Ask Greg

Recent Questions

Question: Why did you stop using inches of hg for barometric pressure? I understand inches of hg but have no reference to what mb mean? — Tommy Warren

Answer: We use both of those units at times. On our regular weather broadcasts as well as the "current conditions" section of our web site, you'll see pressure measurements shown in inches of mercury. It is more customary in tracking tropical cyclones, however (hurricanes, tropical depressions and tropical storms) to use millibars (mb) and we sometimes will use that unit there, or you may see that unit in stories based on National Hurricane Center advisories. You can easily convert between various units for pressure by knowing that the same, equivalent "standard" sea level pressure can be given as 760 mm Hg, 14.7 psi (pounds per square inch), 29.92 inches Hg, 1013.25 millibars, 101.325 kiloPascals, or 1 atmosphere.
Jul. 13, 2014 | Tags: general meteorology, instruments

Question: Re the July 2 dew point question: I've explained dew point with a small bottle labelled 'cold air' and filled, and a large bottle, labelled 'warm air' and partially filled. They can see how warm air holds more water (vapor). I've also stressed that dew point is calculated, not measured. — Chris

Answer: That can be a nice approach to visually explaining the concept of saturation, but you have to be careful about saying explicitly that warm air can "hold" more water vapor than cold air. It is true that at lower temperatures, more water vapor returns to a liquid form and vice versa, but that is a function of the behavior of water vapor molecules and the surfaces they condense onto (or evaporate from) with varying temperature, not the air surrounding them, which has plenty of empty space that could accommodate even more no matter how many happen to be there already. For more background on this point, see the discussion at www.ems.psu.edu/~fraser/Bad/BadClouds.html.

Regarding how dew point is determined, you may not really want to stress that it is a calculated quantity only. While dew point can be calculated based on a measurement of air temperature and relative humidity (which can be determined directly using instruments such as a hair hygrometer or a resistance hygrometer), it is also true that dew point itself can and has been routinely measured directly using instruments such as the chilled mirror hygrometer and the dewcell.
Jul. 12, 2014 | Tags: humidity/dew point, instruments

Question: Where do I find the hurricane computer model map? The only map can now find is the interactive tracking map. On the old website, the various maps were listed on the interactive map site. — Mary Beth

Answer: We assume you're asking about the multiple lines showing the paths of the storm center calculated by several different computer models. That plot is still available on the interactive map, but does take a couple of steps to display. To turn those on, just click the box at the right side of the map labeled "weather," then mouse over the "tropical" box. There you will see radio buttons that you can toggle to display or hide several related elements, including the computer models. After you've selected the items you'd like to display, click the small x just above "weather" to close that menu and view the updated interactive map.
Jul. 11, 2014 | Tags: hurricanes, maps & codes, wral.com

Question: I have heard tornadoes only move north, northeast or east. — Russ Barnes

Answer: A sizable majority of the tornadoes in the United States move in those directions, with most moving generally toward the northeast because the conditions that most favor the formation of tornadoes often involve the presence of an upper-level trough to the west that causes the parent thunderstorms to move northeast. However, it is possible, and has been observed, for tornadoes to move in any direction, depending on the configuration of the large and smaller scale weather patterns in place when they form. We can find two recent examples in our own state. On July 3rd, as Hurricane Arthur moved toward the southern Outer Banks, a tornado pushed northwest across Martin county, while a few hours earlier another tornado tracked westward across Duplin County, damaging several homes in Rose Hill.
Jul. 10, 2014 | Tags: general meteorology, tornadoes

Question: The condensation process can be found to occur when a parcel of air is rising, descending, or static? — Jasmine

Answer: In the atmosphere, condensation occurs by far most often when a parcel of air is rising. This is the manner in which most of our clouds are formed, and from which most of our precipitation derives. The great majority of the time, when air is descending it warms up and dries out, causing evaporation to occur and often leading to fair skies. It's a little more complex in the case of static air that is neither rising nor sinking. In that case, whether condensation occurs or not depends on whether the amount of water vapor in the air is increasing, decreasing or steady, and whether the temperature of the parcel is increasing, decreasing or steady.
Jul. 9, 2014 | Tags: general meteorology, humidity/dew point

Question: I watch the weather frequently and often see that "this frontal system is moving at 35 miles an hour. The front may be in TN at the time and will arrive in our viewing area within a few hours. How can any system move several hundred miles in a short period of time when it is only traveling 35 miles an hour? I thought it may be because of the earth's rotation but could not find anywhere to validate that hypothesis. — Steve

Answer: If a frontal system is indeed moving at 35 mph and maintains that rate of speed, then it could not arrive several hundred miles away in just a few hours, but we wonder if you may have taken something like a half-day or day as a shorter time than we really intended. At that speed for example, the front would travel over 400 miles in half a day and well over 800 in a day.

Of course, there are also situations in which fronts can slow down or accelerate significantly, which can affect arrival times at downstream locations. In addition, it is occasionally the case that fronts appear to "jump" from one location to another. This is not literally a matter of the same front speeding from one location to the next, but instead a new development of a front (called "frontogenesis") that may occur well downstream of the position of another front that is dissipating (a process called "frontolysis").
Jul. 8, 2014 | Tags: fronts & airmasses

Question: Why are they calling Author a cyclone? We have hurricanes, what has changed? — Bonnie

Answer: At the time you wrote in, the system you're referring had not yet developed tropical characteristics, but was expected to potentially do so in a couple of days. Some of the discussions about the system from our meteorologists and others referred to the system as one that had a good chance to become a tropical cyclone in the future, and noted that if it became a named storm, it would be called Arthur.

In this case, the reference to a tropical cyclone was made in a generic sense, because we could not know at the time whether the system would only become a tropical depression, would intensify to tropical storm status, or would become strong enough to be classified as a hurricane. All three of those storm types can be more broadly referred to as a "tropical cyclone." A "cyclone" more generally is any closed weather circulation in which the winds rotate in the same sense as the nearest pole when viewed from above. In the case of the northern hemisphere, that means winds that circulate in a counter-clockwise manner.
Jul. 7, 2014 | Tags: general meteorology, hurricanes

Question: At 11:42pm on June 21 (a Saturday night) I was west of Cary, north of Apex on the Chatham county line. It was raining here pretty steady and the radar showed absolutely nothing for miles. What gives? — Doug Hilliard

Answer: We assume you were referring to the Dual Doppler 5000 displays available on our web site. Unfortunately, we don't have a way to see exactly what was on the site at that time if that's the product you were checking. We did find archived NWS radar display information for the time that verified a clearly indicated shower was drifting south across the area at that time that normally would show up just fine of Dual Doppler. It seems possible that there was either an outage with the Dual Doppler, or in its communications with our web site at the time. While this is fairly rare, it does give us a chance to note that we have some redundancy on the site in terms of cross-checking information in a situation like this. In addition to our standard Dual Doppler 5000 displays, we also have a live streaming Dual Doppler 5000 sweep that you can check, regional radar displays that are based on a mosaic of National Weather Service radar data (found in our "Map Center" section) and the interactive iControl radar display tool, which can be panned and zoomed to the view you find most useful. Like the regional radar maps, iControl displays radar data that is taken from the NWS network. Thus, if there is an outage of NWS radar data in our area, you can usually find Dual Doppler 5000 imagery to fill in, and if that is not available for some reason, there is usually NWS-based radar imagery available as a backup.
Jul. 6, 2014 | Tags: weather radar, wral.com

Question: Why does thunder sound like a distant, long rumble in a war zone when I'm in Wake Forest but a sharper, quicker, louder sound in Cumberland County? — Maddie Britt

Answer: The differences you describe typically aren't so dependent on your physical location in the sense of one town or another, but rather on how far you are from the lightning flash that produces the thunder, and the orientation of the lightning channel relative to your position. When lightning is far away, the higher sound frequencies tend to be absorbed by the air and ground, leaving a dispersed series of longer wavelengths (with lower pitch) to rumble by, while nearby strikes send a more compact sound front that has just transitioned from a shock wave and thus passes quickly, has a higher pitched crack or sometimes tearing sound, and can often shake the building you're in. Regarding the orientation of the lightning channel, if it happens to lined up so that the entire channel is about the same distance from your location, then there will be one rather quickly passing crack or rumble of thunder, when if the channel, which can be miles long, is oriented so that one end is much closer to you than the other, you will hear thunder from the varying distances along the channel arrive and pass through over a notably longer stretch of time.
Jul. 5, 2014 | Tags: lightning, thunderstorms

Question: Thanks for putting the heat indexes on the current conditions page! — John Davis

Answer: We're glad to have it back there. It was inadvertently left off that page when the web site was redesigned a while back, but it has been our practice in the past to include it, and wind chill in the colder months, when conditions warrant showing either value. In addition to the listing with current conditions, we also have maps of heat index and wind chill available through our "Map Center" page, by clicking "Other Maps," then "Temperatures" and "More Views."
Jul. 4, 2014 | Tags: apparent temperature, maps & codes, wral.com

Questions 51 - 60 of 4325.


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.