Chapter 4Chapter 4

Heat, Temperature, and Heat, Temperature, and Atmospheric CirculationAtmospheric Circulation

Weather StudiesWeather Studies Introduction to Atmospheric ScienceIntroduction to Atmospheric Science

American Meteorological SocietyAmerican Meteorological Society

Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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Case-in-PointCase-in-Point Death Valley – Hottest and driest place in Death Valley – Hottest and driest place in

North AmericaNorth America– 134134°°F in 1913F in 1913

22ndnd highest temperature ever recorded on Earth highest temperature ever recorded on Earth

– Summer 1996Summer 1996 40 successive days over 12040 successive days over 120°°FF 105 successive days over 110105 successive days over 110°°FF

– Causes:Causes: Topographic settingTopographic setting Atmospheric circulationAtmospheric circulation Intense solar radiationIntense solar radiation

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What are the causes and consequence of heat What are the causes and consequence of heat transfer within the Earth-atmosphere system?transfer within the Earth-atmosphere system?

TemperatureTemperature– One of the most common and important weather One of the most common and important weather

variables used to describe the state of the atmospherevariables used to describe the state of the atmosphere– HeatHeat

Related to temperatureRelated to temperature How?How? How is heat transferred?How is heat transferred? How does heat affect atmospheric circulation?How does heat affect atmospheric circulation?

This chapter will answer these questionsThis chapter will answer these questions

Driving QuestionDriving Question

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Distinguishing Temperature and HeatDistinguishing Temperature and Heat All matter is composed of molecules or particles in continual All matter is composed of molecules or particles in continual

vibrational, rotational, and/or translational motionvibrational, rotational, and/or translational motion– The energy represented by this motion is called kinetic energyThe energy represented by this motion is called kinetic energy

TemperatureTemperature– Directly proportional to the average kinetic energy of atoms or Directly proportional to the average kinetic energy of atoms or

molecules composing a substancemolecules composing a substance Internal energyInternal energy

– Encompasses all the energy in a substanceEncompasses all the energy in a substance Includes kinetic energyIncludes kinetic energy Also includes potential energy arising from forces between Also includes potential energy arising from forces between

atoms/moleculesatoms/molecules

Heat is energy in transitHeat is energy in transit– When two substances are brought together with different kinetic When two substances are brought together with different kinetic

energy, energy is always transferred from the warmer object to the energy, energy is always transferred from the warmer object to the colder onecolder one

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Temperature ScalesTemperature Scales

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

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Temperature Scales and Heat UnitsTemperature Scales and Heat Units Temperature scales measure the degree of Temperature scales measure the degree of

hotness or coldnesshotness or coldness Calorie – amount of heat required to raise Calorie – amount of heat required to raise

temperature of 1 gram of water 1 Celsius degreetemperature of 1 gram of water 1 Celsius degree– Different from “food” calorie, which is actually 1 Different from “food” calorie, which is actually 1

kilocaloriekilocalorie

Joule – more common in meteorology todayJoule – more common in meteorology today– 1 calorie = 4.1868 joules1 calorie = 4.1868 joules

British Thermal Units (BTU)British Thermal Units (BTU)– The amount of energy required to raise 1 pound of The amount of energy required to raise 1 pound of

water 1 Fahrenheit degreewater 1 Fahrenheit degree– 1 BTU = 252 cal = 1055 J1 BTU = 252 cal = 1055 J

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Measuring Measuring Air TemperatureAir Temperature ThermometerThermometer

– Liquid in glass tube typeLiquid in glass tube type Liquid is mercury or alcoholLiquid is mercury or alcohol

– Bimetallic thermometerBimetallic thermometer Two strips of metal with different Two strips of metal with different

expansion/contraction ratesexpansion/contraction rates– Electrical resistance Electrical resistance

thermometerthermometer Thermograph – measures and Thermograph – measures and

records temperaturerecords temperature Important propertiesImportant properties

– AccuracyAccuracy– Response timeResponse time

Location is importantLocation is important– VentilatedVentilated– Shielded from weatherShielded from weather

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Heat TransferHeat Transfer Temperature gradientTemperature gradient

– A change in temperature over distanceA change in temperature over distance Example – the hot equator and cold polesExample – the hot equator and cold poles

Heat flows in response to a temperature gradientHeat flows in response to a temperature gradient– This is the 2This is the 2ndnd law of thermodynamics law of thermodynamics

Heat flows toward lower temperature so as to eliminate the Heat flows toward lower temperature so as to eliminate the gradientgradient

Heat flows/transfers in the atmosphereHeat flows/transfers in the atmosphere– RadiationRadiation– ConductionConduction– ConvectionConvection– Phase changes in water (latent heat)Phase changes in water (latent heat)

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RadiationRadiation Radiation is both a form of energy and a Radiation is both a form of energy and a

means of energy transfermeans of energy transfer Radiation will occur even in a vacuum such Radiation will occur even in a vacuum such

as spaceas space Absorption of radiation by an object causes Absorption of radiation by an object causes

temperature of object to risetemperature of object to rise– Converts electromagnetic energy to heatConverts electromagnetic energy to heat

Absorption at greater rate than emissionAbsorption at greater rate than emission– Radiational heatingRadiational heating

Emission at greater rate than absorptionEmission at greater rate than absorption– Radiational coolingRadiational cooling

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Conduction and ConvectionConduction and Convection ConductionConduction

– Transfer of kinetic energy of atoms Transfer of kinetic energy of atoms or molecules by collision between or molecules by collision between neighboring atoms or moleculesneighboring atoms or molecules

– Heat conductivityHeat conductivity Ratio of rate of heat transport across an Ratio of rate of heat transport across an

area to a temperature gradientarea to a temperature gradient Some materials have a higher heat Some materials have a higher heat

conductivity than othersconductivity than others

– Solids (e.g., metal) are better Solids (e.g., metal) are better conductors than liquids, and liquids conductors than liquids, and liquids are better than gases (e.g. air)are better than gases (e.g. air)

– Conductivity is impaired by trapped Conductivity is impaired by trapped airair Examples – fiberglass insulation and Examples – fiberglass insulation and

thick layer of fresh snowthick layer of fresh snow

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Conduction and ConvectionConduction and Convection

A thick layer of snow is a good insulator because of air A thick layer of snow is a good insulator because of air trapped between individual snowflakes. As snow settles, trapped between individual snowflakes. As snow settles, the snow cover’s insulating property diminishesthe snow cover’s insulating property diminishes

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Conduction and ConvectionConduction and Convection ConvectionConvection

– Consequence of differences in Consequence of differences in air densityair density

– Transport of heat within a Transport of heat within a substance via the movement of substance via the movement of the substance itselfthe substance itself For this to occur, the substance For this to occur, the substance

must generally be liquid or gasmust generally be liquid or gas

– This is a very important This is a very important

process for transferring heat in process for transferring heat in the atmospherethe atmosphere

– The convection cycleThe convection cycle Ascending warm air expands, cools Ascending warm air expands, cools

and eventually sinks back to groundand eventually sinks back to ground

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Water absorbs or Water absorbs or releases heat upon releases heat upon phase changesphase changes– This is called latent This is called latent

heatheat Latent heatingLatent heating

– This is the movement This is the movement of heat from one of heat from one location to another due location to another due to phase changes of to phase changes of waterwater Example – evaporation Example – evaporation

of water, movement of of water, movement of vapor by winds, vapor by winds, condensation elsewherecondensation elsewhere

Phase Changes of WaterPhase Changes of Water

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Temperature Temperature change caused change caused by input/output of by input/output of a specified a specified quantity of heat quantity of heat varies from varies from substance to substance to substancesubstance

Specific heatSpecific heat– The amount of The amount of

heat required to heat required to raise 1 gram of a raise 1 gram of a substance 1 substance 1 Celsius degreeCelsius degreeNote – Water has a higher specific heat than Earth Note – Water has a higher specific heat than Earth

substances. This is an important aspect of weathersubstances. This is an important aspect of weather.

Thermal Response and Specific HeatThermal Response and Specific Heat

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Specific HeatSpecific Heat

Specific heat is the reason the sand is hotter than the waterSpecific heat is the reason the sand is hotter than the water

Consider the Consider the role specific role specific heat playsheat playsIn continentalIn continentalvs. maritimevs. maritimeclimates – see climates – see next slidenext slide

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Maritime vs. Continental ClimateMaritime vs. Continental Climate A large body of water A large body of water

exhibits a greater exhibits a greater resistance to temperature resistance to temperature change, called thermal change, called thermal inertia, than does a inertia, than does a landmasslandmass

Places immediately Places immediately downwind of the ocean downwind of the ocean experience much less experience much less annual temperature annual temperature change (maritime change (maritime climate) than do locations climate) than do locations well inland (continental well inland (continental climate)climate)

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Heat Imbalance: Atmosphere vs. Heat Imbalance: Atmosphere vs. Earth’s SurfaceEarth’s Surface

At the Earth’s surface, absorption of solar radiation At the Earth’s surface, absorption of solar radiation is greater than emission of infrared radiationis greater than emission of infrared radiation

In the atmosphere, emission of infrared radiation In the atmosphere, emission of infrared radiation to space is greater than absorption of solar to space is greater than absorption of solar radiationradiation

Therefore, the Earth’s surface has net radiational Therefore, the Earth’s surface has net radiational heating, and the atmosphere has net radiational heating, and the atmosphere has net radiational coolingcooling

But, the Earth’s surface transfers heat to the But, the Earth’s surface transfers heat to the atmosphere to make up for the lossatmosphere to make up for the loss

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Heat Imbalance: Atmosphere vs. Heat Imbalance: Atmosphere vs. Earth’s SurfaceEarth’s Surface

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Heat Imbalance: Atmosphere vs. Heat Imbalance: Atmosphere vs. Earth’s SurfaceEarth’s Surface

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Some of the absorbed Some of the absorbed solar radiation is used solar radiation is used to vaporize water at to vaporize water at Earth’s surfaceEarth’s surface

This energy is This energy is released to the released to the atmosphere when atmosphere when clouds formclouds form

Large amounts of heat Large amounts of heat are needed for phase are needed for phase changes of water changes of water compared to other compared to other substancessubstances

Latent HeatingLatent Heating

Latent heat of fusion

Latent heat of vaporization

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Sensible HeatingSensible Heating

Heat transfer via conduction and convection Heat transfer via conduction and convection can be sensed by temperature change can be sensed by temperature change (sensible heating) and measured by a (sensible heating) and measured by a thermometerthermometer

Sensible heating in the form of convectional Sensible heating in the form of convectional uplifts can combine with latent heating uplifts can combine with latent heating through condensation to channel heat from through condensation to channel heat from Earth’s surface into the troposphereEarth’s surface into the troposphere– This produces cumulus cloudsThis produces cumulus clouds– If it continues vertically in the atmosphere, If it continues vertically in the atmosphere,

cumulonimbus clouds may formcumulonimbus clouds may form

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Describes how the Describes how the energy received at the energy received at the Earth’s surface is Earth’s surface is partitioned between partitioned between sensible heating and sensible heating and latent heatinglatent heating

Bowen ratio = [(sensible Bowen ratio = [(sensible heating)/(latent heating)] heating)/(latent heating)]

At the global scale, this At the global scale, this is [(7 units)/(23 units)] = is [(7 units)/(23 units)] = 0.30.3

Bowen RatioBowen Ratio

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Heat Imbalance: Tropics vs. Middle Heat Imbalance: Tropics vs. Middle and High-Latitudesand High-Latitudes

We have seen in previous We have seen in previous chapters how the Earth’s chapters how the Earth’s surface is unevenly surface is unevenly heated due to higher solar heated due to higher solar altitudes in the tropics altitudes in the tropics than at higher latitudesthan at higher latitudes– This causes a temperature This causes a temperature

gradient, resulting in heat gradient, resulting in heat transfertransfer

– Poleward heat transport is Poleward heat transport is brought about through:brought about through: Air mass exchangeAir mass exchange StormsStorms Ocean currentsOcean currents

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Gulf Stre

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Role of Gulf Stream in Poleward Role of Gulf Stream in Poleward Heat TransportHeat Transport

The ocean contributes to The ocean contributes to poleward heat transport poleward heat transport via wind-driven surface via wind-driven surface currents and deeper currents and deeper conveyor-belt-like currents conveyor-belt-like currents that traverse the lengths that traverse the lengths of the ocean basinsof the ocean basins

Warm surface currents Warm surface currents like the Gulf Stream are a like the Gulf Stream are a heat source for the heat source for the atmosphere – they flow atmosphere – they flow from the tropics into from the tropics into middle latitudes and middle latitudes and supply heat to the cooler supply heat to the cooler mid-latitude tropospheremid-latitude troposphere

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The Ocean Conveyor Belt SystemThe Ocean Conveyor Belt SystemContributes to Heat Transfer from Low Latitudes to High LatitudesContributes to Heat Transfer from Low Latitudes to High Latitudes

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Why Weather?Why Weather? Imbalances in radiational heating/cooling create Imbalances in radiational heating/cooling create

temperature gradients betweentemperature gradients between– The Earth’s surface and the troposphereThe Earth’s surface and the troposphere– Low and high latitudesLow and high latitudes

Heat is transported in the Earth-atmosphere system to reduce Heat is transported in the Earth-atmosphere system to reduce temperature differencestemperature differences

A cause-and-effect chain starts with the sun, and A cause-and-effect chain starts with the sun, and ends with weatherends with weather

Some solar radiation is absorbed (converted to Some solar radiation is absorbed (converted to heat), some to converted to kinetic energyheat), some to converted to kinetic energy– Winds are caused by this kinetic energy, as well as Winds are caused by this kinetic energy, as well as

convection currents and north-south exchange of air convection currents and north-south exchange of air massesmasses

The rate of heat redistribution varies by seasonThe rate of heat redistribution varies by season– This causes seasonal weather and air circulation This causes seasonal weather and air circulation

changeschanges

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Variation of Air TemperatureVariation of Air Temperature Radiational controls – factors that affect local Radiational controls – factors that affect local

radiation budget and air temperature:radiation budget and air temperature:– Time of day and time of the yearTime of day and time of the year

Determines solar altitude and duration of radiation receivedDetermines solar altitude and duration of radiation received

– Cloud coverCloud cover– Surface characteristicsSurface characteristics

The annual temperature cycle represents these The annual temperature cycle represents these variationsvariations– The annual temperature maximums and minimums do The annual temperature maximums and minimums do

not occur at the exact max/min of solar radiation, not occur at the exact max/min of solar radiation, especially in middle and high latitudesespecially in middle and high latitudes The atmosphere takes time to heat and coolThe atmosphere takes time to heat and cool

– Average lag time in U.S. = 27 days. Can be up to 36 days with the Average lag time in U.S. = 27 days. Can be up to 36 days with the maritime influencemaritime influence

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Variation of Air TemperatureVariation of Air Temperature Daily temperature cycleDaily temperature cycle

– Lowest temperature usually occurs just after sunriseLowest temperature usually occurs just after sunrise Based on radiation alone, minimum temperature would occur Based on radiation alone, minimum temperature would occur

after sunrise when incoming radiation becomes dominantafter sunrise when incoming radiation becomes dominant– Highest temperature usually occurs in the early to Highest temperature usually occurs in the early to

middle afternoonmiddle afternoon Even though peak of solar radiation is around noon, the Even though peak of solar radiation is around noon, the

imbalance in favor of incoming vs. outgoing radiation continues imbalance in favor of incoming vs. outgoing radiation continues after noon, and the atmosphere continues to warmafter noon, and the atmosphere continues to warm

Dry soil heats more rapidly than moist soilDry soil heats more rapidly than moist soil– Less energy is used to evaporate water if little water is Less energy is used to evaporate water if little water is

presentpresent– More energy is therefore used to warm the Earth, and More energy is therefore used to warm the Earth, and

consequently, the atmosphereconsequently, the atmosphere– Relative humidity also affects the ability of evaporation Relative humidity also affects the ability of evaporation

to occurto occur

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Variation of Air TemperatureVariation of Air Temperature

Annual Temperature Cycle Daily Temperature Cycle

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Variation of Air TemperatureVariation of Air Temperature The Urban heat islandThe Urban heat island

– Lack of moisture and greater concentration of Lack of moisture and greater concentration of heat sources in cities lead to higher heat sources in cities lead to higher temperaturestemperatures Runoff is in sewersRunoff is in sewers Much soil is built over or paved overMuch soil is built over or paved over More solar energy is available to heat the air, as less More solar energy is available to heat the air, as less

is used for evaporationis used for evaporation City surfaces also generally have a lower albedoCity surfaces also generally have a lower albedo

– Less reflection yields more absorption and conversion to Less reflection yields more absorption and conversion to heatheat

Heat sources include motor vehicles, space heaters, Heat sources include motor vehicles, space heaters, etc.etc.

– Best developed at night when the air is calm and Best developed at night when the air is calm and the sky is clearthe sky is clear

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Variation of Air TemperatureVariation of Air Temperature Why is it so cold when snow is on the Why is it so cold when snow is on the

ground?ground?– Snow has a relatively high albedoSnow has a relatively high albedo

Less energy absorbed by the surface and converted Less energy absorbed by the surface and converted to heatto heat

– Snow reduces sensible heating of overlying airSnow reduces sensible heating of overlying air Some of the available heat is used to vaporize snowSome of the available heat is used to vaporize snow

– Snow is an excellent infrared radiation emitterSnow is an excellent infrared radiation emitter Nocturnal radiational cooling is extremeNocturnal radiational cooling is extreme

– Especially when skies are clearEspecially when skies are clear– Cooling is enhanced with light winds or calm conditionsCooling is enhanced with light winds or calm conditions

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Variation of Air TemperatureVariation of Air Temperature Cold and warm air advectionCold and warm air advection

– Air mass advectionAir mass advection Horizontal movement of an air mass from one Horizontal movement of an air mass from one

location to anotherlocation to another Cold air advectionCold air advection

– Horizontal movement of colder air into a warmer areaHorizontal movement of colder air into a warmer area– Arrow “A” on the next slideArrow “A” on the next slide

Warm air advectionWarm air advection– Horizontal movement of warmer air into a colder areaHorizontal movement of warmer air into a colder area– Arrow “B” on the next slideArrow “B” on the next slide

Significance of air mass advection to local Significance of air mass advection to local temperature depends on:temperature depends on:

– The initial temperature of the air new massThe initial temperature of the air new mass The degree of modification the air mass receives as it The degree of modification the air mass receives as it

travels over the Earth’s surfacetravels over the Earth’s surface

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Variation of Air TemperatureVariation of Air Temperature