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Answer: Rain drops reach a terminal velocity that represents a balance between the force of gravity and the effects of air resistance, or drag, on the falling drop. The speed of a drop depends on its size, mass and shape, and whether the air it is falling through has any updrafts or downdrafts. Generally, very small drizzle-sized drops fall at about 5 mph, while large drops reach about 25 mph.
Sep. 15, 2009 | Tags: general meteorology, rain

Answer: We aren't aware of any webcams for the lake, but we do have the levels for Falls, Kerr, Jordan and Gaston lakes on our site. You can find a table with the most recent levels reported by the National Weather Service by clicking on the "Lakes and Rivers" link in the Weather Data section, or the "Recreational" link under Maps & Forecasts. Beneath the table there is a "Lake Level Trends" link where you can find graphs showing how the lake levels have varied compared to normal over the past month, 90-days, one year, and so on.
Sep. 14, 2009 | Tags: cool sites

Answer: The variable nature of precipitation from one location to another, especially in the warmer months, makes any such comparison subject to uncertainty. However, to see if your gauge is in the ballpark with surrounding sites, you can check readings from the Rocky Mount-Wilson airport at www.wunderground.com/history/airport/KRWI/2009/8/31/MonthlyHistory.html (note you can switch to daily or weekly views and select different dates). You could also check readings from volunteers with gauges in Nash and Wilson counties at www.cocorahs.org/state.aspx?state=nc, or compare your totals to gauge-corrected radar estimates from water.weather.gov/?loctype=WFO&loc=wfoRAH.
Sep. 13, 2009 | Tags: cool sites, instruments, rain

Answer: If we make the assumption that the forecasts are all "correct" and also that whether measurable rain occurs each day is independent of what happens on the other days, your choice of a 50% probability of precipitation for each of the seven days allows us to simplify the treatment of this question, since it becomes analogous to a series of coin flips. That is, each consecutive coin flip has a 50% chance of coming up heads, while each consecutive day has the same chance of producing rain. Taken as a group, there is only one outcome that includes rain all seven days (like heads on all seven coin flips), one that includes no rain at all the entire week, seven outcomes that result in one day with rain, 21 different combinations involving two days with rain, and so on for a total of 128 different possible combinations. The probability of no rain at all then, is one in 128, or about .8%, while the probability that it rains one or more days is 100% minus that number, or 99.2%. In practice, this may be unrealistically high due to the imperfection of weather forecasts and the fact that weather systems may prevent the day to day results from being truly independent. However, it does illustrate that since the forecasts indicates conditions that would on average produce measurable rain one out of every two times that the same pattern exists, a group of seven such days should have a good chance of resulting in precipitation.
Sep. 12, 2009 | Tags: general meteorology, rain

Answer: The idea behind finding a ditch or other significant terrain depression to take cover in is not that it is necessarily guaranteed to be safe, but that lacking any better alternatives, it is greatly preferable to being fully exposed to the intense winds of the tornado and especially to any debris (wood, glass, metal and so on) being carried at high speed by those winds. The hope is that the debris and the strongest wind will pass over your head if you can find a suitable spot to shelter in.
Sep. 11, 2009 | Tags: preparedness, tornadoes

Answer: The answer may disappoint you both, but on the other hand there's no winner or loser this way! Comparing average summer temperatures ("summer" in this case defined as June, July and August) we find that the RDU airport reading for summer 2008 was 79.4 degrees, while the 2009 average was... 79.4 degrees as well. The normal summer temperature based on the 30 year period from 1971 through 2000 is 76.9 degrees, so both years came in 2.5 degrees on the warm side of that figure.
Sep. 10, 2009 | Tags: heat, past weather

Answer: As you note, when we show the "live sweep" version of our Dual Doppler or Fayetteville Doppler radar systems, the precipitation echoes detected by the radars update as the sweep passes across a given area. However, lightning strikes are not detected by our radar systems, but by a network of direction-finding electromagnetic sensors operated by a company called Vaisala. We acquire that lightning information through our data vendors, and simply overlay it on the image you see using our radar display software.
Sep. 9, 2009 | Tags: instruments, lightning, weather radar

Answer: Relative humidity (RH) is a fraction expressing how close the air in question is to being saturated with water vapor. When air is saturated, any additional water vapor or any lowering of temperature, will result in some of the vapor condensing into liquid water. So, 50% RH means that the air contains about half the water vapor required for saturation, 95% means that it is almost saturated, and so on. RH depends on both the amount of water vapor in the air and the temperature, so it can change even if the amount of water vapor stays the same, or it can remain the same even if the amount of water vapor changes. Dew point is the temperature at which air would reach saturation (that is, RH would climb to 100%) if you cooled it to that level. Dew point depends only on how much water vapor is in the air, so that a higher dew point means more moisture and vice versa. For some additional discussion and examples, see www.wral.com/weather/blogpost/3423262/.
Sep. 8, 2009 | Tags: humidity/dew point

Answer: The National Climatic Data Center makes a downloadable publication called "Climatic Wind Data for the United States" available on its web site. This includes monthly average wind direction and speed for several locations in each state. See www.ncdc.noaa.gov/oa/documentlibrary/wind/wind1996.pdf for that product. You can also access "wind roses" showing a more detailed monthly distribution of wind directions and speeds for several cities in our state at ftp://ftp.wcc.nrcs.usda.gov/downloads/climate/windrose/north_carolina/.
Sep. 7, 2009 | Tags: cool sites, past weather, winds

Answer: That choice seems to derive from the gradual quantification of the Beaufort wind scale, which originally (1805) described winds with a series of 12 force levels that did not include wind speeds, but instead related to the impact on sailing vessels. Force 12 was defined as a hurricane. Later on, modifications to the scale added descriptions of sea states and wind effects that could be observed on land, and eventually an empirical equation was developed that related wind speeds in meters per second to Beaufort force level. When one enters 11.5 (lower bound of the "Force 12" category) into that equation, the result rounds off to 33 m/s, which likewise rounds off to 64 knots or 74 mph. In practice, the threshold is a de facto 75 mph, as assessments and advisories regarding tropical cyclone wind speeds, whether in knots or mph, are almost always rounded to the nearest 5, a reasonable approach given the combination of potential measurement errors and the spatial and temporal variability of the winds being sampled.
Sep. 6, 2009 | Tags: hurricanes, maps & codes