Monday, December 5, 2011

Types of Precipitation

I concluded my show season for 2011 as I packed up the booth in Marquette last night. Thanks to my friend, Jimmy, who found a college student to help me tear down the booth and load everything into my car. Also, thanks to Jimmy and Helen for feeding and housing me while I have been in Marquette.

Over the weekend we have received many types of precipitation here in the northern Upper Peninsula. So since I did not get a chance to take any photographs, today I'll include information about the types of precipitation. Precipitation can fall as either liquid or frozen types. Liquid forms of precipitation include rain and drizzle. Rain or drizzle which freezes on contact within a subfreezing air mass becomes freezing rain or freezing drizzle. Frozen forms of precipitation include primarily snow, sleet, and hail. Precipitation intensity is determined either by rate of fall, or by visibility restriction.

Rain is by far the most common type of precipitation in our atmosphere. Rain takes place when drops of liquid water fall all the way to the surface of the Earth. Rain often takes one of two main forms. These two forms are showers and drizzles. A shower lasts just a brief period of time, and usually are made up of large heavy drops. Drizzles generally last much longer, and are made up of smaller finer droplets of water.

Liquid precipitation in the form of rain develops when air becomes saturated. This can occur in three ways. These methods include convective, stratiform, and orographic rainfall. Convective precipitation is generally more intense, and of shorter duration than stratiform precipitation.

Convective precipitation occurs when air rises vertically through the self-sustaining mechanism of convection. When moist atmosphere becomes heated more than its surroundings, there can be significant upward motion. Convective rain, or showery precipitation, occurs from convective clouds (cumulonimbus or cumulus). It falls as showers with rapidly changing intensity for a relatively short time, as convective clouds have limited horizontal extent.

Stratiform precipitation occurs when large masses of air rise slant-wise over another mass of air. Stratiform rainfall is caused by frontal systems surrounding lows, which form when warm and often tropical air meets cooler air. Stratiform precipitation falls out of nimbostratus clouds. When masses of air with different moisture and temperature characteristics meet: warmer air overrides colder air. The warmer air is forced to rise and if conditions are right the air becomes saturated causing precipitation. Warm fronts are followed by extended periods of light rain and drizzle, because, after the warm air rises above the cooler air (which sinks to the ground), it gradually cools due to the air's expansion while being lifted, which forms clouds and leads to precipitation. Cold fronts occur when a mass of cooler air dislodges a mass of warm air. This type of transition is sharper, since cold air is more dense than warm air. The rain duration is less, and generally more intense, than that which occurs ahead of warm fronts.

Orographic precipitation is similar, except the upwards motion is forced when a moving airmass encounters a rising slope. This is caused when masses of air are pushed by wind up the side of elevated land formations, such as large mountains. The lift of the air up the side of the mountain is cooled and condenses causing precipitation. In mountainous parts of the world subjected to relatively consistent winds rain usually prevails on the windward side of a mountain than on the leeward (downwind) side. Moisture is removed by orographic lift, leaving drier air on the descending leeward side.

The main type of frozen precipitation is snow. Snow forms when water vapor turns directly into ice without passing through a liquid state. This happens as water condenses around an ice crystal.

Another type of frozen precipitation is sleet. Sleet refers to a mixture of snow and rain, as well as raindrops that freeze on their way down. Unlike snow, the raindrops pass through a liquid form before freezing. The result is that they are not light and fluffy.

Freezing rain takes place when water droplets become super-chilled. They do not freeze in air, but rather freeze the instant they strike an object such as a road, or car. The result can make roads very slippery, and can cause car doors to become frozen shut.

Another type of frozen precipitation is hail. Hail forms in a complex dance between moisture and wind. Deep within cumulonimbus clouds ice crystals form and begin to fall towards the Earth’s surface. As this happens, wind gusts pick up the ice crystals pushing them back up into the clouds. As they begin to again fall down, they continue growing in size. Again, a wind gust might catch the growing hail back up into the cloud. This process may be repeated several more times, until the hail stone becomes so large that it is too heavy for the wind to carry, causing it to fall towards the Earth.

Some precipitation records include:



1 comment:

  1. i think that somebody is locked inside the car with sleet all over it.