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Question: I have heard that people with migraines have more of these debilitating headaches when a low pressure system is moving through. Do you know anything about that? — Ashley

Answer: Traveling lows and frontal passages can bring rapid changes in pressure, temperature and humidity, and such changes have been widely reported to induce headaches and/or joint pain in sensitive individuals, including some migraine sufferers. Pinning down the mechanisms for the pain has proven difficult in scientific studies, however, because the detailed response to similar weather patterns varies notably from one person to another.
Jan. 30, 2015 | Tags: weather & health

Question: How do you determine whether you report "mostly cloudy" vs "partly sunny" or "partly cloudy" vs "mostly sunny?" — Elaine Hayes

Answer: It's all defined based on the fraction of the sky covered by opaque clouds, usually based on eights of the sky (called "oktas"). The scale runs from 0 oktas (clear or sunny), 1-2 oktas (mostly sunny or mostly clear), 3-5 oktas (partly cloudy or partly sunny), 6-7 oktas (mostly cloudy) and 8 oktas (cloudy). For partly cloudy and partly sunny, the definitions used by the National Weather Service are equivalent, each describing a sky condition in which opaque clouds obscure between three and five eighths of the sky, with partly cloudy usable for day and night, while partly sunny only applies in the daytime. Some meteorologists tend to informally use partly sunny when a bit more clouds than sun are expected, and vice versa toward the brighter end of that range. A similar day versus night distinction is applicable for mostly clear and mostly sunny.
Jan. 29, 2015 | Tags: clouds, maps & codes

Question: Where might I find a graph of the monthly (or weekly, or bi-weekly, or whatever) average relative humidity across the year in the Triangle? — Tom

Answer: One of the better visualizations and descriptions of average relative humidity and how it varies through the year that we've seen is at a web site called WeatherSpark. Just scroll down to the "humidity" section. You'll also find a "Dew Point" section just below that. The address is

Jan. 28, 2015 | Tags: cool sites, humidity/dew point, normals

Question: During your intro to the 11:00 weather on Friday, Jan 9, you said, in connection with a change in the weather, that there would be a new "regime" (if I heard you correctly). I know you are a lover of words (and puns) as am I. I didn't understand your use of the word "regime" in that context. Doesn't regime mean a ruling government? — Anonymous

Answer: The definition you noted is one of several that can be found for the word, which, as with many others, has more than a single meaning. Another definition cited by the Merriam-Webster dictionary, for example, is "a characteristic behavior or orderly procedure of a natural phenomenon or process." That probably fits better with use of the term in a weathercast, where we may be referring, as an example, to a large-scale upper flow pattern that for some time has left us with below-normal temperatures and frequent disturbances crossing the region, to be followed by a shift in the overall pattern to one that involves several days or a week of warmer and quieter weather.
Jan. 27, 2015 | Tags: general meteorology, maps & codes

Question: Where can I, on WRAL's weather site, find out what the record high and record low was for the day before in Raleigh? I'm sure it's there somewhere but it's not obvious. — Candy Chitty

Answer: Just click the "Almanac" link from our main weather page. Once you're on the almanac page, you'll see the records for the current day. To find the records for the previous day you can use the "Get Historical Data" section just below. You can either enter the date for the previous day in the box and click the "Send" button, then look for the records in the summary section that results, or you can skip entering a date and just click "send." If you do this you will retrieve data for the current day, but you'll see a link for "previous day" and can use that to go back.
Jan. 26, 2015 | Tags: cool sites, records/extremes,

Question: Sun rises in east and sets in the west. Does it vary much with the time changes? I know the variances probably won't be much and it sounds like a crazy question, but I'm gonna build a new home and this time I don't want the sun killin' my eyes. — David Hicks

Answer: As a matter of fact, the direction in which the sun rises and sets, and the maximum heights it reaches above the horizon, do vary quite a bit throughout the year, which leads to the seasons we experience. The most northerly sunrise occurs in late June on the summer solstice, at an azimuth of 60 degrees, while the most southerly sunrise (and the lowest solar altitude later in the day) falls in late December on the winter solstice, at a southeasterly azimuth of 119 degrees. On the vernal and autumnal equinoxes in March and September, of course, the sun rises very nearly due east, at an azimuth of 90 degrees. Sunsets occur at corresponding directions to our west, most northerly on the summer solstice and most southerly on the winter solstice.
Jan. 25, 2015 | Tags: astronomy

Question: What was the reason for the dense fog we observed the morning of January 15th? I was surprised to see it at all considering that the temperature outside was a few degrees below freezing. — Eric Wilke

Answer: The surface visibility at RDU dropped to about 1/8th of a mile for a while that morning, as the temperature dipped to 32 degrees. One factors that helped encourage the fog to form included wet soil left over from heavy rains early in the week, followed by the very light rain and drizzle at times on Tuesday and Wednesday. By Thursday morning, soundings of the atmosphere taken with radiosondes showed that much of the atmosphere had dried out, allowing for radiative cooling that can encourage low clouds and fog near the surface, and winds were light to calm due to the weak pressure gradients associated with a surface ridge of high pressure extending over the area from the west. Soundings indicated that while much of the column above was quite dry, the amount of absolute humidity in the air actually increased a bit in the lowest few hundred feet above the surface, with near-saturated air below and a shallow temperature inversion above. Even a little bit of occasion light wind to mix this warmer, moister air in with the slight colder air trapped near the ground brought lots of tiny water droplets to near the surface and discouraged any evaporation of those droplets, keeping the fog in place until mid-morning when temperatures began to creep up and therefore reduce the relative humidity.
Jan. 24, 2015 | Tags: past weather, visibility/fog/dust

Question: I noticed your weather team used a ground temperature map this morning that I thought was particularly helpful on this icy day. Is this map available on your website? I can't seem to locate it. — Steve

Answer: We can't be absolutely certain, but we think you're probably asking about the maps that Brian Shrader showed in a few of his Traffic discussions that showed areas where there was a likelihood of ice or snow accumulation on roads versus areas that were likely to resist freezing due to warmer temperatures. That product is available on a section of the NWS Weather Prediction Center website, at To see the particular maps Brian showed that day, look about halfway down along the right hand side of the page and click the button marked "Accumulation on Roads." Be sure to read the explanatory information in the lower left portion of the map page.
Jan. 23, 2015 | Tags: cool sites, winter weather

Question: Checking various websites, I have found a 3-week variation of when the Raleigh area sees it's last frost during the springtime (April 11th up to April 29th). In your opinion, which is it? — David Cameron

Answer: Data from the RDU airport for the current "normals" calculation period (1981-2010) shows the average last freeze date in Spring is April 6th, but you also have to keep in mind that there has historically been about a 10-day standard deviation associated with that. Further statistics show the earliest "last freeze" there has been on March 12th, but there is still a 90% chance that more freezes will occur after March 19th. That historical chance of additional freezes drops below 50 percent after April 6th and falls below 10 percent after April 19th, with the latest freeze on record for the site being May 10th. Generally, the average last freeze date is a few days earlier for areas southeast of RDU and vice versa. You also have to keep in mind that while freeze dates can be tracked pretty accurately, frost can occur under certain conditions with measured air temperatures above freezing, so the last date that frost occurs on average is a little more fuzzy than the last date with freezing air temperatures.
Jan. 22, 2015 | Tags: dew/frost, normals, records/extremes

Question: Which model was correct Jan 9th? You were watching to see. — Johnnie G

Answer: You're probably referring to the idea that the European model was projecting low temperatures for the Triangle area in the upper teens for a few days leading up to the 8th of January, while a number of other models suggested a colder morning dipping into the lower teens. There was a post about it beforehand in the WeatherCenter Blog, which you can see at In the end, the temperature at RDU that morning actually fell to 11 degrees, closer to the forecast from the Global Forecast System (GFS) and North American Mesoscale (NAM) models than to the warmer European model forecast.
Jan. 21, 2015 | Tags: past weather

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