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Answer: On a fair night with light winds and no frontal passages, the earth's surface loses heat by way of outgoing infrared radiation, and in turn cools the air in the lower atmosphere. This cooling process continues until incoming radiation from the rising sun begins to exceed the outgoing radiation, and the temperature starts to climb again. While there can be exceptions, the lowest temperature often occurs a little after sunrise as opposed to actually happening before the dawn.
Nov. 22, 2009 | Tags: cold, general meteorology

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: It's hard to know exactly what you might mean by "things" in this question. If one had to reduce a cloud to two physical components, perhaps you could say clouds are, at their most basic, made up of water and cloud condensation nuclei, as typical clouds in the atmosphere are composed of water droplets or ice crystals, and the tiny particulates of various kinds that can serve as nucleation points upon which the water can condense (as a liquid droplet) or deposit (as an ice crystal) given the correct combination of humidity, temperature and in some cases vertical motion.
Nov. 20, 2009 | Tags: clouds, general meteorology

Answer: There isn't a real obvious link back to an ancient Latin or Greek term as is the case with many modern words, but some etymology resources do cite a Greek word, aithria, that today would mean "good weather" and a later, more general term for weather, kairos. It isn't clear whether those words led in some direct way to later European terms like the Old Saxon word wedar, the Old Norse term vethr, or the Old High German and Old English words wetar and weder, but one can certainly relate that group of terms pretty well to the current form of the word in English, along with the current German wetter and Dutch weder.
Nov. 19, 2009 | Tags: general meteorology, past weather

Answer: Standard sea level pressure can be expressed in a number of different ways. For example, 14.7 pounds per square inch, 760 mm Hg, 29.92 inches Hg, 1013.25 millibars, 101.325 kiloPascals, 1 atmosphere and so on. Since they all represent the same pressure, you can use those values to convert between units.
Nov. 14, 2009 | Tags: general meteorology, normals

Answer: The answer all depends on how much water vapor is in the air already, as indicated by the dew point. Water can evaporate as long as the temperature of the air above it or surrounding it (in the case of suspended drops) has a dew point lower than the temperature of the water. If the water and air are at the same temperature, then as long as the temperature is higher than the dew point, water can evaporate. On the other hand, water vapor will condense into liquid water if the temperature of the air containing the water vapor is cooled to the dew point or below, or can condense onto an adjacent surface if that surface is cooled to the dew point or below.
Nov. 11, 2009 | Tags: general meteorology, humidity/dew point

Answer: Sound waves traveling horizontally through the atmosphere can be and usually are refracted, or bent, upward or downward depending on the vertical profiles of temperature (and the associated variations in density) and wind. Depending on the shape of the temperature profile and the origin point of the sound, it can be a simple matter of bending the sound upward and away from the surface so that sound can't be heard very far from the origin, or a simple matter of bending downward at a curvature close to that of the earth's surface so the sound can be heard very far away. There can also be more complex scenarios that create concentric shadow and amplification zones in which you might, for example, find yourself in a location where no sound is heard, while someone a mile closer to the source and someone a mile farther away hears it more loudly than usual. Conditions appeared to be conducive for a temperature inversion to form on the night you mentioned, but we would have to have detailed vertical profiles above the fairgrounds to work out any of the specifics of how the sound may have been distributed.
Nov. 10, 2009 | Tags: general meteorology

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: The jet stream can indeed be an important feature in marking the the evolution of upper level pressure patterns and can act as a steering feature for lower-level air masses and pressure systems, in addition to channeling smaller "jet streaks" that produce vertical motions that can cause clouds and precipitation to form or dissipate. The intensity, location and organization of the jet stream itself is influenced by the distribution of temperature, humidity and density of air at and below jet stream altitudes, which are in turn influenced by the temperature of land and water surfaces. The distribution and gradients of these temperatures, humidities and densities can also be affected by short-term, small to medium scale heating and cooling associated with condensation and evaporation of water in clouds and precipitation, and to large scale, slow-changing factors like the El Nino-Southern Oscillation. As you can see, it all sets up an interconnected, chicken and egg-like scenario (referred to in science as a "non-linear" system) in which the jet stream influences the movement and behavior of air masses and traveling storm systems, and is in turn itself influenced by that movement and behavior.
Oct. 31, 2009 | Tags: fronts & airmasses, general meteorology

Answer: A ridge in the atmosphere is an elongated area having barometric pressure greater than the pressure at the same elevation in most directions away from the ridge axis. On a weather map, ridges can be identified by a series of sharply curved isobars, with the isobars "pointing" toward areas of lower pressure, or by an area in which isobars stretch between two or more high pressure centers. The appearance of isobars on a weather map in these cases is analogous to the appearance of ridges in topography on a contour map of ground elevations.
Oct. 19, 2009 | Tags: fronts & airmasses, general meteorology