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Answer: Typically, we are looking at ridges and troughs as visualized on a flat piece of paper or computer screen. In order to visualize them in that manner, one usually uses contour lines or color-filled contours. There are two main ways used to accomplish this for pressure. First, one can pick a certain altitude and plot contours of equal atmospheric pressure at that altitude. This is commonly done for "surface" pressure, but showing contours of pressure at an altitude of mean sea level. For upper air charts, however, it is common to choose a pressure level, say 500 millibars or 300 millibars, and plot contours showing the height of that pressure "surface." Because of the near-hydrostatic relationship between pressure and altitude, this makes maps that appear very similar to equivalent pressure-contour maps centered on the altitude ranges appropriate to those pressures. When done in this way, you can imagine each pressure level as a 3-dimensional "sheet" in the atmosphere, in which the 500 millibar surface, for example, is located at a higher altitude along a ridge or within a high center than at other surrounding locations. The height contours used to show the pattern of highs and lows, ridges and troughs would then correspond nicely to the way an elevation contour map illustrates variations in topography of the land and sea surfaces.
Nov. 21, 2009 | Tags: general meteorology, maps & codes

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: Quite a few tropical cyclones form in the southern hemisphere, although the numbers are much higher in the north. Most southern hemisphere storms occur over the western South Pacific and across the southern Indian Ocean between Africa and Australia. On the other hand, they are extremely rare over the South Atlantic. You can get a visual sense of their worldwide distribution using a map of all tracks from 1945-2006 at upload.wikimedia.org/wikipedia/en/0/0f/Tropical_cyclones_1945_2006.png.
Sep. 24, 2009 | Tags: cool sites, hurricanes, maps & codes

Answer: The size of the "squares" would depend on the spacing interval chosen for the map in question. We frequently use maps with a spacing of every 5 degrees latitude and longitude and occasionally include a second set of lines with one degree spacing. The question then becomes, how much distance is associated with each degree? For latitude, each degree is roughly 60 nautical miles or 69 statute miles. For longitude, the same numbers apply at the equator, but since longitude lines all converge at the poles, the distance between them decreases with increasing latitude and is equal to 60 nautical miles times the cosine of the latitude. At 35 degrees north for example, a degree of longitude covers about 43 nautical miles, or 49 statute miles. For a concise discussion of longitude and latitude, see www.nationalatlas.gov/articles/mapping/a_latlong.html.
Sep. 19, 2009 | Tags: hurricanes, maps & codes

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

Answer: If you click the "Hurricanes" link at the upper right hand portion of our main weather page, and then scroll down the right hand side of that page, you'll find a "printable tracking map" image that will take you to a pdf format tracking map covering the Atlantic basin. The map includes a list of all the storm names for the 2009 season.
Sep. 5, 2009 | Tags: hurricanes, maps & codes

Answer: The terms have definitions that are based on the fraction of the sky covered by opaque clouds, and are not interchangeable as you've listed them. For example, a clear sky would have 0 eighths (or "oktas") of cloud cover, while mostly sunny would have 1-2 oktas, and partly cloudy would be 3-5. Moving to the other end of the scale, partly sunny is also defined as 3-5 oktas, while mostly cloudy is 6-7 oktas and of course overcast would be 8 oktas. You'll note partly sunny and partly cloudy have the same fraction of coverage. Technically, those two are interchangeable though informally some meteorologists tend to use partly cloudy for the brighter end of that middle range and vice versa.
Aug. 17, 2009 | Tags: clouds, maps & codes

Answer: That is a National Weather Service forecast zone designation. In our state, each county comprises a single zone, with a couple of exceptions where counties are broken into two zones due to a wide range of possible weather conditions. NC zone 075 is Moore County. For a list of other counties and their zone numbers in the eastern U.S., see www.weather.gov/mirs/public/prods/reports/pdf/forecast_zones/10-507_zones-er.pdf, and you can retrieve a map of NC with zone numbers annotated from www.weather.gov/mirs/public/prods/maps/pfzones_list.htm.
Jul. 19, 2009 | Tags: cool sites, maps & codes