Hail is the crystalized particles or drops of water pieces of ice which sometimes has a regular shape and is often seen in the shape of oval.Pyramid,sphere,and semisphere. The outer surface of the hail is sometimes smooth and sometimes rough and bumpy. Hailstones usually get their final shape in several stages.When the first step in the production of hail,like that of the rain is formed,as a result of a vertical movement of a current of air upwards,the tiny particles of water in the current quickly reach the freezing point and immediately freez.As these frozen particles move downwards,other water and ice drops attach to them and hence they grow.But when they reach the lower levels of clouds,because of the rising of a current of the air inside the clouds which take place as a result of certain processes,the frozen particles move upwards again.This cycle repeats until the current of the air upwards can support the weight of the particles,at this time the hails leave the clouds and the precipitation happens.
Hail generally takes place in clouds of cumulonimbus type,because these clouds have a very high vertical extension and inside them swift horizontal movements,whose speed sometimes gets to twelve to thirty m/s,are seen.These are suitable environments for the formation and the development of hailstones. Hailstones are divided into three groups according to size and shape :
1.smooth hail frost is made of white icy particles which are relatively round and opaque ; their structure is like that of snow and they are often in the shape of a cone , their diameter is varies from 2 to 5 millimeters.The smooth hail cracks and smashes as soon as it gets to the ground.In most cases this kind of hail appears in the form of shower in zero degrees temperatures along with snow or before the snow.
2.tiny hail icicle is made of spherical semi-transparent particles which on rarely conic and their diameter varies from 2 to 5 millimeters.This kind of hail forms when air temperature is several degrees below zero on the centigrade scale.
3.hail is formed from icy particles whose diameter varies from 5 to 50 millimeter and sometimes even more because of the frequent cycles and steps in which hail develops,it consists of transparent,semi-transparent, opaque and milky layers.It often hails in the form of showers in spring. Each hailstone usually consists of three sections :
A - a white core which is usually like a snowball,B - a great number of white fiber-like layers which are seen around the core,C - a thin froze shell which surrounds the hailstone this shell is sometimes missing.
Hail temperature is usually below zero degrees centigrade,but it might occasionally reach that , it which case it carries some water.It usually hails in spring or other transitional seasons of the year,because during these seasons severe atmospheric disturbances make the condition suitable for the formation of hail.
Adapted from the book : "Water"
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