Learning Objectives

Each statement represents the primary learning objective for the corresponding major heading within the chapter. After you complete the chapter, you should be able to:

14.1 Define air mass. Describe the classification and weather associated with different air masses.

14.2 Compare and contrast typical weather associated with a warm front and a cold front. Describe an occluded front and a stationary front.

14.3 Summarize the weather associated with the passage of a mature midlatitude cyclone. Describe how airflow aloft is related to cyclones and anticyclones at the surface.

14.4 List the basic requirements for thunderstorm formation and locate places on a map that exhibit frequent thunderstorm activity. Describe the stages in the development of a thunderstorm.

14.5 Summarize the atmospheric conditions and locations that are favorable to the formation of tornadoes. Discuss tornado destruction and tornado forecasting.

14.6 Identify areas of hurricane formation on a world map and discuss the conditions that promote hurricane formation. List the three broad categories of hurricane destruction.

Concept Checks

 14.1 Define air mass. What is air-mass weather? On what basis are air masses classified? Compare the temperature and moisture characteristics of the following air masses: cP, mP, mT, and cT. What causes lake-effect snow? Which air mass is associated with nor’easters?

14.2 Compare the weather of a typical warm front with that of a typical cold front. Why is cold-front weather usually more severe than warm-front weather? Describe a stationary front and an occluded front.

14.3 Briefly describe the weather associated with the passage of a mature midlatitude cyclone in the Northern Hemisphere when the center of low pressure is about 200 to 300 kilometers (125 to 200 miles) north of your location. If the midlatitude cyclone described in Question 1 took 3 days to pass your location, on which day would temperatures likely be warmest? On which day would they likely be coldest? What winter weather might be expected with the passage of a mature midlatitude cyclone when the center of low pressure is located about 100 to 200 kilometers (60 to 125 miles) south of your location? Briefly explain how flow aloft aids the formation of cyclones at the surface.

14.4 Briefly compare and contrast midlatitude cyclones, hurricanes, and tornadoes. How are thunderstorms related to each? What are the basic requirements for the formation of a thunderstorm? Where are thunderstorms most common on Earth? In the United States?Summarize the stages in the development of a thunderstorm.

14.5 What general atmospheric conditions are most conducive to the formation of tornadoes? During what months is tornado activity most pronounced in the United States? Name the scale commonly used to rate tornado intensity. How is a rating on this scale determined? Distinguish between a tornado watch and a tornado warning.

14.6 What factors influence where and when hurricane formation takes place? Distinguish between the eye and the eye wall of a hurricane. How do conditions differ in these zones? What are the three broad categories of hurricane damage?

Concepts In Review Weather Patterns and Severe Weather

14.1 Air Masses

Define air mass. Describe the classification and weather associated with different air masses.

Key Terms: air mass, air-mass weather, source region, polar (P) air mass, arctic (A) air mass, tropical (T) air mass, continental (c) air mass, maritime (m) air mass, lake-effect snow, nor’easter

• An air mass is a large body of air, usually 1600 kilometers (1000 miles) or more across, that is characterized by a sameness of temperature and moisture at any given altitude. When this air moves out of its region of origin, called the source region, it carries these temperatures and moisture conditions elsewhere, perhaps eventually affecting a large portion of a continent.

• Air masses are classified according to the nature of the surface in the source region and the latitude of the source region. Continental (c) designates an air mass of land origin, with the air likely to be dry; a maritime (m) air mass originates over water and, therefore, will be relatively humid. Polar (P) and arctic (A) air masses originate in high latitudes and are cold. Tropical (T) air masses form in low latitudes and are warm. According to this classification scheme, the four main types of air masses are continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT).

• Continental polar (cP) and maritime tropical (mT) air masses influence the weather of North America most, especially east of the Rocky Mountains. Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States.

QUESTION: Identify the source region associated with each letter on this map. One letter is not associated with a source region. Which one is it?

14.2 Fronts

Compare and contrast typical weather associated with a warm front and a cold front. Describe an occluded front and a stationary front.

Key Terms: front, overrunning, warm front, cold front, stationary front, occluded front

• Fronts are boundary surfaces that separate air masses of different densities, one usually warmer and more humid than the other. As one air mass moves into another, the warmer, less dense air mass is forced aloft in a process called overrunning.

• Along a warm front, a warm air mass overrides a retreating mass of cooler air. As the warm air ascends, it cools adiabatically to produce clouds and, frequently, light to moderate precipitation over a large area.

• A cold front forms where cold air is actively advancing into a region occupied by warmer air. Cold fronts are about twice as steep as and move more rapidly than do warm fronts. Because of these two differences, precipitation along a cold front is generally more intense and of shorter duration than precipitation associated with a warm front.

QUESTION: Identify each of these symbols used to designate fronts. On which side of each symbol are the warm air and the cool air?

14.3 Midlatitude Cyclones

Summarize the weather associated with the passage of a mature midlatitude cyclone. Describe how airflow aloft is related to cyclones and anticyclones at the surface.

Key Term: midlatitude (middle-latitude) cyclone

• The primary weather producers in the middle latitudes are large centers of low pressure that generally travel from west to east, called middle-latitude cyclones. These bearers of stormy weather, which last from a few days to a week, have a counterclockwise circulation pattern in the Northern Hemisphere, with an inward flow of air toward their centers.

• Most middle-latitude cyclones have a cold front and frequently a warm front extending from the central area of low pressure. Convergence and forceful lifting along the fronts initiate cloud development and frequently cause precipitation. The particular weather experienced by an area depends on the path of the cyclone.

• Guided by west-to-east-moving jet streams, cyclones generally move eastward across the United States. Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In cyclones, divergence aloft supports the inward flow at the surface.

14.4 Thunderstorms

List the basic requirements for thunderstorm formation and locate places on a map that exhibit frequent thunderstorm activity. Describe the stages in the development of a thunderstorm.

Key Term: thunderstorm

• Thunderstorms are caused by the upward movement of warm, moist, unstable air. They are associated with cumulonimbus clouds that generate heavy rainfall, lightning, thunder, and occasionally hail and tornadoes.

• Air-mass thunderstorms frequently occur in maritime tropical (mT) air during spring and summer in the middle latitudes. Generally, three stages are involved in the development of these storms: the cumulus stage, mature stage, and dissipating stage.

QUESTION: Which stage in the development of a thunderstorm is shown in this sketch? Describe what is occurring. Is there a stage that follows this one? If so, describe what occurs during that stage.

14.5 Tornadoes

Summarize the atmospheric conditions and locations that are favorable to the formation of tornadoes. Discuss tornado destruction and tornado forecasting.

Key Terms: tornado, Enhanced Fujita intensity scale (EF scale), tornado watch, tornado warning, Doppler radar

• Tornadoes are violent windstorms that take the form of a rotating column of air called a vortex that extends downward from a cumulonimbus cloud. Many strong tornadoes contain smaller internal vortices. Because of the tremendous pressure gradient associated with a strong tornado, maximum winds can approach 480 kilometers (300 miles) per hour.

• Tornadoes are most often spawned along the cold front of a midlatitude cyclone or in association with a supercell thunderstorm. Tornadoes also form in association with tropical cyclones (hurricanes). In the United States, April through June is the period of greatest tornado activity, but tornadoes can occur during any month of the year.

• Most tornado damage is caused by the tremendously strong winds. One commonly used guide to tornado intensity is the Enhanced Fujita intensity scale (EF scale). A rating on the EF scale is determined by assessing damages produced by the storm.

• Because severe thunderstorms and tornadoes are small and short-lived phenomena, they are among the most difficult weather features to forecast precisely. When weather conditions favor the formation of tornadoes, a tornado watch is issued. The National Weather Service issues a tornado warning when a tornado has been sighted in an area or is indicated by Doppler radar.

14.6 Hurricanes

Identify areas of hurricane formation on a world map and discuss the conditions that promote hurricane formation. List the three broad categories of hurricane destruction.

Key Terms: hurricane, eye wall, eye, Saffir–Simpson hurricane wind scale, storm surge

• Hurricanes, the greatest storms on Earth, are tropical cyclones with wind speeds in excess of 119 kilometers (74 miles) per hour. These complex disturbances develop over tropical ocean waters and are fueled by the latent heat that is liberated when huge quantities of water vapor condense.

• Hurricanes form most often in late summer, when ocean-surface temperatures reach 27°C (80°F) or higher and thus are able to provide the necessary heat and moisture to the air. Hurricanes diminish in intensity when they move over cool ocean water that cannot supply adequate heat and moisture, move onto land, or reach a location where large-scale flow aloft is unfavorable.

• The Saffir–Simpson wind scale ranks the relative intensities of hurricanes. A 5 on the scale represents the strongest storm possible, and a 1 indicates the lowest severity. Damage caused by hurricanes has three main causes: (1) storm surge, (2) wind, and (3) heavy rains and inland flooding.

QUESTION: This coastal scene shows hurricane destruction. Which one of the three basic classes of damage was most likely responsible for this destruction? What is your reasoning?