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Answer: Climate records from the Raleigh-Durham airport do not show any snowfall in October, with the earliest trace of snow there reported on November 2nd, 1954, while the earliest measurable snow there was on November 6th, 1953, when .6" was recorded. A different station in Raleigh with a much longer period of record did record a trace of snow as early as October 24th, back in 1910.
Nov. 7, 2009 | Tags: past weather, records/extremes, snow

Answer: Since fog is composed of tiny droplets, or in the case of ice fog tiny frozen crystals, separated by significant amounts of air, it could not and does not form a solid mass in the air, even when the particles themselves are frozen. The point regarding the air being less than freezing, though, is that water needs an appropriate surface upon which to initiate freezing at 32 degrees F, and droplets suspended in air will not typically freeze at that temperature unless they encounter something called an ice nucleus. Even then, with small droplets the ice nuclei are usually most effective at causing the droplets to solidify at temperatures below about 14 degrees F.
Nov. 6, 2009 | Tags: clouds, cold, visibility/fog/dust

Answer: Records for those locations are posted at the Southeast Regional Climate Center Web site, available at www.sercc.com/climateinfo/historical/historical_nc.html. They show that for the Rocky Mount Experimental Station, with records stretching from 1914-2009, the highest reading was 106 degrees F on June 27, 1954 and the coldest was -8 degrees on January 21, 1985. The station at Scotland neck maintained records from 1872-1995. During that period, it was as hot as 105 on June 21, 1933 and as cold as -4 on February 1st, 1936.
Nov. 5, 2009 | Tags: cool sites, past weather, records/extremes

Answer: There are a variety of ways to specify probability forecasts, but what is typically meant with a "40% chance of rain," for example, is that any given place within the forecast area to which that forecast applies, and within the period for which that forecast is valid, should receive measurable rain (defined as one hundredth of an inch or more) about 4 times out of every ten that the 40% forecast is used. That pretty well equates to your second question. Usually, the probability does not explicitly address what percentage of time it will rain during the forecast period, or what percentage of the area will be covered, although those considerations may be implicit with some types of weather systems. It can be important to note what time periods, areas and precipitation amounts are being referred to. A typical NWS forecast includes a probability that at least one-hundredth of an inch will occur in a twelve-hour period generally labeled as "Today," "Tonight," "Wednesday," Wednesday night" and so on. On the other hand, we sometimes show a product on the air that uses a contour map of probabilities that measurable rain will fall within the three hours leading up to the time shown, and we sometimes show the same map but for the chance that rainfall will exceed one-tenth of an inch during the three-hour period. One would expect that for a given weather pattern, the chance of measurable rain would be lower for a three-hour period than for a twelve-hour span, and likewise, the chance of measurable rain would likely be higher than the chance of one-tenth of an inch or more over the same period.
Nov. 4, 2009 | Tags: general meteorology, maps & codes, rain

Answer: Climate data from the Raleigh-Durham airport seems to indicate your observations are right on target, especially for October. In September, the reported sunrise-to-sunset sky cover averaged 60%, very close to the "normal" value of 59%. However, while the normal number of cloudy days for the month, 11.4, matches the 11 that were observed, when you look at the number of "partly cloudy" days (16, versus a normal of 9.1) and "fair" days (3 compared to a normal of 9.5) you can see that cloud cover appeared to be more prevalent than average. In October, average sky cover was 70% compared to a normal of 49%, and there were 17 cloudy days (normal 11.1), 9 partly cloudy days (normal 7.1) and 5 fair days (well below the normal 12.8). Note that here "fair" indicates 2/10 or less of the sky obscured by opaque clouds, "cloudy" indicates 8/10 or more and "partly cloudy" covers the rest.
Nov. 3, 2009 | Tags: clouds, past weather

Answer: Climate statistics for the Raleigh-Durham airport indicate an average of about 73 days each year with a low temperature at or below freezing, while there are about 4 days per year with a high temperature at or below freezing. As a very simplistic first approximation, this would suggest about 69 freeze-thaw cycles. Of course, whether a specific location or material freezes solidly or thaws completely can be impacted by its location, exposure to open sky or lack thereof, its exposure to wind that might reduce the time required for the item to rise or fall to a changed ambient temperature, and the amount of time on a given day that the air temperature spends significantly above or below freezing.
Nov. 2, 2009 | Tags: cold, normals, records/extremes

Answer: While not exactly the same in long-term averages, the two cities' weather with respect to the type of events you asked about are close enough that other considerations would probably be more important to you, thanks to something of a balance between Raleigh's more northerly latitude and Charlotte's more westerly longitude. In terms of winter weather, both cities average about 4-5 "snowfall events" and about 4-6 "sleet and freezing rain events" per year, with some overlap between those two sets of numbers due to the tendency for winter storms here to produce multiple and rather variable precipitation types. Raleigh averages a little more snow overall, at 7.6" annually versus 5.8" for Charlotte. Thunderstorm days for the region run about 40-50 per year, with Charlotte averaging about 2-3 more days than Raleigh. Most of this information is summarized in various portions of the State Climate Office web site at www.nc-climate.ncsu.edu.
Nov. 1, 2009 | Tags: cool sites, normals, snow, thunderstorms

Answer: The jet stream can indeed be an important feature in marking the the evolution of upper level pressure patterns and can act as a steering feature for lower-level air masses and pressure systems, in addition to channeling smaller "jet streaks" that produce vertical motions that can cause clouds and precipitation to form or dissipate. The intensity, location and organization of the jet stream itself is influenced by the distribution of temperature, humidity and density of air at and below jet stream altitudes, which are in turn influenced by the temperature of land and water surfaces. The distribution and gradients of these temperatures, humidities and densities can also be affected by short-term, small to medium scale heating and cooling associated with condensation and evaporation of water in clouds and precipitation, and to large scale, slow-changing factors like the El Nino-Southern Oscillation. As you can see, it all sets up an interconnected, chicken and egg-like scenario (referred to in science as a "non-linear" system) in which the jet stream influences the movement and behavior of air masses and traveling storm systems, and is in turn itself influenced by that movement and behavior.
Oct. 31, 2009 | Tags: fronts & airmasses, general meteorology

Answer: That all depends on what you mean by "just 'the humidity'." In a general sense, humidity is any measure of the amount of water vapor present in a given space. For example, there are quantities called "specific humidity" (a ratio of mass of water vapor to mass of moist air in a given volume), "absolute humidity" (mass of water vapor per unit volume, in other words a concentration value or the density of water vapor), and as you noted, "relative humidity." Relative humidity (RH) is a ratio of the actual amount of water vapor in a given volume of air to the amount of water vapor that would be required to saturate the air. The saturation value depends on the temperature, as it takes a greater amount of water vapor to reach saturation at higher temperatures and vice versa. When saturation is reached, addition of further water vapor, or any cooling of the air, will result in condensation of some of the water vapor into liquid water. So, an RH of 70%, for example, means that for the given temperature, the air contains 70% of the water vapor that would be required to saturate it. Adding water vapor at the same temperature would raise the RH and vice versa, while increasing the temperature while holding the amount of water vapor constant would lower the RH and vice versa. In many weather reports, if you just hear the word "humidity," it is RH that is being discussed, but there may be exceptions.
Oct. 30, 2009 | Tags: humidity/dew point

Answer: According to a climatology of severe weather for central NC produced by our local NWS office, the Raleigh county warning area (covering 31 counties) averages about 5 tornadoes per year. Comparing tornado rates for different parts of the country requires balancing the area involved in the estimates, and a few different methods have been applied in various studies and technical reports. For example, a method estimating the number of tornadoes occurring within 25 nautical miles of any given point shows a value of .4-.8 per year for central NC versus about 1.0-1.4 per year over central OK in the heart of "tornado alley." A similar statistic for F2 and stronger tornadoes shows about 5-15 per century for our state versus about 35-40 there. Another estimate of the fraction of land surface disturbed by tornadoes each year in NC is about 2.1 x 10^-4, compared to around 4.4 to 4.8 x 10^-4 in the most active regions of the country. Finally, an analysis of tornadoes from 1950-2003 that counted tornado events within 2-degree latitude by 2-deg longitude boxes across the U.S. showed 303 in a box including our area, while boxes in the central plains states ranged from around 700 to as high as 942. Taken all together, then, we see that the "tornado alley" areas have about 2-3 times as many tornadoes over a given area as we do overall, but if you exclude the weak F0 and F1 tornadoes and look in particular at the more powerful F2 intensity and higher storms, those are about 4-5 times more likely in states like Oklahoma than they are around here.
Oct. 29, 2009 | Tags: normals, severe weather, tornadoes