Project Atmosphere Canada

Module 1 / Hazardous Weather

Project Atmosphere Canada

Project Atmosphere Canada (PAC) is a collaborative initiative of Environment Canada and the Canadian Meteorological and Oceanographic Society (CMOS) directed towards teachers in the primary and secondary schools across Canada. It is designed to promote an interest in meteorology amongst young people, and to encourage and foster the teaching of the atmospheric sciences and related topics in Canada in grades K-12.

Material in the Project Atmosphere Canada Teacher's Guide has been duplicated or adapted with the permission of the American Meteorological Society (AMS) from its Project ATMOSPHERE teacher guides.

Acknowledgements

The Meteorological Service of Canada and the Canadian Meteorological and Oceanographic Society gratefully acknowledge the support and assistance of the American Meteorological Society in the preparation of this material.

Projects like PAC don't just happen. The task of transferring the hard copy AMS material into electronic format, editing, re-writing, reviewing, translating, creating new graphics and finally format- ting the final documents required days, weeks, and for some months of dedicated effort. I would like to acknowledge the significant contributions made by Environment Canada staff and CMOS members across the country and those from across the global science community who granted permission for their material to be included in the PAC Teacher's Guide.

Eldon J. Oja
Project Leader Project Atmosphere Canada
On behalf of Environment Canada and the Canadian Meteorological and Oceanographic Society

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher.
Permission is hereby granted for the reproduction, without alteration, of materials contained in this publication for non-commercial use in schools or in other teacher enhancement activities on the condition their source is acknowledged. This permission does not extend to delivery by electronic means.

Published by Environment Canada
© Her Majesty the Queen in Right of Canada, 2001

Cat. no. En56-172/2001E-IN
ISBN 0-662-31474-3

Contents

Overview

Thunderstorms

Basic understandings
Narrative
Activity
Answer key

Hurricanes

Basic understandings
Narrative
Activity

Winter storms

Basic understandings
Narrative
Activity
Answer key

Overview

Weather is variable, from the gentle breezes of a balmy evening to the heavy wind and rain of a late afternoon thunderstorm. Hazardous weather, such as thunderstorms, hurricanes, and winter storms - the subjects of this guide - can cause property damage, bodily injury or even death. With the necessary information, proper preparation, and sensible reactions, most people can protect themselves and to some extent their property from the ravages of most kinds of hazardous weather.

Hazardous weather may affect anyone, anywhere in the world at any time. The table (Relative Comparison of Weather Risks) below shows the annual approximate numbers of various types of severe weather for several countries world-wide. Canada is certainly no slouch when it comes to dramatic weather.

Hazardous weather in Canada results in dozens of lives lost each year with total property losses typically reaching into the hundreds of millions of dollars. These are ample reasons why everyone should keep track of the weather and understand what to do if severe weather occurs.

Environment Canada has the responsibility of warning people in this country of the possibility of a severe weather-related event. Warning programs have been developed to inform the public of the weather hazard and to help initiate appropriate adaptive measures. Prompt response to such information can save lives, reduce injuries, and lessen property damage.

** Data courtesy of NOAA, NWS     * Data courtesy of EC-MSC

Note that the U.S.A., Canada and Australia get many more tornadoes than other parts of the world. Warm humid air at low levels and much drier air aloft that cools off rapidly with height are key ingredients. Countries with very dry interiors and that have frequent flows off very warm oceans / seas meet these criteria. The Gulf of Mexico and the dry western plains are the factors that give Canada / U.S. more tornadoes.

Cautionary Note: Due to the potential for significant variations in the criteria and definitions of each weather risk used by the various countries a linear comparison of the data in the table above cannot be made.

Basic understandings - Thunderstorms

Thunderstorm Development

1. A thunderstorm is a localized storm cloud producing thunder, lightning and often gusty winds, heavy rain and hail.
2. Thunderstorms occur when warm, humid air is lifted by surface heating, upslope flow, or a front.
3. Most thunderstorms are composed of individual cells which exhibit three stages. These cells start with what is called the cumulus stage. It is characterized by upward motion. Unstable moist air rises and cools and water vapour condenses to form cumulus clouds.
4. In the mature stage of thunderstorm cell development, the cloud reaches its maximum vertical development and falling precipitation creates a downdraft. This stage is associated with thunder and lightning as well as the possibility of severe weather.
5. A mature thunderstorm cell is characterised by both updraft and downdraft. As the upward motion builds the cloud, it may produce an overshooting top, which penetrates into the stratosphere, and forms an anvil cloud blowing downwind due to the strong winds in the upper atmosphere.
6. The downward-moving air caused by falling precipitation results in the outflow spreading away from the cloud base at the Earth's surface. This sometimes results in a squall line forming as much as three hundred kilometres ahead of the front.
7. The dissipating stage of a thunderstorm cell occurs when the precipitation-induced downdraft cuts off the supply of warm humid air and the cloud eventually sinks and evaporates away.
8. The typical thunderstorm cell exists for about thirty minutes, although some may last longer.
9. The gust front is the boundary separating the cool air outflow from the warm, moist air feeding the updraft of the thunderstorm cell. As the gust front forms it may result in severe localized downdrafts called downbursts or microbursts. The rapid changes in wind speed and direction of these downdrafts can be deadly threats to aircraft.

Thunderstorm Forecasting

10. Severe thunderstorms commonly threaten life and property with lightning, strong gusty winds, heavy rain or hail.
11. If thunderstorms occur in areas with restricted drainage, flash floods may result.
12. Thunderstorms may also generate funnel-shaped tornadoes, with their violent rotating winds and hazardous pressure drops.
13. Weather satellites and radars have greatly improved the observation of thunderstorms. Such improvements are used to analyze storm structure and provide advanced warning to minimize the danger associated with such storms.
14. Weather satellites enable forecasters to observe from above the initial development, movement, and severity of thunderstorms.
15. Infrared satellite imagery enables meteorologists to identify the most intense part of the storm by observing the coldest cloud top temperatures, which coincide with the region of strongest updraft in the cloud cluster.
16. With radar, the meteorologist can determine where the storm is moving, hence where the weather is most likely to cause damage.
17. Doppler radar systems enable the estimation of actual air motion within the storm by measuring the phase shift of the radar return from the wind-driven precipitation.
18. Environment Canada issues a severe thunderstorm watch when there is a threat of a severe thunderstorm developing for a specified area and time.
19. A severe thunderstorm warning is issued when a severe thunderstorm is imminently expected to occur or has actually been observed in that area.
20. The hazardous weather associated with thunderstorms can develop very rapidly and with little advance warning. Consequently, a vigilant watch and awareness of local and broad-scale weather conditions is essential to ensure adequate preparedness and response.

Narrative - Thunderstorms

What is a thunderstorm?

The thunderstorm is usually a storm composed of one or more cells. Each cell is a few kilometres in diameter and develops from clouds that grow rapidly upward and produce thunder and lightning. A thunderstorm often brings heavy precipitation, such as rain or hail, as well as strong gusty winds. Sometimes thunderstorms can become quite violent and may generate flash floods or tornadoes.

What causes thunderstorms?

Thunderstorms can occur when warm, humid air is lifted upward. The air rises either by mechanical lifting, such as when a cold, dense air mass undercuts warm, moist air, or by thermal lifting due to solar heating of the Earth's surface. The rising air expands and cools, and water vapour contained in the air condenses to form cloud water droplets. As the air continues upward, individual cloud towers can become a towering thunderstorm cloud. Once an individual thunderstorm cell reaches maturity, downdrafts, caused by falling precipitation, eventually destroy the cloud. Normally a thunderstorm is composed of more than one cell, and as one cell dies, usually within twenty to thirty minutes, another may develop nearby.

Stages of a Thunderstorm

Thunderstorms have three stages of development:

Cumulus Stage

initial stage of cloud development, as warm, humid air rises and water vapour condenses to form the cloud; characterized by upward motion throughout the cloud

Mature Stage

cloud reaches maximum vertical development, precipitation starts to fall, creating a downdraft; this is the stage with the most violent weather and the occurrence of thunder and lightning

Dissipating Stage

precipitation-induced downdraft is observed throughout the cloud; the cloud sinks and evaporates away.

The typical thunderstorm cell exists for about thirty minutes, although some may last longer.

Illustration of warm, moist air being mechanically lifted over a cold front, resulting in the formation of cumulonimbus clouds (thunderstorms)

Illustration of the three stages of development of a thunderstorm: cumulus, mature and dissipating stages. Arrows denote motions within the thunderstorm.

Structure of a Thunderstorm

Thefigure (Illustration of a mature thunderstorm) below displays a mature thunderstorm, which is composed of:

Updraft

the region of upward motion responsible for building the cloud

Overshooting Top

the uppermost part of the cloud above the updraft core which may penetrate into the stable stratosphere above

Anvil

the top of the cloud blowing downwind due to strong winds at higher altitudes

Downdraft

downward-moving air associated with falling precipitation

Outflow

downdraft air spreading away from the cloud base as it reaches the Earth's surface below

Gust Front

the boundary separating the cool air outflow and the warm, moist air feeding the updraft

Illustration of a mature thunderstorm. Arrows denote direction of air flow in and around the thunderstorm.

Squall Line of Thunderstorms With Associated Warm & Cold Fronts.

Weather Accompanying Thunderstorms

Ordinarily, the most severe thunderstorms form along narrow bands called squall lines, in the warm, moist air ahead of an approaching cold front.

Examples of weather accompanying thunderstorms include

Lightning

the visible electrical discharge which occurs in mature thunderstorms due to large voltage differences within the cloud, between clouds, or between the cloud and the ground below. The heat produced by the stroke causes the air to expand explosively, creating a shock wave which is heard as thunder. Lightning kills an average of 7 people every year in Canada.

Damaging winds and wind shear

the cool air outflow from a thunderstorm, which may achieve wind speeds great enough to damage objects in its path. Wind shear is an abrupt change in the wind (speed or direction) with distance. Strong wind shears exist between the updraft and downdraft areas.

Hail

particles of ice, ranging in size from a pea to a softball or larger, that form in the updraft of thunderstorms as liquid water drops are forced upward to regions of freezing temperatures. Hail storms cause hundreds of millions of dollars in damage to crops and property annually in Canada.

Flash Floods

floods that arise rapidly with little or no advance warning. Such flooding often is associated with slow-moving thunderstorm systems bringing heavy rains to a limited area, especially in regions that are unable to handle the volumes of water because of terrain features (such as canyon walls and hills), soil composition or improper drainage. Lives are occasionally lost to flash floods in Canada.

Tornadoes

violent, rapidly rotating columns of wind that descend from the bases of thunderstorms and come in contact with the Earth's surface. These strong, rotating winds can cause considerable damage and loss of life.

Severe Thunderstorm Detection and Forecasting

Technology has greatly improved the ability to observe and predict the occurrence and movement of thunderstorms. Such advances as weather satellites and Doppler radars provide valuable information about where thunderstorms might develop and move, which greatly assists in providing adequate warning to minimize the damage and risk to people associated with such storms.

Tools for Observing Thunderstorms

Weather satellites enable forecasters to observe the initial development of cumulonimbus clouds, providing a "birds-eye' view of the location of such storms as they build upward though the atmosphere. Infrared satellite imagery enables meteorologists to identify the most intense part of the storm. For example, the coldest cloud top temperatures usually coincide with the region of strongest updraft in the thunderstorm.

Weather radar is perhaps the best tool for tracking a severe thunderstorm. The radar emits microwave energy, which produces an image of the interior of the storm. The radar beam strikes precipitation particles, which reflect energy (return signal) back to the radar antenna with an intensity proportional to the strength of the storm. In the figure (Illustration of Southwestern Ontario Radar) below, the contoured area corresponds to the return signal from an approaching storm. By evaluating and tracking the return signal, the meteorologist is able to determine where the storm is moving, hence where the weather is most likely to cause damage.

Illustration of Southwestern Ontario Radar showing an area of showers and thunderstorms

Watches and Warnings

Thunderstorms can produce a variety of severe weather events that can damage property or cause bodily injury. Environment Canada issues severe weather watches and warnings to advise the public of the approach of a severe thunderstorm and to minimize damage to property and loss of life.

A severe thunderstorm watch is issued when there is a threat of a severe thunderstorm developing in a specified area over a certain period of time.

A severe thunderstorm warning is issued when a severe thunderstorm is imminently expected to occur or has been observed (visually or by radar) in a given location. One should take immediate steps to avoid imminent danger.

Hazardous weather associated with thunderstorms can develop very rapidly. Events such as flash floods or tornadoes may occur with little advance warning. Thus, a vigilant watch and awareness of local and broad-scale weather conditions is essential to ensure adequate preparedness and response to such weather hazards.

Recent weather maps, forecasts, satellite images and radar images from across Canada are available on the internet and can be viewed through Environment Canada's web site at:

http://weatheroffice.ec.gc.ca

Activity - Thunderstorms

Suggested Activities

1. Which country listed in the Relative Comparison of Weather Risks table has the greatest variety of hazardous weather and need for timely weather forecasts, watches and warnings?
2. Of the threats listed, which is of greatest concern in your local area? What should you, your family, and your community do to try to adequately prepare and respond to the threat or threats?
3. What was the most recent hazardous weather you and/or your community faced? What was done or could have been done to lessen its effects?
4. Does your family, school, and community have a plan for all types of hazardous weather that might occur? What has been done or should be done? Who should do it?
5. What are the basic safety rules individuals should follow in facing the different kinds of hazardous weather? (Contact Environment Canada or Emergency Preparedness Canada for specific information on safety rules and hazardous weather preparedness and response.)

Activity - Tracking the Grand Valley Tornado

After completing this exercise, you should be able to:

• describe the motion of a severe tornado, including the width of its path, average speed and direction
  
• list appropriate actions to take to save lives when tornadoes threaten, including those that address the special threats of mobile-home living

Introduction

During the late afternoon and evening of May 31 1985 a powerful cold front moved across Southern Ontario and the Ohio Valley. A total of 88 people were killed by some 40 odd tornadoes. The figure (Track of all tornadoes) shows the track of all these storms.

In Canada there were 9 separate tornado tracks. The map (Track of the 3 main tornadoes) details the 3 largest storms. The northern track through Barrie killed 8 people and caused very extensive damage.

The middle track known as the Grand Valley - Tottenham storm killed 4 people and is the longest tornado track on record in Canada.

The Grand Valley and Barrie tornadoes both reached F4 on the Fujita scale. Damage brought by tornadoes is ranked by the Fujita scale which runs from F0 with winds up to 120 km/h with light damage possible, to F5 with winds in the neighbourhood of 500 km/h and damage described as "incredible" with almost total above-ground destruction. F4 storms have wind speeds up to 400 km/h and can nearly flatten even the most well built home.

Tornadoes are dangerous storms. This activity will demonstrate some of the characteristics of severe tornadoes and appropriate preparedness and response actions you should make when the threat of tornadoes occurs.

Activity

Attached is a map (Track of the 3 main tornadoes) of the 3 longest tornado tracks from May 31 1985. Concentrate on the longest track… the Grand Valley tornado

Answer the following questions based on the map information.

1. For how long a time period was the tornado on the ground?
2. Towards what general direction (E.g. Southeast, Southwest, Northwest, Northeast) did the tornado move?
3. What was the length of the tornado path?
4. What was the average ground speed of the tornado?
5. The width of the Grand Valley tornado was consistently around 200 metres. What was the total area damaged by this tornado.
6. Looking at these 3 tornadoes what was the total area damaged. Significant tornado damage was observed in 5 counties. As a rough guess what was the percentage of the total area of these counties damaged by tornadoes.
7. Use safety rules and information provided by the Environment Canada or Emergency Preparedness Canada to determine where you should take shelter when a tornado threatens. What special precautions should mobile home parks and residents take to reduce severe weather threats?

Track of all tornadoes associated with the May 31, 1985 outbreak

Track of the 3 main tornadoes across Southern Ontario on May 31 1985. The middle track is the Grand Valley tornado

Answer Key

Activity - Tracking the Grand Valley Tornado

1. About 1 hour.
2. East-northeast.
3. 105 km
4. Close to 100 km/h
5. 100 km/h long x 200 m wide = 20 square km
6. About 50 or 60 square km

Basic understandings - Hurricanes

Hurricane Development

1. A hurricane is an intense rotating storm system that forms over warm tropical waters typically in the late summer or early fall.
2. Hurricanes are circular in shape, ranging from 300 to 1,000 km across, and have winds over 118 km/h within 50 kilometres of the centre.
3. The formation of a hurricane requires a low pressure disturbance over a large expanse of warm water. The evaporation of this water will intensify the resulting storm.
4. This formation must be far enough from the Equator so that winds will circulate around a centre of low pressure due to the Coriolis force. The Coriolis force is too weak near the Equator to create the needed rotation.
5. Certain wind patterns at various altitudes are also needed to ensure that the developing hurricane will not simply blow apart.
6. The hurricanes that strike North America form in the tropical North Atlantic and Caribbean and move on a westerly to northerly track, steered by the prevailing winds. They strike the mainland on either the Gulf or Atlantic coasts.
7. The tropical disturbance stage of hurricane development is characterized by a collection of thunderstorms forming in the easterly flow over warm tropical waters with only a slight rotation.
8. The tropical depression stage is a well-defined centre of low pressure with winds of 37 to 62 km/h.
9. The tropical storm stage is characterized by an intense centre of low pressure and winds of between 63 and 117 km/h.
10. The hurricane stage occurs when the wind speed exceeds 117 km/h.

Hurricane Features

11. The major feature within a hurricane is the eye, a small region of relatively calm and clear air in the centre, 15 kilometres or so across.
12. The eye is surrounded by the eye wall where the weather is most severe with high winds and heavy precipitation.
13. Feeding into the wall cloud region are spiral rain bands often embedded with vigorous thunderstorms.
14. The forward movement of hurricanes is slow, typically 15 to 25 km/h in the lower latitudes.
15. The path of the hurricane is determined by the complex interactions with wind currents aloft and the existing large-scale weather patterns. The resulting path can be erratic and difficult to forecast.
16. ln a hurricane, the observed wind speed is largely determined by the combined forward and spinning motions of the storm. On one side of the eye, the wind speed is increased by the forward motion of the storm and on the other side, the wind speed is decreased by this motion.
17. Since hurricanes get their energy from the evaporation of warm tropical water, as they move over colder water or land, they lose their energy source and weaken in intensity.

Hurricane Hazards

18. The low pressures and high winds associated with hurricanes create huge mounds of water called storm surges which cause 90% of all hurricane deaths. Large-scale evacuations of people from low-lying areas can prevent the massive loss of life due to such flooding.
19. Severe thunderstorms and tornadoes are often associated with the convective activity in hurricanes.
20. Hurricane winds have been recorded at speeds up to 300 km/h. Beyond the direct damage caused by such winds, wind-driven waves on top of the storm surge compound the flooding problem by battering and eroding the coastal landscape and structures.
21. The Canadian Hurricane Centre begins issuing bulletins about tropical storms or hurricanes when one is forecast to enter Canada's Response Zone within 72 hours. (see map "Canada’s response zone for hurricane") — as well, there is a lot of other information about hurricanes on the Canadian Hurricane Centre's web site: http://www.ns.ec.gc.ca/weather/hurricane)

Narrative - Hurricanes

What is a Hurricane?

A hurricane is an intense rotating storm system that forms over tropical waters. The typical hurricane is roughly circular in shape and ranging from 300 to 1,000 km across. Winds of hurricane speed — 118 km/h and higher — are confined to a relatively small area typically within a few tens of kilometres of the centre of the storm's path.

What Causes Hurricanes?

Hurricane formation requires the following:

• an initial low pressure disturbance over a large expanse of warm water; evaporation of this water will produce thunderstorm clouds which can intensify the resulting storm.
  
• a location far enough from the Equator so that winds will circulate around a centre of low pressure. The Coriolis force (a consequence of the Earth's rotation) is the source of this circulation. There is no Coriolis force at the Equator; the Coriolis force increases as latitude increases.
  
• certain wind patterns at various altitudes, to ensure that the developing storm will not simply blow apart.

Hurricanes usually form in late summer or early fall. Many hurricanes that strike North America form in the tropical waters of the Atlantic Ocean or Caribbean Sea, move on a westerly to northerly track, steered by the prevailing wind direction, and strike the mainland on either the Gulf or Atlantic coasts.

Stages of Hurricane Development

Tropical Disturbance

The first stage is a collection of thunderstorms forming in the easterly flow over warm tropical waters with only slight rotation.

Tropical Depression

Next the storm develops a well-defined centre of low pressure with winds of 37 to 62 km/h.

Tropical Storm

Next the storm becomes an intense centre of low pressure and carries winds of between 63 and 117 km/h.

Hurricane

When the wind speeds are 118 km/h and higher, the storm is considered a hurricane.

Hurricane Structure

Here are the major features within a hurricane (see diagram "Illustration of Vertical section of a hurricane" below).

Eye

The eye is the small region of relatively calm and clear air in the centre; the eye may be only a few tens of kilometres across.

Eye Wall

The eye is surrounded by clouds that make up the eye wall; here the weather is most severe with high winds and heavy precipitation.

Spiral Bands

Feeding into the wall cloud region are spiral bands of clouds, often composed of strong thunderstorms.

Illustration of Vertical section of a hurricane, displaying the eye, eye wall and surrounding spiral rain bands

Motion of Hurricanes

The forward movement of hurricanes is relatively slow, usually up to about 15 to 25 km/h in the lower latitudes. The path of the hurricane is determined by the complex interactions with wind currents aloft and the existing large-scale weather patterns. The resulting paths can be erratic and difficult to forecast. The diagram below shows the paths of several hurricanes.

Illustration of typical paths of motion of hurricanes in the Atlantic Ocean, Caribbean Sea and Gulf of Mexico.

The hurricane is a system of winds rapidly spiralling into the low pressure centre (counterclockwise in the Northern Hemisphere) as it moves slowly forward. On the right side of the storm's track, the storm's forward motion reinforces the wind. On the other side of the track, the air and storm motions compete, reducing wind speeds. Consequently, storm damage is usually most severe north and east of the eye's landfall and can vary considerably over distances as small as 50 kilometres.

Energy Source

Hurricanes get their energy from evaporation over large expanses of warm tropical water with water temperatures greater than 26^oC. Evaporation from warm water surfaces produces water vapour that carries tremendous amounts of energy into the growing storm. Subsequent condensation of this water releases this energy and intensifies the storm.

As hurricanes move over colder water or land, they lose their warm-water energy source and weaken in intensity.

Hurricane Damage

Major damage due to a hurricane is caused by two factors:

Storm Surge

Many people are surprised to learn that 90% of hurricane deaths are due to high water rather than high winds. Due to the low pressure and strong winds, hurricanes create a huge mound of water called a storm surge, especially in shallow coastal waters. As the surge sweeps ashore, the high water can flow right over sea walls and destroy protective sand dunes; when the surge coincides with high tide, the increase in water level can be as much as 6 metres. Large-scale evacuations of people from low-lying areas, such as the Gulf Coast or the Outer Banks of North Carolina, prevent massive loss of life due to such flooding.

Wind Damage

Hurricane winds have been recorded at speeds up to 300 km/h.. Beyond the damage caused directly by such winds, wind- driven waves on top of the storm surge compound the flooding problem by battering and eroding coastal features.

Hurricane and tropical Storm Bulletins

These are issued by the Canadian Hurricane Centre when a tropical storm or hurricane is forecast to enter Canada's Response Zone within 72 hours.

Canada’s response zone for hurricane and tropical storm bulletins

Activity - Hurricane

Activity - Track of Hurricane Diana

Upon completion of this exercise you should be able to:

• describe how a hurricane can be tracked
  

• observe the unpredictable path of a hurricane

• use the track of a hurricane to plan a disaster relief program

The map (Hurricane Diana) provided shows the area adjacent to Wilmington, North Carolina (population approximately 50,000). This region has extensive private and resort beachfront development.

Use the map (Hurricane Diana) below to plot the track of Hurricane Diana from the positions given in the accompanying table (Hurricane Diana September 11-13, 1984). Each position shows the centre of the storm for the time indicated in the chart. These positions are given as latitude and longitude.

Start by plotting all positions (beginning with the location at 1800 EST on 11 September); label each position with the corresponding number from the table. Connect the points with a smooth curve to show the path of Hurricane Diana.

* Landfall (on an island SE of Southport, NC)

Diana organized as a tropical storm off the Bahamas on September 8th, 1984. It then approached the Florida coast before turning northward to parallel it. Diana reached hurricane strength while approaching North Carolina on the 11^th, then weakened and made landfall. Finally, Diana wandered back off-shore on the 14^th, intensified before heading toward Newfoundland and extinction. Wind damage, beach erosion, and 18 inches of rain produced $78 million damage in the Wilmington, NC area.

(Source: Storm Data, September, 1984, Vol. 26, No. 9, NOAA, National Climatic Data Centre)

1. 1. If you were a meteorologist, what would you tell people living along the coast in the map area?
2. If you were in charge of emergency management for the land area shown on the map, what action would you take, if any? Consider that it will take a few hours to alert people in your exposed areas and allow them to evacuate, if needed. Your decisions can affect many lives.
3. What types of emergency personnel and supplies will be needed if widespread damage occurs?

Additional Activities:

1. When a tropical storm or hurricane is reported, monitor radio and television for information on the storm's progress. Plot the position of the storm's centre on a classroom map or tracking chart. Also mark the coastline along which hurricane watches and warnings have been issued.
2. Invite persons who have lived through hurricanes to speak about their experiences to the class.
3. What can individuals, families, and communities in coastal areas do to meet the hurricane threat? If you live in a coastal area, what are the preparedness and response plans of your family and community?

Basic understandings - Winter storms

Winter Storm Development

1. Winter storms are large-scale disturbances associated with low-pressure areas called mid-latitude cyclones.
2. Winds blow counterclockwise as seen from above (in the Northern Hemisphere) around the centre of the low pressure system.
3. Winter storms occur when warm, humid air interacts with colder air along the frontal boundary separating the two air masses. The two contrasting air masses provide energy, which permits the storm to intensify.
4. Winds speeds increase as the storm strengthens. The warm, moist air is lifted upward, producing widespread areas of cloudiness and precipitation along the frontal surfaces in the vicinity of the developing cyclone.
5. The normal lifetime of a winter cyclone is about three to five days.
6. Steered by the direction of the upper air flow, winter storms tend to move from west to east.

Winter Storm Hazards

7. Winter storms produce strong winds, heavy precipitation (rain, freezing rain, or snow) and cold temperatures. Hazardous winter weather includes freezing rain, snow, blizzards, and bitterly cold temperatures.
8. Cold temperatures feel more extreme when there is wind. The wind chill factor combines the effect of both temperature and winds to determine the cooling rate and the equivalent temperature with no wind.
9. Environment Canada issues a variety of severe winter weather watches and warnings to alert the public to the approach of winter storm conditions.
10. Winter storm dangers include being stranded outside while exposed to the elements, having breakdowns in transportation systems due to accidents, and losing access to basic necessities and services.

Narrative - Winter storms

What is a Winter Storm?

A winter storm is a large-scale disturbance, often hundreds of kilometres across, associated with a low-pressure system (called a cyclone) that develops along a front during the cooler part of the year. Winter storms can produce strong winds, heavy precipitation (rain, freezing rain, ice pellets or snow) and cold temperatures.

What Causes Winter Storms?

Winter storms occur when relatively warm, humid air interacts with colder air along the frontal boundary separating the two air masses. Initially, the front is slow-moving or stationary. The formation and evolution of a deep low pressure centre and the associated circulation is referred to as cyclogenesis. It is a complex process involving upper level divergence as well as near-surface processes. The two contrasting air masses provide energy to this rotating system, permitting the storm to intensify with time. Wind speeds increase as the storm strengthens. The warm, moist air is lifted upward, producing widespread areas of clouds and precipitation along the frontal surfaces in the vicinity of the developing cyclone.

Surface map depiction of a winter storm system, showing the low pressure centre (L), cold and warm fronts, and a accompanying weather (legend gives explanation of symbols)

Structure and Movement of a Winter Storm

The figure (Illustration of the vertical structure of a winter storm) below displays the structure of a mature stage of a winter storm, with major features noted at both the surface and at upper levels (6 to 10 km). The primary storm is associated with the surface position of the low (L) centre and the accompanying cold and warm fronts. At upper levels, the storm normally is associated with an upper level trough, a low pressure region which forms a distinct southerly "dip" in the upper air flow, which generally lags the surface low pressure centre. The surface high (H) pressure behind the cold front brings colder temperatures, clear skies and fair weather. The entire storm system moves(arrow on surface chart labelled Movement) with the upper level steering wind currents (noted by the direction of the arrow of the jet stream).

Illustration of the vertical structure of a winter storm. The bottom view depicts the surface features, and the top view shows upper level features of the system (adapted from Ahrens)

Weather Accompanying Winter Storms

Some of the hazardous weather conditions that accompany winter storms area:

Heavy snow

snowfalls greater than 15 cm in 12 hours usually cause significant problems.

Blowing Snow

wind-driven snow that reduces visibility and causes significant drifting

Blizzards

winds exceeding 50 km/h with snow and blowing snow reducing visibility to near zero; high wind chills.

Snowsqualls

narrow bands of very heavy snow that blow in off the Great Lakes, Gulf of St. Lawrence, and other bodies of water. Heavy snow and near zero visibilities are generally associated.

Ice pellets

raindrops that freeze before reaching the ground. This normally occurs when the rain forms along a warm front and descends through a layer of air with temperatures just below freezing.

Freezing Rain

rain falls through a layer or onto a surface with a temperature just below freezing, causing a layer of ice to form on the surface.

Winter Weather Advisories and Warnings

Environment Canada issues a variety of special weather statements, advisories, watches and warnings to alert the public to the approach of winter storm conditions.

A special weather statement / winter weather advisory is issued when winter weather conditions are expected to cause significant inconveniences.

A winter storm watch is a “heads up” that winter severe weather is expected in the near future.

A winter storm warning is issued when severe winter weather conditions have begun or are about to begin in a given area.

A blizzard warning is issued when snow and strong winds will combine to produce blinding snow (near zero visibility), deep drifts, and life-threatening wind chill.

What is Wind Chill and the Wind Chill Index?

Wind chill is the cooling effect of the wind in combination with low temperatures. When it is windy, we feel colder because our skin temperature is lower. This sensation of cold is what the wind chill index quantifies: as such, the index is not a real temperature and is expressed without units, even though it is based on the Celsius temperature scale.

Environment Canada's wind chill index estimates the temperature which, with a wind of about 5 km/h, would give your face a sensation of cold similar to that caused by the actual temperature with the wind. Wind chill also estimates the risk of your getting frostbite (a severe injury caused by cold), according to these approximate thresholds:

• Wind chill below -25: risk of frostbite in prolonged exposure.
  
• Wind chill of -35: frostbite possible in 10 minutes (warm skin, suddenly exposed; shorter time if skin is cool at the start).
  
• Wind chill of -60: frostbite possible in less than 2 minutes (warm skin, suddenly exposed; shorter time if skin is cool at the start).

Major Health Hazards

The major health problems associated with winter storms are overexposure and overexertion. Overexposure implies that parts of the body are not properly protected from the cold temperatures and/or strong winds, leading to frostbite or hypothermia. Overexertion results from the strain of working too hard in cold temperatures, and can lead to heart failure.

Activity - Winter storms

Activity - Major Winter Snowstorm

Upon completing this activity, you should be able to:

• analyse the snowfall pattern of a winter storm
  
• track the path of the cyclone
  
• determine the relationship between the major storm and corresponding hazardous weather it produced

Introduction

During the period of 25-27 January 1987, a major winter storm moved across the mid-Atlantic States. This particular storm brought heavy snowfalls to the area, with Washington, D.C. receiving about ten inches or 25 cm of snow during this period.

Major snowstorms are generally associated with winter storm systems that move across the country. The purpose of this exercise is to demonstrate the relationship between the areas of heavy snowfall and the track of the winter cyclone.

Activity

1. On the snowfall map (25-27 January,1987 snowfall) provided, draw lines connecting points with 5 inches of snow. Where a 5 is not shown, interpolate between the values provided to locate the line. Next draw the line for 10 inches of snow and finally the line for 15 inches of snow.
2. Observe the three isolines which you have just drawn with a contouring interval of 5 inches and note where the heaviest snowfalls occurred. Also note the amount of change in the snowfall between locations that are very close together. (Remember those times when the weather forecast was for heavy snowfall and there was barely a light dusting? Perhaps the snow was quite heavy only 80 kilometres away!)
3. Examine the four panels of surface weather analyses (Four panels of surface weather analyses of winter storm) below. Locate the positions of the surface low pressure centres (L) in each panel. On the snowfall chart, plot the positions of each low pressure centre by placing an "L" with the date and time.
4. What relationship do you note between the track of the cyclone centre and the heaviest snowfall? Why would you expect this relationship to occur? (Keep in mind the necessary conditions for snow to occur.)

Four panels of surface weather analyses of winter storm on January 27, 1987 (Adapted from Kocin and Uccellini, 1990)

Activity - Operation Icestorm

Upon completing this activity, you should be able to:

• describe the atmospheric conditions that can result in freezing rain
  
• trace the life history of freezing raindrops

Introduction

In January 1998 a series of storms passed through Eastern Ontario, Southern Quebec and the northeastern United States. Over a six-day period, freezing rain resulted in 5 to 10 centimetres of ice accumulating on everything. Millions of trees, thousands of power and telephone poles were downed by the weight of the ice. Millions of people were without electricity for prolonged periods, lasting for several weeks in some rural areas. The great ice storm of 98 will be long remembered.

Ice storms occur when a unique set of atmospheric temperatures and moisture conditions come together to produce freezing rain. Freezing rain differs from ordinary rain in a very important way. When drops of freezing rain strike a surface, some of the water immediately freezes. Quickly, freezing rains make roads and sidewalks slick and dangerous. Coatings of ice silently thicken on objects, adding weight as the glaze builds. Gradually, tree limbs and wires bend and droop from their increasing burdens. Should icing continue, tree limbs begin to break and fall while electric and telephone wires snap, lights go out, furnaces stop, and many people find themselves in threatening situations.

The atmospheric data used in this activity to investigate freezing rain were collected by weather instruments carried aloft by balloons. These instrument packages,called radiosondes, rise through the atmosphere and take temperature, humidity, and other measurements. These observations are then transmitted back to the Earth's surface. The humidity measurement they make is reported as dew point. Dew point is the temperature to which the air must be cooled to become saturated. Whenever the air temperature and the dew point have identical values, the air is saturated. Saturation is a condition which has to be met in order for a cloud to exist. In turn, clouds must be present in order for precipitation to occur.

Activity

The radiosonde data (Eastern Ontario Southern Quebec) are representative of the atmospheric conditions during the January 98 ice storm. Plot the values on the accompanying graph. Place a dot (•) to show temperature at a particular altitude. Use a cross (x) to show the dew point at the same height. If the temperature and dew point values are the same, draw a small circle around the dot ( ). After plotting all the data, connect adjacent temperature values with solid straight lines to show the temperature pattern with altitude, and use dashed straight lines for adjacent dew point values. The combination of temperature and humidity patterns that results is termed a "sounding". It depicts atmospheric conditions in the atmosphere above the reporting station at the time the data were collected.

Use the graph with plotted data to answer the following questions:

1. Were there clouds above the area when these data were collected? What assumption must you make in interpreting the data to answer this question? (Refer back to the Introduction section if you need help answering this).
2. Locate the top and bottom of any existing cloud layer. Draw on the graph horizontal lines that cut across the sounding at the highest and lowest points at which saturated air was reported, i.e., temperature and dew-point values were the same. Since precipitation typically falls from relatively thick layers of cloud at least a few hundred metres thick, were the clouds thick enough to produce precipitation?
3. Shade lightly with your pencil the area enclosed by the vertical 0-degree line on the graph and that part of the plotted sounding showing temperatures of 0 degrees and higher. This shading highlights a layer of air in which temperatures are above freezing. Label the layer, "WARM". Describe in your own words the conditions above the area in terms of layers of air with above-freezing and freezing temperatures.
4. Assume that precipitation was occurring and that it originated as ice particles in the upper reaches of the existing cloud layer. What will prevent these particles from reaching ground level as snow?
5. For freezing rain to occur as it was in Eastern Ontario at the time of observation, raindrops must fall through a relatively shallow layer of freezing air immediately above the Earth's surface. According to the table of data given to you, how thick was this layer above Eastern Ontario ?

The slope of a cold front is steeper than that of a warm front because of the friction between the cold air and surface.

Answer Key

1. One has to assume that cloud is present when the upper air data indicates the presence of saturated air aloft (layers where the temperature and dew-point are the same). The data provided clearly indicates saturation aloft and therefore cloud aloft.
2. The upper air reports indicate that the clouds were about 3 km thick, therefore definitely thick enough to produce precipitation.
3. The saturated air was below freezing above 2.2 km, as well as below .4 km. Between these two levels the temperature was above freezing.
4. When these ice particles fall into the above freezing layer, they will melt and turn to rain before reaching the ground.
5. The layer of below freezing temperatures near the ground was about 500 metres.

The Green Lane^TM,
Environment Canada's World Wide Web Site.