Transcript AMS Weather Studies

AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 4
Heat, Temperature, &
Atmospheric Circulation
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Driving Question
 What are the causes and consequences of heat transfer
within the Earth-atmosphere system?
 This chapter covers:
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Distinguishing temperature and heat
Heat transfer processes
Thermal response and specific heat
Heat imbalances
How does heat affect atmospheric circulation?
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Case-in-Point
Extreme Heat of Death Valley, CA
 Death Valley – Hottest and driest
place in North America
 134°F in 1913
 2nd highest temperature ever recorded on Earth
Cooperative Weather observing
station at Furnace Creek Ranch
 Summer 1996
 40 successive days over 120°F
 105 successive days over 110°F
 Causes:
 Topographic setting
 Atmospheric circulation
 Intense solar radiation
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Distinguishing Temperature & Heat
 All matter is composed of molecules or particles in continual
vibrational, rotational, and/or translational motion.
 Energy represented by this motion is called kinetic energy.
 Temperature
 Directly proportional to the average kinetic energy of atoms or molecules
composing a substance
 Internal energy
 Encompasses all the energy in a substance
 Includes kinetic energy
 Includes potential energy, arising from forces between atoms/molecules
 Heat is energy in transit
 When two substances are brought together with different kinetic energy,
energy is always transferred from warmer object to colder
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Distinguishing Temperature & Heat
 Temperature Scales
 Absolute zero
 Temperature at which
theoretically all molecular
motion ceases
 No electromagnetic
radiation is emitted
 Absolute zero
= -459.67°F
= 273.15°C
=0K
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Distinguishing Temperature & Heat
 Temperature scales measure degree of hotness or coldness
 Calorie
 Amount of heat required to raise temperature of 1 gram of water 1 Celsius
degree
 Different from “food” calorie, which is actually 1 kilocalorie
 Joule
 More common in meteorology today
 1 calorie = 4.1868 joules
 British Thermal Units (BTU)
 Amount of energy required to raise 1 pound of water 1 Fahrenheit degree
 1 BTU = 252 cal = 1055 J
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Distinguishing
Temperature & Heat
Liquid-in-glass
thermometer
 Thermometer
 Liquid-in-glass thermometer
 Uses mercury or alcohol
 Bimetallic thermometer
 Two strips of metal with different expansion/contraction rates
 Electrical resistance thermometer
 Thermograph
 Measures and records temperature
The change of
temperature
during the passage
of a cold front as
determined by an
electronic
thermometer.
Bilmetallic thermometer
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Distinguishing Temperature & Heat
 Shielding temperature sensors
 Important properties
 Accuracy
 Response time
 Location is important
 Ventilated
 Shielded from weather
Enclosure for the NWS electronic
temperature sensor
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Heat Transfer Processes
 Temperature gradient
 Change in temperature over distance
 Example: the hot equator and cold poles
 Heat flows in response to a temperature gradient
 This is the 2nd law of thermodynamics
 Heat flows toward lower temperature so as to eliminate the gradient
 Heat flows/transfers in the atmosphere
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Radiation
Conduction
Convection
Latent heat – phase changes in9 water
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Heat Transfer Processes
 Radiation
 Both a form of energy and a means of energy transfer
 Occurs even in a vacuum, such as space
 Absorption of radiation by an object causes the
temperature of object to rise
 Converts electromagnetic energy to heat
 Radiational heating
 Absorption at greater rate than emission
 Radiational cooling
 Emission at greater rate than absorption
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Heat Transfer Processes
 Conduction
 Transfer of kinetic energy of atoms or
molecules by collision between
neighboring atoms or molecules
 Heat conductivity
 Rate of heat transport across an area to
a temperature gradient
 Some materials have a higher heat
conductivity than others
 Solids (metal) are better conductors
than liquids
 Liquids are better than gases (air)
 Conductivity impaired by trapped air
 Examples: fiberglass insulation, thick
layer of fresh snow
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Heat Transfer Processes
A thick layer of snow is a
good insulator because
of air trapped between
individual snowflakes.
As snow settles, the
snow cover’s insulating
property diminishes.
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Heat Transfer Processes
 Convection
 Consequence of differences in air
density
 Transport of heat within a substance via
movement of substance itself
 Substance must liquid or gas
 Very important process for transferring
heat in atmosphere
 The convection cycle
 Ascending warm air expands, cools and
eventually sinks back to ground
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Heat Transfer Processes
 Latent heating
 Movement of heat from one location to another due to phase
changes of water
 Example: evaporation of water, movement of vapor by winds,
condensation elsewhere
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Thermal Response and Specific Heat
 Temperature change caused by
input/output of a quantity of heat
varies among substances
 Specific heat
 The amount of heat required to
raise 1 gram of a substance
1 Celsius degree
The contrast in specific heat
is one reason why the sand
is hotter than the water.
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Thermal Response and Specific Heat
 Thermal inertia
San Francisco, CA, has a maritime climate while
St. Louis, MO, has a continental climate.
 Resistance to a change in
temperature
 Large body of water exhibits
greater resistance to
temperature change than land
because of difference in specific
heat
 Maritime climate
 Immediately downwind of the
ocean experience much less
annual temperature change
 Continental climate
 Locations well inland experience
greater annual temperature
change
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
 At Earth’s surface
 Absorption of solar radiation is greater than emission of IR
 In atmosphere
 Emission of IR radiation to space is greater than absorption of
solar radiation
 Therefore,
 Earth’s surface has net radiational heating
 Atmosphere has net radiational cooling.
 So, Earth’s surface transfers heat to the atmosphere, making up
difference
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
 Latent Heating
 Some absorbed solar radiation used
to vaporize water at Earth’s surface.
 Energy released to the atmosphere
when clouds form
 Comparatively, large amounts of heat
needed for phase changes of water
 Sensible Heating
 Heat transfer via conduction and
convection that can be sensed by
temperature change and measured
by a thermometer
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
 Sensible heating, in the form of
convectional uplifts, can combine with
latent heating, through condensation, to
channel heat from Earth’s surface into
the troposphere
 Produces cumulus clouds
 If it continues vertically,
cumulonimbus clouds form
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
 Bowen Ratio
 Describes how energy received at
the Earth’s surface is partitioned
between sensible heating and
latent heating
 Bowen ratio =
[(sensible heating)/(latent heating)]
 At the global scale, this is
[(7 units)/(23 units)] = 0.3
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 Surface energy budget
through the course of a year at
Yuma, AZ and Madison, WI.
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R = net radiation absorbed
H = sensible heating
LE = latent heating
G = storage
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Heat Imbalance: Tropics vs.
Middle and High Latitudes
 Earth’s surface unevenly heated
due to higher solar altitudes in
the tropics than higher latitudes
 Causes a temperature gradient,
resulting in heat transfer
 Poleward heat transport
 Air mass exchange
 Storms
 Ocean currents
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Heat Imbalance: Tropics vs.
Middle and High Latitudes
 Heat transport by air mass exchange
 North-south exchange of air masses transports sensible heat from the
tropics into middle and high latitudes
 Air mass properties of depend on source region
 Modify as they move
 Heat transport by storms
 Tropical storms and hurricanes are greater contributors to poleward heat
transport than middle latitude cyclones
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Heat Imbalance: Tropics vs.
Middle and High Latitudes
 Heat transport by ocean circulation
 Contributes via wind-driven surface
currents and thermohaline circulation
 Thermohaline circulation is densitydriven movement of water masses
 Transports heat energy, salt, and
dissolved gases over great distances
and depths
The Gulf Stream flows along the East Coast
from Florida to the Delaware coast.
 Meridional overturning circulation
(MOC)
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 At high latitudes, surface waters cool
and sink, then flow southward as cold
bottom water
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Why Weather?
 Imbalances in radiational heating/cooling create temperature gradients
 Earth’s surface the troposphere
 Low and high latitudes
 Heat transported in the Earth-atmosphere system to reduce temperature
differences
 Cause-and-effect chain starts with the Sun, ends with weather
 Some solar radiation is absorbed (converted to heat), some to
converted to kinetic energy
 Causes winds, convection currents, and north-south exchange of air masses
 Rate of heat redistribution varies by season
 Causes seasonal weather and air circulation changes
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Variation of Air Temperature
 Radiational controls
 Factors that affect local radiation budget and air temperature
 Time of day and time of the year
 Solar altitude and duration of radiation
 Cloud cover
 Surface characteristics
 Annual temperature cycle represents these variations
 Annual temperature maximums and minimums do not occur at exact
max/min of solar radiation, especially in middle and high latitudes
 The atmosphere takes time to heat and cool
 Average lag time in US = 27 days
 Up to 36 days with the maritime influence
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Variation of Air Temperature
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Variation of Air Temperature
 Daily temperature cycle
 Lowest temperature usually occurs just after sunrise
 Based on radiation alone, minimum temperature would occur after
sunrise when incoming radiation becomes dominant
 Highest temperature usually occurs in the early to middle
afternoon
 Even though peak of solar radiation is around noon, imbalance in favor of
incoming vs. outgoing radiation continues so the atmosphere also
continues to warm
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Variation of Air Temperature
Daily Temperature Cycle
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Variation of Air Temperature
 Surface cover
 Dry soil heats more rapidly than moist
 Less energy used to evaporate water
 Especially in drought, energy used only to heat soil, soil becomes hotter
 Relative humidity also affects evaporation
 Snow
 High albedo
 Less energy absorbed by the surface or converted to heat
 Snow reduces sensible heating of overlying air
 Some of the available heat is used to vaporize snow
 Snow is an excellent infrared radiation emitter
 Nocturnal radiational cooling is extreme
 When skies are clear, or light winds or calm conditions
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Variation of Air Temperature
 Air mass advection
 Horizontal movement of an air mass
from one location to another
 Cold air advection (A)
 Horizontal movement of colder air into
a warmer area
 Warm air advection (B)
 Horizontal movement of warmer air
into a colder area
 Significance of air mass advection
to local temperature
 Initial temperature of the air new mass
 Degree of modification the air mass as
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travels over the Earth’s surface
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Variation of Air Temperature
 Urban heat island effect
 City of warmth surrounded by cooler air
 In a city:
 Relative lack of moisture
 Absorbed heat raises temperature (not for
evaporation)
 Greater concentration of heat sources (cars,
air conditioners, etc)
 Multiple reflections and lower albedo
 Building materials conduct heat more readily
than soil and vegetation
 Develops best on nights when air is calm
and sky is clear
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Variation of Air Temperature
Providence, RI
Buffalo, NY
Satellite-produced maps of Providence, RI (top) and Buffalo, NY (bott0m) highlighting the
role that differences in development patterns/vegetation cover can have on a city’s urban
heat island. Providence has a significantly stronger heat island signature.
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