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Weather Questions tagged “fronts & airmasses”

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Question: At any given time, are there an equal number of high pressures and low pressures on our planet? — Brady

Answer: That's a tough question in the sense that depending on how sensitive you choose to be in analyzing a map of surface pressures, you can considerably increase or decrease the numbers of any "centers" (local maxima or minima of pressure) are identified. In general, surface high pressure areas tend to be larger in size than low pressure centers, which are often smaller but with more intense gradients of changing pressure around them, so it seems reasonable to expect that there are probably more lows than highs. However, regardless of the numbers of centers, it remains the case that given a roughly finite (in a practical sense) amount of atmosphere in place around the earth, all the lows, highs and pressure gradients in between average out to single mean sea level pressure that remains constant.
Sep. 10, 2014 | Tags: fronts & airmasses, general meteorology

Question: Sometimes I see that there are high pressure systems over the Atlantic Ocean, like the Bermuda High. Since high pressure systems are most associated with a dry air mass, how are these systems able to form over the oceans? — Michael

Answer: The Bermuda high is part of a semi-permanent high pressure zone called the "subtropical ridge" that circles the globe with its axis more or less centered along 30 degrees north latitude (with a similar counterpart over the southern hemisphere). The high pressure area is somewhat more variable in parts of the world where it cross land, and more steady in nature over ocean areas, though it does tend to shift north in the summer and vice versa.

While high pressure systems are generally associated with fair skies and a lack of precipitation, those are effects of the high pressure system rather than causes for it. The subtropical ridge forms as a part of the general circulation of the planet, in particular due to heating and rising of moist air near the equator in a region called the intertropical convergence zone. Air that rises in that belt around the planet has to eventually spread out and flow horizontally, some going north and some going south. As it does so it cools, becomes more dense and sinks along the northern edge of a cross-sectional, overturning circulation called the "Hadley Cell" and the southern edge of a midlatitude overturning circulation called the "Ferrel Cell." The converging air from these circulations aloft warms and dries out due to sinking and compression, leaving much of the area under the ridge dry much of the time. For this reason, many desert areas around the globe are located beneath or near the subtropical ridges. The flow along the periphery of these ridges also plays a significant role in steering tropical cyclones equatorward of the ridges and mid-latitude low pressure systems poleward of their locations.
Aug. 22, 2014 | Tags: fronts & airmasses, general meteorology

Question: I have noticed that it is unseasonably cold this weekend. What is the cause of it? — Austin

Answer: You were writing about the weekend of August 2nd and 3rd, and probably thinking of Friday the 1st as well. That entire period featured an upper level trough over the eastern United States that brought the jet stream a good deal farther south than it usually resides this time of year. This had the dual effect of allowing relatively cool air of Canadian origin to funnel south into the area, and also kept a frontal boundary to our southeast active in producing periods of thick cloud cover and rainfall, both of which helped hold temperatures well below the seasonal normal at that time of highs around 90. On Friday and Saturday, highs held in the 70s, including a high of 70 degrees at RDU on Saturday August 2nd that was a new record for the date, breaking the old "coolest high" record of 71 set back in 1916. Our coolest high ever in August, though, was in the upper 50s!
Aug. 11, 2014 | Tags: fronts & airmasses, general meteorology, past weather, records/extremes

Question: The cold front coming through today is supposed to bring the second major serious July cool down. I notice that a few hours ahead of the cold front the dew points have already fallen from lower 70's to mid-60's. I usually thing of cold fronts having a combination of drop in dew points, wind shift, and rising barometer. How can dew point fall ahead of the front? — Dave Crotts

Answer: Your question refers to a front that moved through on Monday July 27th, and it highlights the fact that our concepts of cold front and other primary weather system features are often oversimplified and streamlined for presentation in the limited time or space that we have to work with on TV, radio and the web. Cold fronts can take on a variety of forms and organization, and encompass a range of complexity. In the case of the one you're asking about, there was a cold front aloft that preceeded the surface front by several hours. The front aloft provided most of the lift that produced some sprinkles and light showers early in the morning, and it left us with considerably drier air above the humid layer that had dominated at the surface. Mixing associated with the band of showers brought some of that drier air to the surface and reduced dew points, while the transition to cooler and even drier air lagged behind and passed through with little but a few patched of mid-level cloud cover, followed by the highs in the mid 80s and dew points in the low and mid 50s that we experienced the next day.
Aug. 7, 2014 | Tags: fronts & airmasses, general meteorology, past weather

Question: I watch the weather frequently and often see that "this frontal system is moving at 35 miles an hour. The front may be in TN at the time and will arrive in our viewing area within a few hours. How can any system move several hundred miles in a short period of time when it is only traveling 35 miles an hour? I thought it may be because of the earth's rotation but could not find anywhere to validate that hypothesis. — Steve

Answer: If a frontal system is indeed moving at 35 mph and maintains that rate of speed, then it could not arrive several hundred miles away in just a few hours, but we wonder if you may have taken something like a half-day or day as a shorter time than we really intended. At that speed for example, the front would travel over 400 miles in half a day and well over 800 in a day.

Of course, there are also situations in which fronts can slow down or accelerate significantly, which can affect arrival times at downstream locations. In addition, it is occasionally the case that fronts appear to "jump" from one location to another. This is not literally a matter of the same front speeding from one location to the next, but instead a new development of a front (called "frontogenesis") that may occur well downstream of the position of another front that is dissipating (a process called "frontolysis").
Jul. 8, 2014 | Tags: fronts & airmasses

Question: Why do we have all-day rains in the winter and not in the summer? Is it the pressure? — Mary Holt

Answer: There are exceptions to the rule, such as the passage of tropical cyclones or their remnants, but as you note it is often the case that summertime in our area is characterized by shower or thunderstorm activity that pops up and rapidly dies down or moves on, and is often scattered in nature, while some wintertime systems are more prone to produce widespread areas of lingering light or moderate rain.

The main culprit for the difference is the position of the jet stream and the way it influences the behavior of smaller low pressure systems and frontal boundaries. During the winter, those kinds of systems are often present in our area and at times can stall or move very slowly across the region, keeping skies gray and precipitation likely. During the summertime, these kinds of weather patterns tend to shift north away from our area, moving into the northern U.S. or Canada and leaving us in a broad area of warm temperatures and fairly high humidity. Without the fronts and low pressure areas nearby to act as focusing mechanisms, we are more subject to precipitation coming in the form of isolated to widely scattered convective cells or clusters that are most likely to spring up on a hit and miss basis during or a little after the warmest part of the day, often in the later afternoon and evening.

So, it isn't really the value of the pressure itself, but the way the pressure is organized, and how this changes between the colder and warmer parts of the year as the jet stream migrates southward and northward, respectively.
May. 7, 2014 | Tags: fronts & airmasses, general meteorology, rain

Question: Does every cold front bring a chance of rain? — Bryce

Answer: A cold front represents a concentrated gradient of temperature that will result in cooler air replacing warmer air as it moves across an area. Sometimes the front is accompanied by abundant moisture, an intense upper level disturbance or a surface wave of low pressure on the front that enhances the likelihood of precipitation ahead of, along or behind the front, but on some occasions the front is lacking in moisture and no other nearby features exist to help generate precipitation. In those cases, the chance of precipitation may be near zero, and you'll hear it referred to as a "dry cold front."
Feb. 3, 2014 | Tags: fronts & airmasses, rain

Question: In the Seattle area there is a very localized weather phenomenon known as the Puget Sound Convergence Zone. Are there any similar phenomenon here in North Carolina? — A. Emory

Answer: That phenomenon involves west to northwest winds flowing in a split pattern around the Olympic mountains and then reconverging just east of the mountains in such a way that air is forced upward to create clouds and at times a near-stationary narrow band of showers, storms or snow. There are a number of similar topographic influences in our state, having a variety of size scales, but most without a regionally well-known name of that sort. In southwestern NC, for example, there is a very pronounced couplet involving moist southerly winds that produce copious upslope precipitation over Transylvania county and surroundings, with a small region averaging 90 inches of rain per year, highest in the eastern U.S. - only about 30-50 miles north from there over parts of Buncombe county and the French Broad River Valley, a "rain shadow" associated with the same moist southerly flow results in an area averaging a much drier 37 inches per year, driest in not only the state but also much of the southeastern U.S.

Another example is the "Piedmont Trough," a line of slight lower pressure that can develop in the late Spring to early Fall months on hot days that is associated with differences in soil type between the Sandhills and Piedmont and the adjacent coastal plain region just to the east. While it is a fairly subtle feature, the circulations it can generate can initiate or enhance shower and thunderstorm formation or intensity along it and nearby.

Another example can be found during the warmer half of the year when seabreezes developing along the coast occur in localized convergence zones where the coastline is concave or pointed relative to the Atlantic. A south-facing shore adjacent to an east-facing one, for instance, can result in a convergence of sea-breeze winds coming from the south meeting those blowing from the east, encouraging the early and enhanced development of showers and storms in that area relative to other places nearby. This effect can be noticeable at times around Cape Lookout and Cape Fear.
Oct. 25, 2013 | Tags: fronts & airmasses, general meteorology

Question: What is it about Roxboro that results in lower reported temps than other nearby areas such as Oxford, Clarksville, South Boston, Danville, etc? I know Oxford usually reports 2 degrees or lower than Raleigh. I understand that. — Susie

Answer: Average temperatures reported by different weather stations depend on many factors that can require a detailed study to nail down as to all the reasons involved in the way they compare to one another on a regional basis. The most basic reasons behind the difference between Raleigh and Roxboro, for example, likely involves the difference in elevation (RDU is 435 feet above sea level while the Roxboro airport is at 610 feet, a difference that would account for about one-half to one degree F during the daytime based on typical vertical temperature lapse rates) and the difference in latitude and longitude, with Roxboro farther north and west, and therefore rather frequently located farther within, and closer to the core of, cooler airmasses that extend into the area in the wake of cold frontal passages and farther away from relatively warmer and more humid airmasses that sometimes push into the state from the southeast. Other variations that are more difficult to generalize about, especially with regard to some of the other stations you listed, depend on local topography and land use patterns involving soil type, vegetation, presence or absence of nearby urban or suburban areas or sizable bodies of water, and how those features are located with regard to prevailing wind directions.
Oct. 13, 2013 | Tags: fronts & airmasses, general meteorology

Question: I've seen frontal boundaries depicted on weather maps for years as lines with half circles and triangles attached. What do they mean exactly? — Terry

Answer: The lines depict boundaries between airmasses having different characteristics, especially in regards to temperature and moisture content. In general the triangles point away from colder air and toward warmer air, and the half-circles point in the opposite direction. So, a line with a series of triangles (shown in blue if in color) is a cold front, indicating that the air behind that line is colder than the air ahead of it and is advancing to replace the warmer airmass. A warm front (shown in red if in color) likewise is indicated with a series of half circles pointing toward colder air that is being replaced by an advancing warm front. There are a couple of other options that use these symbols. A stationary front, which is more or less a stalled boundary, uses alternating triangles and half-circles that point in opposite directions, while an occluded front, meaning an area where a cold front has overtaken and merged with a warm front such that two colder but separate airmasses are in contact, is shown by alternating triangles and half-circles all pointing in the same direction, that in which the front is advancing. If in color, the occluded front is shown in purple.
Aug. 10, 2013 | Tags: fronts & airmasses, general meteorology

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