Freezing rain
Encyclopedia
Freezing rain is the name given to rain
that falls when surface temperatures are below freezing
. The raindrops become supercooled
while passing through a sub-freezing layer of air, many hundred feet (or meters), just above the surface, and then freeze
upon impact with any object they encounter. The resulting ice, called glaze
, can accumulate to a thickness of several centimeters. The METAR
code for freezing rain is FZRA.
A storm that produces a significant thickness of glaze ice
from freezing rain is often referred to as an "ice storm". Freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power line
s. It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the airfoil
and flight control surfaces.(See atmospheric icing
.)
Usually freezing rain is associated with the approach of a warm front
when cold air, at or below freezing temperature, is trapped in the lower levels of the atmosphere as warmth streams in aloft. This happens, for instance, when a low pressure system moves from the Mississippi River
Valley toward the Appalachian Mountains
and the Saint Lawrence River
Valley of North America
, in the cold season, and there is a strong high pressure system sitting further east. The warm air from the Gulf of Mexico
is often the fuel for freezing precipitation.
Freezing rain develops as falling snow encounters a layer of warm air usually around 800 mbar (800 hPa
) level, then the snow completely melts and becomes 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 (0 °C (32 °F)). However, the drops themselves do not freeze, a phenomenon called supercooling
(or forming "supercooled drops"). When the supercooled drops strike ground, power lines, tree branches, aircraft, or anything else below 0 °C (32 °F), they instantly freeze, forming a thin film of ice, hence freezing rain.
Surface observations by manned or automatic stations are the only direct confirmation of freezing rain. One can never see directly freezing rain, rain or snow on weather radar
s, Doppler
or conventional. However, it is possible to estimate the area covered by freezing rain with radar
s indirectly.
The intensity of the radar echoes (reflectivity
) is proportional to the form (water or ice) of the precipitation
and its diameter. In fact, rain has much stronger reflective power than snow but its diameter is much smaller. So the reflectivity of rain coming from melted snow is only slightly higher. However, in the layer where the snow is melting, the wet flakes still have a large diameter and are coated with water so the returns to the radar is much stronger.
The presence of this brightband indicates that there is a warm layer above ground where snow melts. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing. This artifact can be located, like on the image at left, with a cross-section through radar data. The height and slope of the brightband will give clues to the extent of the region where melting occurs. Then it is possible to associate this clue with surface observations and numerical models prediction to produce output such as the ones seen on television weather programs that divide radar echoes into rain, mixed and snow precipitations.
. When the ice layer exceeds 0.2 inch (0.508 cm) , tree limbs with branches heavily coated in ice can break off under the enormous weight and fall onto power lines. Windy conditions, when present, will exacerbate the damage. Power lines coated with ice become extremely heavy, causing support poles, insulators and lines to break. The ice that forms on roadways makes vehicle travel dangerous. Unlike snow, wet ice provides almost no traction, and vehicles will slide even on gentle slopes. Because freezing rain does not hit the ground as an ice pellet but still as a rain dropplet, it conforms to the shape of the ground, or object such as a tree branch or car. This makes one thick layer of ice
, often called glaze.
Freezing rain and glaze on a large scale is called an ice storm
. Effects on plants can be severe, as they cannot support the weight of the ice. Trees may snap as they are dormant and fragile during winter weather. Pine trees are also victims of ice storms as their needles will catch the ice, but not be able to support the weight. One particularly severe ice storm struck eastern Canada
and northern parts of New York
and New England
in the North American ice storm of 1998
.
equipment to fly in freezing rain of any intensity, and heavy freezing rain can overwhelm even the most sophisticated deicing systems on large airplanes. Icing can increase an aircraft's weight but not typically enough to cause a hazard. The main danger comes from the ice changing the shape of its airfoils.
This will reduce lift and increase drag. All three factors increase stalling speed and reduce aircraft performance, making it very difficult to climb or even maintain level altitude.
An aircraft can most easily evade freezing rain by moving into warmer air — under most conditions, this requires aircraft to descend, which it can usually do safely and easily even with a moderate accumulation of structural ice. However, freezing rain is accompanied by a temperature inversion aloft, meaning that aircraft actually need to climb to move into warmer air — a potentially difficult and dangerous task with even a small amount of ice accumulation. In 1994, American Eagle Flight 4184
encountered heavy air traffic and poor weather that postponed the arrival of this flight at Chicago
's O'Hare International Airport
, where it was to have landed en route from Indianapolis, Indiana
. The ATR-72, a twin-engine turboprop carrying 68 people, entered a holding pattern 65 miles (104.6 km) southeast of O'Hare. As the plane circled, the freezing rain formed a ridge of ice on the upper surface of its wings, eventually causing the aircraft's autopilot to suddenly disconnect and the pilots to lose control. The ATR disintegrated on impact with a field below, all passengers and crew were killed.
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
that falls when surface temperatures are below freezing
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...
. The raindrops become supercooled
Supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
while passing through a sub-freezing layer of air, many hundred feet (or meters), just above the surface, and then freeze
Freezing
Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting....
upon impact with any object they encounter. The resulting ice, called glaze
Glaze ice
Glaze ice or simply glaze is a smooth, transparent and homogenous ice coating occurring when freezing rain or drizzle hits a surface. It is similar in appearance to clear ice, which forms from supercooled water droplets...
, can accumulate to a thickness of several centimeters. The METAR
METAR
METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting....
code for freezing rain is FZRA.
A storm that produces a significant thickness of glaze ice
Glaze ice
Glaze ice or simply glaze is a smooth, transparent and homogenous ice coating occurring when freezing rain or drizzle hits a surface. It is similar in appearance to clear ice, which forms from supercooled water droplets...
from freezing rain is often referred to as an "ice storm". Freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power line
Power Line
Power Line is an American political blog, providing news and commentary from a conservative point-of-view. It was originally written by three lawyers who attended Dartmouth College together: John H. Hinderaker, Scott W. Johnson, and Paul Mirengoff...
s. It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
and flight control surfaces.(See atmospheric icing
Atmospheric icing
Atmospheric icing occurs when water droplets in the atmosphere freeze on objects they contact. This can be extremely dangerous to aircraft, as the built-up ice changes the aerodynamics of the flight surfaces, which can increase the risk of a subsequent stalling of the airfoil...
.)
Mechanism
Warm front
A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient...
when cold air, at or below freezing temperature, is trapped in the lower levels of the atmosphere as warmth streams in aloft. This happens, for instance, when a low pressure system moves from the Mississippi River
Mississippi River
The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...
Valley toward the Appalachian Mountains
Appalachian Mountains
The Appalachian Mountains #Whether the stressed vowel is or ,#Whether the "ch" is pronounced as a fricative or an affricate , and#Whether the final vowel is the monophthong or the diphthong .), often called the Appalachians, are a system of mountains in eastern North America. The Appalachians...
and the Saint Lawrence River
Saint Lawrence River
The Saint Lawrence is a large river flowing approximately from southwest to northeast in the middle latitudes of North America, connecting the Great Lakes with the Atlantic Ocean. It is the primary drainage conveyor of the Great Lakes Basin...
Valley of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, in the cold season, and there is a strong high pressure system sitting further east. The warm air from the Gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...
is often the fuel for freezing precipitation.
Freezing rain develops as falling snow encounters a layer of warm air usually around 800 mbar (800 hPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
) level, then the snow completely melts and becomes 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 (0 °C (32 °F)). However, the drops themselves do not freeze, a phenomenon called supercooling
Supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
(or forming "supercooled drops"). When the supercooled drops strike ground, power lines, tree branches, aircraft, or anything else below 0 °C (32 °F), they instantly freeze, forming a thin film of ice, hence freezing rain.
Observations
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
s, Doppler
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
or conventional. However, it is possible to estimate the area covered by freezing rain with radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
s indirectly.
The intensity of the radar echoes (reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...
) is proportional to the form (water or ice) of the precipitation
Precipitation (meteorology)
In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...
and its diameter. In fact, rain has much stronger reflective power than snow but its diameter is much smaller. So the reflectivity of rain coming from melted snow is only slightly higher. However, in the layer where the snow is melting, the wet flakes still have a large diameter and are coated with water so the returns to the radar is much stronger.
The presence of this brightband indicates that there is a warm layer above ground where snow melts. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing. This artifact can be located, like on the image at left, with a cross-section through radar data. The height and slope of the brightband will give clues to the extent of the region where melting occurs. Then it is possible to associate this clue with surface observations and numerical models prediction to produce output such as the ones seen on television weather programs that divide radar echoes into rain, mixed and snow precipitations.
Effects
At surface
Freezing rain often causes major power outages by forming glaze iceGlaze ice
Glaze ice or simply glaze is a smooth, transparent and homogenous ice coating occurring when freezing rain or drizzle hits a surface. It is similar in appearance to clear ice, which forms from supercooled water droplets...
. When the ice layer exceeds 0.2 inch (0.508 cm) , tree limbs with branches heavily coated in ice can break off under the enormous weight and fall onto power lines. Windy conditions, when present, will exacerbate the damage. Power lines coated with ice become extremely heavy, causing support poles, insulators and lines to break. The ice that forms on roadways makes vehicle travel dangerous. Unlike snow, wet ice provides almost no traction, and vehicles will slide even on gentle slopes. Because freezing rain does not hit the ground as an ice pellet but still as a rain dropplet, it conforms to the shape of the ground, or object such as a tree branch or car. This makes one thick layer of ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
, often called glaze.
Freezing rain and glaze on a large scale is called an ice storm
Ice storm
An ice storm is a type of winter storm characterized by freezing rain, also known as a glaze event or in some parts of the United States as a silver thaw. The U.S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least of ice on exposed surfaces...
. Effects on plants can be severe, as they cannot support the weight of the ice. Trees may snap as they are dormant and fragile during winter weather. Pine trees are also victims of ice storms as their needles will catch the ice, but not be able to support the weight. One particularly severe ice storm struck eastern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and northern parts of New York
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...
and New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
in the North American ice storm of 1998
North American ice storm of 1998
The North American ice storm of 1998 was a massive combination of five smaller successive ice storms which combined to strike a relatively narrow swath of land from eastern Ontario to southern Quebec to Nova Scotia in Canada, and bordering areas from northern New York to central Maine in the...
.
Aircraft
Freezing rain is an extreme hazard to aircraft, as it causes very rapid structural icing. Most helicopters and small airplanes lack the necessary deicingDeicing
For snow and ice control on roadways and similar facilities, see Snow removalDe-icing is defined as removal of snow, ice or frost from a surface...
equipment to fly in freezing rain of any intensity, and heavy freezing rain can overwhelm even the most sophisticated deicing systems on large airplanes. Icing can increase an aircraft's weight but not typically enough to cause a hazard. The main danger comes from the ice changing the shape of its airfoils.
This will reduce lift and increase drag. All three factors increase stalling speed and reduce aircraft performance, making it very difficult to climb or even maintain level altitude.
An aircraft can most easily evade freezing rain by moving into warmer air — under most conditions, this requires aircraft to descend, which it can usually do safely and easily even with a moderate accumulation of structural ice. However, freezing rain is accompanied by a temperature inversion aloft, meaning that aircraft actually need to climb to move into warmer air — a potentially difficult and dangerous task with even a small amount of ice accumulation. In 1994, American Eagle Flight 4184
American Eagle Flight 4184
American Eagle Flight 4184 was an American Eagle ATR 72 that crashed after flying into unknown icing conditions on October 31, 1994. Control was lost and all aboard were killed.-History:...
encountered heavy air traffic and poor weather that postponed the arrival of this flight at Chicago
Chicago
Chicago is the largest city in the US state of Illinois. With nearly 2.7 million residents, it is the most populous city in the Midwestern United States and the third most populous in the US, after New York City and Los Angeles...
's O'Hare International Airport
O'Hare International Airport
Chicago O'Hare International Airport , also known as O'Hare Airport, O'Hare Field, Chicago Airport, Chicago International Airport, or simply O'Hare, is a major airport located in the northwestern-most corner of Chicago, Illinois, United States, northwest of the Chicago Loop...
, where it was to have landed en route from Indianapolis, Indiana
Indianapolis, Indiana
Indianapolis is the capital of the U.S. state of Indiana, and the county seat of Marion County, Indiana. As of the 2010 United States Census, the city's population is 839,489. It is by far Indiana's largest city and, as of the 2010 U.S...
. The ATR-72, a twin-engine turboprop carrying 68 people, entered a holding pattern 65 miles (104.6 km) southeast of O'Hare. As the plane circled, the freezing rain formed a ridge of ice on the upper surface of its wings, eventually causing the aircraft's autopilot to suddenly disconnect and the pilots to lose control. The ATR disintegrated on impact with a field below, all passengers and crew were killed.
See also
- List of ice storms
- Freezing drizzleFreezing drizzleFreezing drizzle is drizzle that freezes on contact with the ground or an object at or near the surface. Its METAR code is FZDZ. When freezing drizzle accumulates on land it creates an icy layer of glaze...
- HailHailHail is a form of solid precipitation. It consists of balls or irregular lumps of ice, each of which is referred to as a hail stone. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms...
- Ice pelletsIce pelletsIce pellets are a form of precipitation consisting of small, translucent balls of ice. Ice pellets usually are smaller than hailstones. They often bounce when they hit the ground, and generally do not freeze into a solid mass unless mixed with freezing rain...
- Black iceBlack iceBlack ice, sometimes called glare ice or clear ice, refers to a thin coating of glazed ice on a surface.While not truly black, it is virtually transparent, allowing black asphalt/macadam roadways to be seen through it, hence the term "black ice"...