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Question: What causes the formation of a High/Low pressure system? — Vicki
Answer: There are two very basic underlying causes for the formation of high and low pressure areas, and the fact that they often organize into centers with winds that flow around them. At the root of it all is uneven heating of the earth's surface by solar radiation. The heating is uneven due to the variety of surfaces receiving that radiation, from bodies of water to vegetation to deserts to ice, and also due to the fact that the earth is spherical, so that parts of the earth are receiving very direct sunlight at the same time that others are receiving much weaker, shallow angle sunlight. This uneven heating causes horizontal density differences between warmer, less dense air and areas of cooler, more dense air, and sets up a pressure difference that causes air to be forced from the more dense, higher pressure locations toward the less dense, lower pressure areas. That moving air of course, is the wind, and that's where the second organizing principle comes in. The fact that the earth is rotating leads to an apparent force (called the Coriolis Effect) on the moving air that causes it to deflect to the right in the northern hemisphere, and vice versa, until it comes into roughly a balance with the pressure gradient force and, near the surface, with frictional drag. Acting together, these forces lead to air that flows away from high pressure areas but turns to circle those highs in a clockwise manner (northern hemisphere) and likewise flows around and into low pressure areas in a counterclockwise manner. At this point, a third direction of flow is also involved, and that is the fact that air can't simply keep piling up toward the low, and instead tends to rise there (and sink near the high pressure centers). Of course, the atmosphere is three-dimensional and we've only described the outlines of flow near the surface. That surface flow influences and is influenced by wind speed and directions at middle and high altitudes as well, where traveling and sometimes stationary high pressure and low pressure centers, troughs and ridges also result. That's a very simplified overview, but hopefully gives you an idea of how it all gets started.
Jan. 24, 2016 | Tags: coriolis, fronts & airmasses, general meteorology
Question: Is it true that a stagnant high pressure cell in the NW US has created a ridge rerouting the jet stream and preventing the polar vortex from bringing storms and rain into CA and OR and other west coast states? Is the rerouting of the winds and resulting high pressure cell being caused by the orbit of the earth undergoing its normal cycle of changes in relation to the sun causing warming of the arctic and northern ice regions? Got these from a documentary that may not be factual so wanted to get your thoughts on it. — Tom Harrison
Answer: Without the context of seeing the documentary and knowing what time frame it was referring to, it's a bit difficult to give a full answer to your question, but we can note that through the end of November and the first week or so of December (leading up to the time you submitted your question), there was initially a persistent surface high pressure area and upstream ridge that principally affected the southwest Canada/northern and central U.S. Rockies region. After a few days, however, this pattern at least partially broke down before another surface high was established farther inland more southerly, centered over the central/southern Rockies. Since that time, there have been additional upper level troughs traveling in from the west and breakdowns of the surface high. The overall tendency of the pattern has been to leave much of central and southern CA with normal to somewhat below normal precipitation amounts, while northern CA and much of OR/WA have seen precipitation amounts through the period largely in the above normal (125-200%) range.
Dec. 27, 2015 | Tags: fronts & airmasses, general meteorology, past weather
Question: What is a "cold front", "warm front" and a "gust front?" — Qwest Cockman
Answer: The first issue here is to note that a "front" of any sort is generally a boundary between two airmasses having significantly different properties, usually in the form of temperature, moisture content and/or density. If a frontal boundary is moving in such a way that colder air advances and takes the place of warmer air, then it is a cold front. Should that same boundary reverse course so that warmer air replaces colder air, it is called a warm front. A stationary front, then, is a front that doesn't move very much in either direction. A gust front is a thunderstorm-related term that refers to the boundary between downdraft air from a storm that encounters the ground and spreads outward away from the storm, and the ambient air surrounding the base of the storm that it replaces. Effectively, most gust fronts are miniature cold fronts, as the air flowing out from the storm is usually markedly cooler and denser than the warm, moist air outside the storm.
Nov. 9, 2015 | Tags: fronts & airmasses, general meteorology
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
Questions 1 - 10 of 53.
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51