降水
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降水是指在大氣中冷凝的水汽以不同方式下降到地球表面的天气现象。大气中的水气几乎全部集中于对流层中，温度越高，大气可以容纳的水汽含量就越多，反之就越少。一定温度下，当空气不可容纳更多的水汽时，称为饱和空气。当饱和空气中的水汽和温度相匹配时，不会出现水汽凝结现象，但当空气达到过饱和状态时，则会产生多余的水汽并发生水汽凝结。
过饱和空气的形成主要是由于空气的上升运动，造成气温下降，形成过饱和水汽；加上吸湿性较强的凝结核的作用，水汽凝结成云，来自云中的云滴，冰晶体积太小，不能克服空气的阻力和上升气流的顶托，从而悬浮在空中。当云继续上升冷却，或者云外不断有水汽输入云中，使云滴不断地增大，以致于上升气流再也顶不住时候，才能从云中降落下来，形成雨、雪、雹等降水天气。
目录 |
[编辑] 降水类型
- 对流雨：近地面空气强烈受热，引起空气的对流运动，湿热空气在上升过程中，随气温的下降，水汽凝结成的降水叫对流雨。特点是强度大，历时短，范围小，还常伴有暴风，雷电，故又称热雷雨。
- 地形雨：暖湿气流在运行的过程中，遇到地形的阻挡，被迫沿着山坡爬行上升，从而引起水汽凝结而形成降水，称为地形雨。地形雨一般只发生在山地迎风破，背风坡气流存在下沉或者下滑，温度不断增高，形成雨影区，不易形成地形雨。
- 锋面雨：冷暖性质不同的气团相遇，使得暖气团被迫抬升而形成的降水天气。
- 气旋雨：如台风雨等等。
[编辑] 空氣飽和的過程
空氣包含水分，並以每公斤乾空氣一克作為量度單位，但現在大多普遍以相對濕度的形式表達，例如百份之五十。空氣的溫度取決空氣本身可以容納多少水分才達至飽和，而暖的空氣比冷的空氣可以容納更多的水分。亦由於空氣有如此特性，冷卻空氣可以使其飽和。而露點溫度則為使既定的空氣份量達到水分飽和所需的溫度。當然，增加空氣中的水分也可以使空氣飽和。
[编辑] 冷卻機制
- Lift (Daytime Heating, Convective, Orographic)
- Conductive Cooling (warm air moves over a cool surface)
- Radiational Cooling (heat radiates off into space at night)
- Evaporative Cooling (Air Temperature falls as liquid water uses the energy to changes phase to vapour)
[编辑] 增加水分
- Precipitation falling from above (Stratus forming in the rain under a higher cloud)
- Daytime Heating Evaporates water from the surface of oceans/lakes
- Drier air moves over open water (Snow streamers off the Great Lakes in winter)
[编辑] 降水的形成
[编辑] 冷凝
降水由溫暖而潮濕的空氣所組成。當空氣冷卻時，水蒸氣開始變成凝結核，逐漸地成為雲朵。當水點變得夠大的時候，有兩個過程的發生會可以導致降水。
[编辑] 合併
合併 occurs when water droplets fuse to create larger water droplets, or when water droplets freeze onto an ice crystal. Air resistance typically causes the water droplets in a cloud to remain stationary. When air turbulence occurs, water droplets collide, producing larger droplets. As these larger water droplets descend, coalescence continues, so that drops become heavy enough to overcome air resistance and fall as rain. Coalescence generally happens most often in clouds above freezing.
[编辑] 白吉龍–芬地生過程
白吉龍–芬地生過程 occurs when ice crystals acquire water molecules from nearby supercooled water droplets. As these ice crystals gain required mass, they begin to fall. this generally acquires more mass than coalescence when occurring between the crystal and neighboring water droplets. This process is temperature dependent, as supercooled water droplets only exist in a cloud that is below freezing. In addition, because of the great temperature differential between cloud and ground level, these ice crystals may melt and become rain as they fall.
[编辑] 途徑
[编辑] 鋒的活動
天氣系統裡，空氣緩慢上升﹝以每秒厘米的速度計算﹞會形成成層的或者大規模的降水，例如沿著冷鋒與暖鋒之前的位置。相似的上升也可以在熱帶氣旋的風眼外圍位置，以及中緯度氣旋的雲帶頭部(comma head)見到。
[编辑] 對流雲降水
對流雨所引起的降水來自不同的對流雲，比如積雨雲和濃積雲。It falls as showers, with rapidly changing intensity and at one moment only over patches, as convective clouds have limited horizontal extent. Convective precipitation is most important in the tropics. Graupel and hail always indicate convection. In midlatitudes, convective precipitation is associated with cold fronts (often behind the front), squall lines, and warm fronts with significant available moisture.
[编辑] 地形性降水
地形性降水 occurs on the windward side of mountains and is caused by the rising air motion of a large-scale flow of moist air across the mountain ridge, resulting in adiabatic cooling and condensation.
In mountainous parts of the world subjected to relatively consistent winds (for example the trade winds), a more moist climate usually prevails on the windward side of a than on the leeward (downwind) side. Moisture is removed by orographic lift, leaving drier air (see katabatic wind) on the descending (generally warming), leeward side where a rain shadow is observed.
Orographic precipitation is well known on oceanic islands, such as the Hawaiian Islands, where much of the rainfall received on an island is on the windward side, and the leeward side tends to be quite dry, almost desert-like, by comparison. This phenomenon results in substantial local gradients of average rainfall, with coastal areas receiving on the order of 500 to 750 mm per year (20 to 30 inches), and interior uplands receiving over 2.5 m per year (100 inches). Leeward coastal areas are especially dry 500 mm per year (20 inches)at Waikiki), and the tops of moderately high uplands are especially wet ~12 m per year (~475 inches) at Wai'ale'ale on Kaua'i).
In South America, the Andes mountain range blocks most of the Pacific moisture that arrives in that continent. On the leeward side of the Andes is the Atacama Desert of Chile. It is also blocked from moisture by moutains to its west as well. Not coincidentially, it is the driest place on earth. The Rocky mountain range creates the same affect in North America forming the 大盆地沙漠、莫哈維沙漠與索諾蘭沙漠。
[编辑] 熱帶性活動
Tropical rainfall is large air masses several hundred miles across with low pressure at the centre and with winds blowing around the centre in either a clockwise direction (Southern hemisphere) or anti-clockwise (Northern hemisphere). Precipitation arises when a warm front is formed by an advancing mass of warm air, which moves up an inclined surface of retreating cold air and is chilled in the process of being lifted up resulting in rainfall. (Manning 1997)
The Great Sandy Desert has nearly all its rain during from monsoonal thunderstorms or the occasional tropical cyclone rain depression. Thunderstorm days average 20-30 annually through most of the area (Burbidge 1983) although the desert has fairly high precipitation rates due to the high rates of evaporation this area remains an arid environment with vast areas of sands.
Other areas of the world, which see these rare precipitation events in drylands, are Northwest Mexico, South West America, and South West Asia. In North America in the Sonoran and Chihuahuan desert have received some tropical rainfall in the last 10 years. Tropical activity is rare in all deserts but what rain does arrive here is important to the delicate ecosystem existing.
[编辑] 降雨特徵
[编辑] 大小與形狀
當大氣層中的水分粒子凝結成小水點，雨因而便會形成。雨水可以是任何形狀，不過直經約為9毫米。However, they do tend, if turbulence does not interfere, toward an aerodynamically stable shape (tear-drop) because this affords the least surface resistance to movement.
[编辑] 強度與長短
通常強度與降雨時間的長短成反比，換句話說，高強度的暴風雨很可能會持續較短的時間，而低強度的暴風雨會持續比較長的時間。
[编辑] 強度與面積
We can expect a less intense rainfall over a large area than we can over a small area.
[编辑] 強度與水滴大小
高強度的暴風雨比低強度的暴風雨有更大的水滴大小。
[编辑] 降雨量度
The standard way of measuring rainfall or snowfall is the standard rain gage, which can be found in 4-inch/100 mm plastic and 8-inch/200 mm metal varieties.[1] The inner cylinder is filled by 25 mm/1 inch of rain, with overflow flowing into the outer cylinder. Plastic gages will have markings on the inner cylinder down to 0.25 mm/0.01" resolution, which metal gages will require use of a stick designed with the appropriate 0.25 mm/0.01" markings. After the inner cylinder is filled, the amount inside it is discarded, then filled with the remaining rainfall in the outer cylinder until all the fluid in the outer cylinder is gone, adding to the overall total until the outer cylinder is empty. These gages are winterized by removing the funnel and inner cylinder and allowing the snow/freezing rain to collect inside the outer cylinder. Once the snowfall/ice is finished accumulating, or as you approach 300 mm/12", one can either bring it inside to melt, or use luke warm water to fill the inner cylinder with in order to melt the frozen precipitation in the outer cylinder, keeping track of the warm fluid added, which is subsequently subtracted from the overall total once all the ice/snow is melted.
Other types of gages include the popular wedge gage (the cheapest rain gage and most fragile), the tipping bucket rain gage,[2] and the weighing rain gage.[3] The wedge and tipping bucket gages will have problems with snow. Attempts to compensate for snow/ice by warming the tipping bucket meet with limited success, since snow may sublimate if the gage is kept much above freezing. Weighing gages with antifreeze should do fine with snow, but again, the funnel needs to be removed before the event begins. For those looking to measure rainfall the most inexpensively, a can that is cylindrical with straight sides will act as a rain gage if left out in the open, but its accuracy will depend on what ruler you use to measure the rain with. Any of the above rain gages can be made at home, with enough know-how.[4]
Once someone has a device to measure precipitation, various networks exist across the United States and elsewhere where rainfall measurements can be submitted through the internet, such as CoCoRAHS[5] or GLOBE.[6] If a network is not available in the area where one lives, the nearest local weather office will likely be interested in the measurement.[7] An important use of precipitation data is for forecasting of river flows and river water quality using hydrological transport models such as SWMM, SHE or the DSSAM Model.
[编辑] 重現期
The likelihood or probability of an event with a specified intensity and duration, is called the 重現期 or frequency. The intensity of a storm can be predicted for any return period and storm duration, from charts based on historic data for the location.
[编辑] 泛濫頻率
現時沒有一個可靠而準確的辦法去預計when the next flood will arrive or how big it will be, but past flooding events can help provide some information as to what to expect.
[编辑] 十年一遇的風暴
「十年一遇的風暴」這個名詞用來形容a rainfall event which is rare and is only likely to occur once every 10 years, so it has a 10% likelihood any given year. The amount of rain which will fall will be greater and cause the flooding to be worse than for a 1 in 1 year event。
[编辑] 百年一遇的風暴
「百年一遇的風暴」(也參見百年一遇的洪災)形容a rainfall event which is extremely rare and which will occur with a likelihood of only once in a century, so has a 1% likelihood in any given year. The amount of rain which will fall will be extreme and cause far greater flooding to be worse than for a 1 in 1 year event。
As with all “probability” events, it is still possible to have two 「百年一遇的風暴」 in the same year。
[编辑] 參見
[编辑] 參考資料
- ↑ http://www.crh.noaa.gov/iwx/program_areas/coop/8inch.php
- ↑ http://www.oardc.ohio-state.edu/williams.2157/Tipping_Bucket_Rain_Gage.htm
- ↑ http://www.weather.gov/glossary/index.php?letter=w
- ↑ http://school.discovery.com/lessonplans/activities/weatherstation/itsrainingitspouring.html
- ↑ http://cocorahs.org
- ↑ http://www.globe.gov/fsl/welcome/welcomeobject.pl
- ↑ http://www.nws.noaa.gov
[编辑] 外部連結
| 氣象學資料與變數 |
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氣壓 | 斜壓度 | 雲 | 水蒸汽 | 風 | 降水 | 閃電 | 對流 | 对流有效位能 | 對流抑制指數 | 露點 | 風寒指數 | 熱濕指數 | 濕度 | 溫度 | 位溫 | 相當位溫 | 海面溫度 | 太陽輻射 | 天氣圖 | 能見度 | 渦度 |
