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The Impact of Storms on the Landscape

The Impact of Storms on the Landscape

  • Storms are phenomena that are more limited than the broad-scale wind and pressure systems.
  • They are transient and temporary.
  • Storms involve the flow of air masses as well as a variety of atmospheric disturbances.
  • They have short-run and long-run impacts.
  • In some parts of the world they have major influence on weather, some on climate.
  • Long-run storms include both positive and negative impacts on a landscape.
  • Positive: promote diversity in vegetative cover, increase size of lakes and ponds, and stimulate plant growth.

Air Masses – a large parcel of air that has relatively uniform properties in the horizontal dimension and moves as an entity. Such extensive bodies are distinct from one another and compose the troposphere.


An air mass must meet three requirements:

  1. Must be large (horizontal and vertical).
  2. Horizontal dimension must have uniform properties (temperature, humidity, and stability).
  3. Must be distinct from surrounding air, and when it moves it must retain that distinction (not be torn apart).


Formation occurs if air remains over a uniform land or sea surface long enough to acquire uniform properties.

Source Regions—parts of Earth’s surface that are particularly suited to generate air masses because they are:

  • Extensive
  • Physically uniform
  • Associated with air that is stationary or anticyclonic


Because the source region determines the properties of air masses, it is the basis for classifying them.

Use a one- or two-letter code.

Movement and Modification

Some air masses remain in the source region indefinitely.

Movement prompts structural change.

Thermal modification—heating or cooling from below

Dynamic modification—uplift, subsidence, convergence, turbulence

Moisture modification—addition or subtraction of moisture

Moving air mass modifies the weather of the region it moves through.

North American Air Masses

Physical geography of the U.S. landscape plays a critical role in air mass interactions.

No east–west mountains to block polar and tropical air flows, so they affect U.S. weather/climate.

North–south mountain ranges in the west modify the movement, and therefore the characteristics, of Pacific air masses.

  • Maritime tropical (mT) air from the Atlantic Ocean and the Caribbean Sea/Gulf of Mexico strongly influences the climate east of the Rockies in the United States, southern Canada, and much of Mexico.
  • Primary source of precipitation; also brings periods of uncomfortable humid heat in summer.
  • Continental tropical (cT) air has an insignificant influence on North America, except for bringing occasional heat waves and drought conditions to the southern Great Plains.
  • Equatorial (E) air affects North America only through hurricanes.

Fronts – a zone of discontinuity between unlike air masses where properties of air change rapidly, narrow but three-dimensional.

  • Typically several kilometers wide (even tens of kilometers wide).
  • Functions as a barrier between two air masses, preventing their mingling except in this narrow transition zone.
  • Though all primary physical properties are involved in a front, temperature provides the most conspicuous difference.
  • Fronts lean, which allows air masses to be uplifted and adiabatic cooling to take place.
  • Some lean so much, they are closer to horizontal than vertical.
  • Always slopes so that warmer air overlies cooler air.
  • Fronts move in association with the direction of the more active air mass, which displaces the less active.

Cold front—the leading edge of a cool air mass actively displacing a warm air mass.

  • Brings cold air.
  • Leads to the rapid lifting of warm air, which makes it unstable and thus results in blustery and violent weather along the front.
  • Weather maps show ground-level position of a cold front (usually has a protruding “nose”); clouds and precipitation tend to be concentrated along and immediately behind the ground-level position.

Warm front—the leading edge of an advancing warm air mass.

  • Brings warm air.
  • Results in clouds and precipitation, usually broad, protracted, and gentle, without much convective activity.
  • Unstable rising air can result in showery and even violent precipitation.
  • Weather maps show the ground-level position of a warm front; precipitation usually falls ahead of this position.

Stationary front—the common boundary between two air masses in a situation in which neither air mass displaces the other.

Occluded front—a complex front formed when a cold front overtakes a warm front.

Major Atmospheric Disturbances

Two types of disturbances: stormy and calm. Both types have common characteristics.

  • Smaller than the components of general circulation, but extremely variable in size.
  • Migratory and transient.
  • Relatively brief in duration.
  • Produce characteristic and relatively predictable weather conditions.

Midlatitude Disturbances

Many kinds of atmospheric disturbances are associated with the midlatitudes, which are the principal battleground for tropospheric phenomena.

Midlatitude cyclones and midlatitude anticyclones are more significant because of size and prevalence.

Tropical Disturbances

Low latitudes are characterized by monotony,  with the same consistent weather.

The only breaks in this pattern are provided by transient disturbances such as hurricanes.

Localized Severe Weather

Occurs in many parts of the world.

Constitutes short-lived but severe weather phenomena such as thunderstorms and tornadoes.

Midlatitude Cyclones —large migratory low-pressure system that occurs within the middle latitudes and moves generally with the westerlies; also called lows or wave cyclones, and depressions.

Probably the most significant of all atmospheric disturbances.

Basically responsible for most day-to-day weather changes.

Bring precipitation to much of the world’s populated regions.


  • A typical mature midlatitude cycle is 1600 kilometers (1000 miles) in diameter and has an oval shape.
  • Patterns of isobars, fronts, and wind flow in the Southern Hemisphere are mirror images of those in the Northern Hemisphere.

In the Northern Hemisphere:

  • Circulation pattern converges counterclockwise.
  • Wind-flow pattern attracts cool air from the north and warm air from the south and creates two fronts.
  • These two fronts divide the cyclone into a cool sector north and west of the center and a warm sector south and east.
  • Size of sectors varies with location: on the ground, the cool sector is larger, but in the atmosphere, the warm sector is more extensive.
  • Warm air rises along both fronts, causing cloudiness and precipitation, which follows patterns of cold and warm fronts.
  • Much of the cool sector is typified by clear, cold, stable air, whereas air of the warm sector is often moist and tends toward instability, so the cool sector may have sporadic thunderstorms. May have squall fronts of intense thunderstorms.
  • Weather Changes With a Passing Front
  • With the passage of a cold front, the following changes typically occur:
  • The temperature decreases sharply.
  • Winds shift from southerly ahead of the front to northwesterly following it (in the Northern Hemisphere).
  • The pressure falls as the front approaches and then rises after it passes.
  • Generally clear skies are replaced by cloudiness and precipitation at the front.


Midlatitude cyclones move throughout their existence.

Average rate of movement is 30–45 kilometers per hour (20–30 miles per hour).

Cyclonic wind circulation of these midlatitude cyclones have winds that generally converge counterclockwise (in the Northern Hemisphere) into the center of the storm from all sides.

The cold front generally moves faster than the storm’s center.

Life Cycle – Origin to maturity typically takes 3 to 6 days, then another 3 to 6 days to dissipate.

Cyclogenesis—birth of cyclones.

  • Most common cause believed to be upper-air conditions in the vicinity of the polar-front jet stream.
  • Most begin as waves along the polar front.
  • Cyclogenesis can also occur on the leeward side of mountains.
  • Often bring heavy rain or snowstorms to the northeastern United States and southeastern Canada.


  • After cyclonic circulation is well developed, occlusion begins.
  • After an occluded front is fully developed, the cyclone dissipates.

Weather Changes With the Passing of a Midlatitude Cyclone


As a cold front passes, temperature drops abruptly.


Pressure falls as the front approaches, and as the front passes the pressure rises steadily.


Winds in the warm sector come from the south. Once the front passes, winds shift and come from the west or northwest.

Clouds and Precipitation

As a cold front approaches, clear skies are replaced by cloudiness and precipitation.

After the front passes, the conditions clear.

Occurrence and Distribution

  • Occur at scattered but irregular intervals throughout the zone of the westerlies.
  • Route of a cyclone is likely to be undulating and erratic, but it generally moves west to east.

Midlatitude Anticyclones —an extensive migratory high-pressure cell of the midlatitudes that moves generally with the westerlies.


  • Typically larger than a midlatitude cyclone, but also moves west to east.
  • Travels at the same rate, or a little slower, than a midlatitude cyclone.
  • Is prone to stagnate or remain over the same region (while cyclones do not).
  • Can cause a concentration of air pollutants.

Relationships of Cyclones and Anticyclones

  • Cyclones and anticyclones alternate with one another in an irregular sequence.
  • Often a functional relationship between the two.
  • Can visualize an anticyclone as a polar air mass with the cold front of a cyclone as its leading edge.

Easterly Waves —a long but weak migratory low-pressure system in the tropics between 5° and 30° of latitude.

  • They are usually several hundred kilometers long and nearly always oriented north–south.
  • They drift westward on the trade winds.
  • Convergent conditions behind the wave generate thunderstorms and cloudiness.
  • Sometimes intensify into hurricanes.
  • Bring characteristic weather of the trade winds with them.

 Tropical Cyclones: Hurricanes

Tropical cyclone—a storm most significantly affecting the tropics and subtropics, which is intense, revolving, rain-drenched, migratory, destructive, and erratic. Such a storm system consists of a prominent low-pressure center that is essentially circular in shape and has a steep pressure gradient outward from the center.

Tropical cyclones provide the only break in weather in low latitudes.

Also called:

  • Hurricanes  in North and Central America
  • Typhoons  in the western North Pacific
  • Baguios  in the Philippines
  • Tropical cyclones  in the Indian Ocean and Australia

With diameters of between 160 and 1000 kilometers, tropical cyclones are smaller than midlatitude cyclones.

Three categories of tropical cyclones:

  1. Tropical depression—winds of 33 knots (61 kilometers, or 38 miles, per hour) or less.
  2. Tropical storm—winds between 34 and 63 knots (63 and 118 kilometers, or 39 and 73 miles, per hour)
  3. Hurricane—winds of 64 knots (119 kilometers, or 74 miles, per hour) or more; can double and even triple that minimum.

World Meteorological Society (WMO) is responsible for monitoring tropical storms globally.

Several local warning centers operate regionally:

National Hurricane Center (Miami, FL) monitors the North Atlantic and northeastern Pacific.

Central Pacific Hurricane Center (Hawaii) monitors the north-central Pacific.

Japan Meteorological Agency (Japan) monitors the northwestern Pacific.


A hurricane pulls in warm, moist air for fuel, and this air rises and cools adiabatically.

This causes condensation and in turn releases heat, which further increases the instability of the air.

Not characterized as midlatitude cyclones.

Dissimilar air masses are not pulled together.

All air in a hurricane is warm and moist.

Eye of a Hurricane

  • Eye—the nonstormy center of a tropical cyclone, which has a diameter of 16–40 kilometers (10–25 miles). In the eye there are no updrafts, but instead a downdraft that inhibits cloud formation.
  • Eyewall—peripheral zone at the edge of the eye where winds reach their highest speed and where updrafts are most prominent.
  • Weather pattern within a hurricane is symmetrical.
  •  Comprised of bands of dense cumulus and cumulonimbus clouds called spiral rain bands.

Eyewall replacement—the process in which a new wall of storms surrounds the wall of storms circling the hurricane’s eye. When this occurs, the inner wall disintegrates so the new wall replaces it. This process tends to weaken the storm.


Form only over warm oceans and where there is no significant wind shear.

The Coriolis effect plays key role: it’s at a minimum at the equator, and no hurricane has been observed to form within 3° of the equator or cross over it.

Rare to have a hurricane closer than 8° to 10° from equator.

The exact mechanism of formation is not clear, but they always grow from some preexisting disturbance.


Most common in the North Pacific basin (origination in the Philippines and west of southern Mexico and Central America).

West central portion of the North Atlantic basin, extending into the Caribbean, and Gulf of Mexico is third in prevalence.

Totally absent from the South Atlantic and from the southeastern part of the Pacific.

Absent apparently because the water is too cold and because high pressure dominates.

General pattern of movement is highly predictable.

About one-third travel east to west without much latitudinal change.

About two-thirds start off on an east–west path and then curve poleward.

Exception occurs in the southwestern Pacific Ocean north and northeast of New Zealand, where the general circulation pattern steers hurricanes, so they travel west to east.

Average hurricane lasts a week; those that remain over tropical oceans can live up to four weeks.

Dies down over continents because energy source of warm, moist air is cut off.

Dies down in midlatitudes because of cooler environment.

In midlatitudes, can diminish in intensity but grow in size and become a midlatitude cyclone.

Damage and Destruction

High seas, or a storm surge, cause the most damage.

Storm size is key to how much damage is caused, then physical configuration of landscape and population size and density of affected area.

Saffir–Simpson Hurricane Scale has been established to rank the intensity of hurricanes.

Ranges from 1 to 5, with 5 being the most severe 

Heavy Rain and Flooding

Strong hurricanes can inflict heavy damage from flooding.

In 2011, Hurricane Irene brought extensive flooding to the northeastern United States and southeastern Canada.

Hurricanes and Climate Change

The 2005 hurricane season in the North Atlantic was the most active on record.

Included 28 named storms.

Three of the most powerful hurricanes measured in terms of minimum atmospheric pressure in the eye.

The 2010 and 2011 seasons tied for third overall, with 19 named storms.

Connection between ocean temperature and hurricane formation and connection between global warming and ocean temperature generates question:

Will global warming increase hurricane activity?

There has been a general increase in the annual number of hurricanes in the North Atlantic in the past 25 years.

Some meteorologists attribute the increase to a multidecadal cycle of hurricane activity that has been well documented since the early 1990s (the Multi-Decadal Signal).

Combination of high sea-surface temperatures (SSTs), low wind shear, and expanded upper-level westward flow of the atmosphere off North Africa.

Underlying components of the Multi-Decadal Signal are not completely understood, but the recent increase in hurricane frequency can likely be explained without tying it to global warming.

However, hurricane intensity may be tied to global warming.

Reflected in the potential relationship between higher SSTs and hurricane intensity.

The 2013–2014 Fifth Assessment Report of the Intergovernmental Panel on Climate Change concluded that it was virtually certain (99–100 percent probability) that we will observe an increase in tropical cyclones during the century as a result of increasing global temperatures.

Localized Severe Weather

Thunderstorm—violent convective storm accompanied by thunder and lightning, usually localized and short lived.

Vertical air motion, considerable humidity, and instability combine to create towering cumulonimbus clouds, so thunderstorms are always associated with this combination.

Frequently occur in conjunction with other kinds of storms.

For example, hurricanes, tornadoes, fronts (especially cold fronts), midlatitude cyclones, and orographic lifting.

Associated with other mechanisms that can trigger unstable uplift.

Mechanism triggers uplift of warm, moist air.

  1. Cumulus stage—updrafts prevail and clouds grow. Rise to above freezing level, where supercooled water droplets and ice crystals coalesce, then fall. Initiate a downdraft.
  2. Mature state—updrafts and downdrafts coexist as the cloud continues to enlarge (but precipitation is leaving the bottom of the cloud). Most active time.
  3. Dissipating state—downdrafts dominate and turbulence ceases.

Virtually unknown poleward  of 60˚ of latitude.


More than 8.5 million lightning bolts occur daily worldwide.

Most frequently, lightning occurs as exchanges between adjacent clouds or between the upper and lower portions of the same cloud; it also occurs as an electrical connection of ionized air from cloud to ground.

The sequence that leads to lightning discharge is known, but the mechanism for electrification is not.


Large cumulonimbus cloud experiences a separation of electrical charges.

Positively charged particles are mostly high in the cloud, while negatively charged particles tend to concentrate at the base.

Growing negative charge in the base attracts a growing positive charge on Earth’s surface immediately below the cloud.

An insulating barrier lies between the cloud base and surface.

Contrast between the cloud base and surface builds to tens of millions of volts and overcomes the insulating barrier.

A finger of negative current flicks down from the cloud and meets a positive charge darting upward from the ground, causing lightning.

Cause is unknown; different hypotheses.

Most popular hypothesis: updrafts carry positively charged particles to the top, while falling ice pellets gather negative charges and transport them downward.

Thunder—an instantaneous expansion of air caused by the abrupt heating that a lightning bolt produces. This expansion creates a shock wave that becomes a sound wave.

Can time the distance that lightning is away because of the different rates thunder and lightning travel at (speed of sound vs. speed of light).

Five-second interval equals about a mile; three-second interval equals about a kilometer.

Tornado—a localized cyclonic low-pressure cell surrounded by a whirling cylinder of wind spinning so violently that a partial vacuum develops within the funnel.

Has the most extreme pressure gradients known (as much as a 100-millibar difference between the tornado center and the air immediately outside the funnel).

Extreme pressure difference produces winds of extraordinary speed.

How fast are winds?

No one knows because tornadoes blow to bits anemometers (instrument for measuring speed). Maximum estimates range from 320 to 800 kilometers (200 to 500 miles) per hour.

Greatest damage tends to be from a combination of strong winds, flying debris, and extreme updrafts.

Old advice of opening windows during tornado event is no longer recommended because it may increase chance of injury from flying debris.

Tornado Formation

Exact mechanism of formation is unknown.

Usually develops in warm, moist, unstable air associated with midlatitude cyclones.

High wind shear (horizontally rotating air) may cause strong updrafts to form in a supercell thunderstorm.

The rotating air may then be tilted vertically, forming a mesocyclone.

About 50 percent of all mesocyclones formed result in tornadoes.

Most often develops along a squall line that precedes a rapidly advancing cold front, or along the cold front.

Spring and early summer are favorable for development because there’s considerable air-mass contrast present in the midlatitudes at that time.

Most occur in midafternoon, at time of maximum heating.

More than 90 percent of all reported tornadoes occur in the United States.

Reflects optimum environmental conditions.

Relatively flat terrain of the central and southeastern United States provides uninhibited interaction of Canadian cP and Gulf mT air masses.


The strength of a tornado is described using the Enhanced Fujita Scale (EF)  (Table 7-3).

Scale is based on estimates of 3-second gust wind speeds.

Percentage of U.S. tornadoes that fall within the five categories are:

EF 0–1 (light or moderate): 69 percent

EF 2–3 (strong or severe): 29 percent

EF 4–5 (devastating or incredible): 2 percent

Annual U.S. tornado death toll has decreased because of better forecasting.

Waterspouts  occur over ocean; have less pressure gradient, gentler winds, and reduced destructive capability.

Severe Storm Watches and Warnings

Storm watch  is an advisory issued for a region where, over the next 4 to 6 hours, the conditions are favorable for the development of severe weather.

Storm warning  is issued by a local weather forecasting office when a severe thunderstorm or tornado has actually been observed

Focus: Conveyer Belt Model of Midlatitude Cyclones

    1. Satellite and weather balloon measurements have revealed that midlatitude cyclones involve more than just surface fronts and a low-pressure center; they also tend to include several well-defined channels of air called “conveyor belts” (Figure 7-A).
    2. The Warm Conveyor Belt
      1. The midlatitude cyclone’s surface low draws air northward from the southeastern portion of the cyclone. The air to the southeast tends to be warm and moist because those are characteristics of the air mass where it originates.
      2. A warm conveyor belt develops from this air, which starts at the surface, but because it is less dense, it eventually rises up and over the cooler air to the north of the warm front.
      3. This can bring moisture and even snow as it contributes moisture to the cold conveyor belt.
    3. The Cold Conveyor Belt
      1. Just north of the warm front, cooler, drier surface air moves westward toward the cyclone’s central low, forming the cold conveyor belt.
      2. Like the warm conveyor belt, some of this cold air can rise to merge with the general westerly flow at upper levels.
        1. However, the cold conveyor belt can also split, with the rest of the air turning cyclonically and rising as it moves toward the low-pressure center.
        2. In winter, this often produces the midlatitude cyclone’s heaviest snowfall just northwest of the low.
      3. The Dry Conveyor Belt
        1. On the western side of a typical midlatitude cyclone, convergence in the upper troposphere produces descending air, some of which swirls counterclockwise into the cyclone’s low, forming the dry conveyor belt.
        2. This air from the upper troposphere is much drier than the air in the other conveyor belts because it is farther from the surface and thus farther from sources of moisture.
        3. Few clouds, if any, can form in this dry air, which often produces a “dry slot” of air just behind the cold front that is lacking in clouds compared with other areas nearby.
          1. The dry conveyor belt gives the cyclone a “comma” shape by separating clouds defining the comma’s head, formed primarily by the cold conveyor belt, from clouds defining the comma’s tail, formed primarily by the warm conveyor belt.

Global Environmental Change: Are Tornado Patterns Changing?

    • The most active part of 2011 was from April 25 to April 28.
      • Produced 343 confirmed tornadoes and killed 321 people.
      • Four of these tornadoes produced damage up to the EF-5 category—the first EF-5 tornadoes anywhere on Earth since 2008.
    • Post-storm damage surveys are the main evidence of tornadoes and their strengths, though storm spotters and the public provide information as well.
      • Doppler weather radar alone does not confirm tornadoes, but the U.S. Doppler network, installed in the 1990s, and advances since that deployment have improved warnings and detection.
      • The number of recorded tornadoes has increased, but to see whether the actual number of tornadoes has increased, researchers tend to analyze trends in larger tornadoes.
      • The overall number of large tornadoes hasn’t changed significantly, but outbreaks such as the one in 2011 are becoming more common.
        • Outbreaks don’t happen every year, but when they do, they’re deadly—by the end of 2011 tornadoes had killed 551 people in the United States, the largest annual total in 66 years of modern records.
      • Thunderstorms tend to be more frequent and powerful when the surface is hot and humid.
        • Global climate change is simultaneously increasing surface temperatures, increasing evaporation rates, and decreasing the number of days with adequate wind shear for tornadic formation.
        • Observed changes, which also fit global climate changes, in tornado patterns are fewer days with tornadoes but on days when there is enough wind shear, increased tendency for outbreaks.

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Geog 001

1.  Introduction to Earth