Understanding Weather and Climate€¦ · Understanding Weather and Climate Seventh Edition Frode...

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Chapter Lectures

Redina L. Herman

Western Illinois University

Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 1


• The Permanent Gases

Composition of the Atmosphere


• Variable Gases

Composition 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.

Layers of the Atmosphere


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 2


• Energy radiated by substances occurs over a wide

range of wavelengths.

Characteristics of Radiation


The Solar Constant


The Causes of Earth’s Seasons

• Earth’s Revolution and Rotation


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 3


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


• 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.



• 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



Free Convection

Forced Convection



The Global Energy Budget


• 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 albedo

I solar constant / 4

T = [(1-α)I/σ]-4

T = -18 °C


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 4


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).

• 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.

p = ρ R T p Pressure

ρ Density

R Gas constant


• 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.

The Distribution of Pressure


• Hydrostatic Equilibrium – Gravity balances strong vertical pressure gradients to create

hydrostatic equilibrium.

– Local imbalances create various up- and downdrafts

• 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.

The Distribution of Pressure

∆p/ ∆z = -ρg


• 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.

Forces Affecting the Speed and Direction

of the Wind

Fc = 2Ωsin(φ)v

Force/mass (acceleration) Ω Earth’s rotation rate

φ Latitude

v velocity


• The Coriolis Force

Forces Affecting the Speed and Direction

of the Wind


• Cyclones

Anticyclones, Cyclones, Troughs, and

Ridges


• Anticyclones

Anticyclones, Cyclones, Troughs, and

Ridges


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 5


• Evaporation and Condensation

Water Vapor and Liquid Water


• 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.

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.

Indices of Water Vapor Content


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


• Effect of Curvature

Factors Affecting Saturation and

Condensation

Larger drops have less curvature than smaller ones.


• Effect of Solution

Factors Affecting Saturation and

Condensation

Small droplets require higher RHs to remain liquid.


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 temperature-

dependent 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.


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.


• 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.



• Adiabatic Processes

Dry adiabatic cooling.



• 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.



Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 6


• 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.

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 Warm front example


• 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.



• Localized convection: Occurs when differential heating

at the surface causes air to lift. The air expands and

cools as it lifts, causing cloud development.



• 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.

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.



• 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).



Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 7


• Introduction

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.



• 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.



• 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.




– 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.



Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 8


Single-Cell and Three-Cell Models


Semipermanent Pressure Cells


• 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.

The Upper Troposphere


• Monsoons

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.

Major Wind Systems


• Sea and Land Breezes

Major Wind Systems


• Valley and Mountain Breezes

Major Wind Systems


O→A

A→O

Normal / LaNina situation

El Nino situation partly reversed

Ocean–Atmosphere Interactions: ENSO


Ocean–Atmosphere Interactions: ENSO


NAO

-

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.




NAO+ NAO-


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 9


• 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.

Air Masses and Their Source Regions


• Source Regions

Air Masses and Their Source Regions


• Continental Polar (cP) and Continental Arctic (cA)

Air Masses

Air Mass Formation


• Midlatitude Cyclone

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.

Fronts


• Warm Fronts

Fronts


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 14


• Introduction

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.



• 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.


Department of Geosciences

Loss agricultural production due to

ozone Tropospheric ozone: Crops


• Effect of Atmospheric Stability

– Inversions can trap pollutants near the Earth’s surface.

Atmospheric Conditions and Air Pollution


Chapter Lectures

Redina L. Herman


Understanding

Weather and

Climate

Seventh Edition

Frode Stordal

University of Oslo

Chapter 15


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.

The Köppen System

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