Foundations of Earth Science

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Heating the Atmosphere
Chapter 11 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition

Distinguish between weather and climate.
Name the basic elements of weather and climate.
Focus Questions 11.1

Weather
Occurs over a short period of time
Constantly changing
Climate
Averaged over a long period of time
Generalized, composite of weather
Focus on the Atmosphere

Elements of weather and climate
Properties that are measured regularly:
Air temperature
Humidity
Type and amount of cloudiness
Type and amount of precipitation
Air pressure
Wind speed and direction
Weather and Climate

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List the major gases composing Earth’s atmosphere.
Identify the components that are most important to understanding weather and climate.
Focus Questions 11.2

Air is a mixture of discrete gases
Major components of clean, dry air
78% Nitrogen (N)
21% Oxygen (O2)
Argon and other gases
0.04% Carbon dioxide (CO2)
Composition of the Atmosphere

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Water vapor
Up to 4% of air’s volume
Forms clouds and precipitation
Greenhouse gas
Aerosols
Tiny solid and liquid particles
Water vapor can condense on solids
Reflect sunlight
Color sunrise and sunset
Variable Components

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Ozone
Three atoms of oxygen (O3)
Distribution not uniform
Concentrated between 10 and 50 km above the surface
Absorbs harmful UV radiation
Variable Components

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Interpret a graph that shows changes in air pressure from Earth’s surface to the top of the atmosphere.
Sketch and label a graph that shows atmospheric levels based on temperature.
Focus Questions 11.3

Atmospheric pressure is the weight of the air above.
Average sea level pressure is 1000 millibars or 14.7 psi
Pressure decreases with altitude
Half of atmosphere is below 3.5 mi (5.6 km)
90% of atmosphere is below 10 mi (16 km)
Pressure Changes

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Four atmospheric layers based on temperature
Troposphere
Lowermost layer
Temperature decreases with increasing altitude
Environmental lapse rate
Average 6.5C per km or 3.5F per 1000 feet
Thickness varies
Average height is about 12 km
Outer boundary is the tropopause
Temperature Changes

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The environmental lapse rate is variable
Actual environmental lapse rate for any particular time and place
Measured with a radiosonde
Attached to a balloon and transmits data by radio
Temperature Changes

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Stratosphere
12 to 50 km
Temperature increases at top
Outer boundary is the stratopause
Mesosphere
50 to 80 km
Temperature decreases
Outer boundary is the mesopause
Thermosphere
No well-defined upper limit
Fraction of atmosphere’s mass
Gases moving at high speeds
Temperature Changes

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Explain what causes the Sun angle and length of daylight to change during the year.
Describe how these changes produce the seasons.
Focus Questions 11.4

Earth’s motions
Rotation on its axis
Once every 24 hours
Circle of illumination is the line separating Earth’s lighted half from dark half
Orbital motion around the Sun
Seasons
Result of:
Changing Sun angle
Changing length of daylight
Earth–Sun Relationships

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Seasons
Caused by Earth’s changing orientation to the Sun
Axis is inclined 23.5º
Axis is always pointed in the same direction
Earth–Sun Relationships

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Special days (Northern Hemisphere)
Summer solstice: June 21–22
Sun’s vertical rays located at Tropic of Cancer
23.5º N latitude
Winter solstice: December 21–22
Sun’s vertical rays located at Tropic of Capricorn
23.5º S latitude
Autumnal equinox: September 22–23
Sun’s vertical rays located at equator (0º latitude)
Spring equinox: March 21–22
Sun’s vertical rays located at equator (0º latitude)
Earth–Sun Relationships

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Distinguish between heat and temperature.
List and describe the three mechanisms of heat temperature.
Focus Questions 11.5

Heat is synonymous with thermal energy
Temperature refers to the intensity, or degree of “hotness”
Heat is always transferred from warmer to cooler objects
Energy, Heat, and Temperature

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Mechanisms of heat transfer
Conduction
Molecular activity
Convection
Mass movement within a substance
Radiation (electromagnetic radiation)
Gamma waves, X-rays
Ultraviolet,visible, infrared
Microwaves and radio waves
Energy, Heat, and Temperature

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Laws of Radiation
All objects emit radiant energy
Hotter objects radiate more total energy per unit area than colder objects
Hotter objects radiate more short-wavelength radiation than cooler objects
Good absorbers of radiation are good emitters as well
Energy, Heat, and Temperature

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Sketch and label a diagram that shows the paths taken by incoming solar radiation.
Summarize the greenhouse effect.
Focus Questions 11.6

Incoming solar radiation
Atmosphere is largely transparent to incoming solar radiation
Atmospheric effects
Reflection
Albedo (percent reflected)
Scattering
Absorption
Most visible radiation reaches the surface
About 50% absorbed at Earth’s surface
Heating the Atmosphere

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Radiation from Earth’s surface
Earth re-radiates longer wavelengths
Terrestrial radiation
Terrestrial radiation is absorbed by
Carbon dioxide and water vapor
Lower atmosphere is heated from Earth’s surface
Heating of the atmosphere is termed the greenhouse effect
Heating the Atmosphere

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Summarize the nature and cause of the atmosphere’s changing composition since around 1750.
Describe the atmosphere’s response and some possible future consequences.
Focus Questions 11.7

CO2 levels are rising
Industrialization of the past 200 years
Burning fossil fuels
coal, natural gas, and petroleum
Deforestation
Present CO2 level is 30% higher than its highest level over at least the past 800,000 years
Human Impact on Global Climate

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Important impacts of human-induced global warming:
Rise in sea level
Shift in large-storm paths
Changes in precipitation distribution
Changes in occurrence of severe weather
Stronger tropical storms
Increasing frequency and intensity of heat waves and droughts
Gradual environmental shifts
Human Impact on Global Climate

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Calculate five commonly used types of temperature data.
Interpret a map that depicts temperature data using isotherms.
Focus Questions 11.8

For the Record: Air Temperature Data
Temperature measurement
Daily maximum and minimum
Other measurements
Daily mean temperature
Daily range
Monthly mean
Annual mean
Annual temperature range

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Isotherms used to examine distribution of air temperatures over large areas
Line that connects points of the same temperature
iso = equal, therm = temperature
Easy to visualize temperature gradient
For the Record: Air Temperature Data

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Discuss the principal controls of temperature.
Use examples to describe their effects.
Focus Questions 11.9

Temperature control – any factor that causes temperature to vary from place to place and from time to time
Receipt of solar radiation
Differential heating of land and water
Land heats more rapidly than water
Land gets hotter than water
Land cools faster than water
Land gets cooler than water
Why Temperatures Vary: The Controls
of Temperature

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Specific heat is the amount of energy needed to raise the temperature of 1 gram of a substance
1 degree Celsius
Why Temperatures Vary: The Controls
of Temperature

Why Temperatures Vary: The Controls
of Temperature

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Other important controls
Altitude
Geographic position
Cloud cover
Albedo
Why Temperatures Vary: The Controls
of Temperature

Why Temperatures Vary: The Controls
of Temperature

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Interpret the patterns depicted on world maps of January and July temperatures.
Focus Question 11.10

Temperature maps
Temperatures are adjusted to sea level
January and July used for analysis
Represent temperature extremes
World Distribution of Temperature

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Global temperature patterns
Temperature decreases poleward from tropics
Isotherms exhibit latitudinal shift with seasons
Warmest and coldest over land
World Distribution of Temperature

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Global temperature patterns
Southern Hemisphere
Isotherms are straighter
Isotherms are more stable
Isotherms show ocean currents
Annual temperature range
Small near equator
Increases with an increase in latitude
Greatest over continental locations
World Distribution of Temperature

Moisture, Clouds,
and Precipitation
Chapter 12 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition

Summarize the six processes by which water changes from one state of matter to another.
For each, indicate whether energy is absorbed or released.
Focus Questions 12.1

Three states of matter
Solid—ice
Liquid—water
Gas—water vapor
To change state, heat must be either absorbed or released
Water’s Changes of State

Heat energy
One calorie is the heat necessary to raise the temperature of one gram of water one degree Celsius
Latent heat
Stored or hidden heat
Not derived from temperature change
Heat exchanged between water and surroundings
Important in atmospheric processes
Water’s Changes of State

Processes
Melting
Solid is changed to a liquid
80 calories per gram added
Latent heat of melting
Freezing
Liquid is changed to a solid
Heat is released
Latent heat of fusion
Water’s Changes of State

Processes
Evaporation
Liquid is changed to gas
600 calories per gram added
Latent heat of vaporization
Condensation
Water vapor (gas) is changed to a liquid
Heat energy is released
Latent heat of condensation
Water’s Changes of State

Water’s Changes of State

Processes
Sublimation
Solid is changed directly to a gas
680 calories per gram of water are added
Deposition
Water vapor (gas) changed to a solid
Heat is released
Water’s Changes of State

Water’s Changes of State

Write a generalization relating air temperature and the amount of water vapor needed to saturate air.
Focus Question 12.2

Humidity—amount of water vapor in the air
Saturated air
Air filled to capacity with water vapor
Capacity is temperature dependent
Warm air has a much greater capacity
Water vapor adds pressure
Vapor pressure
Humidity: Water Vapor in the Air

Humidity: Water Vapor in the Air

Measuring humidity
Mixing ratio
Mass of water vapor in a unit of air compared to the remaining mass of dry air
Measured in g/kg
Relative humidity
Ratio of the air’s actual water vapor content compared with the amount of water vapor required for saturation
(at that temperature and pressure)
Humidity: Water Vapor in the Air

Humidity: Water Vapor in the Air

Measuring humidity
Relative humidity
Expressed as a percent
Saturated air
Content equals capacity
Has 100% relative humidity
Relative humidity can be changed in two ways
Changing the air temperature
Lowering the temperature raises the relative humidity
Dew point temperature
Temperature to which a parcel of air would need to be cooled to reach saturation
Humidity: Water Vapor in the Air

Humidity: Water Vapor in the Air

Two types of hygrometers are used to measure humidity:
Psychrometer
Compares temperatures of wet-bulb thermometer and dry-bulb thermometer
Greater difference = lower relative humidity
If air is saturated, both thermometers read the same temperature
Electric hygrometer
Reads the humidity directly
Humidity: Water Vapor in the Air

Humidity: Water Vapor in the Air

Describe adiabatic temperature changes.
Explain why the wet adiabatic rate of cooling is less than the dry adiabatic rate.
Focus Questions 12.3

Adiabatic temperature changes
Air is compressed
Motion of air molecules increases
Air warms
Descending air is compressed
Air expands
Air parcel does work on the surrounding air
Air cools
Rising air expands
The Basis of Cloud Formation: Adiabatic Cooling

Adiabatic rates
Dry adiabatic rate
Unsaturated air
Rising air expands & cools at 1°C/100 m
Descending air compresses and warms at 1°C/100 m
Wet adiabatic rate
Cloud formation begins at condensation level
Air has reached the dew point
Condensation is occurring and latent heat is being liberated
Sensible heat released by condensing water reduces cooling rate
Rate varies from 0.5°C to 0.9°C/100 m
The Basis of Cloud Formation: Adiabatic Cooling

The Basis of Cloud Formation: Adiabatic Cooling

List and describe the four mechanisms that cause air to rise.
Focus Question 12.4

Orographic lifting
Elevated terrains act as barriers
Result can be a rainshadow desert
Processes That Lift Air

Processes That Lift Air

Frontal wedging
Cool air acts as a barrier to warm air
Fronts are part of middle-latitude cyclones
Processes That Lift Air

Convergence
Air flows together and rises
Processes That Lift Air

Localized convective lifting
Unequal surface heating causes pockets of air to rise because of their buoyancy
Processes That Lift Air

Describe how atmospheric stability is determined.
Compare conditional instability with absolute instability.
Focus Questions 12.5

Stability of air determines:
Type of clouds that develop
Intensity of the precipitation
Stable air
Resists vertical displacement
Cooler and denser than surrounding air
Wants to sink
Unstable air
Warmer than surrounding air
Wants to rise
The Weathermaker: Atmospheric Stability

Types of stability
Environmental lapse rate—how temperature above parcel changes with height
Stable air
No adiabatic cooling
Widespread clouds with little vertical thickness
Precipitation is light to moderate
Absolute stability
Environmental lapse rate less than wet adiabatic rate
The Weathermaker: Atmospheric Stability

The Weathermaker: Atmospheric Stability

Absolute instability
Acts like a hot air balloon
Rising air
Warmer and less dense than surrounding air
Rises until it reaches altitude with same temperature
Adiabatic cooling
Environmental lapse rate greater than dry adiabatic rate
Clouds are often towering
Conditional instability
Atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel
The Weathermaker: Atmospheric Stability

The Weathermaker: Atmospheric Stability

Name and describe the 10 basic cloud types, based on form and height.
Contrast nimbostratus and cumulonimbus clouds and their associated weather.
Focus Questions 12.6

Condensation
Water vapor changes to a liquid and forms dew, fog, or clouds
Water vapor requires a condensation surface
On the ground
Grass, a car window, etc.
In the air are tiny bits of particulate matter called condensation nuclei
Dust, smoke, ocean salt crystals, etc.
Condensation and Cloud Formation

Clouds
Made of millions and millions of
Minute water droplets, or
Tiny crystals of ice
Classification based on form
Cirrus
High, white, thin
Stratus
Sheets or layers that cover much of the sky
Cumulus
Globular cloud masses
Nimbus – major producer of precipitation
Condensation and Cloud Formation

Clouds classified based on height
High clouds
Above 6000 m
Cirrus, cirrostratus, cirrocumulus
Middle clouds
2000 to 6000 m
Altostratus and altocumulus
Low clouds
Below 2000 m
Stratus, stratocumulus, and nimbostratus (nimbus means “rainy”)
Condensation and Cloud Formation

Condensation and Cloud Formation

Clouds of vertical development
From low to high altitudes
Called cumulonimbus
Often produce rain showers and thunderstorms
Condensation and Cloud Formation

Identify the basic types of fog.
Describe how each forms.
Focus Questions 12.7

Fog is a cloud with its base at or near the ground
Considered an atmospheric hazard
Most fogs form because of
Radiation cooling, or
Movement of air over a cold surface
Fog

Fogs caused by cooling
Radiation fog
Earth’s surface cools rapidly
Forms during cool, clear, calm nights
Advection fog
Warm, moist air moves over a cool surface
Upslope fog
Humid air moves up a slope
Adiabatic cooling occurs
Fog

Fog

Evaporation fogs
Steam fog
Cool air moves over warm water
Water has a steaming appearance
Frontal fog, or precipitation fog
Forms during frontal wedging when warm air lifted over colder air
Rain evaporates to form fog
Fog

Fog

Fog
[insert Figure 12.25 here]

Describe the Bergeron process.
Explain how it differs from the collision-coalescence process.
Focus Questions 12.8

Cloud droplets
<20 micrometers (0.02 millimeter) in diameter Fall incredibly slowly Formation of precipitation Bergeron process Temperature in the cloud is supercooled Ice crystals collect water vapor Large snowflakes form and fall to the ground or melt and turn to rain How Precipitation Forms © 2017 Pearson Education, Inc. 59 How Precipitation Forms © 2017 Pearson Education, Inc. 60 Formation of precipitation Collision-coalescence process Warm clouds Large hygroscopic condensation nuclei Large droplets form Droplets collide with other droplets during their descent How Precipitation Forms © 2017 Pearson Education, Inc. 61 How Precipitation Forms © 2017 Pearson Education, Inc. 62 Describe the atmospheric conditions that produce sleet, freezing rain (glaze), and hail. Focus Question 12.9 © 2017 Pearson Education, Inc. 63 Rain, drizzle, and mist Rain Droplets have at least a 0.5 mm diameter Drizzle Droplets have less than a 0.5 mm diameter Mist Smallest droplets able to reach the ground Snow Ice crystals, or aggregates of ice crystals Forms of Precipitation © 2017 Pearson Education, Inc. 64 Sleet and glaze Sleet Small particles of ice in winter Occurs when warmer air overlies colder air Rain freezes as it falls Glaze, or freezing rain Impact with a solid causes freezing Forms of Precipitation © 2017 Pearson Education, Inc. 65 Forms of Precipitation © 2017 Pearson Education, Inc. 66 Forms of Precipitation © 2017 Pearson Education, Inc. 67 Forms of Precipitation © 2017 Pearson Education, Inc. 68 Hail Hard rounded pellets Concentric shells Most diameters range from 1 to 5 cm Formation In large cumulonimbus clouds Layers of freezing rain are caught in violent up- and down-drafts Pellets fall when they become too heavy Forms of Precipitation © 2017 Pearson Education, Inc. 69 Forms of Precipitation © 2017 Pearson Education, Inc. 70 Forms of Precipitation © 2017 Pearson Education, Inc. 71 Rime Forms on cold surfaces Freezing of supercooled fog Freezing of cloud droplets Forms of Precipitation © 2017 Pearson Education, Inc. 72 List the advantages of using weather radar versus a standard rain gauge to measure precipitation. Focus Question 12.10 © 2017 Pearson Education, Inc. 73 Rain Easiest form to measure Measuring instruments Standard rain gauge Uses a funnel to collect rain Cylindrical tube measures in cm or inches Tipping-bucket gauge Two compartments capable of holding 0.025 cm each Bucket fills and tips, then other bucket begins to fill Each bucket tip is recorded on a graph Radar also used to measure rain Measuring Precipitation © 2017 Pearson Education, Inc. 74 Measuring Precipitation © 2017 Pearson Education, Inc. 75 Measuring Precipitation © 2017 Pearson Education, Inc. 76 Snow has two measurements: Depth Water equivalent General ratio is 10 snow units to 1 water unit Varies widely Measuring Precipitation © 2017 Pearson Education, Inc. 77 The Atmosphere in Motion Chapter 13 Lecture Natalie Bursztyn Utah State University Foundations of Earth Science Eighth Edition

Define air pressure.
Describe the instruments used to measure these weather elements.
Focus Questions 13.1

Air pressure is the force exerted by weight of air above
Weight of the air at sea level
14.7 psi or 1 kg/cm2
Decreases with increasing altitude
Units of measurement
Millibar (mb)
Standard sea level pressure is 1013.2 mb
Inches of mercury
Standard is 29.92 inches of mercury
Understanding Air Pressure

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Instruments for measuring
Barometer
Mercury barometer
Invented by Torricelli in 1643
Uses a glass tube filled with mercury
Aneroid barometer
“Without liquid”
Uses an expanding chamber
Barograph
Continuously records the air pressure
Understanding Air Pressure

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Discuss the three forces that act on the atmosphere to either create or alter winds.
Focus Question 13.2

Wind – horizontal movement of air
Out of areas of high pressure
Into areas of low pressure
Controls of wind
Pressure gradient force (PGF)
Isobars
Lines of equal air pressure
Pressure gradient
Pressure change over distance
Factors Affecting Wind

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Coriolis effect
Apparent deflection in wind direction due to Earth’s rotation
Deflection to the right in Northern Hemisphere
To the left in Southern Hemisphere
Friction
Only important near the surface
Acts to slow the air’s movement
Factors Affecting Wind

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Upper air winds
Generally blow parallel to isobars
Geostrophic winds
Jet stream
“River” of air
High altitude
High velocity (120 to 240 kph)
Factors Affecting Wind

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Contrast the weather associated with low-pressure centers (cyclones) and high-pressure centers (anticyclones).
Focus Question 13.3

Cyclone
A center of low pressure
Pressure decreases toward the center
Winds associated with a cyclone
In the Northern Hemisphere
Inward (convergence)
Counterclockwise
In the Southern Hemisphere
Inward (convergence)
Clockwise
Highs and Lows

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Anticyclone
Winds associated with an anticyclone
In the Northern Hemisphere
Outward (divergence)
Clockwise
In the Southern Hemisphere
Outward (divergence)
Counterclockwise
Associated with subsiding air
Usually bring “fair” weather
Highs and Lows

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Summarize Earth’s idealized global circulation.
Describe how continents and seasonal temperature changes complicate the idealized pattern.
Focus Questions 13.4

Caused by unequal surface heating
3 pairs of atmospheric cells redistribute heat
Idealized global circulation
Equatorial low pressure zone
Rising air
Abundant precipitation
Intertropical convergence zone
Subtropical high pressure zone
Subsiding, stable, dry air
Near 30º latitude
Location of great deserts
General Circulation of the Atmosphere

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Air traveling to equator from subtropical high produces the trade winds
Air traveling to poles from subtropical high produces the westerly winds
General Circulation of the Atmosphere

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Subpolar low-pressure zone
Warm and cool winds interact
Polar front: an area of storms
Polar high-pressure zone
Cold, subsiding air
Air spreads to equator and produces polar easterly winds
Polar easterlies collide with the westerlies along the polar front
General Circulation of the Atmosphere

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Influence of continents
– Seasonal temperature differences disrupt the
Global pressure patterns
Global wind patterns
– Influence is greatest in N. Hemisphere
Monsoon
Seasonal change in wind direction occurring over land
During warm months
– Air flows onto land
– Warm, moist air from the ocean
Winter months
– Air flows off the land
– Dry, continental air
General Circulation of the Atmosphere

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The Westerlies
Complex pattern
Air flow is interrupted by cyclones
Cells move west to east in the N. Hemisphere
Create anticyclonic and cyclonic flow
Paths of cyclones and anticyclones are associated with the upper-level airflow
General Circulation of the Atmosphere

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Produced from temperature differences
Small scale winds
Land and sea breezes
Mountain and valley breezes
Chinook and Santa Ana winds
Local Winds

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Land and sea breezes
A sea breeze develops because cooler air over the water moves toward the land
Reaches greatest intensity during the mid- to late afternoon
At night it reverses, and a land breeze develops
Local Winds

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Mountain and valley breezes
Air on mountain slopes is heated more than air at the same elevation over the valley floor
Glides upslope and generates a valley breeze
Cool air is denser than warm air and drains downslope into the valley as a mountain breeze
Local Winds

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Chinook and Santa Ana winds
Chinooks
Warm, dry winds moving down the east slopes of the Rockies
Santa Anas
Chinook like wind that occurs in southern California
Local Winds

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Describe the instruments used to measure wind.
Explain how wind direction is expressed using compass directions.
Focus Questions 13.6

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Two basic measurements:
Direction
Winds are labeled from where they originate
North wind blows from the north
Instrument for measuring wind direction is the wind vane: direction indicated by either
Compass points
Scale of 0º to 360º
Prevailing wind comes more often from one direction
Speed
Often measured with a cup anemometer
Measuring Wind

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Changes in wind direction
Associated with locations of
Cyclones
Anticyclones
Often bring changes in
Temperature
Moisture conditions
Measuring Wind

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Discuss the major factors that influence the global distribution of precipitation.
Focus Question 13.7

Regions influenced by high pressure experience relatively dry conditions
Regions influenced by low pressure receive ample precipitation
Tropical regions (equatorial low) are the rainiest
Subtropical deserts (subtropical high) are arid
Global Distribution of Precipitation

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Other factors influencing precipitation
Nature of the air
Moisture capacity
Latitude
Distribution of continents and oceans
Distribution of mountains
Global Distribution of Precipitation

Weather Patterns and Severe
Weather
Chapter 14 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition

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

Air mass characteristics
Large body of air
1600 km (1000 mi) or more across
Several kilometers thick
Similar temperature at any given altitude
Similar moisture at any given altitude
Move and affect a large portion of a continent
Air mass weather
Air Masses

Air Masses

Source region
Where an air mass acquires its properties
Classification of an air mass
Two criteria used to classify air masses:
By the latitude of the source region
Polar (P) and Arctic (A)
High latitudes: cold
Tropical (T)
Low latitudes: warm
By the nature of the surface in the source region
Continental (c)
Form over land: dry
Maritime (m)
Form over water: humid
Air Masses

Five basic types of air masses
Continental polar (cP)
Continental arctic (cA)
Continental tropical (cT)
Maritime polar (mP)
Maritime tropical (mT)
Air Masses

Air Masses

Air masses and weather
cP and mT air masses important in North America
North America (east of the Rocky Mountains)
Continental polar (cP)
From northern Canada and interior of Alaska
Winter: Brings cold, dry air
Summer: Brings cool relief
Air Masses

Air masses and weather
North America (east of the Rocky Mountains)
Continental polar (cP)
Responsible for lake-effect snows
cP air mass crosses the Great Lakes
Air picks up moisture from the lakes
Snow occurs on the leeward shores
Air Masses

Air Masses

Air Masses
Satellite image of lake-effect snow storm

Air masses and weather
North America (east of the Rocky Mountains)
Maritime tropical (mT)
From the Gulf of Mexico and the Atlantic Ocean
Warm, moist, unstable air
Brings precipitation
Continental tropical (cT)
Southwest and Mexico
Hot, dry
Maritime polar (mP)
Brings precipitation to the western mountains
Occasional influence: causes the Northeaster
Air Masses

Air Masses

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

Fronts—boundaries that separates air masses of different densities
Air masses retain their identities
Warmer, less dense air forced aloft
Cooler, denser air acts as wedge
Overrunning
Fronts

Warm front
Warm air replaces cooler air
Shown on a map by a line with red semicircles
Small slope (1:200)
Clouds become lower as the front nears
Slow rate of advance
Light-to-moderate precipitation
Fronts

Fronts

Cold front
Cold air replaces warm air
Shown on a map by a line with blue triangles
Twice as steep (1:100) as warm fronts
Advances faster than a warm front
Associated weather is often violent
Intensity of precipitation is high
Duration of precipitation is short
Weather behind the front is dominated by
Cold air mass
Subsiding air
Clearing conditions
Fronts

Fronts

Stationary front
Flow of air on both sides of the front is almost parallel to the line of the front
Surface position of the front does not move
Occluded front
Active cold front overtakes a warm front
Cold air wedges the warm air upward
Weather is often complex
Precipitation is associated with warm air being forced aloft
Fronts

Fronts

Summarize the weather associated with the passage of a mature mid latitude cyclone.
Describe how aloft is related to cyclones and anticyclones at the surface.
Focus Questions 14.3

Primary weather producer in the middle latitudes
Idealized weather
Middle-latitude cyclones move eastward across the United States
First signs of their approach are in the western sky
Require two to four days to pass over a region
Largest weather contrasts occur in the spring
Changes in weather associated with the passage of a middle-latitude cyclone
Changes depend on the path of the storm
Midlatitude Cyclones

Midlatitude Cyclones

Weather associated with fronts
Warm front
Clouds become lower and thicker
Light precipitation
After the passage of a warm front:
Winds become more southerly
Temperatures warm
Midlatitude Cyclones

Cold front
Wall of dark clouds
Heavy precipitation
Hail and occasional tornadoes
After the passage of a cold front:
Winds become more northerly
Skies clear
Temperatures drop
Midlatitude Cyclones

Midlatitude Cyclones

Role of air aloft
Cyclones and anticyclones
Generated by upper-level air flow
Maintained by upper-level air flow
Typically are found adjacent to one another
Midlatitude Cyclones

Midlatitude Cyclones

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

Features
Cumulonimbus clouds
Heavy rainfall
Lightning
Occasional hail
Occurrence
2000 in progress at any one time!
100,000 per year in the United States
Most frequent in Florida and eastern Gulf Coast region
Thunderstorms

Thunderstorms

Stages of development
All thunderstorms require
Warm air
Moist air
Instability (lifting)
High surface temperatures
Most common in afternoon and early evening
Thunderstorms

Thunderstorms

Stages of development
Continuous supply of warm air and moisture
Each surge causes air to rise higher
Updrafts and downdrafts form
Eventually precipitation forms
Gusty winds, lightning, hail
Heavy precipitation
Cooling effect of precipitation marks the end of thunderstorm activity
Thunderstorms

Thunderstorms

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

Tornadoes
Tornado – local storm of short duration
Features:
Rotating column of air that extends down from a cumulonimbus cloud
Low pressure inside
Winds approach 480 km (300 mi) per hour
Smaller suction vortices can form inside stronger tornadoes

Tornadoes

Tornadoes
Occurrence and development
Associated with severe thunderstorms
Product of interaction between thunderstorm updrafts and tropospheric winds
Average of 1297 in the United States between 2000 and 2014
Most frequent from April through June

Tornadoes
Occurrence and development
General atmospheric conditions
Occur most often along a cold front
Associated with huge thunderstorms called supercells
Cold, dry cP air meets warm, humid mT air
Greater contrast = more intense storm

Tornadoes

Tornadoes
Characteristics
Diameter 150–600 m (500–2000 ft)
Speed 45 km (30 mi) per hour
Can cut a 10 km (6 mi) long path
Max winds over 500 km (310 mi) per hour
Intensity measured by the Fujita intensity scale, or EF-scale

Tornadoes
Tornado forecasting
Difficult to forecast
Tornado watch
To alert public to the possibility of tornadoes
Issued when the conditions are favorable
Covers 65,000 km2 (25,000 mi2)
Tornado warning
Issued when a tornado is sighted or indicated by weather radar
Use of Doppler radar helps increase the accuracy by detecting the air motion

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

Hurricanes
Most violent storms on Earth
Hurricane
Intense centers of low pressure
Form over tropical oceans
Characterized by intense convective activity and strong cyclonic circulation

Hurricanes
To be called a hurricane:
Wind speed >119 km (74 mi) per hour
Rotary cyclonic circulation
Form between 5º and 20º latitudes
Wind speeds reach 300 kph
Generate 50-foot waves at sea
Typhoons in the western Pacific
Cyclones in the Indian Ocean
North Pacific has the greatest number per year

Hurricanes

Hurricanes
Parts of a hurricane
Eyewall
Near the center
Rising air
Intense convective activity
Wall of cumulonimbus clouds
Greatest wind speeds
Heaviest rainfall

Hurricanes

Hurricanes
Eye
At the very center
About 20 km (12.5 mi) diameter
Precipitation ceases
Winds subsides
Air gradually descends and heats by compression
Warmest part of the storm

Hurricanes
Hurricane formation and decay
Energy from condensing water vapor
Develop most often in late summer
Form in all tropical waters except the South Atlantic and Eastern South Pacific
Other tropical storms
Tropical depression
Winds do not exceed 61 km (38 mi) per hour
Tropical storm
Winds 61–119 km (38–74 mi) per hour

Hurricanes

Hurricanes
Diminish in intensity as:
They move over cooler ocean water
They move onto land
The large-scale flow aloft is unfavorable

Hurricanes
Hurricane destruction
Factors that affect amount of hurricane damage
Strength of storm (the most important factor)
Size and population density of the area affected
Shape of the ocean bottom near the shore
Saffir–Simpson scale ranks the relative intensities of hurricanes

Hurricanes

Hurricanes
Hurricane destruction
Categories of hurricane damage
Storm surge
Large dome of water 65 to 80 km (40 to 50 mi) wide sweeps across the coast where eye makes landfall
Wind damage
Inland flooding from torrential rains

Hurricanes

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