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Weather Questions tagged “winds”(remove tag filter)
Question: Has Raleigh or Wake County ever experienced blizzard conditions? If so, when and what storm? — Chris F
Answer: "Blizzard" has a fairly specific definition in meteorological circles, with a couple of fuzzy aspects, and to check on an occurrence you really need hourly observations, a database of which has been available since the 1940s from RDU. The Southeast Regional Climate Center ran a set of queries on that database to see if we'd met those criteria, and found the answer was no, although there were three storms that at least came within shouting distance.
A blizzard means that winds are either sustained at or frequently gusting to 35 mph or higher, and that visibility is frequently below one-quarter mile in snow or blowing snow, and that those conditions exist for a period of three hours or more. Three storms that came fairly close in terms of lots of wind and poor visibility in snow and blowing snow (but with winds that never became quite strong enough to qualify as a blizzard) occurred on 10 Feb 1973, 2 Mar 1980 and 25 Jan 2000.
Mar. 2, 2014 | Tags: past weather, snow, winds, winter weather
Question: In your Jan 4 answer, in the first sentence of the second paragraph, it seems like the first use of "negative phases" should be "positive phases." — Mary
Answer: You are exactly right, and thanks for getting in touch to point out the typo! It's been corrected in the original posted answer, and for those who didn't catch it the first time, here is the question and answer Mary was referring to...
"Can you explain what the NAO and AO are and how they impact our winter weather?"
Those are two climate indices called the "North Atlantic Oscillation" and "Arctic Oscillation," respectively. In each case the state of the oscillation is measured based on the intensity of semi-permanent high or low pressure areas compared to their average climatalogical values. In the case of the Arctic Oscillation, whether it is positive or negative depends on whether the high pressure area aloft typically in place over the north polar region is weaker or or stronger than average. When it is weaker, we have a positive AO and vice versa. The NAO, on the other hand, depends on the intensity of both a semi-permanent low pressure area near Iceland and a semi-permanent high pressure area to its south that is typically located in the vicinity of the Azores. When both systems are stronger than average, the NAO is considered positive and when they are weaker than average, the NAO is negative.
These two indices are not entirely independent from one another and in both cases, positive phases correlate with strong jet stream winds across the northern U.S. and Canada that tend to keep the coldest arctic air moving eastward rather than spreading deeply into the United States, while their negative phases tend to have the opposite effect, allowing deeper and more sustained cold outbreaks here. The positive phases both tend to relate to warmer winter temperatures and somewhat greater amounts of precipitation (mainly rain) for our area, while the negative phases make less precipitation likely overall, but due to the colder air often involved, increase the potential for wintry weather.
You can get a better sense of all this by looking over some graphics and additional details on a nice page at the State Climate Office of NC web site. Just go to www.nc-climate.ncsu.edu/climate/patterns/NAO.html for those illustrations.
Jan. 22, 2014 | Tags: general meteorology, winds, winter weather
Question: How can you have a Severe Thunderstorm without any Thunder? — Sarah
Answer: On rare occasions, conditions are such that heavy convective showers can occur in an airmass that is only modestly unstable and with fairly shallow moisture, so that the vertical extent of the showers is insufficient to produce the depth of mixed liquid and frozen cloud particles needed to separate electrical charges enough to produce a lot of lightning. However, if very strong winds are present rather low in the atmosphere, the downdrafts associated with heavy showers may carry a lot of momentum to the surface, producing gusts that reach severe levels (58 mph or greater).
Often in these situations, whether some lightning will be generated or not is a close call, and the impacts of high wind gusts are the same as severe thunderstorms, so the National Weather Service will use severe thunderstorm warnings to alert the public about the imminent possibility of a brief period of damaging winds. In cases where winds are not directly associated with convective showers or storms, and may be more frequent or long-lasting, high wind warnings will be issued instead.
Jan. 18, 2014 | Tags: severe weather, winds
Question: Do you have any data on the coldest wind chill experienced in Raleigh? By looking at wind chill chart and specific weather viewing site for Raleigh on Weather Underground for January 20, 1994 and January 21, 1985 (record low temp of -9), it looks like we have had wind chills in the -20 to -30 degree range. — Dave Crotts
Answer: Yes, as far as we can determine those are the two days with the coldest wind chill readings at RDU. In 1994, the coldest was on January 19, when the temperature dipped to 3 degrees with a 17 mph sustained wind in place, giving a chill value of -17 degrees. On January 21, 1985 we had less wind (estimated around 10 mph at the time of the minimum temperature of -9) leading to a wind chill of -27 degrees.
Jan. 13, 2014 | Tags: past weather, records/extremes, winds
Question: What is the height of the anemometer at RDU? I assume it's the standard 10m, but can't find the station metadata to confirm. Also, daily average wind speed readings go back to 1984 - was the anemometer always at 10m? — Hunter
Answer: We also could not find confirming documentation regarding the height of the anemometer at that time. However, the standard for surface weather observations for airports has been 10 meters since long before that time, and with only a few unusual allowances, the Federal Standard for Siting Meteorological Sensors at Airports (FCM-S4-1994) requires them to be sited between 9 and 10 meters above ground level. We checked a few local climate data summaries that included wind information from RDU going back to and beyond 1984 and did not see any notes to the effect that the wind data was obtained at a non-standard height.
Nov. 22, 2013 | Tags: instruments, maps & codes, winds
Question: Why doesn't the temperature drop at night when it is windy? When I go outside and the wind is blowing it makes me cold! — Maj Kerr, Wake Forest
Answer: Air near the surface cools at night largely because it loses heat through conduction to the ground and other solid objects beneath it, which in turn lose their heat by radiating it away as electromagnetic waves at infrared wavelengths. This cooling process, in the absence of wind, causes a shallow layer of air in contact with the surface to be cooled, that layer absorbs some heat from a shallow layer farther up, and so on. However, with little or no wind the air near the surface becomes notably cooler than air a few feet or tens of feet up. This is called an inversion, and it results in a stable layer of air that doesn't mix vertically very much. Under these conditions, the cooling surfaces will continue to lower the temperature of a fairly shallow layer of air nearby and the temperature can drop quite steeply.
If instead a fairly strong wind is blowing, any air that is in contact with the surface is quickly replaced by air that is mixed down from a greater altitude by turbulent eddies caused by friction between the moving air and objects at the surface (hills, buildings, trees, etc). This brings a frequently refreshed supply of warmer air down to the surface, which reduces the effective rate of cooling that would occur if the wind was calm. In effect, when there is no wind the radiatively cooled surfaces only have to absorb heat from a shallow layer of air, but when the wind is blowing they absorb offsetting heat from a much deeper layer of air and thus stay warmer than they would otherwise.
You mentioned how the wind makes it feel colder. That is a separate effect commonly known as wind chill. In that case, the moving air is carrying away heat that is generated by your metabolism. In the absence of wind, your body heat will warm a thin layer of air near itself so that additional heat loss from the skin is reduced. However, if the wind is blowing this warmed air is quickly carried away, there is a new supply of cooler air constantly in contact with your body, and the rate of heat loss is increased, making it "feel" colder that it would with less wind. Just to be clear, this is only the case for air that has a lower temperature than your skin. Also, the moving air isn't any cooler than it would be otherwise because of the wind, it just feels that way because it continuously carries away body heat.
Nov. 20, 2013 | Tags: apparent temperature, general meteorology, winds
Question: Why do the winds at the surface continue to blow in elevated thunderstorms, no downdrafts? — Matt
Answer: An elevated thunderstorm is one with a base that is separated from the earth's surface in the sense that the unstable layer of the atmosphere within which the storm initiates begins well above the surface. This often involves storms that form in warm air that exists above a colder, more stable airmass below, usually associated with a slow-moving, gently sloped frontal boundary. As you noted, in some cases the stable air below the base of the thunderstorm can act to limit downdrafts from the storm reaching the surface, although this is not universally the case and depends on how strong the downdraft is and how stable and deep the underlying cooler airmass is. Even in cases where the downdrafts do not reach the surface, there can be winds that are driven by pressure gradients independent of the thunderstorms. In other words, if there is a moderately strong synoptic pressure gradient in place at the surface, winds will blow based on that forcing and not on the effects of the storms themselves.
Nov. 13, 2013 | Tags: thunderstorms, winds
Question: What causes the wind to die down when the sun goes down? — Belinda Phelps
Answer: During the day, heating at the surface creates pockets and eddies of rising and sinking air, and that turbulent mixing brings stronger winds aloft down near the ground. As heating decreases late in the day and into the night, the surface begins to cool, vertical mixing diminishes, and surface friction becomes more dominant, slowing the winds near the surface, which may "decouple" from those at greater altitude. This effect is most noticeable when few clouds are present so that radiative cooling of the surface proceeds most rapidly.
Oct. 20, 2013 | Tags: general meteorology, winds
Question: You ran a story on 6/25/13 about how the polar jet stream is often the root cause of unusual weather patterns. Where can I find a map of the jet stream on any given day? I haven't been able to find one on your website. — Kirk
Answer: There are lots of resources for finding observed and forecast representations of the jet stream, or streams, since they are not necessarily discrete or continuous. Examples include www.aviationweather.gov/adds/winds/, where you can select Flight Level 300 and step forward in time for a forecast, and squall.sfsu.edu/crws/jetstream.html. A search for "jet stream map" will turn up others.
Jul. 7, 2013 | Tags: cool sites, winds
Question: What are the historical data for derecho storms in the Triangle area? — Cecilia Romascanu
Answer: Derecho is a term used to describe a well-organized bowed or wavy line of thunderstorms that produces an extensive series of damaging wind gusts stretching over a distance of at least 240 miles, with most of the winds being of the straight-line variety rather than tornadic in nature. Most derechos occur between April and August, and they are considerably more frequent in a broad band from around the northeast half of Texas up into the Midwest than they are in most of our state. A study of derecho climatology that focused on "medium to high impact" events involving wind gusts mainly 75 mph and greater indicated that the chance of such a derecho in a given year ranged from a little over 50% toward the western end of the state to less than 20% along the coast, with the Triangle area at a little under 30%. This indicates recurrence intervals of about once every 2 years in the west, once every 5-6 years in the east and once in 3-4 years for the Triangle. Keep in mind that these are historical averages and not guarantees that there will be a certain amount of time that passes between one derecho event and the next.
Jun. 20, 2013 | Tags: normals, past weather, winds
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Published: 2007-10-09 14:40:00
Updated: 2013-08-13 13:37:27
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