals grow does have to be below 32 degrees, and in fact to get a decent snow usually requires there be a moist layer that is in the range of about -4 to 14 degrees F. However, those crystals, and the snowflakes that are often composed of multiple crystals stuck together, can sometimes survive falling through a shallow layer of air that is above freezing near the surface. In that way, at ground level it can indeed snow at temperatures above 32 degrees, and on rare occasion it can do so with surface readings well into the 40s. This usually requires the lowest levels of the atmosphere to have a strong "lapse rate," meaning that the temperature drops unusually rapidly with increasing altitude. While snow with surface air temperatures in the 40s is fairly uncommon, it isn't unusual at all for snow to fall with temperatures in the mid 30s, several degrees above freezing.
Mar. 28, 2015 | Tags: general meteorology, snow
Question: Degree-days would be a good way to quantify just how cold Feb was this year, but I never see degree days posted and I couldn't find recent data on the web. Do you have this number and how does it compare to the average and record for Raleigh? — Doug Dowling
Answer: We do have those numbers, and you can look back at degree day totals using the "get historical data" section of our "Almanac" page. This year brought one of the coldest Februaries on record, and this was reflected in the degree day totals. There were no cooling degree days for February 2015 in data from RDU, and there were 836 heating degree days (HDD, base 65). This compares to a normal of 575 degree days for the month. The greatest number of HDD for Raleigh since records have been kept beginning in 1887 was 901 in 1895, with the lowest being 346 in 1890. February 2015 ranked 5th highest out of 128 on the list of February HDD totals.
Mar. 27, 2015 | Tags: cold, normals, past weather, records/extremes
Question: Do school systems ask you for advice on whether to delay or cancel classes? — Brian
Answer: On rare occasion they do, but generally government organizations of that sort will have contact with their local National Weather Service Forecast Offices, or will gather forecast, watch and warning information (along with reports on driving conditions for primary, secondary and neighborhood roads) from several sources, and make their own determination that includes consideration of inclement weather plans they already have in place.
Mar. 26, 2015 | Tags: preparedness, winter weather
Question: During extreme weather, WRAL may pre-empt regular programing to provide live, street-level reporting of the event. Is that type of broadcast streamed to WRAL.com so that it can be seen live on computer? — Doug
Answer: They are indeed. For example, if we break into programming because of a tornado warning, the usual protocol would be for the same signal to be streamed to the web site for as long as we remain on the air. A link to the stream should be available on the home page and in the "Video" section of the web site, as well as through the WRAL News App for mobile platforms. This can have the advantage of providing a live feed of the coverage even if power is out to your home computer or if hard-wired internet service is interrupted (assuming cell-based data coverage is available and your phone or tablet is charged).
Mar. 25, 2015 | Tags: preparedness, severe weather, wral.com
Question: The device that records the "official" temperature; where is it? Is it urban/rural? Is it high/low (from the ground)? Surrounded by concrete/asphalt/nature? Suspended/attached to a structure? In shade/full sunlight? What specific environment is it in? — Bill Keene
Answer: An automated surface observing station (ASOS) unit located at the Raleigh-Durham airport currently serves as the station of record for Raleigh. The instruments are all joused or sited according to standards meant to measure atmospheric quantities in a repeatable, consistent way from site to site (for example, wind measurements are taken at a height of 10 meters above ground, temperature and dew point readings in a shaded, ventilated enclosure about 4-6 feet off the ground, all instruments located well away from buildings and wooded areas and preferably in an open, nominally grass-covered area, etc). Details about the station location, history, elevation, etc, along with a series of photos intended to illustrate its surroundings, can be found at www.ncdc.noaa.gov/homr/#ncdcstnid=20013893&tab=LOCATIONS. Note the tabs above the map that link to various pieces of information, and also the camera symbol a ways below the map that accesses the photo series.
Mar. 24, 2015 | Tags: cool sites, instruments, maps & codes
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51
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