Transcript Precipitation ppt

Precipitation typically forms high in the atmosphere
where the temperature is below freezing. As ice
crystals form aloft and fall toward the surface, they
collect each other to form large snowflakes. If
ground temperature is above 32 F, the freezing level
must be located somewhere above the ground. As
the falling snow passes through the freezing level
into the warmer air, the flakes melt and collapse
into raindrops. During the summer months, it is not
uncommon for the freezing level to be found at a
level above cloud base.
When the air temperature at the ground is less than 32 F,
the snowflakes do not melt on the way down and therefore
reach the ground as snow.
Occasionally, we observe snow reaching the ground even though
the outside temperature is above freezing. This occurs when a
very thin layer of warm air is found near the surface.
In the case of a cold front, a colder, denser air mass lifts the
warm, moist air ahead of it. As the air rises, it cools and its
moisture condenses to produce clouds and precipitation. Due to
the steep slope of a cold front, vigorous rising motion is often
produced, leading to the development of showers and
occasionally severe thunderstorms.
In the case of a warm front, the warm, less dense air rises up
and over the colder air ahead of the front. Again, the air cools
as it rises and its moisture condenses to produce clouds and
precipitation. Warm fronts have a gentler slope and generally
move more slowly than cold fronts, so the rising motion along
warm fronts is much more gradual. Precipitation that develops
in advance of a surface warm front is typically steady and more
widespread than precipitation associated with a cold front.
Freezing Rain supercooled droplets freezing on impact Ice storms
can be the most devastating of winter weather phenomena and
are often the cause of automobile accidents, power outages and
personal injury. Ice storms result from the accumulation of
freezing rain, which is rain that becomes supercooled and freezes
upon impact with cold surfaces. Freezing rain is most commonly
found in a narrow band on the cold side of a warm front, where
surface temperatures are at or just below freezing.
The diagram below shows a typical temperature profile for freezing
rain with the red line indicating the atmosphere's temperature at
any given altitude. The vertical line in the center of the diagram is
the freezing line. Temperatures to the left of this line are below
freezing, while temperatures to the right are above freezing.
Freezing rain develops as falling snow encounters a layer of warm air
deep enough for the snow to completely melt and become rain. As
the rain continues to fall, it passes through a thin layer of cold air
just above the surface and cools to a temperature below freezing.
However, the drops themselves do not freeze, a phenomena called
supercooling (or forming "supercooled drops"). When the
supercooled drops strike the frozen ground (power lines, or tree
branches), they instantly freeze, forming a thin film of ice, hence
freezing rain.
Freezing rain is dangerous because it is almost invisible on smooth
surfaces and consequently, people are often unaware of its presence.
Sidewalks become extremely slick when covered with freezing rain,
increasing the likelihood of someone slipping and injuring
themselves. Automobile accidents are more likely to occur during an
ice storm because of the icy roads.
The weight of ice can damage telephone poles and wires, cutting
power and lines of communication to millions of people. During one
severe ice storm in New England in 1921, ice that accumulated on
the wires between two telephone poles was estimated to weigh
over 4 tons. Ice can accumulate up to 4-6 inches deep during the
most intense events, forcing businesses to shut down and greatly
restricting commuters due to the ice-covered roads.
During the 1921 New England ice storm mentioned earlier,
Worcester Parks Recreation Department estimated that 7,500 to
8,000 trees were completely destroyed and that an additional
5,000 to 7,000 were going to die from severe damage.
Devastation to a forest by an ice storm can be as severe as the
damage caused by large tornadoes.
Ice storms also have devastating effects on livestock and birds.
Grazing areas covered with ice can cause many livestock to slip and
fall, while ice build-up on their nostrils can cause them to suffocate.
Birds have been found suffocated, their beaks and nostrils having
been frozen shut. Birds have also been found frozen to trees or
unable to fly due to the weight of the ice on their wings.
In most cases, freezing rain results from the process of warm
moist air "overrunning" colder air. Perhaps the most common
overrunning scenario occurs as warm moist air flows up and over
a warm front associated with a midlatitude cyclone. The rising air
cools, the water vapor condenses, producing a narrow band of
freezing rain ahead of the front. This band is typically less than
50 kilometers (30 miles) wide and is represented by region #1
(shaded in orange) in the diagram below. This band is often
wrapped around and behind the low pressure center by
counterclockwise winds flowing around the cyclone. Some of the
most devastating ice storms occur in association with this narrow
band of freezing rain.
A second area of freezing rain is typically found behind the cold
front, (region #2 shaded in orange in the diagram above). Freezing
rain develops as southerly winds at upper levels push warm moist
air up and over the cold front, producing precipitation that falls
into the colder air. Freezing rain associated with the cold front is
usually very light and scattered, and in rare cases, even observed
ahead of the front.
Forecasting Freezing Rain the importance of temperature profiles
Freezing rain is one of the most difficult events to forecast. The smallest
variations in temperature (even only tenths of a degree) can mean the
difference between rain, freezing rain, sleet or snow. Freezing rain
occurs less frequently than other winter weather events and falls in very
narrow bands, usually not more than 50 kilometers wide. When
attempting to forecast a freezing rain event, sounding data is very useful
for examining vertical temperature profiles of the atmosphere, which
are indicative of what type of precipitation (if any) will likely occur.
There are four types of soundings associated with
the four different types of precipitation
(mentioned above). In the following diagrams, the
blue line represents the temperature profile of the
atmosphere and the black line represents the 0C
isotherm (a line of equal temperature). When the
blue line is to the right of the black line, it means
the atmospheric temperature is warmer than 0C,
but when the blue line is to the left of the black
line, it means the atmospheric temperature is
colder than 0 degrees C.
The entire temperature profile
near the ground is above freezing
so all ice particles completely
melt and reach the ground as
rain
A shallow layer of cold
air lies below a layer of
warmer air, which
completely melts all ice
particles as they pass
through. When the
raindrops enter the
shallow layer of cold air,
they supercool and
freeze instantly on
contact.
The warm layer is very
shallow so ice crystals
only partially melt as they
pass through. Once they
enter the cold layer
below, they freeze again
and strike the ground as
ice pellets, or sleet.
The entire sounding is
completely below
freezing so the
precipitation reaches the
ground as snow.
Upper Air Soundings useful when forecasting for
freezing rain Soundings are the most important tool
for identifying potential freezing rain regimes. Three
types of soundings can lead to freezing rain and the
most common consists of a shallow layer of cold air
at the surface with a depth of about 600 meters
(1,800 feet).
A Sleet is more difficult to forecast than freezing rain because it
develops under more specialized atmospheric conditions. It is
very similar to freezing rain in that it causes surfaces to become
very slick, but is different because its easily visible.