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Weather Systems
Introduction
Weather in any region is influenced by the atmospheric changes that occur when masses of air with contrasting
properties interact. The characteristics of air masses vary with location ranging from dry and cold to warm and
humid. The daily clash of air masses over North America generates our common weather patterns characterized
by high and low-pressure systems bounded by warm and cold fronts. These frontal systems are relatively
narrow, curvilinear zones that mark a transition from one air mass to another. Weather experienced over much
of the central and eastern U.S. is the result of the west-to-east migration of regional-scale low pressure systems,
termed mid-latitude cyclones, and their associated warm and cold fronts. Mid-latitude cyclones affect much of
the continental landmass for up to a week at a time. Meteorologists attempt to predict the path of these midlatitude cyclones and their frontal systems by monitoring their associated atmospheric conditions such as
moisture, temperature, pressure, and wind direction. Using these characteristics they can predict the potential
weather for two to five days in the future. However, these dynamic systems are subject to change, and the shortterm, relatively accurate forecast becomes a long-term calculated guess as the forecast extends beyond two or
three days.
Air Masses
Air masses represent large regions (1,000s km2) of the lower troposphere with relatively uniform properties
(temperature, moisture content). The characteristics of individual air masses are dependent upon the attributes
of a source area and the modification of the air mass that occurs as a result of movement from the source
region. Weather in any region is influenced by the changes that occur in the air mass over time and the
interactions that occur at fronts, the boundaries between contrasting air masses.
Source Areas
An air mass develops when the atmosphere is located above a relatively uniform land or water surface for
several days. The lower atmosphere assimilates some of the properties of the underlying surface. Air masses are
identified by their temperature (polar/tropical) and the character of the underlying surface
(continental/maritime). The latter property is a proxy for moisture content. Air masses that develop above
oceans contain much more moisture than those formed over land.
The distribution of air masses is relatively intuitive. Arctic and polar air masses are located at high latitudes
(+50o) in the Northern Hemisphere and tropical air masses are located closer to the equator (Fig. 7). Continental
air masses are found over land, maritime air masses over ocean. The boundaries between individual air masses
vary with seasons. Polar air creeps further south during winter and retreats northward during summer.
Meteorologists use a form of scientific shorthand to label the most common types of air masses For example a
maritime tropical air mass (warm, moist air formed over the ocean) is identified by mT (m = maritime; T =
tropical). The characteristics of five types of air masses are summarized below.
• cA - continental Arctic/Antarctic air forms at high latitudes around the poles above permanently snowcovered ground (or pack ice). These air masses are characterized by extremely cold, dry air that may sweep
south across Canada and produce days of bone-chilling cold temperatures over much of the central and eastern
U.S. during winter.
• cP - continental polar air forms over the northernmost portions of North America, Europe, and Asia. It shares
its basic characteristics (cold, dry) with cA air, without the exceptional cold temperatures. High "lake effect"
snowfalls in the upper Midwest are the result of the dry cP air masses picking up moisture as they cross the
warmer waters of the
• cT - hot, dry continental tropical air forms over continental interiors such as the dry lands of northern
Mexico and southwestern U.S. This air mass disappears from North America in winter but brings scorching
summers to the southwest. It will decay as it moves east, absorbing moisture and cooling as it goes.
• mP - maritime polar air masses form in the northern Atlantic and Pacific Oceans and are characterized by
cool, moist air that affects states bordering the Atlantic shore of the Northeast and most of the Pacific coastline.
Temperatures at the ocean surface are less extreme than on land (less cold) so mP air is warmer than cP air.
• mT - high temperatures and high humidity distinguish the maritime tropical air masses that move inland
from the tropical Pacific, Gulf of Mexico, and tropical Atlantic Ocean. The mT air brings hot, humid summers
to southeastern states and can form at any time during the year.
Think about it . . .
Create a concept map that summarizes the characteristics of the principal air masses and their influence
on weather patterns in North America.
Frontal Systems
Frontal systems represent the meteorological battle that ensues when air masses of contrasting properties clash
along their boundaries. As air masses move across Earth's surface they inevitably interact to create relatively
narrow, curvilinear zones that mark a front, a transition from one air mass to another. Advancing frontal
systems bring clouds and precipitation and are accompanied by changes in moisture, temperature, pressure, and
wind direction. The clash between cP and mT air masses over the Great Plains and Midwest is the most
common source of frontal systems in the U.S. (For more on the causes of this phenomenon, see the section,
Mid-latitude Cyclones.) Weather conditions change in a predictable sequence as warm and cold fronts pass over
an area.
Cold Front
Cold, dense continental polar air replaces moist, warm maritime tropical air across the cold front. People living
downwind from the front experience decreasing temperature and humidity and increasing atmospheric pressure
with the passage of the cold front. Warm air will always rise over cooler air so both the cold and warm fronts
are inclined toward the warm air mass. Warm air is pushed up and over the advancing cold front, causing
relatively rapid cooling and condensation that results in the development of tall cumulonimbus clouds that host
heavy but relatively short-lived precipitation. Rapidly advancing cold fronts may be marked by the growth of a
squall line of thunderclouds.
Warm Front
Changes following the passage of the warm front are more benign than the storms that travel with the cold
front. Friction at Earth's surface causes the warm front to slope gently (~½ degree inclination) toward the warm
air mass. Warm, humid air is transported upward over a distance of approximately 1,000 km (625 miles). The
first signal of an approaching warm front is the appearance of light, upper-level clouds (cirrus, cirrostratus). Up
to 12 hours later, the high clouds will be replaced by lower nimbostratus with associated light to moderate
precipitation. Rain associated with a warm front may last longer than precipitation that accompanies a cold front
because the warm front typically moves more slowly and extends over a larger area. Temperatures and humidity
rise and winds typically shift direction (from south to southwest) with the passage of the warm front.
Occluded Front
The cold front moves more rapidly than the warm front (~ 10 km per hour faster) and will eventually close the
gap between the fronts, forcing the intervening warm air upward generating additional precipitation. An
occluded front is represented by a combination of warm and cold front symbols. The occluded front juxtaposes
two bodies of cold air; the warmer of the two masses is forced up and over the other. Occluded fronts may be
marked by the occurrence of nimbostratus clouds.
Think About It…
List the cloud types that appear with the different types of fronts.
Thunderstorms
Thunderstorms form where warm, humid air is forced upward to altitudes of up to 15 km. Condensation
occurs as the air cools, releasing latent heat and ensuring that the rising air remains unstable (warmer than
surrounding air). Thunderstorms may occur as relatively isolated, short-lived events or as longer-duration
severe storms depending upon the conditions that cause the air to rise. Isolated afternoon thunderstorms, or
cells, are commonplace in warm summer months where moist maritime tropical air masses move over land. The
temperature of the land surface rises to a maximum during the mid-afternoon, warming overlying air parcels
and causing them to become unstable enough to rise, generating scattered thunderstorms. Severe storms, or
supercells, are associated with frontal lifting along the cold front between the continental polar and maritime
tropical air masses in mid-latitude cyclones. These storms are most common during spring and early summer,
when the contrast in temperatures between air masses is greatest. Because thunderstorms are associated with
mid-latitude cyclones it should come as no surprise that storms are most common over the Great Plains and
southeastern U.S. States such as Florida may experience over 100 days a year with thunderstorms whereas such
storms are rare (less than 20 per year) in Pacific Coast states.
There are three stages during the life of a typical thunderstorm that rarely lasts for more than two hours:
1. Cumulus stage - early cloud development when a cumulus cloud expands laterally and vertically as air
enters the cloud mass at all levels. Cloud formation is rapid, requiring approximately 15 minutes to grow
vertically to heights of 10 km (6 miles) Updrafts (~ 4 m/sec ear ground surface to ~ 10 m/sec at high levels)
within the cloud carry humid air to higher, colder levels where condensation occurs.
2. Mature stage - top of cloud cell may be at altitudes up to 15 km (9 miles). Rain and ice formed by
condensation become too large to be supported by updrafts and fall to ground. Falling precipitation generates
friction within rising air, creating a zone of downdrafts. Descending air warms up, resulting in evaporation that
absorbs latent heat and cools the cloud. This stage lasts for 15 to 30 minutes generating rainfall or hail at the
surface accompanied by gusty winds (downdraft).
3. Dissipating stage - cloud formation processes end as moisture is expended and descending air cools the
cloud mass, returning stability to air. The final stage is characterized by diminishing precipitation (light rain) as
the cell dissolves.
Tornadoes
Tornadoes are narrow, funnel-shaped spirals of rapidly rotating air that form in association with
thunderstorms. Like hurricanes and mid-latitude cyclones, tornadoes are near-circular low-pressure systems.
However, the pressure gradient is much more intense for tornadoes.
Pressure differences across mid-latitude cyclones are in the range of 20 to 30 mb (millibars) over hundreds of
kilometers (pressure gradient, 0.02-0.03 mb/km). Hurricanes may experience pressure gradients of more than
100 mb over shorter distances (~0.2-2 mb/km) but large pressure differences in tornadoes occur over distances
measured in hundreds of meters. Extreme pressure gradients of up to ~0.1 to 1 mb/m are possible for
tornadoes, generating the strongest natural winds on Earth with wind velocities of up to 500 km/hr. Tornadoes
are classified using the Fujita Intensity scale which places tornadoes in one of six categories (F0-F5) according
to level of destruction which is taken as a proxy for wind speed. The scale divides tornadoes into three
subgroups: weak (F0, F1); strong (F2, F3), and violent (F4, F5). Wind speed cannot be measured directly
because the high winds that can blast apart whole buildings would make short work of measuring instruments.
Scientists use the level of destruction to gauge estimates of wind speed and thus determine the Fujita value for a
specific tornado. This makes it difficult to rank winds that touch down in sparsely populated areas. The funnel
of the tornado moves more slowly than the winds that give it shape. Funnels are typically less than 600 m
(2,000 feet) wide and average funnel velocities are approximately 50 km/hr, although velocities as high as 200
km/hr (125 mph) have been recorded. Tornado paths follow the direction of movement of their parent
thunderstorms that are in turn associated with east to northeast-directed mid-latitude cyclones. Scientists have
been unable to observe the birth of a tornado because of the difficulty in determining exactly where tornadoes
may originate. One hypothesis on tornado formation considers tornado development in three stages :
• Early stage: Friction slows winds at the ground surface, resulting in increasing wind velocity with elevation
in the lower troposphere. These contrasting vertical wind speeds generate local winds that rotate about a
horizontal axis.
• Updraft stage: Updrafts below thunderstorm cell draw the spiraling horizontal winds upward forming a
mesocyclone (a small cyclone) within the larger storm cloud cell. Mesocyclones may be up to 10 km in
diameter.
• Tornado stage: Rotation within the mesocyclone forms smaller, more intense spiraling winds within a
tornado that extend downward from a cloud base toward the ground surface.
Tornado paths are typically 5 to 25 km (3-16 miles) in length but some larger tornadoes may remain on the
ground for over 100 km (62 miles). Smaller funnels may skip across the surface like a fickle avenger,
destroying one home while leaving neighboring properties undisturbed. Improvements in forecasting methods
have reduced the number of fatalities associated with tornadoes. Approximately two-thirds of U.S. fatalities
occur as a result of tornadoes destroying homes; nonpermanent mobile homes are especially susceptible. The
U.S. is home to the majority of the world's tornadoes, averaging about 1,000 a year. Tornadoes occur when
thunderstorm activity is at an optimum, during the late spring and early summer across much of the nation. The
highest frequency of tornadoes per area occurs over the Great Plains states (Texas, Oklahoma, Kansas, and
Nebraska) and parts of the upper Midwest (Iowa, Indiana, and Illinois), a region that has come to be known as
tornado alley. The timing of tornado activity is tied to seasonal movement of the polar front that drives midlatitude cyclones. Tornadoes move out of the Gulf Coast and southeastern states into the Great Plains as the
front retreats northward in late spring. Summer sees tornado activity shifting to the northern Plains states and
the upper Midwest.
Think about it . . .
Use a Venn diagram to compare and contrast the characteristics of tornadoes and hurricanes.
Hurricanes
Hurricanes are cyclonic storm systems that form over tropical oceans during summer and fall. Also known as
typhoons (Pacific Ocean) and cyclones (Indian Ocean), hurricanes are characterized by high winds (more than
119 km/hr , 74 mph), heavy rainfall (10-25 cm 4-10 inches), and storm surges (sudden rise in sea level) along
coastlines. Hurricanes are smaller (approximately a third to half the size) and less frequent than mid-latitude
cyclones that govern most U.S. weather patterns but they have much more powerful winds. Unlike mid-latitude
cyclones, hurricanes do not originate as a result of perturbations at a boundary between contrasting air masses.
Instead they grow from tropical storms generated by disturbances in the belt of equatorial trade winds.
Hurricanes can result in fatalities and substantial property damages as winds and high seas cause structural
damage to ocean-going vessels and coastal developments, and heavy rainfall leads to flooding as much as 200
km inland from the coast.
Building a Hurricane
Hurricanes develop under a specific suite of conditions including warm surface waters, cyclonic circulation, and
divergent flow in the upper troposphere.
• The initial stage in the development of a hurricane is the formation of a tropical depression (low-pressure
system) where the trade winds converge near the equator. The location of the convergent winds changes with
seasons, lying north of the equator during summer in the Northern Hemisphere and migrating to southern
latitudes during our winter. The rising air cools and condenses to form cumulus clouds that will develop into
cumulonimbus cells if the rising air is sufficiently warm and humid.
• Water temperatures must be at least 27oC and should extend downward for 50 to 65 meters (165-215 feet) to
ensure that colder water won't be drawn to the surface by the developing storm. Warm surface waters typically
straddle the equator but are absent south of the equator in the eastern Pacific and Atlantic Oceans due to oceanic
circulation patterns. The North Atlantic hurricane season officially lasts from June 1 to November 30, and
most U.S. hurricanes strike in August and September.
• Earth's rotation as reflected in the Coriolis Effect imparts a clockwise (Southern Hemisphere) or
counterclockwise (Northern Hemisphere) rotation to the growing storm. The magnitude of the Coriolis Effect
increases with increasing latitude and is zero at the equator. Consequently, the necessary rotation is not
imparted on storms within 5 degrees either side of the equator.
• The inflow of air into the developing low-pressure system must be matched with an outflow of air in the
upper troposphere to maintain the pressure gradient in the developing hurricane. If not, the pressure contrast
decreases and wind speed declines. If all of these conditions are met, a tropical depression (wind speed <37
km/hr) forms and has the potential of growing to a tropical storm (wind speed 63-119 km/hr) before
developing into a hurricane with wind speeds of at least 119 km/hr. Wind speed increases in inverse proportion
to the decrease in pressure in the eye of the hurricane; the lower the pressure the higher the wind velocity.
Rising air in the deepening low pressure system cools and condenses, releasing latent heat and generating a
dense spiral of cumulonimbus clouds punctuated by a central eye characterized by clear skies. Bands of clouds
spiral outward from the vortex that will continue to grow in size and intensity as long as the underlying water
temperature remains above 27oC. Precipitation is concentrated within a radius of approximately 100 km on
either side of the eye, releasing up to 20 billion tons of water per day.
Much of the destruction associated with hurricanes isn't caused by high winds or storm surges but is linked to
the exceptional precipitation events that can unload 60 cm (24 inches) of rain from a single storm system in
just a few days.
Hurricane Measurement
Hurricanes are divided into five categories by wind speed using the Saffir-Simpson hurricane intensity scale.
Destruction associated with major (category 3, 4 and 5) hurricanes includes damage to permanent homes,
widespread coastal flooding, uprooting of trees, toppling of power lines. Anticipation of such damages prompts
evacuation of residents from the area of expected landfall.
Think about it . . .
1. Use the Venn diagram to compare and contrast the characteristics of tornadoes and hurricanes.
2. You work in a team of disaster specialists for the Weather Channel. The channel wants to create its
own scoring system that better evaluates the potential damage from incoming storms. You and your team
are given the assignment to create an evaluation rubric to assess factors that will influence the risk of
potential damage from a future hurricane.
Summary
1. What is an air mass?
2. How are air masses identified?
3. Which air masses have the greatest impact on U.S. weather?
4. What is a frontal system?
5. What is the difference between weather at cold and warm fronts?
6. What is an occluded front?
7. Under what conditions do thunderstorms form?
8. What are the stages of thunderstorm development?
9. Is there any similarity between tornadoes and hurricanes?
10. How do scientists classify tornadoes?
11. How do tornadoes form?
12. Why does the U.S. have more tornadoes than any place on Earth?
13. What are the key conditions needed for hurricane formation?
14. Hurricanes form over warm ocean waters with temperatures of
15. What hazards are associated with a hurricane?