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Weather Questions tagged “fronts & airmasses”

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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

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

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