1940 Armistice Day Storm: Weather
The stories of Harold Hettrick and others who experienced the storm of November 11, 1940 describe an event that weather scientists refer to as a extratropical cyclone, or a midlatitude cyclone. With a further understanding of the science behind these storms and the atmospheric conditions leading up to them, students can more fully appreciate the stories surrounding these events. This step-by-step guide is intended for the teacher to use to describe storms such as the Armistice Day storm of 1940.
How do we make weather forecasts?
What is the extratropical cylcone pattern for November storms in the Upper Midwest?
What is a conceptual model and why do scientists use them?
In what ways can we compare different storms?
- Appreciate the value of a conceptual model.
- Understand the conceptual model of the extratropical cyclone.
- Apply this model to the Armistice Day Storm of 1940.
Preparing to Teach this Lesson
Weather forecasting is a familiar and a fascinating aspect of meteorology. An accurate weather forecast requires knowledge of all the weather variables (temperature, pressure, humidity, winds) and how these variables interact and change over time.
Today, weather forecasts are based on using sophisticated computers to solve complex mathematical equations. But people have been trying to forecast the weather long before the existence of the Weather Channel and television. The most basic forecasting method is to observe weather patterns and develop predictive formulas based on personal experience. People often share this weather wisdom through stories or folk sayings--short rhymes passed down through the generations.
In the field of meteorology, such observations developed a more formal format via the analog method, the predictive method at use in the 1940’s. The analog forecast relies on historical weather events and wind patterns similar to the current conditions. The goal is to identify a weather type that matches today’s weather and to use that historical event in time to forecast tomorrow’s weather. The success of the analog method depends on weather events following a particular sequence of steps.
The Armistice Day Storm of November 11, 1940 was part of a pattern of historical weather events. Only twenty years earlier, meteorologists discovered that these storms, called extratropical cyclones, often evolve according to a simple conceptual model, known as the Norwegian cyclone model. In this simplified model the storm originates on a stationary front that separates cold dry continental polar (cP) air from maritime tropical (mT) air. In the open wave phase, the storm develops strong cold and warm fronts and the storm moves east or northeast. The barometric pressure usually reaches its minimum value during the occluded phase of the storm, when the cold front and the warm front merge to form an occluded front. The winds are usually strongest during this stage because of the strong pressure changes with distance.
On November 7, 1940 a deep low pressure was centered approximately 150 miles west of Tatoosh, Washington. The storm demolished the Tacoma Narrows Bridge in the morning. A video of the collapse of the bridge can be viewed at http://www.civilzone.com/tacoma-br.html .
The storm moved to the east on November 8 as the atmospheric pressure increased. The storm crossed the Rocky Mountains on the 9th of November. Without satellites, storms are difficult to track as they cross the mountains. The rugged terrain disrupts the winds and the changes in altitude across the mountain range make it difficult to locate the center of the low pressure system.
On November 10, the storm organized and intensified over Colorado, just east of the Rockies. The low pressure was approximately 998 mb. The storm moved eastward and during the evening Amarillo, Texas reported 66 mph winds and Oklahoma City, Oklahoma reported 54 mph winds. During the night the storm intensified, or deepened, as the pressure of the low decreased to approximately 994 mb and began to move to the northeast towards the Upper Great Lakes region.
On the morning of November 11, Little Rock, Arkansas reported 63 mph winds. Schools were cancelled in Minnesota. In Saint Paul, a Pepsi-Cola truck rolled over and was abandoned by the driver. A group of school children playing outside discovered the truck and indulged in large quantities of Pepsi. As the storm center moved over Iowa Falls, Iowa it ravaged nearby apple orchards. Around noon the storm center was over La Crosse, Wisconsin. Chicago’s temperature dropped from 63F to 20F! Fifty-foot waves on Lake Michigan sank three steamers, killing 59 crew members. On November 11 the cold front was determined to be traveling in excess of 50 mph! In Grand Rapids, Michigan winds were reported at 80 mph. The central pressure was as low as 967 mb. By the evening the storm had reached in maximum intensity and began to weaken.
The storm continued to move northeastward on November 12 and it continued to weaken while continuing to cause damage. For example, the storm was so fierce on Lake Superior that a number of automobiles were flung over the deck of the steamer Crescent City.
In the end, the storm claimed the lives of approximately 157 people (estimates vary).
Weather satellites and radars did not exist in 1940. Most weather observations were made using weather instruments stationed on the ground. This made it difficult to track and study the storm, especially when it traveled over large mountains.
Wisconsin has experienced a number of strong winter storms in the month of November that were similar to the Armistice Day storm. Another historic November storm occurred in 1975 sinking the Edmund Fitzgerald while it was carrying ore on Lake Superior. Information on this storm can be found in several books and on the web (eg., NOAA's Storm Warning , Weather Wise's Great Lakes Storms , and CIMSS' The Sinking of the SS Edmund Fitzgerald).
Another storm very similar to the one that sank the Edmund Fitzgerald also occurred on November 10-11 1998 (discussed below in Assessments).
These storms demonstrate that the Norwegian cyclone model, though simplified, can help explain most of the characteristics of a real-life, deadly extratropical cyclone. We can also use these three storms to investigate how to use the analog method of weather forecasting.
Introducing conceptual models
Different conceptual models are used in many sciences. Explain to the students that conceptual models are only idealistic representations that often over-simplify the complexities of nature.
Have students draw a face or a person. Select one or more drawings, show them to the class and ask the class if there is anyone who actually has a face that looks like the one drawn. The answer is of course not. Everyone in the class will likely recognize the drawing of a face as exactly that—a face—but should also acknowledge that they have never seen anyone who looks like that. Yet, this simple drawing demonstrates the major features and the overall pattern of a human face: two eyes and ears, a nose, a mouth, eyebrows, and a chin. When you describe a friend to someone, you give details about those features. It is the same with describing the atmosphere. A general description of a few current atmospheric features might allow you to recognize the overall weather pattern.
We’ve learned to draw these simple faces based on observations. For example, we all know from experience that the nose lies above the mouth and between the eyes. To make a simple, conceptual model of storms requires similar observations. Our simple models of fronts and the extratropical cyclone are simplified conceptual models. You will never observe a storm that exactly follows this conceptual model. This, nevertheless, does not undermine the usefulness of conceptual models in science. Using conceptual models is a method scientists routinely use to represent systems in nature.
Introducing air masses
The conceptual model of weather events such as the 1940 Armistice Day storm involve air masses and fronts. The five types of North American air masses are:
- Continental Tropical (cT)
- Maritime Tropical (mT)
- Maritime Polar (mP)
- Continental Polar (cP)
- Arctic (A)
Use the air masses (PDF) image to show, generally, where the five North American air masses originate. Make sure your students understand that in this context, “maritime” means moist air, while "continental" implies air with lower humidity.
(It may be helpful to provide a spatial comparison, so that the student realizes that we are referring to air masses that span entire regions of a continent.)
Once the student understands that there are different masses of air on the continent, some warm and moist, others cold and dry, etc, the student can learn that these air masses move and have rather distinct boundaries.
Although an individual air mass can change over time, different air masses do not mix readily. Therefore, when two air masses come into close contact they retain their separate identities for several days. The transition zone between two different air masses is called a front. Fronts can be hundreds of miles long and exist as long as the air masses they separate remain distinct.
There are four types of fronts: warm, cold, stationary, and occluded.
Norwegian meteorologists around the time of World War I laid the foundation for our concepts of fronts and their movements. They observed large air masses with different temperature and moisture properties that advanced and retreated versus one another. Clashing air masses often led to disruptive weather conditions. The boundary between air masses was called a “front,” analogous to the boundaries used on military maps to separate battling armies. Use the fronts (PDF) image that shows examples of all four types of fronts and the conventional way meterologists depict them on weather maps. To show an example of an active weather map depicting one or more fronts, try the current weather map from The Weather Channel.
Once the concept of a front from a “birds-eye” perspective has been introduced to the students, note that fronts have unique cross-sectional characteristics that will eventually be of great importance in the development of a severe extratropical cyclone. Cross sectional diagrams may easily be drawn on a chalkboard for cold fronts, warm fronts, stationary fronts, and occluded fronts. Use this cross sectional front (PDF) image to illustrate this dimension of a front.
Each type of front brings characteristic weather patterns. For the purposes of studying an event like the Armistice Day storm of 1940, it is important that the students understand how cold fronts and warm fronts interact to produce an occluded front. An occluded front is the result of a cold front catching up to a warm front. The counter-clockwise movement around a low pressure center is called an extratropical or midlatitude cyclone (PDF).
Introducing the midlatitude cyclone
An extratropical or midlatitude cyclone can be thought of as the process of air converging at the surface, rising, then diverging at the tropopause. Display the cross-sectional cyclone (PDF) image of surface convergence, rising air, and divergence aloft. Point out that it makes sense that the air must rise, since it has no other place to go, since when it converges the ground is below.
To illustrate these dynamics, inflate a balloon but do not tie the end. Have a student volunteer come to the front of the class and hold the balloon by the open end, pinching it so that no air can escape. Ask the rest of the class whether they believe there is higher air pressure inside the balloon pushing outward, or outside the balloon pushing inward. The correct answer is inside the balloon pushing outward. Explain that this is similar to the situation where there is a great difference in pressure over some small distance on the surface map. Use the pressure (PDF) image to illustrate. Next, without letting go of the balloon, have the student let the air out of the balloon by loosening his or her grip on the open end of the balloon. Notice how vigorously the air flows out of the balloon from a higher pressure condition toward a lower pressure condition.
In a storm, wind blows from high pressure towards low pressure. The stronger the difference in pressure with distance, or the tighter the packing of the isobars, the stronger the wind. This is why the wind blows so hard during midlatitude cyclones such as the one affecting the upper-Midwest on November 11, 1940.
Life cycle of the midlatitude cyclone
Midlatitude cyclones (also called extratropical cyclones) have a life cycle consisting of birth (cyclogenesis), maturity, and decay (cyclolysis). Sea level pressure can be thought of as the weight of the atmosphere pushing down upon the ground. A midlatitude cyclone is born during cyclogenesis, when the sea level pressure over a location falls because the divergence aloft is greater than the surface convergence.
Have the students imagine a can, open on each end, of cotton balls that represents air molecules. Use the divergence (PDF) to illustrate. If one were to quickly pick cotton balls from the top opening of the can (i.e. large divergence), while only slowly inserting a few cotton balls into the bottom of the can, it is easy to see how this can of “air” becomes lighter. Since a midlatitude cyclone is created by a low pressure center, and sea level air pressure is analogous to the weight exerted by the atmosphere, the students have just demonstrated how a midlatitude cyclone is born. During the cyclogenesis stage of a midlatitude cyclone the cold front begins to “chase” the warm front around the newly created low pressure center. At this point the two fronts are distinct and no occluded front has formed.
The mature stage of a midlatitude cyclone is the relative balance of the surface convergence, rising air, and divergence aloft. Use the balance (PDF) to illustrate. If the students have been introduced to the concept of equilibrium, this is an analogous point. The sea level pressure at the low pressure center is no longer falling, and not yet rising. In the case of the cotton balls, the same number of cotton balls are being pushed in through the bottom of the can (surface convergence) as being taken out of the top (divergence aloft). At this point in the life cyclone of the midlatitude cyclone, the cold front has just begun to catch up to the warm front as they swing around the low pressure center. Since the students are now familiar with the definition of the occluded front, they should recognize that this new formation is called an occluded front. The most precipitation and heaviest winds usually occur in the mature stage of the midlatitude cyclone.
The mature stage of a midlatitude cyclone is the relative balance of the surface convergence, rising air, and divergence aloft (PDF visual). If the students have been introduced to the concept of equilibrium, this is an analogous point. The sea level pressure at the low pressure center is no longer falling, and not yet rising. In the case of the cotton balls, the same number of cotton balls are being pushed in through the bottom of the can (surface convergence) as being taken out of the top (divergence aloft). At this point in the life cyclone of the midlatitude cyclone, the cold front has just begun to catch up to the warm front as they swing around the low pressure center. Since the students are now familiar with the definition of the occluded front, they should recognize that this new formation is thus called an occluded front. The most precipitation and heaviest winds usually occur in the mature stage of the midlatitude cyclone.
During the decay phase of the midlatitude cyclone, the opposite process of the birth phase occurs. The sea level pressure rises as the surface convergence becomes greater than the divergence aloft. Once again using the analogy of the open-ended can of cotton balls, have the students imagine more cotton balls being stuffed in through the bottom than being taken out the top. Use this decay phase (PDF) to illustrate. The can would clearly become heavier and thus the midlatitude cyclone would stop being a low pressure center. During this stage, the fronts are no longer easily distinguished.
To wrap up the study of the life cycle and evolution of a midlatitude cyclone, display three successive transparencies or handouts - one for each stage of the lifecycle of the November 11, 1940 midlatitude cyclone (maps1.pdf, maps2.pdf, maps3.pdf). Each image has a surface map with sea level isobars contoured as well as the fronts and areas of heaviest precipitation. Have the students point out features from the above discussion of the three stages of the life cycle of the midlatitude cyclone.
Inform your students on the interpretation of a wind barb. Have students analyze a surface map with wind barbs plotted for November 11, 1940. Have students comment on what they observe about the prevailing wind pattern about the midlatitude cyclone. What can the students infer about the meaning of “cyclonic” winds, and conversely “anticyclonic” winds?
Introducing watches and warnings
Explain to the students that watches and warnings are not the same thing. When the National Weather Service (NWS) determines that conditions in the atmosphere are favorable for a blizzard or severe blizzard, they will issue a watch. A watch does not mean that the weather event is occurring, only that the public should be vigilant in case it does.
A warning, on the other hand, means that the weather event is occurring at that time. If a severe blizzard warning is issued by the NWS, students should understand this to be an actual state of emergency and they and their families should respond accordingly.
Have the students discuss the role communication plays in quickly distributing watches and warnings to the public. Do they think these same methods of communication existed in 1940? How do they think word spread about the storm then?
On November 10 & 11, 1998, a large weather system developed over the Central and Eastern United States. In the Upper Midwest, the storm brought damaging winds, cold air, and precipitation. In the South, the storm brought strong thunderstorms. Barometric pressures at the low center were measured in the 960 mb range.
This storm was noteworthy for two reasons. The first was its rapid intensification (deepening at a rate of between 1 and 2 mb per hour over a 24-hour period) and its accompanying severe weather (including tornadoes) in the southern U.S. and severe winter weather (blizzard conditions) in the Upper Midwest. The second is its uncanny similarity to the storm that contributed to the sinking of the SS Edmund Fitzgerald. As seen in the image November storm tracks (PDF), the tracks of the two storms are nearly parallel. The 1998 storm was significantly more intense.
Have students plot the path of the 1940 Armistice Day storm on the map.
* Which storm moved the fastest? Which storm moved slowest?
* Which storm had the lowest pressure?
* In what ways were the paths of the storms similar and how were they different?
* How well did the Armistice Day storm represent the Edmund Fitzgerald (1975) storm?
* How well did the 1940 and 1975 storms represent the 1998 storm?
* For how long could an historical analog help you forecast the weather – a day, a week, an hour?
* Ask the students what they were doing in November 1998? Were they affected by this storm?