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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 1
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Composition of the Atmosphere
• The Permanent Gases
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Composition of the Atmosphere
• Variable Gases
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Layers of the Atmosphere
• Layering Based on Temperature Profiles
• Thermal Layers of the Atmosphere
– Four distinct layers of the atmosphere emerge from
identifiable temperature characteristics with height.
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 2
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Characteristics of Radiation
• Energy radiated by substances occurs over a wide
range of wavelengths.
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The Solar Constant
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The Causes of Earth’s Seasons
• Earth’s Revolution and Rotation
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 3
© 2015 Pearson Education, Inc.
Energy Transfer Processes
• Surface–Atmosphere Radiation Exchange
– Water vapor and CO2 are the primary absorbers of longwave
radiation (greenhouse gases).
– The range of wavelengths, 8-15 μm, matches those radiated
with greatest intensity by the Earth’s surface.
– This range of wavelengths not absorbed is called the
atmospheric window.
Atmospheric window
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Energy Transfer Processes
• Conduction
– As the surface warms, a temperature gradient develops in the
upper few centimeters of the ground.
– Temperatures are greater at the surface than below.
– Surface warming also causes a temperature gradient within a
very thin (a few millimeters) sliver of adjacent air called the
laminar boundary layer.
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Energy Transfer Processes
• Convection
– The temperature gradients in the laminar boundary layer
induce energy transfer upward through convection.
– This occurs any time the surface temperature exceeds the
air temperature, typically occurring in the middle of the day.
– At night, the surface cools more rapidly that air and energy
is transferred downward.
– Convection can be generated by two processes in fluids.
• Free Convection
– Mixing related to buoyancy, warmer, less dense fluids rise
• Forced Convection
– Initiated by eddies and other disruptions to smooth, uniform flow
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Energy Transfer Processes
Free Convection
Forced Convection
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The Global Energy Budget
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Energy Transfer Processes
• Net Radiation and Global Temperature
– Earth’s radiation balance is a function of an incoming and
outgoing radiation equilibrium.
– Balances occur on an annual global scale and diurnally over
local spatial scales.
(1-α) I = σ T4
α
I
albedo
solar constant / 4
T = [(1-α)I/σ]-4
T = -18 °C
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Energy Transfer Processes
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 4
© 2015 Pearson Education, Inc.
The Equation of State
• Pressure, temperature, and density are related to
one another and their relationship can be described
through the equation of state (ideal gas law).
p=ρRT
p
ρ
Pressure
Density
R
Gas constant
• The equation of state results in the following:
– At constant temperatures, an increase in air density will
cause pressure to increase.
– Under constant density, an increase in temperature will
also cause an increase in pressure.
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The Distribution of Pressure
• Pressure Gradients
– The pressure gradients provide the movement of air
commonly known as wind.
– The strength of the pressure gradient force determines
the horizontal wind speed.
• Horizontal Pressure Gradients
– Typically, small gradients exist across large areas.
– Concentrated weather features, such as hurricanes and
tornadoes, display larger pressure gradients across small
areas.
• Vertical Pressure Gradients
– Vertical pressure gradients are greater than extreme
examples of horizontal pressure gradients as pressure
always decreases with altitude.
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The Distribution of Pressure
• Hydrostatic Equilibrium
– Gravity balances strong vertical pressure gradients to create
hydrostatic equilibrium.
– Local imbalances create various up- and downdrafts
∆p/ ∆z = -ρg
• The Role of Density in Hydrostatic Equilibrium
– Gravitational force is proportional to mass.
– A dense atmosphere needs greater gravitational force to remain
in balance.
• For warm air, this equates to smaller vertical pressure gradients
leading to hydrostatic equilibrium.
• For cold air, this equates to larger vertical pressure gradients
leading to hydrostatic equilibrium.
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Forces Affecting the Speed and Direction
of the Wind
• The Coriolis Force
– Objects in the atmosphere are influenced by Earth’s rotation.
– Overall, the result is a deflection of moving objects to the right
in the Northern Hemisphere and to the left in the Southern
Hemisphere.
Fc = 2Ωsin(φ)v
Force/mass (acceleration)
Ω
φ
v
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Earth’s rotation rate
Latitude
velocity
Forces Affecting the Speed and Direction
of the Wind
• The Coriolis Force
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Anticyclones, Cyclones, Troughs, and
Ridges
• Cyclones
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Anticyclones, Cyclones, Troughs, and
Ridges
• Anticyclones
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 5
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Water Vapor and Liquid Water
• Evaporation and Condensation
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Indices of Water Vapor Content
• Vapor Pressure
– Saturation vapor pressure is the vapor pressure of the
atmosphere when it is saturated.
The movement of water vapor molecules
exerts vapor pressure on surfaces.
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Indices of Water Vapor Content
• Vapor Pressure
– Saturation vapor pressure is temperature dependent.
– At low temperatures the saturation vapor pressure increases
slowly, but it increases rapidly at higher temperatures. It is not
a linear increase.
Nonlinear increase in saturation vapor
pressure with increase in temperature.
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Processes That Cause Saturation
• Air can become saturated in three ways:
– The addition of water vapor
– Mixing cold air with warm air
– Moist air—by cooling the air to dew point
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Factors Affecting Saturation and
Condensation
• Effect of Curvature
Larger drops have less curvature than smaller ones.
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Factors Affecting Saturation and
Condensation
• Effect of Solution
Small droplets require higher RHs to remain liquid.
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Factors Affecting Saturation
• Ice Nuclei
– Atmospheric water does not normally freeze at 0°C.
– Supercooled water refers to water having a temperature
below the melting point of ice but nonetheless existing in a
liquid state.
– Ice crystal formation requires ice nuclei, a rare temperaturedependent substance similar in shape to ice (six-sided).
• Examples: clay, ice fragments, bacteria, etc.
• Ice nuclei become active at temperatures below -4°C
– Between -10° and -30°C, saturation may lead to ice crystals,
supercooled drops, or both.
– Below -30°C, clouds are composed solely of ice crystals.
– At or below -40°C spontaneous nucleation, the direct
deposition of ice with no nuclei present, occurs.
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Cooling the Air to the Dew or Frost Point
• Diabatic Processes
– Diabatic process involves the addition or removal of energy.
• Example: Air passing over a cool surface loses energy through
conduction.
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Cooling the Air to the Dew or Frost Point
• Adiabatic Processes
– Cloud formation typically involves temperature changes with no
exchange of energy (adiabatic process), according to the
first law of thermodynamics.
– Rising air expands through an increasingly less dense
atmosphere, causing a decrease in internal energy and a
corresponding temperature decrease.
– Parcels expand and cool at the dry adiabatic lapse rate
(DALR), 1°C/100 m.
– Parcels may eventually reach the lifting condensation level,
the height at which saturation occurs.
– Parcels then cool at the saturated adiabatic lapse rate
(SALR), ~0.6°C/100.
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Cooling the Air to the Dew or Frost Point
• Adiabatic Processes
Dry adiabatic cooling.
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Cooling the Air to the Dew or Frost Point
• The environmental (ambient) lapse rate (ELR) refers to
an overall decrease in air temperature with height.
• This rate, which changes from place to place, stems
from the fact that air located farther from surface
heating is typically cooler than that nearer the surface.
A comparison of adiabatic and
environmental cooling rates.
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 6
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Mechanisms That Lift Air
• Orographic uplift: Occurs when a mass of air is
deflected over or around a terrain, usually a hill or a
mountain. This upward movement of air results in
adiabatic cooling. This promotes the development of
clouds and precipitation.
• Rain shadow: Air compresses as it descends down
the terrain and results in little to no precipitation.
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Mechanisms That Lift Air
• Frontal lifting: Occurs when two air masses
converge at the front. This can occur when cold air
advances toward warm air (cold front) or when warm
air advances toward cold air (warm front). Clouds
develop as a result of these two situations.
Cold front example
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Warm front example
Mechanisms That Lift Air
• Convergence: Occurs when there is a horizontal
movement of air into a region. When air converges
along the Earth's surface, it is forced to rise since it
cannot go downward.
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Mechanisms That Lift Air
• Localized convection: Occurs when differential heating
at the surface causes air to lift. The air expands and
cools as it lifts, causing cloud development.
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Static Stability & Environmental Lapse Rate
• Absolutely unstable:
This occurs when a
parcel of air is lifted and
it continues to move
upward regardless of
saturation. If the
environmental lapse rate
(ELR) exceeds the dry
adiabatic lapse rate
(DALR), the air is
absolutely unstable.
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Static Stability & Environmental Lapse Rate
• Absolutely stable: This
occurs when a parcel of
air returns to its original
location after being
displaced. If the
environmental lapse
rate (ELR) is less than
the saturated adiabatic
lapse rate (SALR), the
air is absolutely stable.
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Static Stability & Environmental Lapse Rate
• Conditionally unstable: This
occurs when the
environmental lapse rate
(ELR) is between the dry
adiabatic lapse rate (DALR)
and the saturated adiabatic
lapse rate (SALR). An air
parcel become saturated at
the lifting condensation level
(LCL) and it will become
buoyant if lifted to a critical
altitude called the level of free
convection (LFC).
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 7
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Growth of Cloud Droplets
• Introduction
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Growth of Cloud Droplets
• Growth by Condensation
– Condensation nuclei form most cloud drops but after all the
available condensation nuclei have attracted water, any
further condensation can only occur on existing droplets.
– With so many droplets competing for a limited amount of
water, none can grow very large by condensation.
– Two other processes are responsible for further droplet
growth.
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Growth of Cloud Droplets
• Growth in Warm Clouds
– Condensation nuclei form most cloud drops but after all the
available condensation nuclei have attracted water, any
further condensation can only occur on existing droplets.
– Collision–coalescence causes precipitation of warm clouds.
– Collision–coalescence begins with large droplets, called
collector drops, which have high terminal velocities.
– As the collector drops fall, they overtake smaller droplets in its
path and provides the opportunity for collisions and
coalescence.
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Growth of Cloud Droplets
• Growth in Cold and Cool Clouds
– Cold clouds have temperatures below 0°C and consist of
ice crystals.
– Cool clouds have temperatures above 0°C in the lower
range and subfreezing conditions in the higher range.
– Clouds may be composed of liquid water, supercooled water,
and/or ice.
– The coexistence of ice and supercooled water is critical to
the creation of cool cloud precipitation—the Bergeron
Process.
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Growth of Cloud Droplets
• Growth in Cold and Cool Clouds
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Growth of Cloud Droplets
• Growth in Cold and Cool Clouds
– Riming occurs when liquid water freezes onto ice crystals
producing rapid growth.
– Aggregation occurs when the joining of multiple ice
crystals through the bonding of surface water builds ice
crystals to the point of overcoming updrafts.
– Collision combined with riming and aggregation allows the
formation of precipitation within 1/2 hour of initial
formation.
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 8
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Single-Cell and Three-Cell Models
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Semipermanent Pressure Cells
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Semipermanent Pressure Cells
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The Upper Troposphere
• Troughs and Ridges
– Height contours meander considerably across the globe.
– The bulges of heights extending toward the poles are
called ridges.
– The valley of low heights extending toward the equator is
known as troughs.
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Major Wind Systems
• Monsoons
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Major Wind Systems
• Foehn, Chinook, and Santa Ana Winds
– Foehn winds flow down the side of mountain slopes. Air
undergoes compressional warming. They are initiated
when midlatitude cyclones pass to the southwest of the
Alps.
– Chinooks are similar winds on the eastern side of the
Rocky Mountains and form when low pressure systems
occur east of the mountains.
– Both Foehn and Chinook winds are most common in
winter.
– Santa Ana winds occur in California during the transitional
seasons, especially autumn, when high pressure is located
to the east. The Santa Ana winds often contribute to the
spread of wildfires.
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Major Wind Systems
• Sea and Land Breezes
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Major Wind Systems
• Valley and Mountain Breezes
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Ocean–Atmosphere Interactions: ENSO
A→O
O→A
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Normal / LaNina situation
El Nino situation partly reversed
Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: NAO
NAO
+
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NAO
-
Ocean–Atmosphere Interactions: NAO
• Arctic Oscillation and North Atlantic Oscillation
– The oscillations of the Atlantic Ocean are known as the Arctic
Oscillation (AO) and the North Atlantic Oscillation (NAO).
– The NAO is in a positive phase when the pressure gradient is
greater than normal and negative when it is less than normal.
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Ocean–Atmosphere Interactions: NAO
NAO+
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NAO-
Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 9
© 2015 Pearson Education, Inc.
Air Masses and Their Source Regions
• Introduction
– Air masses contain uniform temperature and humidity
characteristics.
• They affect vast areas.
– Fronts are boundaries between different air masses.
• Fronts are spatially limited and usually linked to midlatitude
cyclones.
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Air Masses and Their Source Regions
• Source Regions
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Air Mass Formation
• Continental Polar (cP) and Continental Arctic (cA)
Air Masses
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Fronts
• Midlatitude Cyclone
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Fronts
• Cold Fronts
– A cold front is a mass of cold air advancing toward warm air.
– Typically associated with heavy precipitation, rain, or snow,
combined with rapid temperature drops.
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Fronts
• Warm Fronts
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 14
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Atmospheric Pollutants
• Introduction
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Atmospheric Pollutants
• Sulfur Compounds
– Sulfur compounds can occur as gaseous or aerosol forms.
– Natural sources: steam vents, volcanic eruptions, sea spray.
– Anthropogenic sources: burning sulfur containing fossil fuels
(particularly coal and oil) and ore smelting.
• Sulfur dioxide (SO2) is a respiratory irritant.
• Forms sulfate aerosols that contributes to acid fog and acid
rain.
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Atmospheric Pollutants
• Photochemical Smog
– Ozone, NO2, formaldehyde, and other gases combine with
solar radiation to form Los Angeles-type photochemical
smog.
– Ozone causes respiratory and heart problems.
– High levels of ozone result in environmental degradation.
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Loss agricultural production due to
ozone
Tropospheric ozone: Crops
Department of Geosciences
Atmospheric Conditions and Air Pollution
• Effect of Atmospheric Stability
– Inversions can trap pollutants near the Earth’s surface.
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Chapter Lectures
Understanding
Weather and
Climate
Seventh Edition
Frode Stordal
University of Oslo
Redina L. Herman
Western Illinois University
Chapter 15
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The Köppen System
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The Köppen System
• A—Tropical. Climates in which the average temperature for
all months is greater than 18°C. Almost entirely confined to
the region between the equator and the tropics of Cancer
and Capricorn.
• B—Dry. Potential evaporation exceeds precipitation.
• C—Mild Midlatitude. The coldest month of the year has an
average temperature higher than –3°C (or 0°C) but below
18°C. Summers can be hot.
• D—Severe Midlatitude. Winters have at least occasional
snow cover, with the coldest month having a mean
temperature below –3°C (or 0°C). Summers are typically
mild.
• E—Polar. All months have mean temperatures below 10°C.
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