الأرصاد الجوية

سابعاً :
العوامل المركبة ذات الفعالية في الغلاف الجوي وتوليد الظواهر الجوية

الجبهات الهوائية

نشاط مقترح

حاول ترجمة الفقرتين الاتيتين حول الجبهة الباردة والدافئة لمزيد من المعلومات علما أن هاتين الفقرتين اخذتا من موقع على شبكة الإنترنت ستجده في قائمة المراجع .

Cold Fronts

Cold fronts are usually associated with depressions. A cold front is defined as the transition zone where a cold air mass is replacing a warmer air mass. At a cold front cold air following warm air undercuts the warm air at a warm front. The air associated with a cold front is usually unstable and conducive to cumulonimbus cloud formation. Because the up thrust is delivered along a boundary between the two air masses, the cumulonimbus form a well-defined line in contrast to the well-spaced clouds forming during thermal convection. Usually, rainfall associated with cold fronts is in the form of heavy deluge. More rain may fall in a few minutes as the cold front passes, the clouds roll by and the air temperature may become noticeably cooler, with temperatures dropping by 5 ْ C or more within the first hour.

On synoptic (weather) charts a cold front is represented by a solid line with triangles along the front pointing towards the warmer air and in the direction of movement. On colored weather maps, a cold front is drawn with a solid blue line.

Warm Fronts

A warm front exists when warm air is rising over cold air. In vertical cross-section, the boundary takes the form of a gradual slope (roughly 1: 100) and lifting is slow but persistent. As the air lifts into regions of lower pressure, it expands cools, and condenses water vapour as flat sheet cloud (altostratus), from which rain can start to fall once cloud has thickened and ***** a weight of 2500 from the ground. Cloud continues to lower towards the boundary at ground level, known as the surface front. This lower level cloud is called stratus or nimbostratus, from which appreciable amounts of rain may fall. Sometimes, nimbostratus cloud may be only a few hundred meters above the ground, and can completely cover hilltops and mountains.

Because frontal systems have a velocity of there own, an observer on the ground will witness a succession of cloud types with cloud gradually thickening before rain arrives. These telltale signs can be used by the observer to predict the onset of bad weather within a few hours. When the surface warm front arrives, there may be a burst of rather heavier rain, and this offers a hopeful sign that a drier interlude is on the way. Clouds will break, rain cease, and there may be a noticeable rise in temperature as the warm air engulfs the observer.

On synoptic (weather) charts a warm front is represented by a solid line with semicircles pointing towards the colder air and in the direction of movement. On colored weather maps, a warm front is drawn with a solid red line.



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تحرير : المدرسة العربية www.schoolarabia.net	إعداد : أ. خالد الشحام
	أ. سليم حمام
	تاريخ التحديث : شباط 2004