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.
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
Many properties of the atmosphere vary dramatically as we move upward from
the surface. Because most of the sun's rays readily pass through the clear atmosphere
to warm the planet's surface, the atmosphere is strongly heated from below.
Thus, the highest temperatures are typically found at the Earth's surface and
decrease as altitude increases. This bottom atmospheric layer of decreasing
temperatures, ranging from 6 to 16 km in depth, is called the troposphere or
Above the troposphere, we find a layer of air whose temperature increases with altitude. The cause of this heating is the absorption of solar ultraviolet radiation by oxygen species and chemical reactions which form and dissociate ozone (the three-atom species of oxygen). Here ozone is naturally formed and destroyed, and several of the components of the process release heat which is then transferred to the surrounding air. The effect of this warming produces a layer of constant temperature topped by a layer of increasing temperatures with altitude. This layer is called the stratosphere or "stable layer". The boundary zone between the troposphere and the stratosphere, where the temperature stops decreasing and becomes constant with height, is termed the tropopause.
Both air pressure and air density decrease with increasing altitude. Air pressure is the weight per unit surface area of an air column extending from the given height to the top of the atmosphere. Therefore, atmospheric pressure is greatest at sea level.
Air is highly compressible, as is readily seen by inflating a tire. Therefore, it is most dense at the bottom of the atmosphere where the weight of the air above compresses it to high densities. At higher altitudes, the air is less dense because of the lesser weight of overlying air at upper levels. The result is that both air pressure and air density initially decreases very rapidly with altitude and then decrease more slowly. Half of all air molecules are found within only 5.5 km of sea level. The next one-quarter of the atmospheric mass is located between 5.5 to nearly 11 km.
Not only do atmospheric properties such as temperature, pressure and density vary with altitude, but so does the nature of the air's motion. On the planetary (or global) scale the winds blowing at middle latitudes in the middle and upper troposphere blow predominantly from the west. These upper-air, prevailing westerlies encircle the globe in a wave-like pattern, undulating north and south as they flow along the latitude belt.
The upper-air winds play an important role in the daily march of weather across the planet. They push air masses from their regions of origin and steer storm systems from one place to another.
Understanding the basic characteristics of these upper-air (tropospheric) westerlies is a prime key to understanding the variability of mid-latitude weather.
After completing this activity, you should be able to:
Figure 1 - Northern Hemisphere depicting upper-air westerlies with troughs and ridges
Figure 3 - Environment Canada 500 hPa Analysis for 12Z Oct 31, 2000. The solid lines on the chart are called 'contours" or iso-lines where the height above sea level of the 500 hPa level are the same. Click on the figure to enlarge for viewing
As World War II was approaching its conclusion, the United States introduced
the first high-altitude bomber, an airplane called the B-29. It could fly at
altitudes well above 6 kilometres. When the B-29s were being put into service
from a Pacific island base, two air force meteorologists were assigned to prepare
wind forecasts for aircraft operations at such altitudes.
To make their prediction, the meteorologists primarily used surface observations and what is known in meteorology as the thermal wind relationship. In plain language, this relationship states that if you stand with your back to the wind, and the air is colder to your left and warmer to your right, the wind speed on your back will get stronger as you ascend in the atmosphere. Using this relationship, the meteorologists predicted a 168-knot wind blowing from the west. Their commanding officer could not believe the forecast, believing the forecast speed much too high. However, on the next day, the B-29 pilots reported wind speeds of 170 knots from the west as predicted! The jet stream, as it would come to be known, was discovered.
Actually atmospheric scientists had theorized the existence of jet streams at least as early as 1937. The bomber pilots just confirmed it. Today, almost every radio and television weathercast mentions the positions of jet streams and their impact on daily and coming weather events.
The jet stream is a narrow current of relatively strong winds concentrated
as in the upper atmosphere. There are two main jet streams found in the global
circulation: the subtropical jet stream and the polar-front jet stream (also
known as the polar jet stream and often just the jet stream).
The subtropical jet stream is found between the tropical and middle latitude atmospheric circulations. Although not as clearly related to surface weather features as its polar counterpart, the subtropical jet sometimes reaches as far north as the southern United States. It is an important transporter of atmospheric moisture into storm systems.
The polar-front jet stream occurs over the polar front, where relatively cold air at higher latitudes comes in contact with warm air from the lower latitudes, and near the tropopause. It has an important influence on the weather of the middle latitudes. This is of special interest to meteorologists because of its influence on the development and maintenance of middle-latitude storm systems which evolve where warm and cold air masses come in contact.
The polar-front jet stream encircles the globe at altitudes between 9 and 13 kilometres above sea level in segments thousands of kilometres long, hundreds of kilometres wide, and several kilometres thick. It generally flows from west to east in great curving arcs as it undulates north and south. It is strongest in winter when core wind speeds are sometimes as high as 400 kilometres per hour.
The polar-front jet stream's location is one of the most influential factors on the daily weather pattern across North America. Meteorologists focus on the nature and position of the polar-front jet stream as they prepare weather forecasts. Changes in the jet stream indicate changes in the movement of weather systems and thus changes in weather.
The jet stream is also of great importance to aviation, as the B-29 pilots quickly found out. Westbound, high-altitude flight routes are planned to avoid the jet-stream head winds, which would slow the aircraft and consume precious fuel. Eastbound flights welcome time-saving tail winds from the jet stream to increase their speed and thus save fuel. However, the jet stream produces strong wind shears, large changes in wind speed over short vertical and horizontal distances, in some locations. The resulting air turbulence experienced in shear zones can be very hazardous to aircraft and passengers.
Using either the Environment Canada 250 hPa analysis from the web or the sample CMC Environment Canada 250 hPa Analysis Chart found in Figure 4, examine the patterns, troughs and ridges drawn on the map from the perspective of:
Figure 4 - CMC Environment Canada 250 hPa Analysis Chart for 12Z Nov 1, 2000
The location of the polar-front jet stream is often closely related to the
daily weather pattern across North America. The following two activities investigate
the causes of jet streams and the relationships of the polar-front jet stream
with surface weather.
Each activity can be stand-alone. One activity does not need to be done before the other. However, Activity 1 requires the construction of two sets of five pressure blocks. There are two sets of instructions for making the pressure blocks. The first suggestion is for the construction of a permanent set of pressure blocks while the second option uses more readily available but less durable material for classroom exercises. Finally, Activity 1 requires more time to complete than Activity 2.
Construction of a Permanent Set of Pressure Blocks
Cut blocks from solid materials such as wood or insulation material. The blocks should all have the same size square bases. The tall blocks should be twice the height of the short blocks. All blocks should weigh the same. Adjust the weight by drilling holes in the short blocks and inserting metal weights. Paint short blocks blue and tall blocks red.
Upon completing this activity, you should be able to:
One of the most important properties of the atmosphere is air pressure. It
is important because differences in air pressure from place to place put air
into motion just as in the case of air rushing out of the open valve of an inflated
tire. Pressure differences at altitudes of nine or more kilometres lead to the
development of high-speed winds, called Jet Streams.
This activity uses sets of blocks to investigate basic understandings about pressure and pressure differences produced by density variations. These understandings are then applied to the atmosphere to introduce the basic causes of jet streams.
To study pressure, we must first define it. Pressure is a force acting on a
unit area of surface Air pressure is the weight (weight is a force) of a column
of air acting on a unit area of horizontal surface, e.g. kilopascal (kPa) is
a pressure unit. To represent the concept of pressure concretely, two sets of
blocks with the following characteristics will be used.
Figure 7 - Upper-Level Exercise Map
Upon completing this activity, you should be able to:
The polar-front jet stream is like a high-speed river of air in the upper atmosphere.
It separates warm and cold regions at the Earth's surface. It may be several
hundred kilometres across from north to south, 1,500 to 3,000 meters thick and
at an altitude of 9,000 to 13,000 meters. The polar-front jet stream generally
flows from west to east, and is strongest in the winter when core wind speeds
are sometimes as high as 400 kilometres per hour. Changes in the jet stream
indicate changes in the circulation of the atmosphere and associated local weather.
The highest upper-level wind speeds are frequently observed at altitudes of
approximately 9 to 13 kilometres above sea level. In Figure
8, the upper air data chart displays plotted data depicting wind speed and
direction observed at 12Z (7 a.m. EST) at altitudes where the air pressure was
250 hectopascals (hPa). At map time, the actual altitudes at which the air pressure
was 250 hPa varied from 9,030 metres to 10,970 metres above sea level. Upper
level data are routinely displayed on constant-pressure charts because of the
usefulness of such charts to meteorologists. The data were acquired by tracking
balloon-borne weather instruments, called radiosondes, which measure and transmit
weather data as they rise through the atmosphere.
Wind information is depicted by "arrows" or wind barbs at locations on the map where radiosondes were launched. On the wind bard, the straight line represents the wind direction while the feathers represent the wind speed. Winds are named for the direction from which they are blowing. Wind speed is reported in knots (1knot equals 1.9 kilometres per hour); each full-length feather represents 10 knots, each half feather stands for 5 knots, and each flag means 50 knots. For example, in Figure 8, the plotted 250 hPa upper air data for The Pas, MB at 12Z on September 13, 2000 depicts a wind from the west-north west with a speed of 60 knots (or 114 km/h).
The wind information at the 250 hPa level received from each radiosonde can
be plotted on a chart and used to analyze the upper air wind patterns and to
locate the jet stream. In Figure 9, you will find the
upper air 250 hPa data for 12Z Ocotber 13, 2000 plotted and also a number of
shaded area. Within the shaded areas, the wind speeds at 250 hPa are 60 knots
or greater. The darker the shading the higher the wind speed, i.e. 60 kts, 90
kts and 120 kts, which will help you identify the jet stream and the jet streak
or jet max (or maximum). In this case, the jet maximums exceed 100 knots. The
dotted lines encircling these shaded areas connects the points where the winds
speed is 60 knots and are referred to as isotachs.
1. Using Figure 8, with a pencil, draw a line or lines to enclose the region(s) where the 250 hPa wind speeds are 60 knots or greater. Lightly shade the enclosed area. Draw a dark, heavy, smooth, curved arrow through the core of highest wind speeds. Add an arrowhead to show wind direction. Note: In Figure 8, you will see 4 stations with a dotted circle around them, which have been flagged by the computer that some aspects of the data set may have an error. So do not use those circled stations in your analysis.
2. The large arrow you drew on your map approximates the location of the existing polar-front jet stream across North America. Now imagine that you are in a gondola attached to a helium-filled balloon that is located over Prince George, BC. Assuming your balloon stays at the 250-hPa level, describe your path as you travel across the country. Over what cities, provinces or US states are you likely to pass over as you cross North America? At what point would you leave the east coast?
3. What is your approximate speed measured with respect to the surface of the Earth?
4. Even though the wind speed is 60 knots or greater, as measured relative
to the ground, an anemometer attached to the gondola shows the wind to be calm.
Figure 8 - CMC Environment Canada 250 hPa upper air chart depicting plotted data for 12Z September 13, 2000. Wind speeds are plotted in knots (1knot equals 1.9 km/h). Click on the figure to enlarge for viewing
Figure 9 - CMC Environment Canada 250 hPa upper air chart depicting plotted data and Jet Stream analysis for 12Z October 13, 2000. Wind speeds are plotted in knots (1knot equals 1.9 km/h). Click on the figure to enlarge for viewing
5. Look at winds on either side of the jet. The winds on either side of the
jet are (slower) (faster) than the jet stream winds and have (the same), (a
6. The polar-front jet stream is like a "river" of high-speed air embedded in the planetary-scale circulation of the atmosphere. The drawings below in figures 10a and 10b illustrate the wavy and westerly (or eastward) flow of air at upper levels in the middle latitudes of the Northern Hemisphere (planetary-scale circulation). The wave pattern can vary considerably in amplitude (latitude range).
a) Indicate which drawing (10a) or
(10b) best matches the upper-air flow of Figure 4.
b) Which drawing (10a) or (10b)
best matches today's the upper air flow as depicted by the 250 hPa analysis
found on the Environment Canada web site?
7. Across North America, storms tend to follow the path of the polar-front
jet stream. In Figure 9, a storm in the Denver area
at map time is likely to be moving towards (the Great Lakes), (Florida).
8. Knowledge of the location of the jet stream is very important to commercial aviation. Explain why at map time on figure 8, an airline flight from Montreal to Vancouver would take considerably more time than a flight from Vancouver to Montreal.