Transcript Document

NAS 125: Meteorology
Heat, Temperature,
and Atmospheric Circulation
Importance of weather
• More than 70 percent of U.S. businesses are sensitive
to temperature and other weather variables.
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Heating and cooling costs
Transportation expenses
Agriculture and forestry
Recreation industry
Employee health and safety (not a priority for some)
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Insurance and derivatives
• Businesses buy insurance to protect themselves
against losses from high-risk, low probability events
(hurricanes, floods, etc.).
• They buy weather derivatives to protect themselves
against losses from low-risk, high probability events
(mild winter for ski resorts, for example).
– Weather derivatives are a recent innovation in business.
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Derivative example
• Snowshoe wants to protect itself from losses due to a
mild winter.
– It purchases weather derivates from a seller (DS).
– Snowshoe and DS agree that contract should be based on
heating degree-days such that, if the number of heating
degree-days is less than an agreed-upon threshold value,
DS pays Snowshoe an amount to make up for the losses
Snowshoe has incurred as a result of the unfavorable skiing
weather.
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Temperature and landscape
• Long-run temperature conditions affect the organic
and inorganic components of the landscape.
– Animals and plants often evolve in response to hot or cold
climates.
– Soil development is affected by temperature, with repeated
fluctuations in temperature being the primary cause of
breakdown of exposed bedrock.
– Human-built landscape is created in response to
temperature considerations.
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Kinetic energy
• Kinetic energy is the energy of motion.
• Heat is the total quantity of kinetic energy in a
substance.
• Temperature is the average amount of kinetic energy
in a substance.
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Measuring heat
• Kinetic energy often measured in calories (cal), the
amount of heat energy required to raise the
temperature of 1 g of water 1 °C
• The joule (J) is another way to measure kinetic
energy.
– 1 cal = 4.187 J; 1 J = 0.239 cal.
• The British Thermal Unit (BTU) is the amount of
energy it takes to raise the temperature of 1 pound of
water 1 °F (from 62 °F to 63 °F).
– 1 BTU = 252 cal = 1055 J
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Measuring temperature, part 1
• There are a number of instruments for measuring
temperature. All work on the principle that most
substances expand when heated, calibrating this
change in volume to measure temperature.
• There are three temperature scales used in the United
States: the Fahrenheit Scale, the Celsius Scale, and
the Kelvin Scale.
– The Fahrenheit scale is used by public weather reports from
the National Weather Service and the news media; few
other countries than United States use it.
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Measuring temperature, part 2
• Three temperature scales (continued):
– The Celsius scale is used either exclusively or
predominately in most countries other than United States,
which uses it for scientific work. It is slowly being
established to supersede the Fahrenheit scale.
• 0 °C = 32 °F
• 100 °C = 212 °F
– The Kelvin scale is used in scientific research, but not by
climatologists and meteorologists. It is similar to the
Celsius scale, but the zero point is set to absolute zero, the
temperature at which all molecular motion ceases.
• 0 K = -273.15 °C = -459.67 °F
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Measuring temperature, part 3
• Temperature data is recorded throughout the world at
thousands of locations, following specific rules for
providing accurate and important raw material for
weather reports and long-run climatic analyses.
– Official temperatures must be taken in shade so measure air
temperature, not solar radiation.
– Official thermometers are usually mounted in an instrument
shelter that shields them from sunshine and precipitation
while providing air circulation.
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Measuring temperature, part 4
• Recording temperature data (continued):
– Thermographs are often used to continuously record
temperature.
• The highest and lowest temperatures are recorded for each 24-hour
period.
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Heating and cooling
• To understand how energy travels from the Sun to
Earth, it’s best to examine how heat energy moves.
– Heat energy moves from one place to another in three
ways:
• Radiation
• Conduction
• Convection
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Radiation, part 1
• Radiation is the process by which electromagnetic
energy emits from an object; radiant energy flows out
of all bodies, with temperature and nature of the
surface of the objects playing a key role in radiation
effectiveness.
– Hot bodies are more potent than cool bodies (and the hotter
the object, the more intense the radiation and the shorter the
wavelength).
– A blackbody is a body that emits the maximum amount of
radiation possible, at every wavelength, for its temperature.
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Radiation, part 2
• If the amount of radiation absorbed is greater than
that emitted, the temperature of the object will rise,
this is radiational heating.
• If the amount of radiation emitted is greater than that
absorbed, the temperature of the object will fall, this
is radiational cooling.
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Conduction, part 1
• Conduction is the movement of energy from one
molecule to another without changes in the relative
positions of the molecules. It enables the transfer of
heat between different parts of a stationary body, or
from one object to a second object when the two are
in contact.
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Conduction, part 2
• Conduction does require molecular movement,
however. Although the molecules do not move from
their relative positions, they do become increasingly
agitated as heat is added.
– An agitated molecule will move and collide against a
cooler, calmer molecule, and through this collision transfer
the heat energy. Thus, heat energy is passed from one place
to another, without the molecules actually moving from one
place to another, just vibrating back and forth from
agitation. (Thus, it’s the opposite of convection.)
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Conduction, part 3
• Conduction ability varies with the makeup of the
objects.
– Heat conductivity is the ratio of the rate of heat transport
across an area to a temperature gradient.
– Metals are excellent conductors in comparison to earthy
materials like ceramics or gases.
• Solids >> liquids >> gases
• Snow is a poor conductor (conversely, a good insulator) because of
air trapped between snowflakes.
– Differences in heat conductivity can make some objects
(good conductors) feel cooler than others (poor
conductors).
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Convection
• Convection is the transfer of heat within a fluid by
motions of the fluid itself.
– Convection is essentially the opposite of conduction.
• Molecules actually move from one place to another,
rather than just vibrating from agitation.
– The principal action in convection is vertical, with less
dense fluids rising and more dense fluids sinking.
• Advection is when a convecting liquid or gas moves
horizontally as opposed to vertically as in convection.
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Water
• Water occurs in all three states of matter:
– Solid (snow, sleet, hail, ice);
– Liquid (rain, water droplets);
– and Gas (water vapor).
• The gaseous state is the most important in driving the
dynamics of the atmosphere.
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Latent heat, part 1
• Latent heat is the energy stored or released when a
substance changes state; it can result in temperature
changes in atmosphere.
• Changes of state:
– Evaporation is when liquid water converts to gaseous water
vapor; it is a cooling process because latent heat is stored.
– Condensation is when gaseous water vapor condenses to
liquid water; it is a warming process because latent heat is
released.
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Latent heat, part 2
• Changes of state (continued):
– Freezing is when liquid water converts to solid water (ice);
it is a warming process because latent heat is released.
– Sublimation is when ice converts to gaseous water vapor; it
is a cooling process because latent heat is stored.
• Latent heating refers to the transport of heat from one
location to another as a result of the changes of state
of water.
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Specific heat
• Specific heat is the amount of energy it takes to raise
or lower the temperature of 1 g of a substance 1
degree C.
• The specific heat of water is 1 cal/g/degree C.
• Water changes its temperature less than most other
substances when it absorbs or radiates a given amount
of energy.
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Water and climate
• Water’s thermodynamic properties explain why
maritime climates have more moderate temperature
ranges than arid climates, and why sweating is so
important to cooling the body.
• Water stabilizes air temperatures by absorbing heat
from warmer air and releasing heat to cooler air.
• Water can absorb or release relatively large amounts
of heat with only a slight change in its own
temperature.
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Land-water interactions
• Since land and water differ in their response to solar
heating, climates can be classified according to a
region’s proximity to water.
– Continental climates occur in areas far from large bodies of
water (such as oceans and seas). They are characterized by
large temperature extremes.
– Maritime climates occur near large bodies of water, thus
have reduced climate fluctuations.
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Radiation budget, part 1
• The global and annual average energy budget (for
every 100 units incoming solar radiation):
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31 units scattered and reflected to space
20 units absorbed by the atmosphere
49 units absorbed at the Earth’s surface
100 units total
• At the Earth’s surface
– 19 units lost due to infrared cooling
– 49 units gained by solar heating
– 30 units net heating
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Radiation budget, part 2
• The atmosphere
– 50 units lost due to infrared cooling
– 20 units gained by solar heating
– 30 units net cooling
• Heat transfer from Earth’s surface to atmosphere
– 7 units sensible heating (conduction plus convection)
– 23 units latent heating
– 30 units net transfer
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Phase changes
• Tremendous amounts of energy are involved in phase
changes of water.
– Latent heat of melting: 80 cal are required to convert 1 g of
frozen water to liquid water at the freezing/melting point
• Temperature remains at 0 °C until all ice melts
– Latent heat of vaporization: varies, depending on initial
temperature of water
• 600 cal required to evaporate 1 g of liquid water at 0 °C
• 540 cal required to evaporate 1 g of liquid water at 100 °C
– Latent heat of sublimation: equals sum of latent heat of
melting plus latent heat of vaporization
• 680 cal required to evaporate 1 g of frozen water at 0 °C
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Sensible heating, part 1
• Heat transfer by conduction and convection can be
measured (sensed) by temperature changes.
• Sensible heating incorporates both conduction and
convection.
– Heating reduces the density of air, causing it to rise above
cooler, denser air.
• Convection thus transports heat from surface to troposphere
• Convection is more important than conduction
because air is a poor conductor of heat.
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Sensible heating, part 2
• Sensible and latent heating often work together.
– As air cools by convection, the water vapor in the air
condenses, thus releasing its latent heat as sensible heat –
and leading to the formation of cumulus clouds.
– The latent heat released by water vapor is converted into
sensible heat in the air. This in turn can lead to stronger
updrafts, as is seen in cumulonimbus clouds.
– By these processes, heat can also be transferred from the
atmosphere to the surface, such as on cold nights when
radiational cooling causes the surface to have a lower
temperature than the air above.
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Bowen ratio
• The Bowen ratio describes how heat energy received
at the Earth is partitioned into sensible and latent
heat.
– Bowen ration = [(sensible heating)/(latent heating)]
• Globally
– Bowen ratio = [(7 units)/(23 units)] = 0.3
• The Bowen ratio varies considerably by region and
surface type.
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Latitudinal differences, part 1
• There is unequal heating of different latitudinal zones
for four basic reasons, angle of incidence, day length,
atmospheric obstruction, and latitudinal radiation
balance:
• The angle of incidence is the angle at which rays
from the Sun strike Earth’s surface; always changes
because Earth is a sphere and Earth rotates on own
axis and revolves around the Sun.
– Angle of incidence is the primary determinant of the
intensity of solar radiation received on Earth.
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Latitudinal differences, part 2
• The angle of incidence (continued).
– Heating is more effective the closer to 90°, because the
more perpendicular the ray, the smaller the surface area
being heated by a given amount of insolation.
• Angle is 90° if Sun is directly overhead.
• Angle is less than 90° if ray is striking surface at a glance.
• Angle is 0° for a ray striking Earth at either pole.
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Latitudinal differences, part 3
• Day length is important because the longer the day,
the more insolation can be received and the more heat
can be absorbed.
– Middle and high latitudes have pronounced seasonal
variations in day length, while tropical areas have little
variation.
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Latitudinal differences, part 4
• Atmospheric obstructions – such as clouds,
particulate matter, and gas molecules – absorb,
reflect, or scatter insolation.
– How much effect they have depends on path length, the
distance a ray must travel.
• Because angle of incidence determines path length, atmospheric
obstruction reinforces the pattern established by the varying angle
of incidence.
• Because they must pass through more atmosphere than high-angle
rays, low-angle rays are subject to more depletion through
reflection, scattering, and absorption.
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Latitudinal differences, part 5
• The latitudinal radiation balance occurs because the
belt of maximum solar energy swings back and forth
through tropics as the direct rays of sun shift
northward and southward in course of a year.
– Low latitudes (about between 28° N and 33° S) receive an
energy surplus, with more incoming than outgoing
radiation.
– There is an energy deficit in latitudes north and south of
these low latitudes.
• This simple latitudinal pattern is interrupted principally by
atmospheric obstruction.
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Heat transfer, part 1
• The tropics would become progressively warmer (and
less habitable) until the amount of heat energy
absorbed equaled the amount radiated from Earth’s
surface if not for two specific mechanisms moving
heat poleward in both hemispheres:
– Atmospheric circulation is the most important mechanism,
accomplishing 75 to 80 percent of all horizontal heat
transfer.
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Heat transfer, part 2
• Heat transfer mechanisms (continued):
– Oceanic circulation (ocean currents) reflect average wind
conditions over a period of several years.
• Current refers to various kinds of oceanic water movements.
• The atmosphere and oceans serve as thermal engines; their currents
are driven by the latitudinal imbalance of heat.
• There is a direct relationship between these two mechanisms:
– Air blowing over ocean is the principal driving force of major surface
ocean currents;
– Heat energy stored by ocean affects atmospheric circulation.
• Waters cooler than the overlying air act as a heat sink.
• Waters cooler than the overlying air act as a heat source.
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Whither weather?
• The sun heats the Earth.
• The variations in heating through time and space
generate radiation imbalances.
• The imbalances generate energy redistribution
mechanisms that are among the fundamental causes
of weather and climate variations.
– Weather systems do not last indefinitely, however, as
kinetic energy is dissipated in the form of frictional heat as
winds blow across the Earth’s surface.
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Air mass advection
• Air mass advection refers to the movement of air
masses from one region to another.
– Cold air advection occurs when wind transports colder air
over a warmer land surface.
– Warm air advection occurs when wind transports warmer
air over a colder land surface.
• The significance of air mass advection depends on the
initial temperature of the air mass and the degree of
modification it undergoes as it is transported.
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