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Answer: As you imply with your question, it is certainly possible to have below normal rainfall while receiving rain in trace or rather small measurable amounts fairly often. This year has fit into that category to some extent. Through the end of October, we had six months with above normal number of days with at least one hundredth of an inch of rain, while four months were below normal. In total during that time, 104 days brought measurable rain, when the normal for the first ten months of the year is 95.
Nov. 12, 2009 | Tags: normals, rain

Answer: There are a variety of ways to specify probability forecasts, but what is typically meant with a "40% chance of rain," for example, is that any given place within the forecast area to which that forecast applies, and within the period for which that forecast is valid, should receive measurable rain (defined as one hundredth of an inch or more) about 4 times out of every ten that the 40% forecast is used. That pretty well equates to your second question. Usually, the probability does not explicitly address what percentage of time it will rain during the forecast period, or what percentage of the area will be covered, although those considerations may be implicit with some types of weather systems. It can be important to note what time periods, areas and precipitation amounts are being referred to. A typical NWS forecast includes a probability that at least one-hundredth of an inch will occur in a twelve-hour period generally labeled as "Today," "Tonight," "Wednesday," Wednesday night" and so on. On the other hand, we sometimes show a product on the air that uses a contour map of probabilities that measurable rain will fall within the three hours leading up to the time shown, and we sometimes show the same map but for the chance that rainfall will exceed one-tenth of an inch during the three-hour period. One would expect that for a given weather pattern, the chance of measurable rain would be lower for a three-hour period than for a twelve-hour span, and likewise, the chance of measurable rain would likely be higher than the chance of one-tenth of an inch or more over the same period.
Nov. 4, 2009 | Tags: general meteorology, maps & codes, rain

Answer: A glance at data for Hammonton on the NWS Precipitation Frequency Data Server confirms that you were directed toward the right ballpark on the 20-hour rainfall return interval, as the resulting graphs appear to place that amount of precipitation over that time frame at about a 3-4 year return period. A similar check for 2.2 inches of rain over a a 2.5 hour span turns up a rather similar return period of about 4-5 years. If you'd like to explore that site yourself, it is located at hdsc.nws.noaa.gov/hdsc/pfds/. Just click on the state of interest and select the location you want to research.
Oct. 9, 2009 | Tags: cool sites, past weather, rain

Answer: In some parts of the world, there are well-defined wet and dry seasons and in some cases people like to keep their rain statistics by resetting seasonal or yearly rain totals to zero at the beginning of, say, July or October. However, in our state precipitation is, on average, fairly evenly distributed throughout the year. That being the case, you would probably want to set your "rain season" to begin on January 1st and end December 31st, so that you can easily check "calendar year" progress, and so rainfall for the year will reset to zero as the next calendar year begins.
Oct. 6, 2009 | Tags: instruments, rain

Answer: The second part of your question is pretty easy to answer, as it isn't extremely unusual to have a day here and there when the entire state is free of cloud cover, with bright blue skies all around. This usually results from a large cool season high pressure system moving across the region. On the other hand, large storm systems can occasionally produce clouds and precipitation covering a majority of the state. However, even in these situations there are often some gaps in the precipitation coverage at any one time, and it would be rather difficult to verify that any particular day in history featured rain everywhere in our state, and all at the same time.
Oct. 5, 2009 | Tags: general meteorology, rain

Answer: The "normal" annual rainfall is just an average over a defined time period, that being the 30 years ending with the most recent "zero" year, and updated every ten years. Based on observed totals from the current "climate normals" period of 1971-2000 at RDU, the average yearly precipitation is 43.1 inches, and the standard deviation about that value is plus or minus 6.4 inches. This indicates that annual rainfall ranges between roughly 37 and 49 inches about 67% of the time, and between 31 and 55 inches about 95% of the time.
Oct. 1, 2009 | Tags: drought, rain, records/extremes, water resources

Answer: When sea water evaporates, nearly all the dissolved salt is left behind in the ocean, so that the rain produced by a hurricane is essentially fresh, although there can be some slight exceptions due to entrainment of salt water droplets scoured from the ocean surface by high winds and either blown directly onto land areas or carried up into some of the rain-producing clouds. To illustrate this, if you allow some sea water to evaporate from a dish or glass, you'll see that a crust of salt is left behind.
Sep. 30, 2009 | Tags: hurricanes, rain

Answer: There are a lot of influences on rain drop size and the distribution of sizes in a given rain event. Some of the more notable factors are the intensity of upward motions in the cloud, the relative humidity and degree of supersaturation available there, the concentration and quality of condensation and/or ice nuclei in the vicinity, and the temperature profile of the air within which the drops are forming. The size of droplets reaching the ground also depends on the temperature and relative humidity of the intervening air between the cloud base and the surface.
Sep. 29, 2009 | Tags: general meteorology, rain

Answer: There are several addresses where you can hunt up rain gauge reports from multiple official and unofficial sites around Wake County. For example, you can go to www.nc-climate.ncsu.edu/cronos and type "Wake" in the search box, then click on the name of any station for more details. For a sizable listing of nearby "personal weather stations," go to www.wunderground.com, type a Wake County town into the search form and click "Go," and scroll to the bottom of the resulting page. For a more graphical rain depiction based on a combination of radar and gauges, see water.weather.gov, click on the "NWS WFOs" button below the map, and select "Raleigh, NC (RAH)." You can then turn on/off county lines, highways, etc and can view rainfall amounts, departures from normal and more over varying time frames.
Sep. 28, 2009 | Tags: cool sites, past weather, rain

Answer: Like many processes in meteorology, it would be nice if there was a straightforward, simple answer to that! However, many other factors come into play in causing precipitation to form or dissipate and in causing pressure to rise or fall, and it is possible in specific circumstances for developing precipitation to result in pressure changes, and for pressure changes to trigger precipitation. Overall, though, it is probably reasonable to say that the more common and dominant relationship of the two you mentioned is for falling pressure to contribute to the formation of rain.
Sep. 18, 2009 | Tags: general meteorology, rain