للعلم
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عند انخفاض درجة الحرارة في السحب إلى ما دون درجة التجمد تتحول قطرات الماء الموجودة في السحب إلى كريات ثلج صغيرة.
عندما تبدأ هذه الكريات بالسقوط تحملها تيارات هوائية صاعدة إلى أعلى داخل السحابة.
فتتراكم عليها طبقة جديدة من الجليد، وبتكرار هذه العملية يزداد وزنها فتسقط على شكل حبات من البرد
ويكون حجم حبة البرد تبعا لعدد المرات التي استطاع فيها التيار الهوائي الصاعد حمل حبة البرد وارجاعها داخل السحابة
العلاقة طردية بين قوة التيارات الهوائية الصاعدة وحجم حبة البرد
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