chapt04_lecture Getis 13e(2).ppt

Introduction to
Geography
Arthur Getis, Judith Getis, &
Jerome D. Fellmann

Physical Geography:
Weather and Climate
Chapter 4

Overview
 Introduction
 Air Temperature
 Air Pressure and Winds
 Ocean Currents
 Moisture in the Atmosphere
 Climate Regions
 Climatic Change

Introduction
 Weather
 State of the atmosphere at a given time and place
 Climate
 Long-term average weather conditions in a place
 Geographers analyze differences in weather
and climate from place to place to understand
how they affect human occupance of the earth

Introduction
 Troposphere is of particular concern
 Atmospheric layer closest to the earth
 Contains virtually all of the air, clouds, and
precipitation of the earth

Air Temperature
 Why do temperatures vary from place to place?
 Need to understand how heat accumulates on the
earth’s surface
 Solar energy is transformed into heat primarily at
the earth’s surface and secondarily in the
atmosphere
 Insolation (amount of incoming solar radiation)
 Solar radiation received at the earth’s surface
 Determined by angle of the sun’s rays and number
of daylight hours

Air Temperature
 Angle of sun’s rays, number of daylight hours
and these modifying variables determine the
temperature at any given location:
 Amount of water vapor in the air
 Cloud cover
 Nature of the surface of the earth (land or water)
 Elevation
 Degree and direction of air movement

Air Temperature
Earth’s Movements
 Rotation
 Revolution

Air Temperature
Earth Inclination
 Axis of the earth tilts at ≈ 23.5°
 Location of highest incoming solar radiation
varies during the year
 Summer Solstice in Northern Hemisphere
(about June 21)
 Northern Hemisphere tilted toward the sun
 Vertical rays of the sun at noon are at 23.5° N

Air Temperature
Earth Inclination
 Winter Solstice in the Northern Hemisphere
(about December 21)
 Northern Hemisphere tilted away from the sun
 Vertical rays of the sun at noon are at 23.5° S
 Spring and fall equinoxes in the Northern
Hemisphere (about March 21 and
September 21, respectively)
 Vertical rays of the sun at noon are at the equator

Air Temperature
Earth Inclination
 Causes variation in length of days and nights
 At the equator
 12 hours of light each day of the year
 Inside the Arctic Circle and Antarctic Circle
 24 hours of daylight/darkness on solstices
 Angle of the sun’s rays above the surface of the
earth
 The larger the angle, the more energy available

Air Temperature: Reflection
and Radiation Emission
 Clouds in the atmosphere and light colored
surfaces on the earth reflect 32% of incoming
solar energy back to outer space
 Radiation cycle
 Shortwave solar radiation is absorbed by the surface
of the earth, thereby heating the surface. Then
radiation is emitted from the earth into the
atmosphere in the form of longwave radiation.
 Earth’s atmosphere is mainly heated by terrestrial
radiation.

 Daily cycle of temperature
 Air temperature rises when incoming solar radiation
exceeds energy lost through reflection and terrestrial
radiation
 Air temperature drops when energy lost through
reflection and terrestrial radiation exceeds incoming
solar radiation

 Water heats and cools more slowly than land
 Marine environment
 Cooler summers, warmer winters
 Land heats and cools more rapidly than water
 Continental environment
 Hotter summers, colder winters

Air Temperature
The Lapse Rate
 Temperature generally decreases as altitude
increases in the troposphere
 Lapse rate
 Average of 3.5° F per 1000 feet (6.4° C per 1000 m)
 Temperature inversion
 Cooler air trapped below warmer air
 Contributes to smog problems

Air Pressure and Winds
 How do differences in air pressure from place to
place affect weather conditions?
 Air pressure = weight of the atmosphere
 Normal sea level pressure = 14.7 pounds or 6.7
kilograms
 Air pressure is highest closest to the earth’s
surface

 Temperature and air pressure relationship (for
equal amounts of cold and hot air):
 Cold air is denser than warm air: high pressure
 Warm air is lighter than cold air: low pressure
 Air pressure is measured by a barometer in
inches of mercury or millibars
 Air pressure at a given location changes as
surfaces heat or cool

 Zones of high and low air pressure exist in
earth’s atmosphere
 Pressure gradient force
 Causes air to flow from high to low pressure areas
 Wind
 Velocity is in direct proportion to pressure
differences
 Convection system
 Circulatory movement of rising warm air and
descending cool air

 Convectional wind effects due to differential
heating and cooling of land and water:
 Land sea breezes
 Day: from sea to land
 Night: from land to sea
 Mountain and valley breezes
 Day: from valley to mountains
 Night: from mountains to valley

 Coriolis effect
 Result of Earth’s rotation
 Earth rotates in a counterclockwise direction when
viewed from the position of the North Pole
 Earth rotates in a clockwise direction when viewed
from the position of the South Pole
 Apparent deflection relative to the earth’s surface
 Northern Hemisphere: wind veers toward the right
Southern Hemisphere: wind veers toward the left
 Coriolis effect and pressure gradient force produce
spiral wind patterns

 Frictional effect of Earth’s surface on movement
of wind
 Decreases wind speed
 Changes wind direction

Global Air-Circulation Pattern
 Equatorial low pressure
 Warm air rises and moves away from Equatorial
region
 Subtropical high pressure
 About 30° N and 30° S of the equator
 Cold air aloft sinks
 When sinking air reaches Earth’s surface it moves
away from the region of high pressure, thereby
creating the trade winds and the westerlies
 Trade winds
 In the tropics

 Westerlies
 In the midlatitudes
 Subpolar low
 Polar easterlies
 Polar high

 Jet streams
 Belts of strong winds in the upper atmosphere
 Flow from west to east in an undulating path
 Bring cold air equator ward and warm air pole ward
 Guide the movement of weather systems
 More pronounced in the winter than in the summer

 Monsoon
 Wind system that reverses direction seasonally
 Produces wet and dry seasons
 Significant effect on parts of southern and eastern Asia
 Farm economy is dependent upon summer monsoon
rains
 Negative impact if rainfall arrives late or rainfall is
less or more than optimum
 May cause disastrous flooding and loss of lives

Ocean Currents
 Surface ocean currents correspond roughly to
global wind direction patterns
 Movement due to winds and differences in
water density
 Direction affected by Coriolis effect
 Direction also influenced by landmasses and shape
of ocean basins
 North Atlantic drift
 Warm current
 Winters in northern Europe are warmer than
expected for its latitude

Ocean Currents
 Ocean currents affect precipitation on adjacent
land areas
 Cold currents
 Dry conditions (coastal deserts)
 Warm currents
 Moist conditions

Moisture in the Atmosphere
 Ascending air expands and cools
 Less able to hold water vapor
 Supersaturated air
 Water vapor condenses around condensation nuclei

 Clouds
 Consist of rain droplets or ice crystals supported by
upward movements of air
 Droplets may coalesce and fall as rain
 Temperatures below freezing - formation of snow
 Form and altitude of cloud depends on:
 Water vapor content, temperature, wind movement

 Clouds (continued)
 Types of clouds:
 Cumulus
 Stratus
 Cirrus
 Cumulonimbus

 Relative humidity
 Percentage measure of the moisture content of the air
 Amount present relative to the maximum that can
exist at the current temperature
 Dew point
 Temperature at which condensation forms

Types of Precipitation
 Convectional precipitation
 Heated, moisture-laden air rises and then cools
below the dew point
 Summer in tropical and continental climates
 Orographic precipitation
 Warm, moisture-laden air is forced to rise over hills
or mountains and is thereby cooled
 Windward side
 Receives a great deal of precipitation
 Leeward side
 Very often dry (rain shadow)

Types of Precipitation
 Cyclonic (frontal) precipitation
 Common to the midlatitudes
 In the tropics – originator of hurricanes and typhoons
 Occurs where cool and warm air masses meet
 Air mass
 Body of air with similar temperature, pressure, and
humidity characteristics throughout
 Forms over a source region
 Front
 Zone of separation between two air masses

Storms
 Midlatitude Cyclone
 Masses of air circulating about a region of low
atmospheric pressure
 Counterclockwise rotation in Northern Hemisphere
 Warm air moves up and over cold air along a front
 Can develop into a storm

Storms
 Hurricane
 Severe tropical cyclone with winds exceeding 74 mph
 Forms in low-pressure zone over warm waters; e.g., in
the Atlantic, Caribbean, or Gulf of Mexico
 Typhoon
 Name for hurricanes in the western Pacific Ocean
 Structure of hurricane
 Eye
 Eye wall
 Surge
 Concentric belts of rising air

Storms
 Blizzard
 Heavy snow and high winds
 Tornado
 Most violent of all storms
 Smallest storm
 Common in Central U.S. in spring and fall
 Spawned in huge cumulonimbus clouds
 Funnel-shaped cloud of whirling winds that spins at
speeds as high as 300 mph
 Enhanced Fujita Scale
 Waterspout

Climate Regions
 Climate of an area is a generalization based on
daily and seasonal weather conditions
 Two most important elements that differentiate
weather conditions are temperature and
precipitation

Climate Regions
 Köppen climate system
 Based on temperature, precipitation, and natural
vegetation criteria
 Six broad categories
 A: tropical
 B: dry
 C: mild midlatitude
 D: midlatitude with severely cold winters
 E: polar
 H: highland

Climate Regions
Tropical Climates (A)
 Generally found between Tropics of Cancer
and Capricorn
 Tropical rainforest (Af)
 Found in equatorial low pressure zone; also along
coasts extending away from the equator
 High temperatures and daily convectional rainfall all
year
 Tall, dense forests

Climate Regions
Tropical Climates (A)
 Tropical savanna (Aw)
 To the north and south of rain forests
 High temperatures
 Heavy convectional rainfall in summer, dry winters
 Forests to grasslands
 Tropical monsoon (Am)
 Significant increase in rainfall when summer
monsoon winds bring water-laden air
 Dense forests

Climate Regions
Dryland Climates (B)
 Hot deserts (BWh)
 Found in subtropical high pressure zone
 Considerable sunshine, high temperatures
 Very little precipitation
 Shrubs in gravelly or sandy environments
 E.g., Sahara, Arabian, Australian and Kalahari
Deserts

Climate Regions
Dryland Climates (B)
 Midlatitude deserts and semideserts (BWk)
 Warm/hot summers and cold winters
 Some convectional or frontal rainfall in summer,
some snowfall in winter
 Extremely dry areas known as cold deserts
 Moderately dry lands known as steppes
 Among most naturally fertile soils in the world
 Vegetation: grasslands, desert shrubs
 Known for hot, dry summers and biting winter
winds which sometimes bring blizzards

Climate Regions
Humid Midlatitude Climates (C)
 Mediterranean (Cs)
 Located in transition zone between subtropical highs
and the westerlies
 Aside from Mediterranean region itself, generally
found on the western coasts of continents in the
middle latitudes; e.g., Southern California, tip of
South Africa, western Australia and central Chile
 Warm/hot summers and mild/cool winters
 Dry summer, frontal precipitation in winter
 Vegetation
 Shrubs
 Small deciduous trees

Climate Regions
 Humid subtropical (Cfa)
 Located on eastern coasts on continents
 Hot, moist summers and moderate, moist winters
 Convectional summer showers, winter cyclonic
storms
 Deciduous and coniferous forests
 May be affected by hurricanes

Climate Regions
 Marine west coast (Cfb)
 Prevailing winds from the sea
 Moderate temperatures in both summer and winter
 Frontal and orographic precipitation

Climate Regions
Humid Continental Climates (D)
 Cyclonic storms are more responsible for rainfall
than are convectional showers
 Colder winters, shorter summers
 Prevailing winds from land
 Location:
 Northern and central US
 Southern Canada
 Most of European portion of Russia
 Northern China

Climate Regions: Subarctic
Climates (Dfc, Dfd, Dwb)
 Subarctic
 Cool/cold, short summers and very cold winters
 Coniferous forest to mosses and lichens
 Tundra
 Treeless area between the Arctic tree line and the
permanently ice-covered zone
 Location
 Alaska
 Northern Canada
 Northern Russia

Climate Regions
Arctic Climates (E)
 Arctic
 Ice cap near the poles
 Extremely cold with light precipitation
 Antarctica and Greenland are icy deserts

Climate Regions
Highland Climates (H)
 Lower temperatures than lowlands at the same
latitude
 Variety of conditions based on:
 Elevation
 Prevailing winds
 Orientation of slope relative to the sun
 Valley, slope, or peak
 Ruggedness

Climatic Change
 Long-term climatic change
 Significant variations over geologic time
 Ice ages
 Medieval warm period and “little ice age”
 May be due to variations in: shape of Earth’s orbit, tilt of
the axis, gyration of the rotation axis
 Short-term climatic change
 Natural processes
 Volcanic eruptions, oceanic circulation, sunspot activity
 Human processes
 Enhanced greenhouse effect

Climatic Change
 Greenhouse effect
 Certain gases in the atmosphere function as an
insulating barrier, trapping infrared radiation
 Global warming
 Caused by human activities that have increased the
amount of greenhouse gases in the atmosphere
 Carbon dioxide: burning fossil fuels, deforestation
 Methane: natural gas and coal mining, agriculture
and livestock, swamps, landfills
 Nitrous oxides: motor vehicles, industry, fertilizers
 Chlorofluorocarbons: industrial chemicals

Climatic Change
 Evidence of global warming (continued)
 20th century was the warmest in 600 years
 Average surface temp rose over 1° F during the
century
 Winter temps in the Arctic have risen about 7° F since
the 1950s
 Loss of Arctic ice cap
 Glaciers are thinning and retreating

Climatic Change
 Consequences of global warming include:
 Rising sea levels
 Changes in temperature and precipitation patterns
 Impact on soils, vegetation, agriculture

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