Download GEOGRAPHY 257 Introduction to Meteorology

Weather Studies
Introduction to Atmospheric Science
American Meteorological Society
Chapter 4
Heat, Temperature, and
Atmospheric Circulation
Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants
Case-in-Point
 Death Valley – Hottest and driest place in
North America
– 134°F in 1913
 2nd highest temperature ever recorded on Earth
– 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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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 = -459.67°F = 273.15°C = 0 K
5
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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 Thermometer
Measuring
Air Temperature
– 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
 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 cover’s 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
Note – Water has a higher specific heat than Earth
substances. This is an important aspect of weather.
– The amount of
heat required to
raise 1 gram of a
substance 1
Celsius degree
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Specific Heat
 Specific heat is the reason the sand is hotter than the water
Consider the
role specific
heat plays
In continental
vs. maritime
climates – see
next slide
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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.
Earth’s Surface
 At the Earth’s 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 Earth’s surface has net radiational
heating, and the atmosphere has net radiational
cooling
 But, the Earth’s surface transfers heat to the
atmosphere to make up for the loss
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Heat Imbalance: Atmosphere vs.
Earth’s Surface
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Heat Imbalance: Atmosphere vs.
Earth’s Surface
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Latent Heating
Latent heat of vaporization
Latent heat of fusion
 Some of the absorbed
solar radiation is used
to vaporize water at
Earth’s 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
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substances
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
Earth’s 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
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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Heat Imbalance: Tropics vs. Middle
and High-Latitudes
 We have seen in previous
chapters how the Earth’s
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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Role of Gulf Stream in Poleward
Heat Transport
 The ocean contributes to
poleward heat transport
via wind-driven surface
currents and deeper
conveyor-belt-like currents
that traverse the lengths
of the ocean basins
 Warm surface currents
like the Gulf Stream are a
heat source for the
atmosphere – they flow
from the tropics into
middle latitudes and
supply heat to the cooler
mid-latitude troposphere 24
The Ocean Conveyor Belt System
Contributes to Heat Transfer from Low Latitudes to High Latitudes
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Why Weather?
 Imbalances in radiational heating/cooling create
temperature gradients between
– The Earth’s 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 to 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
 The Urban heat island
– Lack of moisture and greater concentration of
heat sources in cities lead to higher
temperatures
 Runoff is in sewers
 Much soil is built over or paved over
 More solar energy is available to heat the air, as less
is used for evaporation
 City surfaces also generally have a lower albedo
– Less reflection yields more absorption and conversion to heat
 Heat sources include motor vehicles, space heaters,
etc.
– Best developed at night when the air is calm and
the sky is clear
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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 Earth’s surface
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Variation of Air Temperature
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