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Question: What does the term "unstable air mass" mean? — Bill
Answer: The atmosphere is unstable if vertical displacement of a parcel of air results in that parcel accelerating in the direction it was moved. This is especially important in regards to convective systems like thunderstorms, in which an unstable environment means that air forced upward by a front, a trough, differential heating, topography or some other mechanism may accelerate upward, leading to the strong updrafts that produce cumulonimbus clouds and their associated heavy rain, lightning, hail and gusty winds.
Air becomes unstable when the rate of temperature decrease with height in surrounding air is greater than the rate at which a lifted parcel with cool when it is lifted or warm when it is forced downward. When this is the case, the lifted parcel will be warmer than it's surroundings, and therefore buoyant, while the opposite would happen when forced downward.
Stable air, on the other hand, tends to resist vertical displacement because lifting a parcel leaves it cooler than surrounding air (and therefore negatively buoyant since it will be denser) and forcing it downward leaves it warmer than its surroundings. When displaced, therefore, a parcel in stable air returns to its original altitude in the absence of other forces.
Dec. 30, 2013 | Tags: general meteorology
Question: What is the freezing point? — John
Answer: The freezing point for any material is the temperature at which, when cooled, it changes phase from a liquid to a solid state. The freezing point we're all most familiar with is that of water, which under typical conditions of atmospheric pressure and day-to-day experience freezes at about 32 degrees Fahrenheit, or 0 degrees Celsius. Of course, this freezing point is subject to change if the water contains dissolved impurities, and is also subject to change if the water is in the form of small droplets or is under atypical amounts of pressure.
Dec. 29, 2013 | Tags: cold, general meteorology
Question: When you show the temperature and below it is "feels like" -- what does that mean? — Anonymous
Answer: Any reference to "feels like" that shows a number different than the temperature is generally an "apparent temperature," that is one that takes into account some added factor that can make a person perceive that it is warmer or colder than it really is.
The most common apparent temperatures that you'll run across are the heat index (in which hot, humid conditions can lead to apparent temperature values warmer than the actual reading) and the wind chill (in which moderate to strong winds during cold weather lead to an apparent temperature lower than the actual reading). There are some other apparent temperature formulas out there that also attempt to factor in cloud cover, humidity, sun angle and so on.
Dec. 28, 2013 | Tags: apparent temperature
Question: Has there ever been a White Christmas in state of North Carolina? Not just flurries but a full-fledged snow event. — Walter Rudolph
Answer: In a statewide database of winter storms that reaches back to 1959, most of the events that occurred in the 24-26 December time frame were mixes of snow, sleet and ice with most of the snow confined to the mountains. However, a more widespread snow occurred on 25-26 December in 2010, with roughly the western half of the state seeing measurable snow on Christmas Day (in the range of 4-12 inches for about the western third of the state) and much of central and eastern NC adding between 5 and 13 inches on 26 Dec, with the highest amounts in the general vicinity of Rocky Mount, Wilson and Tarboro. You can check out the winter storm summaries for the dates surrounding Christmas at www.nc-climate.ncsu.edu/white-christmas.php, and there is also a link there to the broader winter storm database for anytime of the year.
Historical records for some individual sites also show .4" of combined snow and sleet in Raleigh on Christmas Day 1947, and .5" in southwest Raleigh (but nothing at RDU) on Christmas Day in 1966.
Dec. 27, 2013 | Tags: past weather, snow
Question: We had a record high yesterday (12/21) and we are supposed to have another today. I heard someone say that this is the first time we have had a record high (in Raleigh) for all of 2013! Is this true? If so, when is the last time we broke a record here? — Bob
Answer: You wrote the morning after we hit 74 degrees to set a new Raleigh record for Dec 21st, and we did indeed go on to reach 78 for a new record on Dec 22nd as well. You heard correctly that, while a few other types of records were set during the year, this was the first record high temperature for Raleigh in 2013.
The most recent previous record high here was set on July 8, 2012 with a high of 105 degrees. Also, that was the end of a really hot spell that brought us seven new record highs in a span of 10 days during late June and early July!
Dec. 26, 2013 | Tags: heat, past weather, records/extremes
Question: Old saying says fogs in August tell how many snows for winter. How many fogs did we have? — Pam Boggs
Answer: It depends on how you define fog. If you include any fog or mist that even modestly reduces visibility, then there were 20 times in August 2013 when that was the case at the RDU airport. If you restrict it to rather thick fogs in which the visibility is held to a quarter-mile or less, then the answer is two. You also have to ask what the old saying means by "snows." Does it include brief flurries that produce no accumulation, or only snows that result in a thick white coating? There isn't really a plausible causal connection between the number of fog days in August and snow days the following winter, but it wouldn't be surprising to find that for some locations those numbers happen to be similar on average. It's unlikely, though, that a statistical correlation would show above normal August fog days are consistently followed by more snow days than usual, or vice versa.
Dec. 25, 2013 | Tags: folklore, snow, visibility/fog/dust
Question: What are the chances of Roxboro getting snow for Christmas? I like snow probably more than you do. I wish for snow every day of the winter. — Betsy
Answer: In a historic sense, the Roxboro area has about the highest chance in our viewing area, but even then when it comes to snow actually falling on Christmas Day, the odds are quite low. Measurable snow has been recorded in 3 years out of 86 available, with three additional years indicating trace amounts of snow. This gives a long-term chance of snow falling on Christmas of about 7% (half that if you restrict it to measurable amounts).
The State Climate Office has a "White Christmas" probability estimator that is a little more relaxed in terms of timing, as it counts snow that either falls or is on the ground anytime in the December 24-26th time frame, but only counts measurable amounts of snow. Using that criteria, the overall chances at Roxboro are about 5.2%. You can use this tool to check the corresponding probabilities at many stations across our state, using the drop-down selector at www.nc-climate.ncsu.edu/white-christmas.php.
Finally, in terms of the chances for this year, they appear very slender indeed, as it looks as if high pressure over the area will leave us cold, but bright and dry for Christmas Day.
Dec. 24, 2013 | Tags: cool sites, normals, past weather, snow
Question: I've heard that if the temperature is below freezing and there's wind that the windshield will not frost over. Is this true? — Donna
Answer: In a strict sense this is not true, but it is the case that in many instances wind can make the formation of frost on a windshield less likely. It can do this in a few different ways.
First, say the temperature of the air and of the windshield are both below freezing in calm conditions, but there is a temperature inversion such that air warmer than freezing exists a few tens or hundreds of feet up. A period of winds can mix the air so that the warmer air comes to the surface and leaves the temperature at the level of the windshield above freezing. This will prevent frost from occurring.
In another scenario, suppose the air temperature is above freezing but the windshield and other upward-facing parts of the car cool below freezing due to radiative heat loss. If the windshield manages to cool to the level of the dew point, then frost can form on it in calm conditions in spite of the surrounding air being a few degrees warmer.. However, in this scenario wind could keep a fresh supply of warmer air blowing against the windshield, offsetting radiative cooling and preventing the windshield from falling below freezing and thus preventing frost.
Finally, consider a case where the air and the windshield air are both below freezing and there is no temperature inversion. In this case the wind can blow and the windshield will remain freezing no matter what. Then, whether frost will form depends not only on these temperatures, but also on the dew point (how much moisture is in the air). If the dew point is well below the temperature, there could be times when calm conditions will allow the windshield and other objects to get much colder than the air - if they reach a temperature that equals the dew point or less frost can form. However, wind will tend to keep the windshield closer to the actual air temperature and may prevent frost if the dew point is quite a bit lower. On the other hand, if the dew point is very close to the temperature, frost may form on the windshield in spite of the wind.
This doesn't cover all the possible combinations by any means (for example, the dew point could be high close to the ground but lower aloft, and wind could mix drier air to the surface, and the opposite could happen as well), but overall in many cases wind will make frost less likely, while in a few cases frost can occur even with the wind blowing.
Dec. 23, 2013 | Tags: dew/frost, general meteorology
Question: As a former resident of Buffalo, NY, transplanted to Roanoke Rapids, I am wondering what the snowfall amount would be here since it has been raining since Saturday. Don't miss the lake effect snow one bit!! — Jolene Dendler
Answer: You wrote in on the 10th of December about significant rain that fell in the preceding days. At the Roanoke Rapids airport, the total through that time was nine-tenths of an inch. Rain episodes and snow episodes don't necessarily correspond in terms of how much precipitation they can produce, but if we were to make the assumption that the same amount of water fell as snow, a rough translation for our area would be about 9 inches, based on an average ratio around here of roughly 10 inches of snow per inch of liquid water equivalent. Note that this is a highly variable ratio, and depending on the particulars of vertical temperature and moisture profiles in a given event, can range from as low as about 3:1 for wet snow and mixed precipitation up to as high as around 20:1 in especially cold air.
Dec. 22, 2013 | Tags: general meteorology, snow
Question: I understand how clouds moving in at night can stop the temp drop, but how can they make the temp rise? (where does this energy come from?) — Bill Current
Answer: Some background helps to set up the explanation here. All objects having any temperature beyond absolute zero emit infrared radiation at an intensity that is proportional to the temperature. The air temperature falls on a typical night because once the sun sets incoming shortwave infrared radiation is no longer heating the ground, which continues to lose energy by emitting longwave infrared radiation (it does this during the day as well, but that loss is more than offset by the energy from the sun). The cooling ground and other surfaces in turn cool the air near the ground through a combination of conduction and convection.
On a clear, calm night, the surface cools rapidly and efficiently because the outgoing radiant energy is largely unopposed by incoming energy from above. If the night begins with clear skies and the ground cools substantially, and then a layer of clouds moves in overhead, the situation changes because the clouds will emit longwave radiation as well, some going upward from the top of the cloud, some going downward from the base. The intensity of the downward radiation is dependent on the height, thickness and temperature of the cloud. If those factors combine in such a way that the downward radiation from the cloud is of greater intensity than the upward radiation from the ground, then the surface temperature will begin to climb. This is most likely when the clouds are fairly low or when the clouds exist in the presence of a temperature inversion that leaves the clouds warmer than the ground is. There may also be cases in which the clouds only partially offset the radiant heat loss from the ground so that temperatures fall more slowly than they would otherwise, but do not necessarily rise.
A number of other complexities can affect the outcome on any given night. Even if the clouds do not entirely balance the heat loss from the surface themselves, for example, they might get it close enough that heat stored deeper in the soil is able to well up to the surface and cause the temperature to rise. Heat is transferred very slowly through the soil so that on clear nights it would not be able to keep up with radiant cooling, but with some cloud cover may be enough to help temperatures creep upward. In addition, the can be some nights when frontal systems lead to warmer air blowing into the region from elsewhere and causing temperatures to rise. At times, this process can bring an increase in temperatures regardless of cloud conditions.
Dec. 21, 2013 | Tags: clouds, general meteorology
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Published: 2007-10-09 14:40:00
Updated: 2013-08-13 13:37:27
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