Transcript and midlatitude wave cyclones

The Motion of Air Masses
An air mass is a large body of air with fairly uniform temperature and
moisture characteristics. Air masses acquire their characteristics from
their source regions.
Temperature is indicated using the upper case (for example T for Tropical
or P for Polar). Moisture is indicated using the lower case (for example, c
for continental (dry) or m for maritime (wet).
The Motion of Air Masses
Global air masses and source regions. An idealized continent,
producing continental (c ) air masses, is shown at the center. It is
surrounded by oceans, producing maritime air masses (m). Tropical
(T) and equatorial (E) source regions provide warm or hot air
masses, while polar (P), arctic (A), and Antarctic (AA) source regions
provide colder air masses of low specific humidity.
The Motion of Air Masses
This map of air masses over North
America shows how some air
masses may move over large
distances and may on occasion
influence weather over a large
proportion of the continent (for
example, the cold continental air
masses from the north). (Data from
U.S. Department of Commerce.)
Cold, Warm, and
Occluded Fronts
A given air mass usually has a sharply defined boundary between itself and a
neighboring air mass. This boundary is termed a front.
In a cold front, a cold air mass lifts a warm air mass aloft. The upward motion sets off a
line of thunderstorms. The front is steep and fast moving.
In a warm front, warm air rides up and over colder air. The front has a shallow gradient
and moves slowly. A notch of cloud is cut away in this diagram to show rain falling
from the dense stratus clouds.
In an occluded front, a warm front is overtaken by a cold front. The warm air is pushed
aloft, and it no longer contacts the ground. Abrupt lifiting by the denser cold air
produces precipitation. This is the process that determines the development of wave
cyclones.
Cold, Warm, and Occluded
Fronts
a).
c).
b).
a). A cold front.
b). A warm front.
c). An occluded front.
Cyclones and Anticyclones
Cyclones and anticyclones are weather systems that involve masses of air moving
in a spiraling motion. In a cyclone, air spirals inward, whereas in anticyclone, air
spirals outward. Most types of cyclones and anticyclones are large features that
move slowly across the Earth's surface, bringing changes in the weather as they
move. These are referred to as traveling cyclones and traveling anticyclones
The circulation of weather systems is determined by their pressure which in turn
determines whether air ascends as in the case of low pressure, or descends as in
the case of high pressure. Where air rises, condensation and cloud formation
results as seen here in the system developing off the west coast of Alaska. In an
anticyclone, outspiraling air descends and is warmed adiabatically leading to clear
conditions.
Cyclones and Anticyclones
Air motion in cyclones and anticyclones.
Wave Cyclones
In the middle and high latitudes, the dominant form of weather
system is the wave cyclone. The wave cyclone is a large
inspiral of air that repeatedly forms, intensifies, and dissolves
along the polar front.
*See movie on wave cyclones in the geodiscoveries section of your text’s
website.
Wave Cyclones
Simplified surface weather maps and cross
sections through a wave cyclone. In the
open stage (left), cold and warm fronts pivot
around the center of the cyclone. In the
occluded stage (right), the cold front has
overtaken the warm front, and a large pool
of warm, moist air has been forced aloft.
Examining a
Wave Cyclone
Wave cyclones not only develop over time, they also move. Generally
mid-latitude wave cyclones track from west to east move from east to
west until they encounter west winds aloft near the Tropics of Cancer
and Capricorn.
Examining a Wave Cyclone
Paths of tropical cyclones and wave cyclones. This world map
shows typical paths of tropical cyclones (red) and midlatitude
wave cyclones (blue). (Based on data of S. Pettersen, B. Haurwitz,
and N. M. Austin, J. Namias, M.J. Rubin, and J-H. Chang.)
Tornadoes
A tornado is a small but intense
cyclonic vortex in which air spirals at
tremendous speed. The dark funnel
cloud results from spiraling updrafts
from thunderstorms, although the
precise details of why some storms
result in tornadoes and others do not
are still unresolved.
How Tornadoes Cause
Damage
The high wind speeds in association with the large volumes of debris carried,
causes the damage associated with tornadoes.
The scale for tornado intensity is referred to as the Fujita scale or F-scale. Weak
tornadoes (0-1) which may result in downed trees include tornadoes with wind
speeds up to 181 km/hr. The F-scale ranges up to 5 where wind speeds exceed 500
km/hr and sees houses thrown around by the strong winds.
*See movies on tornadoes in the geodiscoveries section of your text’s website.
Occurrence of Tornadoes
The occurrence of tornadoes is associated with thunderstorms spawned by
fronts in the midlatitudes of North America. Tornadoes also occur regularly in
Australia and occasionally in other mid-latitude locations.
The highest occurrence of tornadoes occurs in parts of Texas, Oklahoma and
Florida. This movie clip shows the development of tornadoes over Fort Worth
Texas.
*See movie on the occurrence of tornadoes in the geodiscoveries section of your text’s website.
Hurricanes
Tropical cyclones are
known as hurricanes in the
western hemisphere,
typhoons in the western
Pacific off the coast of
Asia, and cyclones in the
Indian Ocean.
*See movie on hurricanes in the
geodiscoveries section of your
text’s website.
Hurricane Gladys from Apollo 7
The Eye of the Hurricane
Winds spiral inward in hurricanes due to low pressures at the center. At
the center an "eye" develops whereby skies become clear and wind
speeds are relatively low. However, outside the eye, tall cumulonimbus
clouds develop as the result of the intense circulation with heaviest
rainfall and highest winds speeds occurring along the outer edge of the
eye (or eye wall). Toward the outer edge of the storm, high cirrus clouds
predominate.
Identify these features on this satellite image of hurricane Mitch.
The Eye of the Hurricane
Anatomy of a hurricane. In this schematic
diagram, cumulonimbus (Cb) clouds in
concentric rings rise through dense
stratiform clouds. Cirrus clouds (Ci) fringe
out ahead of the storm. Width of diagram
represents about 1000 km (about 600 mi).
Redrawn from NOAA, National Weather
Service.
Hurricane Mitch, 10.25.98, NOAA.
Mapping a Hurricane
The mapping of hurricanes is important in order to understand the
relative location of differing weather conditions. However, tracking
hurricanes also sees the use of satellite imagery in order to identify a
storm's potential track. For example, hurricanes in the Atlantic track
from the east toward the northwest until diverted northeast by
westerlies.
Mapping a Hurricane
A simplified weather map of a
hurricane passing over the
western tip of Cuba. Daily
locations, beginning on
September 3, are shown as
circled numerals.
Tracks of typical hurricanes
occurring during August. The
storms arise in warm tropical
waters and move northwest.
On entering the region o
prevailing westerlies, the
storms change direction and
move toward the northeast.