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Weather Questions tagged “fronts & airmasses”

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Question: I want to know if meteorological fronts are only higher latitude phenomenon, because I live in the equator and I haven't seen fronts depicted on the meteorological charts. — Daniel

Answer: "Only" is probably a bit too strong a word, but well-defined large-scale frontal boundaries are very much a mid and high-latitude phenomenon for the most part. The comparatively small variation in incoming solar radiation throughout the year in the tropical locations leads to much weaker horizontal gradients of temperature there, making frontal zones difficult to form and intensify. On rare occasion, especially intense frontal boundaries originating from the mid-latitudes have been observed to penetrate deep into the tropics, and even cross the equator in especially unusual circumstances.
Aug. 7, 2015 | Tags: fronts & airmasses, general meteorology

Question: What is the spin in the clouds in SW NC mountains right now? — Mike Nash

Answer: We had a good number of questions queued up for publication here ahead of your, so we should note the "right now" you're asking about was around 10:30 PM on Monday, May 18th. What we found in looking back through radar, satellite and surface/upper air map archives was that a pre-frontal outflow boundary had induced a fast moving squall line across the lower MS Valley and deep south that had continued southeastward, with some additional storm development in the later afternoon and evening near the northern end of the trough of low pressure associated with that line. With air rushing east and southeast along that line and moving more slowly near the northern end, showers and storms that passed across the southern mountains were induced to swirl in a counterclockwise manner, enhanced by the upward motions in the precipitation area that helped develop a low pressure center in the area, which tracked on toward the northeast later that night and into Tuesday, weakening and leaving behind a northwest to southeast surface trough on the east side of the mountains.
May. 31, 2015 | Tags: fronts & airmasses, past weather, thunderstorms

Question: I thought the barometric pressure always dropped preceding and during a notable frontal passage? I noticed in a recent frontal passage on March 5th that the pressure was rising as the front moved closer. I noticed this at 1:50pm. I live near Angier & it had been raining for about 3 hours or so and I recorded a wind gust up to 30 mph. Is this normal, or had my weather station gone nuts? — B Parrish

Answer: The somewhat simple, classic conceptual model of a cold frontal passage usually would involve pressure that decreases with time until the front passes, at which time the wind would shift, the temperature and humidity would begin to fall and the pressure would start to rise. This does occur a good bit of the time, but it is also true that frontal systems are often considerably more complex in the real atmosphere, and may feature multiple boundaries of pressure, temperature, moisture, etc that are a bit offset, or there may be waves of low pressure traveling along the front, or there may be strong showers that superimpose their own small-scale circulations and pressure variations on top of the larger scale features that are passing through. In the case you're asking about, we noted in the data that both at Raleigh and an airport station not far from you (in Erwin), that the pressure trace was fairly steady prior to the frontal passage and then rose at a decent pace in its wake, at about the same time winds shifted from S/SW to northerly, and just as temperature and dew point began to decrease fairly quickly. We suspect that the overall storm system was either undergoing some "filling," meaning pressures in general were trending up a bit on both sides of the frontal trough, enough to offset the usual fall in advance of the front, or that the motion of the front and associated low pressure center nearby were such that the low pressure center was angling away to the north as the front approached, which again could allow the pressure to hold steady or even climb a bit in advance of the front, followed by an even more rapid increase once the front passed. On top of that, more localized pressure variations due to rain showers passing through your area may have contributed to the pressures you observed climbing some before the frontal passage.
Apr. 24, 2015 | Tags: fronts & airmasses, general meteorology, past weather

Question: Could the term back door cold front be explained? I keep hearing that but it is not clear exactly what that means. — Kent Riedling

Answer: The majority of cold fronts that cross our area, and the eastern U.S. in general, are oriented in a line that runs from north to south or northeast to southeast, and they tend to move through here from the west or northwest. This being the most common mode of arrival, the cold air behind the fronts can be figuratively said to enter through the "front door." Less commonly, a cold front will have more of a west to east orientation, and will push across our area from the north or the northeast, often because of a strengthening cold high pressure center over eastern Canada or New England. When the cold air arrives in this manner, the front that marks its leading edge is called a "back door" front.
Mar. 2, 2015 | Tags: folklore, fronts & airmasses, general meteorology

Question: Explain why large high and low pressure systems don't move from east-to-west across the continental United States. — Carlos

Answer: On some fairly rare occasions, they actually do, but as you note most surface centers of high and low pressure in the midlatitudes, including most of the U.S., tend to move with a component from west to east the majority of the time. the basic reason for this is that they are steered along by mid-level winds that flow predominantly from west to east around the northern hemisphere. This results from greater solar heating near the equator and less near the poles, which sets up an average pressure differential at higher altitudes in which pressures are higher to the south and lower to the north. In the absence of any other factors, this would cause air to flow from south to north at these levels. however, the rotation of the earth causes the moving air to defect to the right of it's original path (in the northern hemisphere) until a rough balance is reached between the Coriolis Force (imposed by the earth's rotation) and the pressure gradient force. This balance results in winds aloft, including jet stream winds, that flow from west to east in the mid-latitudes and help cause surface low and high pressure centers to do the same.
Jan. 1, 2015 | Tags: fronts & airmasses, general meteorology, winds

Question: We are fifth grade students and on a weather review sheet an answer was "bib front." We can not find out what a bib front is. We have searched the internet. — Carol Nelson

Answer: This made us feel a bit like Jeopardy contestants, since we wondered what question might have been answered by the term "bib front." We are familiar with a number of frontal types in meteorology, examples including cold, warm, occluded, coastal, arctic, polar, upper, secondary and so on, but not one starting with "bib." We managed to find a multiple choice review question for 5th graders online that asked "What happens when a cold air mass and a warm air mass bump into each other and stop moving?" The answer to the question would clearly be a stationary front, but the choices also included cold, warm and bib. We suspect the writer of the question was just looking for a filler answer to check for guessing by students who didn't know the answer, and used the fashion-related term "bib front" which seems to apply to some shirt, pant and overall styles.
Dec. 1, 2014 | Tags: fronts & airmasses, general meteorology

Question: On your broadcasts you often talk about where a cold front coming across the state is located. What measures other then wind direction shift and falling dew point are used to gauge the frontal passage? Is the pressure readings from trusted local home weather stations used? — Dave Crotts

Answer: While we do check readings from personal weather station from time to time, there is usually enough detail from official stations located at airports to get a reasonable estimate of the location of a passing cold front, assuming it's a relatively well-organized from that lends itself to straightforward analysis. Fronts can also be diffuse and difficult to locate, or complex in nature with displaced locations for the surface front and the front at higher altitudes, or a front in which the sharpest gradients of temperature, humidity, wind direction and pressure are offset from one another. In a general sense, though, we do watch for organized wind shifts, pressure falling rather rapidly followed by a rapid rise, the location and movement of well-defined troughs of low pressure on surface analyses, and sharp drops in dew point and/or temperature (or in a sort of combined form of those, a variable called equivalent potential temperature) to help identify the location and movement of frontal boundaries.
Nov. 13, 2014 | Tags: fronts & airmasses, general meteorology

Question: We had a strong weather system and cold front headed our way back on October 14th, and surface maps predictions showed the front passing through Triangle by 8 a.m. the next day. However, the WRAL forecast and other public forecasts for the day or so following the front passage showed winds out of the Southwest. I thought most cold fronts brought winds from NW or NE. — Dave Crtts

Answer: Your question does a good job highlighting the fact that while a lot of behavior with weather systems follows reasonably well-behaved patterns, there are substantial exceptions to almost any general rule, and the atmosphere often evolves in a sufficiently complex manner to make generalizations difficult. It is true that most cold frontal passages here result in a shift to winds with a northerly component, but there are situations that diverge from that behavior.

In the case you were asking about, an initial frontal passage was followed by a sharpening upper level trough that dug into a closed upper low positioned to our northwest and helped to generate a new surface low and trough that stretched south across KY and TN, ahead of which winds rapidly shifted to the west and southwest for our area. Situations like this often result in temperatures that only back off a little in the wake of the initial front. In this case, a high of 79 on the 14th was followed by highs that remained in the low to mid 70s the next several days, due to the systems west of us preventing a significant intrusion of colder air.
Nov. 1, 2014 | Tags: fronts & airmasses, general meteorology, past weather

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

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