| Section S | S index | 1201-1209 of 1376 terms |
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sulfur dioxide—Acidic gas, formula SO2, formed in the combustion of many fuels and in the oxidation of naturally occurring sulfur gases. It is the primary sulfur gas emitted from combustion sources and is a precursor to sulfuric acid, which is a major constituent of acid rain.
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sulfur hexafluoride—Extremely unreactive gas, formula SF6, which is of entirely anthropogenic origin. It is used as a dielectric insulator or to provide a chemically inert atmosphere. Due to its inertness, SF6 has a very long atmospheric lifetime (>1000 years) and has been used to estimate the age of stratospheric air. It is also used as a chemical tracer in tropospheric field experiments.
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sulfur rain— Rain of yellowish color caused by foreign matter (e.g., pollen, yellow dust) picked up by raindrops during descent. A dust-filled subcloud layer is required to yield this effect, and the particles must contain sufficient sulfur dioxide to be yellow in color. Compare blood rain, mud rain.
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sultriness—An oppressively uncomfortable state of the weather that results from the simultaneous occurrence of high temperature and high humidity. It is often enhanced by calm air and cloudiness. This term is gaining technical recognition in light of the increased study of human comfort and air conditioning. Sultriness has been expressed in terms of the values of various measures of comfort. Some of the lower limits are water vapor pressure of 14 mm; dewpoint of 19°C (65°F); equivalent temperature of 56°C (133°F); effective temperature of 24°C (75°F); and the following pairs of temperature and relative humidity, 35°C (95°F) and 25%, 30°C (86°F) and 40%, 25°C (77°F) and 65%. See comfort chart, comfort zone.
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sumatra—A squall, with wind speeds occasionally exceeding 13 m s−1 (30 mph), in the Malacca Strait between Malay and Sumatra during the southwest monsoon (April through November). It usually blows from the southwest, sometimes from the west or northwest, raising a heavy sea on the Malay coast. The wind veers and strengthens and a heavy bank or arch of cumulonimbus arcus passes overhead (arched squall) with heavy rain and often thunder. Sumatras usually occur at night; they bring a sudden drop in temperature and are generally due to the descent of air cooled by radiation on the high ground of northern Sumatra. In a few cases they mark an air mass boundary during the advance of the monsoon. They are said to occur simultaneously along a line of 320 km (200 miles) or more that advances in a direction between southeast and northeast at about 9 m s−1 (20 mph).
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summation principle—The principle that the sky cover at any level is equal to the summation of the sky cover in the lowest layer, plus the additional sky cover in all successively higher layers up to and including the layer in question.
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summer monsoon—The summer phase of the annual cycle of winds driven by the land–sea thermal contrast. The heating of the landmasses during summer leads to a deficit of atmospheric pressure over the continents, resulting in an onshore flow. Other factors such as the topography of the land have a considerable effect. The summer monsoon is strongest on the southern and eastern sides of Asia, but it can also occur over northern Australia, parts of Africa, the southwestern United States, and the Mediterranean. See monsoon; compare winter monsoon.
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