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Answer: This is a speculative answer, since we don't know any details of your location and the surrounding topography, but a good guess would be that you are experiencing some form of drainage wind, also known as "katabatic," in which air in contact with a cooling ground surface becomes cooler and more dense, resulting in a downslope flow. If there is a hill or mountainside somewhere close to you that is in a position to be among the first areas to start cooling noticeably via outgoing radiation as the sun sets, air descending that slope may cause the breeze that you are noticing in otherwise calm conditions. Under similar conditions, there would sometimes be a reverse, "anabatic" wind that flows upslope once the sun begins heating the same sloping ground.
Jan. 30, 2013 | Tags: fronts & airmasses, winds

Answer: The storm you mentioned was crossing the northeastern U.S. on the day you wrote (Friday, Dec 21, 2012) and at the time had a central sea level pressure of 981 millibars. To give a sense of scale to this, the average sea level pressure is about 1013 mb, while a typical high pressure center may have a central sea level pressure around 1030 mb. What's going on near the center of a low is air that spirals in toward that center is rising at a rate that depends on how fast air at higher altitudes is being removed from the vertical column of air above the point near the center. If air is being removed more rapidly aloft than it spirals in below, then the surface low deepens, and the pressure becomes lower. In addition, of course, if the low is moving fast, a person in an area that it approaches will experience rapidly falling barometric pressure and vice versa when it moves away.

Some people do report a flare-up of physical symptoms such as joint pain and headaches in these situations. This can be thought of as somewhat similar to the effects of driving or flying through various altitudes, and of course many of us have felt the effect of pressure changes in these situations, often without much discomfort, but sometimes an illness may exacerbate the effects, and some people appear to be much more sensitive than others. The pressure changes involved with a passing low of the sort that was active on the day you wrote, if a person was under a high pressure center one day and the center of the low the next, would be about the same as changing altitude by about 1300 feet or so over that same amount of time. When people dive underwater, a much greater potential pressure change has to be dealt with, as the pressure increases by about 1013 mb for every 33-34 feet of increasing depth below the surface.
Dec. 31, 2012 | Tags: fronts & airmasses, weather & health

Answer: When water evaporates from the surface of the ocean, it leaves the dissolved salt behind so that when the vapor eventually forms cloud droplets, ice crystals or precipitation, it is in relatively pure form. You can observe this first-hand if you put some salt water in a shallow dish and allow it to evaporate. When the water is gone, you should see a lingering film or crust of salt.
Sep. 20, 2012 | Tags: fronts & airmasses, general meteorology

Answer: There are many influences on the atmospheric pressure at a given point at the surface and a given point in time. While the composition of the air, including its water vapor content, is one of them, others, including the temperature of the air and, even more so, motions in the atmosphere that either increase or decrease the amount of mass in the column of air overhead, are more dominant. This generally involves air flow at various altitudes that converges (causing air to "pile up" so that mass increases) or diverges (so that mass is removed from the column). Usually there are levels at which both of these processes are occurring, and whether pressure is low, high, increasing or decreasing depends on the net increase or decrease in mass when all levels are added together. When it comes to moisture alone, the results might surprise you. If you were to keep all other influences constant (no air coming in or out of the column, and the temperature remains the same) humid air would weigh less and exert less pressure than the same amount of dry air. That is because water vapor molecules have a lower molecular weight than that of dry air (made up mostly of diatomic oxygen and nitrogen molecules having a considerably higher molecular weight than water vapor). To very roughly illustrate the concept, imagine the dry air as a pile of 1000 bricks that weighs a certain amount. Then, humid air might be represented by the same pile, but with 10 or 20 or of the bricks replaced by styrofoam blocks of the same size.
Sep. 14, 2012 | Tags: fronts & airmasses, general meteorology, humidity/dew point

Answer: It is true that on occasion a relatively weak summer cold front approaches the area, and due to either the front dissipating or clouds associated with the front moving on after holding temperatures down for a day or two, temperatures can head up shortly after the front passes through or stalls in the area. However, it is also true that some fronts that have moved through during the August have in fact gradually pushed temperatures toward lower values. Observations at RDU, for example, show that we had frequent "90+" days near the beginning of the months, quite a few days in the mid 80s to around 90 near the middle of the month, and in the latter part of the month have had a stretch of days with highs in the upper 70s to low 80s, so some of those fronts are having a cooling effect!
Aug. 28, 2012 | Tags: fronts & airmasses, past weather

Answer: There doesn't seem to be much agreement in scientific literature on the exact relationship of air pressure to arthritis pain. It is possible that changes in pressure on the outside can cause compression or rarefaction of the inside of joints that may be irritating, but it has been difficult for medical scientists to come up with a consistent physical explanation that confirms the exact mechanism, and when people are tested to track pain against values of pressure or changes in pressure, the results tend to vary widely from person to person. Similar problems arise in trying to correlate headache pain with weather-related influences.
Aug. 13, 2012 | Tags: fronts & airmasses, weather & health

Answer: It is frequently the case in summertime that scattered showers and storms that form in the heat of the day track west to east or southwest to northeast due to prevailing wind patterns that time of year. On the night in question, however, we did have a couple of moderately unusual features in place for late June. One was an upper level trough to our northeast that brought northwesterly winds aloft, which caused most individual shower and storm cells to move southeastward. There was also a fairly sharp cold front that moved through from the north overnight, and that boundary served as a focus for shower and storm development so that the line of storms pushed nearly due south. As you noted, this isn't an especially common configuration for us.
Jul. 2, 2012 | Tags: fronts & airmasses, normals, past weather

Answer: It is likely the case that some low pressure swirls or high pressure circulations can be tracked all the way around the globe, but it is also the case that many if not most dissipate, merge with other systems or otherwise become unrecognizable as the same system well before doing so, especially those in the lower level of the atmosphere. As the disturbances encounter changing surface conditions (land versus water, ice versus land, mountains, deserts, etc) that alter the character of the systems, they change to the point that it would be very rare, if ever, that those of us in NC could follow a system that passes through here around the world and see it approach a second time from the west.
May. 7, 2012 | Tags: fronts & airmasses, general meteorology

Answer: Like many other aspects of weather, cold fronts can come from a variety of directions and the weather systems on wither side of them can take on a number of different configurations that impact the progression of wind directions that occur in advance of and in the wake of the frontal passage. The one you asked about crossed our area around 1-2 AM on Friday morning January 13th. There was a rapid drop in temperature, falling dew points, and a sharp wind shift, in this case from S/SW winds ahead of the front to Westerly winds in its wake. This is often the case when the predominant center of surface high pressure behind the front builds into a position south or southwest of us rather than west or northwest, which is a somewhat more common configuration that is more likely to produce northwest or northerly winds behind the front.

Another factor that comes into play when the principle surface high is off to the southwest is that westerly low level flow behind the front often induces a near-stationary surface trough of low pressure in the lee of the mountains (just east of them). This smaller scale feature, called a "lee trough," often results in a period of time where post frontal winds become northwesterly over the foothills and far western Piedmont, but remain west or southwesterly over central and eastern parts of the state. One of those troughs is evident on surface maps on the Friday you were asking about, as is a surface high pressure center positioned over southeast TX and LA.
Jan. 22, 2012 | Tags: fronts & airmasses, general meteorology, past weather

Answer: Atmospheric pressure is just the force exerted per unit area on any object exposed to the air, and results from the weight of the column of air above a particular location up to the top of the atmosphere. By "Mb" you are probably referring to millibars, which is a commonly used unit of air pressure within the meteorological community. The standard sea-level value of air pressure in millibars is 1013.25, which is equal to about 14.7 pounds of pressure per square inch of surface area. Of course, this pressure can vary substantially with the passage of weather systems, and also falls rapidly with height above the surface.
Oct. 8, 2011 | Tags: fronts & airmasses, general meteorology