In the past article, I mainly explained how ice crystals and precipitation forms in warm core and cold core clouds. In this article, I will be explaining what conditions are necessary for snow growth. I will explain the temperatures and humidity levels that are needed for snowfall.
Let me start where I left off in the last article. Most precipitation in the mid-latitude regions results from the growth of ice crystals in the glaciated, and "mixed" (region of the cloud that contains both cloud droplets and ice crystals) regions of the cloud layer. The mixed region of the cloud always occurs from about the freezing level to -40 degrees Celsius. The glaciated part of the cloud will always occur above -40 degrees Celsius. But this is for spring and summer clouds, where temperature are very warm in the low levels. During the winter, clouds are made up of predominately ice crystals and water droplets, which tends to make them higher above the earth's surface. The cloud droplets that do make up at the cloud at it's lower levels are very tiny. Due to the lattice structure of an ice crystal, it loses very little water to evaporation. What this means is that vapor pressure is less around an ice crystal than it is around a cloud droplet (which means that when the air is saturated over an ice crystal, it is supersaturated over a water droplet, above 100% relative humidity). The vapor pressure "gradient" causes water vapor molecules to move from the water droplet to the ice crystal, and depositing on the crystal causing it grow. In this way, since the ice crystals are in relatively less abundance than are water droplets, they grow at the expense of the massive amounts of water, and continue on the process as they shatter. Eventually, if the thermodynamic profiles favor it, the shattered ice crystals will attach themselves to another ice crystal, and fall to the earth as a snowflake. The snowflake will only reach the earth's surface if the entire temperature profile is below or right near freezing temperatures. In the next section, I will talk about the required conditions for snow crystal growth.
In order for snow crystals to grow most rapidly at the expense of the water droplets around them, certain temperatures are required. In the field of cloud physics, much research has been achieved on the required temperatures for the most rapid snow growth. In most research experiments, it was found that dendrites (the most efficient type of ice crystal) that grow from accretion and deposition grow in the areas of the cloud that are in between -10 and -18 degrees Celsius (14 degrees Fahrenheit to 0 degrees Fahrenheit). These dendrites have the shape of a prism with a hexagonal shape. This is not the only condition that needs to be met for the most efficient snow growth. Another condition includes a relative humidity that is near or over 100%, which will allow the ice crystals to maximize growth. It also helps the situation when strong upper level lift is located in the same thermal layer.
Dendrites are not the only way snow can form. Other types include rimed ice, platelets, and the traditional snowflake. Rimed ice crystals usually form when the precipitation layer has temperatures near or around -4 to -6 degrees Celsius. In this case, the ice crystals tend to be very wet, and allow for a better surface for water to "stick" to. These are the very wet, and large snowflakes that we occasionally see during the late winter and late fall months. Platelets are another type of snow crystal, which are usually very, very small. They tend to form during the coldest winter patterns (namely cold lake effect snow events), where cloud temperatures are below -25 degrees Celsius. They create the sight in the air that makes it look foggy. They are usually the snow crystals that blow very easily, and are the main reasons for visibility problems during winter storms. On the otherhand, the traditional snow flake usually forms in relatively warm cloud conditions; Colder than with rimed ice crystals, but warmer than dendritic snowflake conditions. They can easily become bigger, but water vapor does not stick to them as well. Normally, snow events dominated by these type of crystals do not usually bring much accumulation (but it is possible).
Next week, I will be writing an article about the myths in the Arctic, mainly about Arctic sea ice extent, actual observed temperatures, and winter/summer time weather.
Let me start where I left off in the last article. Most precipitation in the mid-latitude regions results from the growth of ice crystals in the glaciated, and "mixed" (region of the cloud that contains both cloud droplets and ice crystals) regions of the cloud layer. The mixed region of the cloud always occurs from about the freezing level to -40 degrees Celsius. The glaciated part of the cloud will always occur above -40 degrees Celsius. But this is for spring and summer clouds, where temperature are very warm in the low levels. During the winter, clouds are made up of predominately ice crystals and water droplets, which tends to make them higher above the earth's surface. The cloud droplets that do make up at the cloud at it's lower levels are very tiny. Due to the lattice structure of an ice crystal, it loses very little water to evaporation. What this means is that vapor pressure is less around an ice crystal than it is around a cloud droplet (which means that when the air is saturated over an ice crystal, it is supersaturated over a water droplet, above 100% relative humidity). The vapor pressure "gradient" causes water vapor molecules to move from the water droplet to the ice crystal, and depositing on the crystal causing it grow. In this way, since the ice crystals are in relatively less abundance than are water droplets, they grow at the expense of the massive amounts of water, and continue on the process as they shatter. Eventually, if the thermodynamic profiles favor it, the shattered ice crystals will attach themselves to another ice crystal, and fall to the earth as a snowflake. The snowflake will only reach the earth's surface if the entire temperature profile is below or right near freezing temperatures. In the next section, I will talk about the required conditions for snow crystal growth.
In order for snow crystals to grow most rapidly at the expense of the water droplets around them, certain temperatures are required. In the field of cloud physics, much research has been achieved on the required temperatures for the most rapid snow growth. In most research experiments, it was found that dendrites (the most efficient type of ice crystal) that grow from accretion and deposition grow in the areas of the cloud that are in between -10 and -18 degrees Celsius (14 degrees Fahrenheit to 0 degrees Fahrenheit). These dendrites have the shape of a prism with a hexagonal shape. This is not the only condition that needs to be met for the most efficient snow growth. Another condition includes a relative humidity that is near or over 100%, which will allow the ice crystals to maximize growth. It also helps the situation when strong upper level lift is located in the same thermal layer.
Dendrites are not the only way snow can form. Other types include rimed ice, platelets, and the traditional snowflake. Rimed ice crystals usually form when the precipitation layer has temperatures near or around -4 to -6 degrees Celsius. In this case, the ice crystals tend to be very wet, and allow for a better surface for water to "stick" to. These are the very wet, and large snowflakes that we occasionally see during the late winter and late fall months. Platelets are another type of snow crystal, which are usually very, very small. They tend to form during the coldest winter patterns (namely cold lake effect snow events), where cloud temperatures are below -25 degrees Celsius. They create the sight in the air that makes it look foggy. They are usually the snow crystals that blow very easily, and are the main reasons for visibility problems during winter storms. On the otherhand, the traditional snow flake usually forms in relatively warm cloud conditions; Colder than with rimed ice crystals, but warmer than dendritic snowflake conditions. They can easily become bigger, but water vapor does not stick to them as well. Normally, snow events dominated by these type of crystals do not usually bring much accumulation (but it is possible).
Next week, I will be writing an article about the myths in the Arctic, mainly about Arctic sea ice extent, actual observed temperatures, and winter/summer time weather.
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