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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Hot Sea Running

17 08 2009

Guest Post from Dan Satterfield

A lot of tropical news this week. The 2009 hurricane season in the Atlantic has stirred to life quickly with two (Update Sunday: 3 !) tropical storms forming on Saturday. It’s not at all unusual to have little hurricane activity until August. The season runs from June 1st to November 30, but the prime season is from Aug, 1st to mid September. American forecasters have an old saying that there will almost always be a hurricane on the weather maps when Labour Day arrives.

From NOAA/NCDC. The bigger the dot the more the temperature was warmer or colder than normal.

These storms form in very warm ocean. The National Climate Data Center (NOAA) released the July global land and ocean temperatures on Saturday. Ocean temps were the warmest on record for July. The land and ocean temps were the 5th warmest on the instrumental record. This follows June 2009 which also came in as warmest.

Another interesting bit of tropical news this week is a new paper published in Nature on hurricanes of the past. One of the great debates in science right now is the question of whether climate change will bring more hurricanes or fewer. The debate has raged between two opposing groups. Kerry Emanuel of MIT has produced interesting evidence that we have seen an increase in hurricanes already due to the warming of the past 50 years.

Chris Landsea of NOAA has produced evidence that we are just detecting more tropical storms, and that there has not been an increase. I had a chance a couple of years ago to hear both of them present at the AMS meeting in San Antonio. I left with the firm conviction that the question remains open. Understand here, that this debate is not about climate change in general.Despite what you read on the Internet, science has moved on from that.

One thing that does seem very certain now is that hurricanes in the warmer world of late this century, will be wetter. Perhaps considerably wetter. The kind of catastrophic flooding we saw in Taiwan this past week, will likely be more common in the future.

Why you ask? Water vapour.

GOES Image of Tropical Storm Bill Early Sunday. from NASA MSFC
GOES Image of Tropical Storm Bill Early Sunday. from NASA MSFC

If the average temperature of the air over the oceans rises 1 degree F, the air can hold 4% more water. (This is one reason why more snow is likely in Antarctica as it warms, not less. A 3C rise in temp. by late this century would bring an increase of around 22% in the amount of water held in the atmosphere! (You won’t see that bit of science on these junk science sites)

Sea surface temperatures are a major factor in hurricane formation. If the sea surface temperature is below about 27C then hurricanes are not likely. Upper level wind shear and atmospheric water vapour are other important ingredients.

Other factors like wind shear in the upper atmosphere act to inhibit hurricanes. The El Nino that develops every 4-7 years in the Pacific, increases the wind shear over the Atlantic, and we usually see fewer storms. Will there be more wind shear in a warmer world? Possibly. Conditions could combine to produce about the same number of storms in the future. (Much wetter ones though)

Micheal Mann of Penn State University is the lead author of a fascinating paper in this weeks NATURE. His team used soil/silt cores in a series of locations to estimate past hurricanes. If a hurricane hits a coastline, the overwash of sea water will leave a deposit that can be identified in the cores. They used these sediment cores to estimate hurricane activity over the last 1500 years. In addition, they used a statistical model that factored in variables like sea surface temperature to estimate storms as well.

Reconstruction of landfalling Atlantic hurricanes. Nature 460, 880-883 (13 August 2009) | doi:10.1038/nature08219;   Atlantic hurricanes and climate over the past 1,500 years  Michael E. Mann, Jonathan D. Woodruff2 et al
Reconstruction of landfalling Atlantic hurricanes. Nature 460, 880-883 (13 August 2009) | doi:10.1038/nature08219; Atlantic hurricanes and climate over the past 1,500 years Michael E. Mann, Jonathan D. Woodruff2 et al

They found that during a period of rather warm Atlantic Ocean water around 1000 years ago, we saw as many hurricanes as we have over the past 15 years. This is a good confirmation that warmer seas, do give more hurricanes and perhaps more intense ones.

Chris Landsea of NOAA argues that the increase in storms over the past century is just an artifact of spotting them more easily with satellites and aircraft. One thing seems likely here, the hurricanes did increase in the past during a period of warmer oceans.

Whether or not a warmer world caused by human means, instead of natural ones, will do the same is still open for debate. The science, however, might just be beginning to tilt in favor of Mann and Emanuel.

Either way, with sea level now rising 3mm per year, and increasing, future hurricanes, will be wetter and cause more destruction. The current thinking is the IPCC will be adjusting their forecast of sea level rise up considerably in the next report.

This back and forth in the peer reviewed literature is how science advances. When we can answer the question of hurricanes in a warmer world, we will have gleamed another piece of fundamental knowledge of how are planet works.

I end with a book recommendation. Kerry Emanuel of MIT is one of the leading experts on hurricanes. He has written a fabulous book called Divine Wind. It combines poetry and science. It’s one of the best general audience  science books ever written.

Note this is a dual post- I wrote it as a guest post on Skywarn 256’s Weather Blog as well.