EPSC 185 Lecture Notes - Lecture 6: Weather Map, Green Mountains, Atlantic Ocean
23 Jun 2018
School
Department
Course
Professor
Ice storms:
What is compelling on the study of ice storms and the need to understand how they form, where
the occur and how to mitigate against these ice storms involve the following points:
-Caused by an accumulation of freezing rain
-Among the most devastating winter weather events
-The devastation can halt air and ground transportation
-Impede the infrastructure of the community by weighing down/snapping down power
lines and trees
Example of power outage: 1998 the great ice storm of quebec. The hydro quebec hyvlolatge
towers were crumpled and destroyed because of the extraordinary amount of weight of the ice.
The accumulation of ice amounted upward of 6 inches. Therefore, it is important to understand
what happens during a freezing rain storm and why these storms occur.
What is freezing rain?
Supercooled water that freezes upon impact
Supercooled water: liquid water can exist at temperatures below 0 degrees celsius
Example: ice area that where freezing is occurring. We have a melting point at 0 degrees
celsius. Although 0 degrees celsius is a melting point, it does not necessarily mean that ice or
liquid water will freeze at that temperature. A number of elements and conditions must exist in
order for liquid water to freeze. Among them being there must be ice nuclei, hydroscopic
dust particles in the atmosphere. These nuclei tend to be most affective at producing ice at
temperatures less than -10 degrees Celsius. Common to see liquid water exist at temperatures
between -1 to -10 degrees celsius. There happen to be very few ice nuclei in certain regions of
the atmosphere that would facilitate the production of freezing rain. When the temperature falls
below 0 we say that the liquid water exists in a supercooled environment. We use the term
supercooled to indicate the state of H20 in a liquid faze at temperatures below 0 degrees
celsius.
Example of what happens when we utilize supercooled water and pour it onto a surface - it
facilities a freezing process.
What happens during a real freezing rain storm is that we effectively have supercooled water
that will freeze upon impact with a relatively cold surface. In there lies the problems with a
destruction of major infrastructure during a major ice storm.
As the freezing rain falls, the liquid water falls onto a solid surface (car windows, roof areas)
tremendous amount of loading to the extent that not only trees are loaded down and limbs
snapping but roofs can fall as well.
-Liquid water can exist at temperatures as low as -40 degrees celsius
-Water needs a nucleus (cloud condensation an impurity) to form a crystal. If the impurity
is absent, no freezing occurs.
-Not a sufficient amount of condensation nuclei in the atmosphere to create freezing until impact
of the ground.
-During a precipitation event, the atmosphere gets cleaned of aerosols and any other particles
that can serve as cloud condensation nuclei and therefore no freezing occurs.
-The supercooled water that becomes ice begins to melt as it is exposed to the warm
environment (room temperature)
-Illustrates the remarkably dangerous process of supercooled water as it impact a cold surface
in the environment in the winter
find more resources at oneclass.com
find more resources at oneclass.com
Where does freezing rain occur ?
-As we will see there are very special locations that facilitate the production of freezing rain
-Map of the north eastern of the US and south eastern part of Canada - what we see is an
example of the locations of freezing rain bands that occurred during the 1998 ice storms. The
bands form in narrow areas and tend to be 10-160 km in width. These very small changes that
occur in the atmosphere (temperature, wind speed, moisture content) can automatically alter the
location, intensity and duration of freezing rain. Its a relatively rare event that occurs over a
small area. The St Laurence river runs in a north east south west direction. These bands of
freezing rain lie along these river valley’s that facilitate the production of cold air at lower levels.
-Synoptic conditions for an ice storm
Synoptic = the large scale environment of the weather patterns
-What happens during an ice storm is that the production of freezing rain tends to occur
on the colder side (north side) of a warm front or to the west (cold side) of a surface
cyclone. Generally about 3/4 or 70-75% occur in this location.
-Additionally, there could also be an ice storm (freezing rain storm) that could occur on the cold
side of the cold frontal system in a dome of cold air associated with a surface high pressure
system. Particularly in the winter time over the continents, high pressure systems tend to be
associated with very cold or arctic air masses that produce very shallow ranges of cold air.
-Many ice storms are also associated with both of the above conditions where we have freezing
rain occurring on a cold side of a cold front or the cold side of a warm front as well.
-The development of a surface cyclone: initially there tends to be a boundary (or a cold frontal
system) that separates warm air from the denser cold air to the north. The front is often
stationary. A low pressure system tends to be associated with a rotating counter clockwise
circulation. As the cyclone continues to develop, the warm air bands will over ride the colder air.
-The warm and moist air over rides the cold air at the surface. Meteorologist call it a warm
conveyer belt. The conveyer belt represents a source, a path where the air is producing
precipitation as it is forced to rise up over the cold air. The cold conveyer belt indicates the
progression of cold, dense air that hugs the ground.
-The particular faze of precipitation is dependent on how the atmosphere is structured in terms
of its temperature in both low levels and upper levels
-In order to develop freezing rain we need several components of the atmosphere, several
processes to go on.
-Spatial scale - represents about 100 km. The diagram indicates that the widths of the rain are
30-40 km. The width vary’s from case to case.
-Summary of synoptic conditions for the formation of freezing rain:
-Mid-latitude cyclone (freezing rain on the cold side of a cold front) vs cold air dome (near the
boundaries of the cold front)
-Low’s and high’s are coupled together
What happens in terms of the temperature structure to make an ice storm?
-In order to define the stability we need to consider a few facts about what typically occurs in the
atmosphere and what may not so typically occur. Normally, the temperature decreases with
increasing elevation. We are developing an inversion of temperature.
-Atmospheric stability: air parcel tweaked one way or the other and accelerates left or right
(towards or away from its position). The stability is very different during an ice storm (stable
conditions). We are in a position of forming an atmospheric inversion as the temperature
increases as we go up in elevation. The cold hair hugs the ground and doesn't allow the
warm air to penetrate near the ground and forces it upwards. This is what is responsible for
the production of precipitation.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
What is compelling on the study of ice storms and the need to understand how they form, where the occur and how to mitigate against these ice storms involve the following points: The devastation can halt air and ground transportation. Impede the infrastructure of the community by weighing down/snapping down power lines and trees. Example of power outage: 1998 the great ice storm of quebec. The hydro quebec hyvlolatge towers were crumpled and destroyed because of the extraordinary amount of weight of the ice. The accumulation of ice amounted upward of 6 inches. Therefore, it is important to understand what happens during a freezing rain storm and why these storms occur. Supercooled water: liquid water can exist at temperatures below 0 degrees celsius. Example: ice area that where freezing is occurring. We have a melting point at 0 degrees celsius.
