The Rules of Formation

5 12 2009

What is rain?
Rain is formed by water vapor molecules as they rise in the atmosphere (by a force such as a low pressure system, cold front, etc). As the molecules rise, they cool and condense and merge together to form a droplet of liquid water. As the droplet gets larger, it gains weight and drops to the ground.

What is Freezing Rain?
Freezing rain is much like sleet – however, the layer of colder air which refreezes the liquid is shallow and very close to ground level. When the liquid passes through this layer, it freezes. The layer can even be shallow enough to where the rain drop will already be splattered on the ground before it freezes.

What is sleet?
Sleet is formed as snowflakes are but travels through a layer of warmer air midway down and melts. When the liquid refreezes, it turns into a tiny bead of ice by the time it hits the ground.

How atmospheric layer temperatures determine precipitation type.

What is Snow?
Snow is formed when liquid vapor molecules freeze and gain weight (ice is heavier than vapor) and fall. Individual snow crystals have 6 sides – many snow crystals form or merge together to form elegant flakes of snow. Their size is determined by the amount of liquid water coating the flakes. In order for snow to form and successfully fall to the ground, temperatures where they form all the way to ground level must remain above freezing.

Dry Snow vs. Wet Snow – The Difference
Dry Snow is formed when temperatures in the troposphere are well below freezing. This is commonplace in the northern US during the winter and more often in Canada. Dry snow can be identified easily – the snowflakes are very small and don’t stick together very well. Try making a large snowball from dry snow and it will fall apart. Wet snow, on the other hand, forms when temperatures in the troposphere are at or just below freezing or there is an unbalanced temperature range in that particular atmospheric layer. Wet snow flakes tend to be larger than dry snow flakes and stick together very well. Wet snow is the best type of snow to use to build snow-men (or snow-women, I won’t discriminate). Wet snow is formed when snowflakes fall and partially melt. This forms a thin layer of liquid water on the snow flake – cold enough to prevent the total melting of the flake but warm enough to not freeze itself. This tends to make the flake sticky. For this reason, wet snow is usually larger. Snowflakes stick together, if wet, as they are falling. This can lead to some snowflakes to appear to look like little snowballs falling from the sky. Some snowflakes have been recorded to reach a half-dollar in size or even larger. The only problem with measuring such immensely sized snowflakes are that they crumble when they hit the ground- eye witness reports have shown flakes to be the size of baseballs or even softballs but it remains unproven. Flakes of great size are infrequent and usually only form when there is a gentle breeze. No wind at all will force the flakes to break apart due to changes in air pressure and resistance. A hard breeze will have the same effect – a shearing effect more or less, but a light wind will keep the flake floating down more slowly but be gentle enough to not shred the flake.

Guide to Collapsing Raindrops

Why are some raindrops large while others are small?
Raindrops merge and grow larger infinitely – all the way until they are on the ground (and even after, research flooding/flash flooding). As they fall, they grow larger until some force breaks them apart – usually air resistance. As the raindrop grows, the resistance from the air forces it to ‘parachute’ itself. The raindrop begins to form a dome shape which eventually ‘pops’ and the raindrop breaks into several smaller raindrops. Small raindrops form from rainclouds that are very high in the atmosphere. They have a longer amount of time to gain speed, merge and break apart. Thus, this is how mist, sprinkles and other tiny drops are formed. Large raindrops are formed from storm clouds which are closer to the ground. They don’t have very far to fall and thus less air resistance to move through. Winds are also a variable in this sense. If winds are strong, the drops will not fall vertically, but at an angle. This slows the drop’s descent somewhat, leaving a little less shearing stress on the drop itself, although strong enough wind can shred the drop itself.

Hope you enjoyed the article and it helped you to better understand why it pours one precipitation sometimes and others at other times. It’s easy to get confused with sleet, snow and freezing rain – especially in our area. Feel free to post any comments, suggestions, questions or concerns. I will respond as soon as I get a chance.

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