Ask Greg

Weather Questions tagged “fronts & airmasses”

(remove tag filter)

Question: What was that crazy thin blue line on the doppler paralleling US-1 from Raleigh to Sanford moving rapidly north-west between 5:30 and 6 pm Monday July 3? All the other local storms seemed to be sitting basically still between Seven Lakes and Fayettville, but this thing caused some intense winds when it passed through Sanford, but only for about 2 minutes. — Todd Lewis

Answer: A bit earlier that afternoon there were intense thunderstorms over northern Cumberland County and Lenoir County that produced intense downdrafts that then spread outward from the storms in a shallow layer of denser air. The dividing line between this denser and cooler air and the ambient warmer and less dense air (as well as between lighter winds outside the layer and a gust front of stronger winds that often accompanies it) is called an "outflow boundary." The change in wind speed and sometimes direction across these boundaries leads to convergence and a resulting concentration of dust, small flying insects and sometimes birds, in addition to creating turbulent variations of density where the differing airmasses are mixing. All of these things combine to create a thin line of weak radar reflectivity that can clearly mark the boundary on radar displays. Since the boundaries are often rather shallow, they are typically only visible fairly close to a radar site, where the beam remains at a fairly low altitude. This one was well-situated with respect to the DualDoppler 5000 and NWS Raleigh NEXRAD radars, and actually followed a curving path northeast across Lee and southern Wake Counties, and then east across southern Wilson County and on into the Greenville area. On occasion, the upward motion caused by the dense layer of air undercutting the surrounding warm, humid and unstable air triggers new convective cells, and this appeared to be the case in a few locations with this outflow, including a cell that popped up over central Lee County at around 5:20 PM after the boundary had proceeded northwest to about the Chatham County border.
Jul. 10, 2017 | Tags: fronts & airmasses, past weather, thunderstorms, weather radar

Question: What barometric pressure determines a high or low; for example, 30" Hg or above is a high and below 30" is a low. — Don E

Answer: Actually, low pressure centers or systems and high pressure centers or systems are defined more by the relative pressure at their locations compared to surroundings (and the related circulations of air that result), rather than by particular values of pressure. In a very rough sense, you could take the idea that standard sea level pressure is 29.92 inches of mercury (equal to 1013.25 millibars), so you could take a pressure value notably higher than that to be high and vice versa. However, again, it is possible for a pressure lower than that to be higher than surroundings, and thus a "high" on a weather map, and likewise the other way around.
Jul. 6, 2017 | Tags: fronts & airmasses, general meteorology, maps & codes

Question: Could you explain how you come up with the dewpoint ? — Danny

Answer: Dew point is measured either by using an instrument that arrives at it directly, such as a chilled mirror dew point hygrometer, or indirectly be deriving it from other quantities. The chilled mirror hygrometer uses a temperature-controlled mirror that has a beam of light directed off of it to a sensor. When the mirror is cooled to the point that water droplets (dew) condense onto it, the beam is partially interrupted and the instrument notes the temperature at which that transition occurs, which is the dew point. You can also measure relative humidity with a few different types of instruments, and by using the temperature (measured with a thermometer at the same time and location) and relative humidity together, the dew point can be calculated. Once we know initial values for dew point at many locations, using the network of surface stations, upper air balloons and other instruments that sample the atmosphere around the world, the data is used to initialize mathematical computer models that assist with projecting the state of the atmosphere into the future. Those future states include dew point at any location as one of the forecast variables, so we can check some model output to determine likely future dew point values and include them on maps, or show them in bar graphs, for example.
Jun. 5, 2017 | Tags: fronts & airmasses, humidity/dew point

Question: I apologize if you have answered this question before but in the cold front passage earlier this week (today is May 5) and the one occurring today, all of the public forecasts show dew points falling but wind continuing to come out of the SW or W. I thought most of our cold fronts came from either NW or NE (back door front). Why would a SW flow not be bringing up warm humid air? — Dave Crotts

Answer: Like many aspects of meteorology, there are typical and common situations, such as the most frequent types of cold frontal passages that you mentioned, and there are complications and exceptions to rules. One that occurs from time to time is the situation you noticed in which a frontal passage, usually accompanied by a low pressure area that intensifies some not too far to our north or northwest as it passes, produces a west or southwesterly flow in the wake of the frontal, but drier and sometimes cooler air that is associated with the front has been pushed a long way south over the nation's midsection or along the Mississippi Valley, before being transported into our state from the west. This can allow even air from the southwest to move in with lower dew points (and thus humidity) than the air it replaces. In many of these cases, the air coming in from the west or southwest is notably drier than the air ahead of the front, but not much cooler, and in some of these cases we actually warm up more in the immediate aftermath of the "cold" front, simply due to a lack of precipitation and much more sunshine once the front has pushed away to the east.
May. 13, 2017 | Tags: fronts & airmasses, general meteorology

Question: I have fibromyalgia and the barometric pressure causes a lot of my pain. My question is why do you not find maps that have the pressure forecast like temperatures? It would be so much easier to prepare for when it puts you in bed. Second question is there any web site that tracks the barometric pressure like that. — Anonymous

Answer: By going to our "Almanac" section and finding the "get historical data" you can find graphs of observed pressure for any day, and expand these if desired to weekly or monthly graphs. When it comes to future barometric pressure changes, you can find forecast maps with surface pressure patterns on them at many university and government web sites, for example the "precipitation and SLP (sea level pressure)" projections under any of several models at One source for a graph, rather than a map, showing projected pressure is the forecast model meteogram page at You can get a 60-hour projection for Raleigh by clicking "RDU" on the map and looking at the bottom graph labeled "pres." Note that the times along the bottom of the graph are in Universal time, so that during DST "00" is 8 PM in Raleigh and "12" is 8 AM. During standard time those two values are 7 PM and 7 AM, respectively.
Apr. 16, 2017 | Tags: cool sites, fronts & airmasses, weather & health

Question: Is it possible for a high pressure system to bring rain? I know that most storms are brought by a low pressure system, but is it possible? — Luke

Answer: This is a good example of how, in order to make weather understandable and more easily communicated, we often greatly simplify what are in fact very complex patterns and processes in the atmosphere. While we commonly associate low pressure with precipitation and high pressure with fair skies (and rightly so in many cases), there are exceptions and there are situations in which interactions of relatively low pressure at one altitude correspond to relatively high pressure at another, sometimes resulting in precipitation. That's a long way of saying that it IS possible for precipitation to occur when there is a high pressure system in place. One common example for us is the "cold air damming" pattern, in which high pressure extends into the state from the north or northeast, creating a north or northeasterly wind flow and bringing a shallow layer of cold, dense air into the region. Sometimes, that air undercuts and lifts warmer, moister air that condenses into clouds and precipitation. In many cases, this only yields sprinkles, drizzle or very light rain, but in some instances the precipitation can be more substantial. Another example would be high pressure building in from the west in the winter, producing strong northwesterly winds for our state. For a lot of us, that is generally a dry pattern. However, it isn't uncommon in the mountains for air flowing up the western slopes within the eastern or northeastern portions of the surface high to be moist enough to produce flurries or snow showers there. One other note is that tropical cyclones often are best organized and set up to intensify if the low pressure center near the surface is lined up nicely with a high pressure center in the upper atmosphere more or less directly above it.
Feb. 10, 2017 | Tags: fronts & airmasses, general meteorology, rain

Question: Why is it that the air sometimes smells noticeably fresher? This evening is one of those times. — K. Schaffer

Answer: There can be a number of reasons, and it varies depending on the kind of "un-fresh" smell that existed before. Sometimes it is a simple matter of the air being stagnant or blowing from the direction of a source of pollutants, and then having the wind direction shift or the speeds pick up such that turbulent mixing brings cleaner air down from a few hundred or few thousand feet above the ground. There can also be a fresh smell in the wake of a rainfall in which the rain captures some of the particles in the air and scours them out, depositing them on the ground. In the case of the evening you wrote in, we had a warm afternoon followed by a cold frontal passage that occurred around mid-afternoon. You may have been noticing the effects of cooler, less humid air flowing in from a different direction and perhaps a mixing effect as well. In the case of this front, we went from a high temperature of 76 that day to a high of 55 the next, indicating a notable airmass change.
Feb. 7, 2017 | Tags: air quality, fronts & airmasses

Question: I got a weather surprise today (January 26) around noon as I was bike riding West on Hillsborough Street near NCSU. I was going to try to experience the passage of the strong cold front that I had been tracking over time on the personal weather stations to the West of us. As the front got to somewhere between Cary and West Raleigh a narrow line of heavy rain and wind similar to a squall line suddenly blew up within roughly a 5-minute period. From looking at myfox8 interactive radar website when I got home it seemed to have blown up around I-540. A lot of times this sudden development happens with summer thunderstorms between Durham and Raleigh or around I-95 well to the East. Can you explain the dynamics of this type of pop-up squall line? — Dave Crotts

Answer: The frontal structure that day appears to have been of the "split front" variety in which a cold front aloft decouples from and surges well ahead of a lagging surface cold front. In these situations, there is usually a band of fairly widespread precipitation that is located along a "warm conveyor belt" of moist air that is acted on in part by upward motions associated with a broad upper-level trough following along behind both frontal boundaries. That band of precipitation is what occurred through the late night/early morning hours on Thursday. The cold front aloft brought a deep layer of cooler, drier air into place that caused some clearing in the wake of the morning rain, but left a fairly moist, shallow layer of warmer air near the surface along and ahead of the surface cold front. By the time that front moved in to the Raleigh area, the break in cloud cover had allowed significant warming at the surface, leading to substantial instability due to the warm surface and cooling airmass aloft. At about the same time, the surface front provided some measure of lift to the low-level air along it (due to drier, cooler, and thus more dense air undercutting the warmer, more humid air) and at about the same time a concentration of higher winds aloft (a subtle jet streak) moved into position to also provide a measure of enhanced upward motion. Where the two features overlapped, we saw a northeast to southwest oriented line of showers develop, and they briefly become rather intense over a small area. In short order, the surface front moved on to the east, the jet streak pulled away northeast, and skies cleared again. You can read briefly about split fronts and see a couple of helpful diagrams at
Feb. 3, 2017 | Tags: cool sites, fronts & airmasses, past weather

Question: If warm air comes from the southwest and the northwest, then where does cold air come from? — Aliyah

Answer: The development of colder or warmer air than is currently in place over a given area is actually much more complex than that, and includes factors such as the presence or absence of clouds and precipitation, whether on the large scale air over the region is rising or sinking, and the extent to which warmer or cooler air than average has been carried especially far north or south by other weather systems upwind of our location. Because of that, it is possible under the right circumstances for warmer air or colder air to flow in from any direction. Even with all that being the case, it is true that in a broad, general sense, warmer air tends to originate south of us and vice versa, so colder air most often moves in with winds having a northerly component (northwest, north, northeast, etc) while warmer air is more likely to move in with winds having a southerly component.
Dec. 22, 2016 | Tags: cold, fronts & airmasses, general meteorology, heat

Question: Will we have another unseasonably warm end of November and December like last year? — Mark Womack

Answer: There isn't a very strong influencing climate signal to base a confident forecast about that on, though a weak La Nina pattern in the Pacific does suggest a slight tilt of odds toward warmer and drier than normal weather overall for December into January. In the shorter term, most models lean toward a range of temperatures not too far off normal for the most part as we head through the first half or so of November (after a rather warm start in the first few days of the month), while the Climate Prediction Center suggests the latter half of November has about equal chances of turning out above, near, or below normal for temperatures.
Nov. 7, 2016 | Tags: fronts & airmasses

Questions 1 - 10 of 69.

Ask Greg Your Question Now!

Please understand that the volume of Ask Greg questions makes it impossible to answer every one or to list them all here. You may find it helpful to search for your own question using the form at the top of this page to see if it has been posted in our database.

When you submit a question you understand that your question and e-mail address will be sent to our editorial staff. Accordingly your question will not be subject to the privacy policy of this site.