nas 125: meteorology heat, temperature, and atmospheric circulation

NAS 125: Meteorology

Heat, Temperature,and Atmospheric Circulation

Rev. 16 February 2006 Heat, Temperature, and Atmospheric Circulation

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Importance of weather

• More than 70 percent of U.S. businesses are sensitive to temperature and other weather variables.– 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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• 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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• 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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• 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):– 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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• 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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• 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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• 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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• 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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nas 125: meteorology heat, temperature, and atmospheric circulation

Documents

measuring temperature

temperature conditions

temperature scales

temperature considerations

heat energy

atmospheric circulation

kinetic energy kinetic

measuring heat kinetic