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Question: Take a look at the WRAL 24-hr rainfall map. What is up with the rainfall in Greenville? It looks like they are reporting rain from Hermine in centimeters rather than in inches. — Dave Salman
Answer: We noticed that, too, and in fact there were a couple of different locations that we were not directly plotting the values on our maps where we saw that color contours in the background had areas that were obviously contaminated with readings that were too high, so that we turned off that background layer for on-air purposes. We aren't absolutely sure what the problem was yet, but have seen instances in the past with reports from the type of automated sensor used at the Greenville airport in which display software adds up rainfall cumulative rainfall totals as if they were sequential un-related values instead, leading to a significant over-calculation of the reported amount. Other nearby stations, along with radar-based estimates, make it appear most areas in the vicinity of Greenville received around 6-8 inches of rain Thursday through Saturday from the combination of a frontal boundary and Hermine.
Sep. 7, 2016 | Tags: instruments, maps & codes, rain
Question: I'm dying on the inside. When will the heat stop? I'm going to melt before autumn comes. — Kayla
Answer: We can't really promise a particular time, but of course we are heading out of summer and into Fall in the coming weeks, so the overall trend will be downward. However, after a bit of a comfortable break due to the combination of a cold front and Hermine, our temperatures appear as if they will run a good bit above normal for much of the week to come, before sinking to more typical levels after the coming weekend - still warm, but most likely lower 80s instead of upper 80s to mid 90s.
Sep. 6, 2016 | Tags: heat
Question: I'm watching the 10:00 pm forecast and they say current temperature is 85. WRAL web site at the exact same time says it's 82.... why? — Toolman
Answer: There are a couple of ways this may have been the case, depending on where you were looking for data on the web site. One involves the fact that we have a weather station on our roof that provides a temperature for our station location, while there is a separate temperature measured at the RDU airport that we use on many of our on-air maps and current conditions graphics. These sensors are far enough apart that different readings are not especially unusual. In addition, while the value from the sensor on our roof is almost continually updated, the automated station at RDU normally sends a new observation once per hour. If temperatures are trending up or down through that period, that can also lead to a discrepancy between the two values. You can check both readings by clicking the "Current Conditions" link on our main weather page.
Sep. 5, 2016 | Tags: instruments, maps & codes, wral.com
Question: Can't a line of storms redevelop if it is in an unstable air mass? — Amy
Answer: When a line of storms exists, there can be a sizable number of ways in which it was generated, ranging from frontal boundaries to outflows from previous storms to passing upper level disturbances, and how persistent the line is can depend on the strength and orientation of any of those factors, along with the 9overall level of instability in the surrounding airmass. In some cases, the surrounding environment may become less supportive of storms and the line will simply decay and the storms will come to an end. Given enough instability, however, and a lack of opposing forces, there are other times when the remnants of a line, or outflow boundaries that were produced by that line, can redevelop and continue to produce newly forming showers and storms.
Sep. 4, 2016 | Tags: general meteorology, thunderstorms
Question: Will it ever, ever rain in Raleigh again?? Will the air in Raleigh ever be cool again? — Candy
Answer: Rain has certainly been less widespread and frequent for the past few weeks, and that general pattern may not change all that much soon. However, by the time you read this response, it is quite possible there will have been some showers and storms associated with a passing cold front, and some rain generated by an interaction of that front and a tropical storm moving by to our southeast. Those systems didn't absolutely guarantee substantial rain everywhere in the vicinity of Raleigh, but they did involve a high probability of having some, and also a good chance of temperatures running notably cooler, with less humidity, for a few days. This may be followed by another round of fairly hot weather, but we'll be heading into the fall over the next few weeks and you can be assured that, with a few ups and downs along the way, we'll trend toward cooler weather.
Sep. 3, 2016 | Tags: general meteorology, heat, rain
Question: When I get a weather alert that says storms or lightning near your current location, how far from the location is the activity? — Patti
Answer: It depends a little on the alerts you have disabled or enabled in your app, but as examples, the lightning alert indicates that lightning has been detected within 10 miles, while the "Storms in area" alert means a thunderstorm is within 25 miles. There is also a "Dangerous Storm Approaching" alert you can enable. That one operates a little differently, as it is based on the potential for the storm to arrive within 15 minutes or less - there isn't a set distance, since that would depend on how fast the storm is moving. Finally, you can also receive warnings that are issued by the National Weather Service. In that case, it means that you fall within a geographic area outlined on a map that they consider to be under a threat for that type of storm (tornado, severe, etc) within the valid time of the warning.
Sep. 2, 2016 | Tags: preparedness, severe weather, wral.com
Question: It's so HOT! When will it start cooling down? — Destiny
Answer: The day this appears on the web site, we should have a cold front moving in from the northwest. We'll likely top 90 again just ahead of it, but then should have a stretch of more comfortable days (70s to mid 80s for highs, with decreasing humidity as well) behind the front and a departing tropical system for the Labor Day weekend. There are some hints that we'll heat back up some the week after, but eventually of course the changing seasons will mean cooler air will arrive more and more often, and stick around longer.
Sep. 1, 2016 | Tags: heat
Question: How is the "heat index" temperature calculated? — Diane
Answer: The idea of heat index is that when humidity levels are elevated and temperatures are high, the body is less able to cool itself by way of the evaporation of sweat, so that in terms of maintaining a normal body temperature, high humidity reduces our ability to do so to a rough equivalent of what it would be at a higher temperature, but lower humidity. The index used in the United States today is derived from work published by Robert G. Steadman in 1979 in a paper titled "The Assessment of Sultriness, Part I." His tables of apparent temperature, based on many factors and with a specific value assigned to each combination of temperature and relative humidity, can be seen, for example, in the graphic at www.nc-climate.ncsu.edu/climate/heat_index_climatology.php.
These values were later reduced to a regression equation that calculates heat index given inputs of temperature and humidity, and that can be adapted to calculate the values based on inputs of temperature and dew point. One version of that equation is shown next to the table at the address above, and it is worth noting that the equation is approximate, so its output will not exactly match all the values in the table.
You can find another, shortened and more approximate version of the equation relating heat index and relative humidity at www.srh.noaa.gov/fwd/?n=heat5. A handy calculator for computing approximate heat index based on knowing either temperature and relative humidity, or temperature and dew point, is available at www.srh.noaa.gov/ffc/html/metcalc.php.
Aug. 31, 2016 | Tags: apparent temperature, cool sites, heat, humidity/dew point
Question: Are the lack of 100+ degree F readings for an RDU year average, or normal, or statistically significant? — Jo La
Answer: At the time of your question, we had not reached the century mark at the RDU airport in 2016 so far, although we did reach 99 degrees on Friday, August 26th. In a historical sense, it is not at all unusual to go through a calendar year without recording any readings of 100-degrees or higher. In fact, looking through 128 years of data for Raleigh, we fail to reach 100 or more about two out of three years, or 66% of the time, as 84 of those 128 years did not include any days that hot. The top 5 years for numbers of 100+ readings were 1999 (12 days), 2012 (9), 2011 (9), 1952 (8) and 2007 (7).
Aug. 30, 2016 | Tags: heat, past weather
Question: When the number of daytime or nighttime hours is specified, is this the time between sunrise and sunset (when the upper edge of the sun is at the horizon) or the time between civil twilight (when the upper edge of the sun is six degree below the horizon)? — Jacob Smith
Answer: "Hours of daylight" is computed using your first suggested definition, that is from the time the upper edge, or "limb" of the sun crosses the horizon on its way up in the morning until the upper limb passes below the horizon at sunset. These times usually assume an observer at sea level and an unobstructed horizon. In addition, a "standard atmosphere" is usually assumed, since downward bending of light rays by our atmosphere causes the sun to appear higher in the sky that it really is when it is at low elevations (when we see the sun the last couple of minutes before sunset or the first couple of minutes after sunrise, it is actually below the horizon and would not be visible if not for this refractive effect).
Aug. 29, 2016 | Tags: astronomy
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Published: 2007-10-09 14:40:00
Updated: 2014-06-24 16:06:51
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