Causes of Precipitation

Final: Precipitation

So, you are making your way up to Wisconsin for a spring camping trip, and the forecast has you a little rattled. Plans were made a few weeks back and you are now seeing first-hand how unpredictable the spring weather can get here in the Midwest. The weekend forecast calls for a good chance of rain, sleet and snow! That may sound like a terrible outing and it very well might be. My hope is that throughout this paper you leave with a better understanding of precipitation and greater fascination in weather.

There are many routes that weather can take, and I believe precipitation behaves in some of the most interesting way when a few variables change. The journey that cloud droplets take to eventually converts into precipitation is one you may not familiar with but is crucial in order of better understanding of weather. There is a whole spectrum of precipitation that range from liquid to freezing to frozen. It’s difficult to talk about rain and snow without bringing up clouds and not all are created equal, different types of clouds serve different purposes.

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We actually have artificial methods being used today that are implemented to help increase or decrease precipitation in some areas. Since precipitation can take on various states from liquid to solid, there are equally as many methods to measure each type of precipitation.

 When it comes to how precipitation is formed there are essentially two theories out there that can help you understand the process. Before I draw out the theories you should know the general overview of clouds. Clouds are the product of water in the atmosphere that has cooled and condensed. I will go into greater detail about the different cloud types later in the paper. The first theory is the Bergeron-Findeisen ice-crystal mechanism which I will refence as the ‘Bergeron process’ for short. In the Bergeron process, precipitation starts at the higher colder part of clouds and the central part of this theory is that you need high enough clouds for this to occur. In that high colder part of a cloud is where you have water vapor, ice crystals and liquid droplets, so water is in all three phases; gas, liquid and solid. Because of hat height, pressure and coldness ice crystals are being made more abundantly and faster than water droplets. The Bergeron process is under the belief that all precipitation begins as ice and as it moves closer to the ground it goes through different layers of air that can either keep it a solid (snow) or melts it (rain). One important side note to keep in mind is that water droplets behaves differently in the atmosphere than on the surface and water droplets don’t freeze at 0 degrees Celsius but at -40 degrees Celsius. Water that remains a liquid when it is below 0 degrees Celsius is named supercooled.

 The second theory on precipitation is the Longmuir’s collision and coalescence theory. The Bergeron process dealt with tall cold clouds and the collision and coalescence theory deals with lower and warmer clouds where water stays more liquid and not as ice crystals or supercooled. The basis of the second theory is fairly straightforward where you have smaller cloud droplets that bump into one another and merge into a larger cloud droplet. To a certain point the water droplets become too big (roughly 3mm in radius) to stay suspended inside a cloud and with the help of friction they fragment back into smaller droplets as they fall down. While those droplets are falling, they are still crashing into other droplets and as they keep breaking, they eventually end up as rainfall.

 There are many different forms of precipitation that you may be unaware of and to keep it simple, I can narrow them down to 3 groups; liquid, solid and mixed. So far, we know that precipitation is water either in the form of liquid or solid that is dropping from clouds. But there is also participation that does not come from the clouds that can be present like dew which is called ground precipitation. For now, we will take a look at liquid precipitation like rain and drizzle. We can tell difference between drizzle by rain droplet size. A normal raindrop will be regularly between 5 mm and 6 mm in diameter while drizzle is fraction of that at around 0.2 mm to .6 mm. I mentioned earlier about supercooling, where water stays a liquid even when being exposed to temperatures below 0 degrees Celsius. That’s is important because some of the most dangerous precipitations that can wreak havoc on road transportation and general safety is freezing rain/drizzle. Freezing rain is pretty hazardous because it passes down from the sky as a super cold liquid and then when it hits a surface like a road, it collects and forms an almost clear thin layer of ice that is tough to spot.

I briefly mentioned ground precipitation like dew, but what is it exactly. Dew is a result of natural condensation that occurs on the earths surface. You most likely know of dew from spotting it on grass in the morning. The dew point is one of the measurements we have that can tell is how much water vapor is in the air. Dew mainly occurs during the warmer months of the year here in Wisconsin because colder air is unable to hold as much water as warm air. A rule of thumb is that a higher dew point means a higher relative humidity and a damper feel the air will have. A high dew point can also be a precursor to bad weather. When there is a lot of water vapor in the atmosphere that can indicate that thunderstorms, will continue to be fed and get to the point where they can produce tornadoes.

 Frozen or solid precipitation is where things really start to get interesting. This is where we see the likes of snow and hail. Just to recap, under the Bergeron process all precipitation starts out as ice crystals (snow) however in some cases it liquefies because of warmer temperature before it hits the surface. There is a type of precipitation called Virga that is basically rain that evaporates and tapers off before hitting the ground. A snow equivalent is called a Fallstreak where the snow falls from clouds and somewhat melts but eventually evaporates before reaching the ground. Hail is stones of ice that normally occur during thunderstorms. They are created when raindrops are sent back upwards to the freezing zones of a cloud and fall back down as ice. Hail size is determined by refencing it to common items, that can range from a pea (1/4 inch diameter) to a grapefruit (4.5 inch diameter). Hail is not the only ice participation; sleet is ice pellets that are small, and ball shaped. You can tell sleet apart because the nature of its shape, makes it spring and as it hits the ground. An unusual type of precipitation is Graupel, which is described as snow cocooned inside ice. Graupel is brittle and breaks down to the touch or even when it reaches the ground. Graupel’s flimsiness is how you tell it apart from other precipitation, for instance hails has multiple layers of ice that makes it sturdy enough to not only hold its shape but also cause damage when hitting a surface. 

 The many different cloud types can be a bit intimidating. A good thing to note is that clouds can be classified into the four groups: high, middle, Low and other. High clouds are part of the cirrus family and because of that they have the prefix “cirro” in their name and exceed 20,000 feet. Within high clouds there are 3 types; cirrocumulus, cirrostratus and cirrus. Cirrocumulus clouds are small, usually white or blue clouds that look like broken ripples and patchy. Cirrostratus clouds look a bit blurred or out of focus but are the most commonly seen from the high cloud family. Cirrus clouds look like thin straggly bands. For the most part high clouds are composed of ice crystals and frozen water droplets. High clouds are thin, due to the cold air where water vapor cannot stay long.

Middle clouds are part of the alto family and their names have “alto” in prefix and are present at 6,500 feet to 20,000 feet. Within medium clouds there are 2 types; altocumulus and

altostratus. Altocumulus clouds are white and gray and usually in a row or band-like shape. Altostratus clouds are a mix of gray and blue in color and look smooth and flat. For the most part medium clouds are made up of ice crystals and water droplets. Medium clouds are where you will see unusual precipitation like virga or fallstreaks.

 Low clouds are part of the stratus family but not given a prefix, but they use “cumulo” and “strato” and exist at 6,500 feet and below.  Within low clouds there are 2 types; cumulus and stratus. Cumulus clouds develop vertically and have rounds tops with flat bottoms. There are 2 types of cumulus clouds from cumulus congestus which are very high but haven’t exhibited the capacity to create thunderstorms. To cumulonimbus which are more matured and now have the ability to generate thunderstorms (thunder and lightning), these clouds have the distinct anvil tops. Stratus clouds develop horizontally, and they look smooth and flat and are a gray color. There are 2 types of stratus clouds from stratocumulus to nimbostratus. Stratocumulus clouds are thin bands of clouds that are gray and white [see fig 2. Pg 2.]. Nimbostratus clouds are just thicker version of stratocumulus clouds, so they have a better ability to keep a steady downpour. Low clouds are made exclusively of water droplets.

 The fourth and last set of clouds, which we will class as other are fairly interesting. Fog for instance is a stratus cloud that is close to the surface. Contrails are a result of condensation from jet engine exhausts that leave behind skinny lines of clouds. Mammatus clouds are unique because they have an underside that has bag-like lumps. There unique shape is because they are not formed by rising air but falling air. Fractus clouds, which include scud or pannus clouds derive from the word “fracto”, which means broken. For example, scud clouds look like funnels and that can be misleading because they are actually not related to any storm system. Shelf clouds and wall clouds appear during storms, where shelf clouds are out in front of a storm and wall clouds are in the rear. While shelf clouds maintain a wedge like shape, wall clouds have a wide range. Lenticular clouds have an exclusive saucer shape and are found fixed above obstacles, like mountains, hills or even man-made structures.

 Cloud seeding is a man-made technique we have in place to influence weather. Cloud seeding is initiated by sending a plane towards cloud formations and releasing seeding agents. Those seeding agents can be dry ice; (solid carbon dioxide – CO2), salt (sodium chloride – NaCl) or silver iodide (AgI). These agents act as additional ice nuclei, meaning that supercooled water droplets that are colliding but not combining, are introduced to additional things to bond with. Natural condensation nuclei are made up of smoke or dust. Eventually enough droplets bond together that they become big enough to fall to the ground. A misconception about cloud seeding is that it can be used to create rain, which is not entirely true. Cloud seeding is used to boost or suppress rain, it needs existing clouds and cannot create rain from scratch. For example, during a drought, the area and air is dry, that lack of moisture doesn’t present good prospects for cloud formation, which is fundamental for precipitation.

 As we have discussed the various types of precipitation, we should also dive into the methods and devices we have to measure it. To measure rainfall, some of the methods available are tipping bucket gauge, standard bucket rain gauge and weighing gauge. A rain gauge is a device that gathers and measures rain over a set period of time. A tipping bucket rain gauge has a funnel above a bucket that goes into a teeter tooter tool that tips every time it reaches 0.01 inches. Whenever it tips, that triggers data to mark water levels. A standard bucket rain gauge has a funnel that sends rainwater to a graduated cylinder that is located inside a larger cylinder, normally 10x the size of inner cylinder. The inner granulated cylinder shows increments of 0.01 inches. A weighing precipitation gauge is like a standard rain gauge only it sits upon a weighing scale. This notes the weight of rainwater and converts that to inches of rain. Snow can be measured in a variety of ways. A straight forward method is by using a snowboard, which is an even flat white platform that collects snow and then measured with a ruler. Snow can also be measured by water with a heated standard bucket rain gauge. It works just like a gauge for rain only it has a heating element that liquefies snow. The water equivalent to snow is normally 10 centimeters is equal to 1 cm of liquid water.  

 Our atmosphere is an amazing playground for water vapor. From cloud types to how we measure rain water, we have covered a lot of bases on precipitation. The Midwest, with its wide range of cool and warm weather and high levels of humidity lead to a variety of diverse precipitation types. Hopefully that on your trip to the Midwest, you spot the many different cloud formation as hints for future weather.

Fig. 1. Close shot water droplets after hitting surface during a morning drizzle.

Fig. 2. Shot of stratocumulus clouds.

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