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Weather Questions tagged “fronts & airmasses”

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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

Question: I live in the Interior of Alaska. There is sometimes a slight breeze at sundown during an otherwise dead calm day. What is it called? — Robin

Answer: This is a speculative answer, since we don't know any details of your location and the surrounding topography, but a good guess would be that you are experiencing some form of drainage wind, also known as "katabatic," in which air in contact with a cooling ground surface becomes cooler and more dense, resulting in a downslope flow. If there is a hill or mountainside somewhere close to you that is in a position to be among the first areas to start cooling noticeably via outgoing radiation as the sun sets, air descending that slope may cause the breeze that you are noticing in otherwise calm conditions. Under similar conditions, there would sometimes be a reverse, "anabatic" wind that flows upslope once the sun begins heating the same sloping ground.
Jan. 30, 2013 | Tags: fronts & airmasses, winds

Question: With this latest massive Midwest snowstorm and the classic "comma shape" of this huge low pressure system, I've often wondered on what exactly is going on at the center of circulation? Can it affect how you feel physically just from a low pressure standpoint and not the weather it generates all round it? — T. Sykes

Answer: The storm you mentioned was crossing the northeastern U.S. on the day you wrote (Friday, Dec 21, 2012) and at the time had a central sea level pressure of 981 millibars. To give a sense of scale to this, the average sea level pressure is about 1013 mb, while a typical high pressure center may have a central sea level pressure around 1030 mb. What's going on near the center of a low is air that spirals in toward that center is rising at a rate that depends on how fast air at higher altitudes is being removed from the vertical column of air above the point near the center. If air is being removed more rapidly aloft than it spirals in below, then the surface low deepens, and the pressure becomes lower. In addition, of course, if the low is moving fast, a person in an area that it approaches will experience rapidly falling barometric pressure and vice versa when it moves away.

Some people do report a flare-up of physical symptoms such as joint pain and headaches in these situations. This can be thought of as somewhat similar to the effects of driving or flying through various altitudes, and of course many of us have felt the effect of pressure changes in these situations, often without much discomfort, but sometimes an illness may exacerbate the effects, and some people appear to be much more sensitive than others. The pressure changes involved with a passing low of the sort that was active on the day you wrote, if a person was under a high pressure center one day and the center of the low the next, would be about the same as changing altitude by about 1300 feet or so over that same amount of time. When people dive underwater, a much greater potential pressure change has to be dealt with, as the pressure increases by about 1013 mb for every 33-34 feet of increasing depth below the surface.
Dec. 31, 2012 | Tags: fronts & airmasses, weather & health

Question: You said on the news Thursday that the Gulf of Mexico and the Atlantic would be open on Monday, increasing the chance of rain. If the moisture is coming from the ocean, why isn't the rain salty? — Larry Price

Answer: When water evaporates from the surface of the ocean, it leaves the dissolved salt behind so that when the vapor eventually forms cloud droplets, ice crystals or precipitation, it is in relatively pure form. You can observe this first-hand if you put some salt water in a shallow dish and allow it to evaporate. When the water is gone, you should see a lingering film or crust of salt.
Sep. 20, 2012 | Tags: fronts & airmasses, general meteorology

Question: If barometric pressure is the weight of the atmosphere pressing down at a particular point... why is this pressure lower during a time when the air is full of moisture? Seems the opposite should be true. — Tony Helms

Answer: There are many influences on the atmospheric pressure at a given point at the surface and a given point in time. While the composition of the air, including its water vapor content, is one of them, others, including the temperature of the air and, even more so, motions in the atmosphere that either increase or decrease the amount of mass in the column of air overhead, are more dominant. This generally involves air flow at various altitudes that converges (causing air to "pile up" so that mass increases) or diverges (so that mass is removed from the column). Usually there are levels at which both of these processes are occurring, and whether pressure is low, high, increasing or decreasing depends on the net increase or decrease in mass when all levels are added together. When it comes to moisture alone, the results might surprise you. If you were to keep all other influences constant (no air coming in or out of the column, and the temperature remains the same) humid air would weigh less and exert less pressure than the same amount of dry air. That is because water vapor molecules have a lower molecular weight than that of dry air (made up mostly of diatomic oxygen and nitrogen molecules having a considerably higher molecular weight than water vapor). To very roughly illustrate the concept, imagine the dry air as a pile of 1000 bricks that weighs a certain amount. Then, humid air might be represented by the same pile, but with 10 or 20 or of the bricks replaced by styrofoam blocks of the same size.
Sep. 14, 2012 | Tags: fronts & airmasses, general meteorology, humidity/dew point

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