Heat, Temperature, and Atmospheric Circulation

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AMS Weather Studies Introduction to Atmospheric
Science, 4th Edition

• Chapter 4
• Heat, Temperature, and Atmospheric Circulation

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Case-in-Point

• Death Valley Hottest and driest place in North
  America
• 134F in 1913
• 2nd highest temperature
• ever recorded on Earth
• Summer 1996
• 40 successive days
• over 120F
• 105 successive
• days over 110F
• Causes
• Topographic setting
• Atmospheric circulation
• Intense solar radiation

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Driving Question

• What are the causes and consequence of heat
  transfer within the Earth-atmosphere system?
• Temperature
• One of the most common and important weather
  variables used to describe the state of the
  atmosphere
• Heat
• Related to temperature
• How?
• How is heat transferred?
• How does heat affect atmospheric circulation?
• This chapter will answer these questions

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Distinguishing Temperature and Heat

• All matter is composed of molecules or particles
  in continual vibrational, rotational, and/or
  translational motion
• The 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
• Also 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 the warmer object to the colder
  one

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Temperature Scales

• Absolute zero is the temperature at which
  theoretically all molecular motion ceases and no
  electromagnetic radiation is emitted
• Absolute zero -459.67F 273.15C 0 K

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Temperature Scales and Heat Units

• Temperature scales measure the 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)
• The amount of energy required to raise 1 pound of
  water 1 Fahrenheit degree
• 1 BTU 252 cal 1055 J

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Measuring Air Temperature

• Thermometer
• Liquid in glass tube type
• Liquid is mercury or alcohol
• Bimetallic thermometer
• Two strips of metal with different
• expansion/contraction rates
• Electrical resistance
• thermometer
• Thermograph measures and
• records temperature
• Important properties
• Accuracy
• Response time
• Location is important
• Ventilated
• Shielded from weather

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Heat Transfer Processes

• Temperature gradient
• A 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
• Radiation
• Conduction
• Convection
• Phase changes in water (latent heat)

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Radiation

• Radiation is both a form of energy and a means of
  energy transfer
• Radiation will occur even in a vacuum such as
  space
• Absorption of radiation by an object causes
  temperature of object to rise
• Converts electromagnetic energy to heat
• Absorption at greater rate than emission
• Radiational heating
• Emission at greater rate than absorption
• Radiational cooling

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Conduction and Convection

• Conduction
• Transfer of kinetic energy of atoms or molecules
  by collision between neighboring atoms or
  molecules
• Heat conductivity
• Ratio of rate of heat transport across an area to
  a temperature gradient
• Some materials have a higher heat conductivity
  than others
• Solids (e.g., metal) are better conductors than
  liquids, and liquids are better than gases (e.g.
  air)
• Conductivity is impaired by trapped air
• Examples fiberglass insulation and thick layer
  of fresh snow

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Conduction and Convection

• A thick layer of snow is a good insulator
• because of air trapped between
• individual snowflakes. As snow settles,
• the snow covers insulating property
• diminishes

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Conduction and Convection

• Convection
• Consequence of differences in air density
• Transport of heat within a substance via the
  movement of the substance itself
• For this to occur, the substance must generally
  be liquid or gas
• This is a very important
• process for transferring heat in the atmosphere
• The convection cycle
• Ascending warm air expands, cools and eventually
  sinks back to ground

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Phase Changes of Water

• Water absorbs or releases heat upon phase changes
• This is called latent heat
• Latent heating
• This is the 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
  specified quantity of heat varies from substance
  to substance
• Specific heat
• The amount of heat required to raise 1 gram of a
  substance 1 Celsius degree

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Thermal Inertia

• Thermal inertia is a resistance to a change in
  temperature
• A large body of water exhibits a greater
  resistance to temperature change than land
  because of difference in specific heat

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Maritime vs. Continental Climate

• A large body of water exhibits a greater
  resistance to temperature change, called thermal
  inertia, than does a landmass
• Places immediately downwind of the ocean
  experience much less annual temperature change
  (maritime climate) than do locations well inland
  (continental climate)

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Heat Imbalance Atmosphere vs. Earths Surface

• At the Earths surface, absorption of solar
  radiation is greater than emission of infrared
  radiation
• In the atmosphere, emission of infrared radiation
  to space is greater than absorption of solar
  radiation
• Therefore, the Earths surface has net
  radiational heating, and the atmosphere has net
  radiational cooling
• But, the Earths surface transfers heat to the
  atmosphere to make up for the loss

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Heat Imbalance Atmosphere vs. Earths Surface
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Latent Heating

• Some of the absorbed solar radiation is used to
  vaporize water at Earths surface
• This energy is released to the atmosphere when
  clouds form
• Large amounts of heat are needed for phase
  changes of water compared to other substances

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Sensible Heating

• Heat transfer via conduction and convection can
  be sensed by temperature change (sensible
  heating) and measured by a thermometer
• Sensible heating in the form of convectional
  uplifts can combine with latent heating through
  condensation to channel heat from Earths surface
  into the troposphere
• This produces cumulus clouds
• If it continues vertically in the atmosphere,
  cumulonimbus clouds may form

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Bowen Ratio

• Describes how the energy received at the Earths
  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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Heat Imbalance Tropics vs. Middle and
High-Latitudes

• We have seen in previous chapters how the Earths
  surface is unevenly heated due to higher solar
  altitudes in the tropics than at higher latitudes
• This causes a temperature gradient, resulting in
  heat transfer
• Poleward heat transport is brought about through
• 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 the middle
  and high latitudes
• The properties of air mass depend on its source
  region
• Air masses modify as they move away from their
  source region
• Heat transport by storm
• Tropical storms and hurricanes are greater
  contributors to poleward heat transport then
  middle latitude cyclones
• Heat transport by ocean circulation
• Contributes via wind-drive surface currents and
  thermohaline circulation
• The thermohaline circulation is the
  density-driven movement of water masses
• Transports heat energy, salt, and dissolved gases
  over great distances and depths
• Meridonal overturning circulation (MOC)
• At high latitudes, surface waters cool, sink and
  flow southward as cold bottom water

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The Gulf Stream flows along the East Coast from
Florida to the Delaware coast
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Why Weather?

• Imbalances in radiational heating/cooling create
  temperature gradients between
• The Earths surface and the troposphere
• Low and high latitudes
• Heat is transported in the Earth-atmosphere
  system to reduce temperature differences
• A cause-and-effect chain starts with the sun, and
  ends with weather
• Some solar radiation is absorbed (converted to
  heat), some is converted to kinetic energy
• Winds are caused by this kinetic energy, as well
  as convection currents and north-south exchange
  of air masses
• The rate of heat redistribution varies by season
• This 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
• Determines solar altitude and duration of
  radiation received
• Cloud cover
• Surface characteristics
• The annual temperature cycle represents these
  variations
• The annual temperature maximums and minimums do
  not occur at the exact max/min of solar
  radiation, especially in middle and high
  latitudes
• The atmosphere takes time to heat and cool
• Average lag time in U.S. 27 days. Can be up to
  36 days with the maritime influence

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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, the imbalance in favor of incoming vs.
  outgoing radiation continues after noon, and the
  atmosphere continues to warm
• Dry soil heats more rapidly than moist soil
• Less energy is used to evaporate water if little
  water is present
• More energy is therefore used to warm the Earth,
  and consequently, the atmosphere
• Relative humidity also affects the ability of
  evaporation to occur

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Variation of Air Temperature
Annual Temperature Cycle
Daily Temperature Cycle
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Variation of Air Temperature

• Why is it so cold when snow is on the ground?
• Snow has a relatively high albedo
• Less energy absorbed by the surface and 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
• Especially when skies are clear
• Cooling is enhanced with light winds or calm
  conditions

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Variation of Air Temperature

• Cold and warm air advection
• Air mass advection
• Horizontal movement of an air mass from one
  location to another
• Cold air advection
• Horizontal movement of colder air into a warmer
  area
• Arrow A on the next slide
• Warm air advection
• Horizontal movement of warmer air into a colder
  area
• Arrow B on the next slide
• Significance of air mass advection to local
  temperature depends on
• The initial temperature of the air new mass
• The degree of modification the air mass receives
  as it travels over the Earths surface

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Variation of Air Temperature

• Cold Air Advection
• Warm Air Advection

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Anthropogenic Influence

• An urban heat island is an example of
  anthropogenic influence on the
• Earths climate
• An urban heat island is a city of warmth
• surrounded by cooler air
• Caused by
• Relative lack of moisture in the city
• More available heat from absorbed radiation is
  used to raise the temperature of city surfaces
  and less for evaporation of water
• Greater concentration of heat sources in a city
  (cars, air conditioners, etc)
• Lower albedo of city surfaces
• Building materials conduct heat more readily than
  soil and vegetation
• Develop best on nights when the air is calm and
  the sky is clear