Precipitation 16/08/2008
Through the process of condensation, minute water droplets, less than 0.05mm in diameter is produced. If the dew point temperature is below freezing, ice crystals and produced. The droplets are very small and weigh very little and are kept buoyant by the rising currents in the atmosphere. It must be understood, that although condensation produces clouds, clouds do not necessarily produce precipitation. As rising air currents are often strong, there has to be a process within the clouds which enables the small water droplets and/or ice crystals to become sufficiently large to overcome the uplifting mechanism and fall to the ground. Precipitation that reaches the surface of the earth can occur in many different forms, including rain, freezing rain, drizzle, snow, ice pellets, and hail. Virga is precipitation that begins falling to the earth but evaporates before reaching the surface. Precipitation is a major component of the water cycle, and is responsible for depositing most of the fresh water on the planet. Approximately 505,000km3 (121,000cumi) of water falls as precipitation each year, 398,000km3 (95,000cumi) of it over the oceans. Given the Earth's surface area, that means the globally-averaged annual precipitation is about 1m (39in), and the average annual precipitation over oceans is about 1.1m (43in).
There are at present two theories which try to explain the rapid growth of water droplets.
Inside a cloud, there is a mass of water droplets. When they first condense they are very small and they float in the air. However, if there is strong uplift of air, the droplets will collide and coalesce (i.e. join together). This is called the collision and coalescence process suggested by Longmuir. Once they reach a certain size, gravity causes the water drops to fall as rain. The stronger the uplift, the higher that water droplets will rise in the cloud – so giving them a longer time to grow. If the temperature falls below 0 degrees Celsius, ice crystals form and rapidly grow with the moisture from the super cooled droplets. When the crystals reach a larger enough size, they fall as snow flakes. However, if the temperature is sufficiently warm at a lower altitude, the snowflakes melt and drop as rain – gaining size as they collide with more droplets. This process is called the ice crystal mechanism suggested by Bergeron and later by Findeisen.
In large cumulonimbus clouds, the droplets can sometimes fall and then be swept back up into the higher part of the cloud several times. If the droplet freezes, a new layer of ice is added each time that uplift occurs. This forms hail and, in some circumstances, hailstones can grow very large. Although most have a diameter less than 10mm, some have been recorded weighing over 1kg. When large hailstones fall there can be damage to crops and property.
Types of Precipitation
Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice. Each of these categories can be further subdivided:
· Liquid precipitation:
· Drizzle
· Rain
· Freezing precipitation:
· Freezing drizzle
· Freezing rain
· Frozen precipitation:
· Snow
· Snow grains
· Ice pellets
· Hail
· Snow pellets/Graupel
· Ice crystals
Rain
Rain occurs when tiny cloud droplets collide to form bigger droplets. This keeps happening until the droplet is two heavy for the air to support it. The droplet then begins to fall, colliding with more cloud droplets as it gains in size. If the liquid water does not encounter a deep layer of sub-freezing air, it will remain liquid and fall to the ground as rain. Sometimes rain will show up on the RADAR but there is no rain reaching the ground. This phenomenon is called virga.
Drizzle
Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.
Hail
Hail is a product of very intense thunderstorms. People commonly mistake hail for sleet but they each form under different conditions. Hail is rarely seen when the surface air temperature is below freezing. It forms as a byproduct of strong updrafts that exist in thunderstorms. The cumulonimbus clouds that are associated with thunderstorms can grow to heights where the temperature is below freezing. Drops of water will rise up with the upward directed wind as they collide with other droplets and grow larger. This will eventually result in the droplet freezing into a hailstone.
The hailstone will "cycle" through the cloud. The updrafts will cause it to rise and gravity will cause it to fall. While this is occurring the hailstone is picking up more moisture and growing in size. Eventually the hailstone will grow too large for the updrafts to keep it suspended and it will then fall to the ground.
The size of hailstones varies with the intensity of the thunderstorm. The stronger the thunderstorm, the stronger the updraft and the longer the hailstone will have to grow in size. Hail size is often described using common spherical objects. Sizes can range from pea-sized to golf-ball sized, but have even been seen as big as a softball or even larger.
Freezing Rain
Freezing rain is probably the most dangerous type of precipitation. There is no noticeable difference between freezing rain and rain so people ignore the fact that is can cause such things as black ice on roadways.
Freezing rain occurs when there is a shallow layer of air at the surface that is below freezing followed by a layer of above freezing air above it. The precipitation will fall through the warmer layer so it will not freeze over. When it hits the sub-freezing layer it will cool but not freeze. Since the surface temperature is below freezing, objects in contact with the air are also below freezing. The super-cooled water droplets will freeze on contact to these surfaces. This can cause problems with ice forming on roadways while it is raining. The following diagram shows the atmospheric temperature profile that can cause freezing rain.
Sleet
Sleet is nothing more than frozen raindrops. Sleet occurs when there is a warm layer of air above a relatively deep sub-freezing layer at the surface. The layer above freezing will allow for liquid precipitation but as the drops hit the cold layer, they will freeze and hit the ground as frozen water droplets. Sleet usually doesn't last long and mainly occurs ahead of warm fronts during winter months. The following chart will show the temperature profile typically associated with sleet.
Snow
Snow occurs when the layer of the atmosphere from the surface of the earth through the cloud is entirely below freezing. The precipitation falls from the cloud as snow and does not melt at all while falling to the ground. The following chart will show the vertical temperature profile associated with snow.
Snow Pellets
A snow pellet is precipitation that grows by supercooled water accreting on ice crystals or snow flakes. Snow pellets can also occur when a snowflake melts about half way then refreezes as it falls. Snow pellets have characteristics of hail, sleet and snow. With sleet (ice pellets), the snowflake almost completely melts before refreezing thus sleet has a hard ice appearance. Soft hail grows in the same way snow pellets can grow and that is ice crystals and supercooled water accreting on the surface. Snow pellets will crush and break apart when pressed. They can bounce off objects like sleet does. Snow pellets have a whiter appearance than sleet. Snow pellets have small air pockets embedded within their structure and have visual remnants of ice crystals unlike sleet. Snow pellets are typically a couple to several millimeters in size.
Ice Crystals
Also called diamond dust. They are small ice crystals that float with the wind.
Graupel
Graupel forms in the same way as hail except the diameter is less than 5 millimeters. It usually grows by soft hail processes.
Definitions
super-cooled water - Water that exists in liquid form but has a temperature below freezing. This water will freeze on contact with any surface.
virga - Raindrops that evaporate before hitting the ground. This is usually associated with rain falling through a dry layer of the atmosphere.