- 行业: Weather
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The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, AMS has a membership of more than 14,000 professionals, ...
The leader which, after the first stroke, typically initiates each succeeding stroke of a multiple-stroke flash lightning. (The first stroke is initiated by a stepped leader. )
The dart leader derives its name from its appearance on photographs taken with streak cameras. The dart leader's brightest luminosity is at its tip which is tens of meters in length, propagating downward at about 107 m s−1. In contrast to stepped leaders, dart leaders do not typically exhibit branching because the previously established channel's low gas density and residual ionization provide a more favorable path for this leader than do any alternative ones.
Industry:Weather
A technique for using the radiosonde observation to determine the presence of liquid water droplets in supercooled clouds in saturated or nearly saturated layers of air. For each reported level in the sounding, the negative value of eight times the dewpoint spread (−8D) is plotted on the pseudoadiabatic chart (or equivalent chart). Where the temperature sounding lies to the left of the −8D curve, liquid droplet clouds are considered to be present, and icing is possible on aircraft flying in the cloud layer.
Industry:Weather
A duststorm caused by cyclonic winds in the vicinity of the River Darling in Australia.
Industry:Weather
A photographic effect in which lightning discharges register as dark instead of light due to multiple exposures caused by successive members of a composite flash. This effect, which results from a characteristic of the exposure versus illumination curve of a photographic emulsion, is sometimes called the Clayden effect after its English discoverer.
Industry:Weather
The process by which the human eye's sensitivity increases in response to marked luminance decreases. If the average luminance on the retina decreases suddenly from daytime photopic levels, a light source's just-detectable threshold luminance (alternatively, its threshold illuminance at the eye) decreases steadily for three to four minutes. This occurs because the eye's cones become much more sensitive during this period, although their sensitivity does not increase afterward. Note that cones are the only photoreceptors found in foveal (or central) vision, but that they coexist with rods outside the fovea. After about seven to ten minutes in very low-light scotopic conditions (0. 1 lux or less), the rods become more sensitive than the cones, and visual sensitivity once again increases, reaching a maximum at about 20–30 minutes. The fully dark-adapted eye is nearly 105 times more sensitive than it is at daytime light levels, and the maximum rod sensitivity is about 103 times the maximum cone sensitivity. Although dark-adapted observers have a much lower threshold illuminance for detection, this is offset by the greatly increased threshold contrast in scotopic conditions. Mesopic vision occurs when both rods and cones contribute to vision at light levels between entirely photopic and scotopic conditions. Cones dominate photopic vision while only rods contribute to scotopic vision. Color vision is not possible in scotopic conditions, and the peak spectral sensitivity of scotopic vision occurs at shorter wavelengths than it does for photopic vision. Thus to the dark-adapted eye, blues and greens tend to look brighter than reds (the Purkinje shift).
Industry:Weather
1. As used in aviation observations prior to 1 September 1956, descriptive of a cloud cover that is composed of predominantly threatening or unusually dark-colored clouds. It was denoted by the symbol “+” preceding the appropriate sky cover symbol. See thin. 2. For observing at night, a person needs two to three minutes to adapt to dark (dark adaptation) before a noninstrumental observation, e.g., visibility at night.
Industry:Weather
Volume of water flow per unit area per unit time passing through a porous medium in the direction of interest. See specific discharge.
Industry:Weather
Warm–cold oscillations during the last glacial period recorded in the oxygen isotope record of the Greenland ice, and also found in biotic and isotopic indices from deep-sea sediments in the North Atlantic. The warm phases of these events correspond to interstadials, persist for a few hundred to a few thousand years, and have very rapid onset and termination (as little as a few decades). The range of temperature change inferred for the regions where the snow was formed that ultimately produced the Greenland ice was several degrees Celsius. Named for the ice core paleoclimatologists Willi Dansgaard and Hans Oeschger, Dansgaard–Oeschger events between 80 000 and 20 000 years ago are grouped in 10 000- to 15 000-year periods of increasing cooling. The ending of each of these groups of events is marked by a major flux of icebergs into the North Atlantic, as evidenced by associated material found in ocean floor sediments. The temperature conditions in Greenland and the North Atlantic region then return to the higher level of the beginning of the group of Dansgaard–Oeschger events. Most of the Dansgaard–Oeschger events that lasted 2000 years or more coincide with warmer conditions in East Antarctica, also revealed by analysis of oxygen isotopes in ice.
Industry:Weather
The side of a tropical cyclone to the right of the direction of movement of the storm in the Northern Hemisphere (to the left in the Southern Hemisphere), where the winds are stronger because the cyclone's translation speed and rotational wind field are additive. The opposite side is termed the navigable semicircle. This terminology originated in the days of sailing ships. It occurred naturally since 1) the dangerous semicircle of the storm has the strongest winds and heaviest seas; 2) a sailing ship on this side tends to be carried into the path of the storm; and 3) if the storm recurves, its center is likely to cross the course of a ship running before the wind.
Industry:Weather
