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Weather Questions tagged “fronts & airmasses”(remove tag filter)
Question: This morning when checking the weather maps I noticed an odd-colored purple frontal boundary stretching from Winnepeg south through much of western Minnesota. Wikipedia tells me it is an occluded front, and being someone who watches the local news the majority of evenings, it struck me as odd that I'd never heard the term before. Is it that I've not been a good weather citizen all these years, or are they uncommon to see? How often do we encounter them in the Piedmont? — Matt in Durham
Answer: Occlusions are quite common overall as a part of the general organization of mid-latitude synoptic storm systems. In somewhat oversimplified terms, they arise where cold fronts catch up to warm fronts, such that the warmer airmass between those two boundaries is lifted off the ground in and along the vicinity of the occluded front. For much of the year, they tend to occur most frequently north or northwest of us, while more typical warm fronts, cold fronts and stationary fronts extending south of the primary low pressure areas affect us. However, during the winter months, we tend to see a few occluded fronts track across our state when surface low pressure systems follow a notably more southerly path than at other times of year. The character of weather associated with them falls somewhere between classic warm front and cold fronts, and can lean closer to either, depending on the distribution of temperature and moisture around a particular storm system. Now that you're aware of them, you'll likely notice them on weather maps rather often, in many cases running just east and/or southeast from slow-moving, deep low pressure centers.
Sep. 30, 2016 | Tags: fronts & airmasses, general meteorology
Question: Why do the prevailing winds change in September and October? — Brian
Answer: In September and October, as we transition out of summer and into the Fall, the jest stream begins to move on average to a more southerly position and as a result we begin to have more frequent passages of the polar front south of our position. However, during this early part of the transition to a colder regime, it is common from fronts to move a short distance south of us and either stall or wash out, while a ridge of surface high pressure centered near New England or off the northeastern coast of the U.S. either stalls for a few days or moves slowly, with the high pressure ridge extending for several days down the eastern seaboard, focused on the east side of the Appalachian mountains. This high pressure position results in winds anywhere from northerly to easterly for several days at a time on a number of occasions during those months, leading to the prevailing direction being northeasterly. That said, as with the remainder of the year, it is possible to have winds from any direction during September and October, but they are somewhere between north and east often enough to average out to northeasterly. For the rest of the year, winds can also be variable, but they are frequently enough between south and west for that to be the predominant direction for all other months. During the summer, for example, winds are often fairly light but strongly favor a southwesterly direction due to the near-stagnant position of the Bermuda High. During the core of winter there is a noticeable tendency for winds to shift to northwesterly behind cold fronts, northeasterly for a short time with high pressure areas northwest of us, and southwesterly as high pressure centers move to our east or southeast and new cold fronts approach from the west or northwest.
Sep. 20, 2016 | Tags: fronts & airmasses, general meteorology, winds
Question: What is the difference between a 'front' and a high/low pressure system? — John Sides
Answer: Pressure systems are characterized by how the atmospheric pressure varies horizontally, such that a high pressure center, for example, is a location where the pressure become lower in all directions away from the center. Low pressure centers are the opposite, such that the pressure is higher in all directions away from the center. It is also possible to have "systems" that aren't centers, such as elongated ridges of high pressure or troughs of low pressure.
A front, on the other hand, is a zone of contrast (a "boundary," of sorts) between airmasses having significantly different temperature and/or moisture characteristics. Strong fronts have sharp differences of temperature, moisture (and usually wind direction) over a short distance, but fronts can also be weak and rather diffuse, with more gradual changes in the character of the airmass. The name and symbol for a front depends largely on its movement. Where colder air replaces warmer air, you have a cold front, and vice versa for a warm front. A frontal boundary can also be stalled in place, or nearly so, in which case it is called a stationary front.
Generally, frontal boundaries exist on the periphery of large high pressure systems, while many (but not all) low pressure systems form along or very close to frontal zones.
May. 25, 2016 | Tags: fronts & airmasses, general meteorology
Question: I noticed tonight around 5pm that it is eerily quiet outside. I know part of that is the cold weather keeping everyone inside, and nearly 0 wind, but I noticed that even the planes overhead were quieter than usual, almost muffled sounding. Is there a set of atmospheric conditions (humidity, temperature?) that tends to abate sounds at the ground? — Michael Whitney
Answer: There are indeed atmospheric conditions that are quite important to the propagation of sound, with temperature (more properly, the gradient of temperature) playing the primary role. The reason for this is that the speed of sound is proportional to temperature, so it travels a little faster in warmer air and vice versa. When there is a sharp change in temperature with height, this can refract, or bend, sound waves so that sounds traveling horizontally are bent upwards when the temperature decreases with height, and downward when there is a temperature inversion, in which temperature increases with height. On Monday afternoon, we had BOTH of those conditions within the lowest few thousand feet of the atmosphere. Very cold air at the surface was capped by much warmer air flowing in from the south just a a thousand or so feet up, and not far above that layer the temperature decreased again at a rapid pace. While part of your observed quiet may have indeed involved relatively little outdoor activity, it could also have been the case that sources of noise at ground level not so far from you were having their sounds waves sharply refracted downward so they didn't make it to your location, while aircraft flying above the height of the "warm nose" above the inversion had sound waves that were bent upward, making it difficult for those to reach your location (assuming they were at some fairly shallow angle rather than passing directly overhead - downward traveling sound waves wouldn't have been affected much). You don't mention whether you were in an area that had received recent snow. Fairly fresh snow on the ground is absorptive of sound waves and can also lead to that sense of "eerie quiet." Note that in cases where there is a more gentle temperature inversion, sound waves can be bent downward at a lesser rate that more or less matches the curvature of the earth. In those situations sound can travel horizontally an unusually long distance without much loss of volume, making it possible to hear sounds at a distance that would typically make them inaudible.
Feb. 21, 2016 | Tags: fronts & airmasses, general meteorology
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
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