Ask Greg

Weather Questions tagged “rain”

(remove tag filter)

Question: What does a double rainbow mean? Thank you. — Pat

Answer: The principal indication given by a double bow is that the raindrops that are refracting and reflecting sunlight are of medium to large size, rather than being very small or tiny droplets. The larger droplets produce narrower and brighter bows, making the secondary bow more visible than it would be with smaller drops. The primary bow, produced by light rays reflected once within the drops, occurs along a circle that is about a 42-degree angle from the center of that circle, while the secondary bow, produced by light rays that reflect twice within the drops, occurs at about a 51-degrees angle. One thing to notice when you see a double bow is that due to the second reflection the colors are reversed - the primary bow is red on the outside edge, while the secondary is red on the inside.The secondary bow is a good deal fainter than the primary, so that when the drops are small and the bow is faint, it may not be noticed at all. There's a lot of great information about rainbows, along with diagrams explaining how they form and photographic examples of the many different forms, at http://www.atoptics.co.uk/bows.htm.
Jul. 14, 2016 | Tags: atmospheric optics, rain

Question: Storm just rolled through and at 9:10PM the temp has dropped to around 74 degrees. On your hour by hour forecast, it is predicted to be around 80 degrees about now. When temperatures drop this time of night due to rain, will the temperatures rise even though there is no sun to heat the air back up? Also, can we expect the overnight temperature to be lower since we are now cooler than expected for this time of night. — Joseph

Answer: The situation you describe has enough variables involved that there isn't a single yes or no answer. As with many situations in weather, "it depends" on a number of factors, and there will be times when the rain-cooled air will regain some of the lost temperature (if, for example, the cooled air is rather shallow and mixing brings warmer air down from above, or the rain-cooled air covers a small horizontal area and advection causes warmer air from another location to flow back over the area that had cooled some, or if the ground had become very warm during the day and some stored heat is released that is able to overcome other cooling effects during the night). On the other hand, absent some of those effects, it is also possible for the temperature to remain near, or to simply fall from, the newly reduced level. The most rapid fall would typically occur if skies clear out after the rain and winds remain light. A similar set of considerations affects whether the rain-cooled air will eventually fall to a lower temperature than it would have in the absence of the rain - in some cases, yes, in some no. On the night you asked about, the temperature at RDU fell from 82 just before the rain to 75 just after (at about 9 PM), then ticked up to 76 by 1 AM before dropping to 74 at 2 AM, only to edge back up to 75 at 3 AM. After that it drifted as low as 72 right around sunrise.
Jul. 13, 2016 | Tags: general meteorology, rain

Question: One of the questions references an almanac and rainfall amounts. Is that Online? — Lyn Triplett

Answer: It is indeed. The particular product showing how rain at RDU compares to "normal" over the past five years is located in the "Almanac" section of our web site. Just look for the link in the upper part of the web page. Then, on the Almanac page, you'll see a link for "RDU rainfall Charts," where you'll find graphs of cumulative rain totals versus cumulative normals for several periods ranging from 30 days to 5 years.
Jul. 12, 2016 | Tags: past weather, rain, wral.com

Question: The local pool certainly seems cooler after a significant rain. Are there any generalizations you can make about the temp of the rain during a typical summer storm? — Nadine

Answer: There are, although the situation is complicated enough that some generalizations are indeed necessary, since the temperature of rain reaching the surface depends on its initial temperature in the formation region aloft, the transfer of energy into the drops (warming) as they fall toward the surface, the cooling of the drops as they evaporate (offset in some cases by condensation of additional moisture onto the drops as they fall through warmer, humid air), the size of the drops, and the presence or absence of hail in the storm (even if the hail melts before reaching the surface. All that said, it is common in summer storms for rain temperatures to range anywhere from mid 50s to around 60 degrees in the presence of significant hail, to upper 60s to mid 70s otherwise. Of course, given that summer storms often develop or roll in when temperatures near the surface are in the 80s or 90s, this can indeed have a sharp cooling effect!
Jul. 9, 2016 | Tags: general meteorology, rain, thunderstorms

Question: Looking at the almanac, NC has had almost 40" above normal rainfall in the past 5 years. Even the 30, 60, 90-day almanac shows consistently higher than normal amount of rainfall here. I thought it was just my imagination but we really are getting a lot more than normal rain here in NC. What's up with that, in your opinion? — D.

Answer: You're correct, but it is really pretty difficult to provide a satisfying answer beyond the fact that "normal" is a 30 year average value (ending with the most recent "zero" year and updated every ten years) that results from years having a fairly wide spread of rainfall amounts, and in which some multi-year longer scale variations are evident when you look at a really long-term plot of annual totals. in the case of the past 5 years, for example, 2015, 14 and 13 all were notably wetter than normal, but the 4 years prior to that were all at or below normal. Historically, in records going back to 1887, we see some stretches with a steady average but large swings year to year, interspersed with a number of stretches of 4-6 years that favor more frequent above (late 1940s to early 50s, mid 1990s to around 2000) or below (1880s to early 1890s, mid 1960s to around 1970) amounts. We'll have to wait a few more years to see how this year and the past three stack up in that historical sense, and whether any further long-term patterns become evident.
Jul. 6, 2016 | Tags: past weather, rain

Question: Hi! I know this is a more specific question, but I'm not sure how to get the answer other than to go to "The Fish!" I am trying to find out what time rain entered the Raleigh/Knightdale area on 6/28/16. — Erica Hinton

Answer: An exact time for a particular spot is touch to pin down due to a few scattered sprinkles and showers that occasionally formed and dissipated out ahead of a more widespread, steady band of rain and showers that morning. Based on radar and surface station observations, however, for Raleigh the first sprinkles and patches of rain tracked in from the west and southwest around 8 AM or just after, while the steadier rain began to pick up closer to 9 AM. In both cases, the timing would have been about 15 to 30 minutes later for Knightdale.
Jul. 2, 2016 | Tags: past weather, rain, weather radar

Question: I need daily rainfall totals for research I am doing near the Arboretum at NC State University. Is there a database that exists with this kind of information? — Joel Burley

Answer: Our primary recommendation for you would be the version of the Applied Climate Information System at xmacis.rcc-acis.org/. There are a couple of functions under the single-station, like "Daily Data Listing" and "Daily Data for a Month," that you may find very helpful. There is a cooperative station at or very near your location of interest. You can access that information in the "Station Selection" drop down by choosing "Raleigh State Univ." If that doesn't appear in the list, make sure the "Change CWA" section has been set to "RDU - Raleigh."
Jun. 26, 2016 | Tags: cool sites, past weather, rain

Question: I noticed for the rolling five year weather chart we are 3 feet above normal. Does this become the "new normal" at some point? — Thomas Best

Answer: It does indeed, if it is in fact part of a longer term pattern that sustains itself. However, by design, "normal" is intended to resist sharp changes that might be overly influenced by short-term stretches of unusual weather. As you noted, if you averaged it out, the past 5-year period runs about 7.3 inches per year above normal. The term "normal" in this case is the 30-year average ending with the most recent "zero" year, so current normal annual rainfall is the average yearly amount for the period 1981-2010. The next update to the normal values will come shortly after we finish out the year 2020, and if we were to continue with the higher rainfall amounts of the past 5 years, it would produce a noticeable jump in the resulting normal value, but the magnitude of that jump would still be restrained a bit by data from the previous two decades. The definition of normal was chosen to balance the need for reasonably current values of normal for various weather parameters against the need for some stability in those numbers and some resistance to undue influence of short-term fluctuations, as noted earlier.
Jun. 5, 2016 | Tags: normals, past weather, rain

Question: What was the amount of rainfall for the 7 days ending with 5-24-16? — Martin Johnson

Answer: We weren't sure of your location, but for the seven days comprising May 18-24, 2016, we checked rainfall totals for the RDU sirport and at two State Climate Office stations around Raleigh (at Reedy Creek Rd and along Lake Wheeler Rd) and found totals there of .71, .98 and 1.1 inches, respectively. We also looked over a contour map of gauge-adjusted radar rainfall estimates for that period from the National Weather Service's Precipitation Analysis Page. The map indicated around .5-1" for much of Wake County, while area to the west varied between .5 and 1.5", and areas to the east around the I-95 corridor generally received amounts in the .1-.5" range. You can check daily archived maps of this sort showing 8AM-8AM precipitation totals at water.weather.gov/precip/.
Jun. 1, 2016 | Tags: cool sites, past weather, rain

Question: When your weather forecast for Durham says "60% probability of rain at 9AM," does that mean that we should expect that on 60% of the days that you make that forecast at least one drop of rain will fall you between 8:30 AM and 9:30 AM regardless of where you are in Durham County, or does it mean that we should expect that on 60% of the days that you make that forecast at least one drop of rain will fall between 8:30 AM and 9:30 AM on every square inch of Durham County, or does it mean that we should expect that regardless of where you are in Durham County it will be raining 60% of the time between 8:30 AM and 9:30 AM on the day for which you make that forecast, or something else? (Note that for some of these options, the probability should change depending on how large a time interval you specify or on how large an area you are making a prediction for.) — Ned Kennington

Answer: The closest of your descriptions to what a probability of precipitation is usually defined as is the one that notes that for a given point in the forecast area, one would expect measurable rain (.01" or more) to occur on about 6 out of ten days with similar conditions, within the time frame for which that forecast is valid. Historically, these kinds of probabilistic forecasts were developed for and applied to periods of time on the order of 6-12 hours, and so there are some issues worth being aware of with applying such numbers on a more granular basis like an hourly forecast. You make a good point that by statistical theory, for example, if a 60% chance applies to a "day" (say 7 AM - 7 PM), most hourly values should be lower than that, and likewise, a series of 60% probabilities over a 6-hour period would imply a 6-hour probability higher than 60%. However, in practice the probability of precipitation is arrived at by a combination of objective guidance techniques based on statistical relationships between model output and past results when those conditions were projected, fraction of model ensemble members that produce measurable precipitation at a given point in time and space versus those that don't (each of which can come from several guidance tools that may suggest notably variable probability values), and the forecaster's theoretical and experience-based sense of how a given pattern tends to play out. The end result is a partly objective, partly subjective sense of confidence by the forecaster (on a 0-100% scale) of the likelihood of measurable precipitation. Typically, the resulting hourly values will top out at the value the forecaster selects for a daily value or nightly value (roughly 12-hour probabilities), so if you see a forecast of a 60% probability for, say, "Tuesday," you might find that the hourly forecasts would show a 60% chance from 8 AM to 11 AM, when the forecaster expected the best chance of rain due to a passing front, and then a 40% chance at noon trending down to 10% by 7 PM. Depending on the distribution of probabilities throughout this period, 60% may be higher or lower than a statistically proper consolidation of all the hourly values. One small additional consideration, just to further the level of complexity, is that during the cool season, with precipitation most associated with traveling weather features that have a probabilistic likelihood of ending up properly organized and located so as to produce precipitation, the probability tends to correspond more to whether precipitation actually forms or not, over a fairly sizable area. On the other hand, during the warm, unstable months with convective showers and thunderstorms, there may be more a question of how much area they will cover (combining coverage at a given time with movement of the cells) so that there may be a high chance that some cells actually form, but the probability will correspond to what percentage of the area is expected to receive rain (will the storms be isolated, scattered, numerous, widespread, etc?).
May. 17, 2016 | Tags: general meteorology, maps & codes, rain, wral.com

Questions 1 - 10 of 227.


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.