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Driving Question
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What are the causes and consequences of heat transfer within the Earth-atmosphere system?
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This chapter covers:
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Distinguishing temperature and heat
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Heat transfer processes
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Thermal response and specific heat
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Heat imbalances
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How does heat affect atmospheric circulation?
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Death Valley – Hottest and driest place in North America
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134°F in 1913
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2nd highest temperature ever recorded on Earth
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Summer 1996
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40 successive days over 120°F
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105 successive days over 110°F
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Causes:
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Topographic setting
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Atmospheric circulation
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Intense solar radiation
Cooperative Weather observing station at Furnace Creek Ranch
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Distinguishing Temperature & Heat
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All matter is composed of molecules or particles in continual vibrational, rotational, and/or translational motion.
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Energy represented by this motion is called kinetic energy.
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Temperature
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Directly proportional to the average kinetic energy of atoms or molecules composing a substance
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Internal energy
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Encompasses all the energy in a substance
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Includes kinetic energy
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Includes potential energy, arising from forces between atoms/molecules
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Heat is energy in transit
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When two substances are brought together with different kinetic energy, energy is always transferred from warmer object to colder
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Distinguishing Temperature & Heat
5
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Absolute zero
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Temperature at which theoretically all molecular motion ceases
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No electromagnetic radiation is emitted
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Absolute zero
= -459.67
° F
= 273.15
° C
= 0 K
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Distinguishing Temperature & Heat

Temperature scales measure degree of hotness or coldness
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Calorie
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Amount of heat required to raise temperature of 1 gram of water 1 Celsius degree
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Different from “food” calorie, which is actually 1 kilocalorie
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Joule
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More common in meteorology today

1 calorie = 4.1868 joules
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British Thermal Units (BTU)
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Amount of energy required to raise 1 pound of water 1 Fahrenheit degree

1 BTU = 252 cal = 1055 J
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Distinguishing
Temperature & Heat
Liquid-in-glass thermometer

Thermometer
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Liquid-in-glass thermometer
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Uses mercury or alcohol
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Bimetallic thermometer
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Two strips of metal with different expansion/contraction rates
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Electrical resistance thermometer

Thermograph
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Measures and records temperature
The change of temperature during the passage of a cold front as determined by an electronic thermometer.
Bilmetallic thermometer
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Distinguishing Temperature & Heat

Shielding temperature sensors

Important properties

Accuracy

Response time
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Location is important
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Ventilated
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Shielded from weather
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Enclosure for the NWS electronic temperature sensor
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Heat Transfer Processes


Change in temperature over distance
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Example: the hot equator and cold poles
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This is the 2 nd law of thermodynamics
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Heat flows toward lower temperature so as to eliminate the gradient
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Radiation
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Conduction
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Convection
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Latent heat – phase changes in water
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Heat Transfer Processes
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Both a form of energy and a means of energy transfer
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Occurs even in a vacuum, such as space
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Converts electromagnetic energy to heat
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Radiational heating
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Absorption at greater rate than emission
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Radiational cooling
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Emission at greater rate than absorption
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Heat Transfer Processes
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Conduction
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Transfer of kinetic energy of atoms or molecules by collision between neighboring atoms or molecules
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Heat conductivity
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Rate of heat transport across an area to a temperature gradient
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Some materials have a higher heat conductivity than others
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Solids (metal) are better conductors than liquids
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Liquids are better than gases (air)
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Conductivity impaired by trapped air
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Examples: fiberglass insulation, thick layer of fresh snow
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Heat Transfer Processes
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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Heat Transfer Processes
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Convection
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Consequence of differences in air density
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Transport of heat within a substance via movement of substance itself
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Substance must liquid or gas
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Very important process for transferring heat in atmosphere
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The convection cycle
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Ascending warm air expands, cools and eventually sinks back to ground
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Heat Transfer Processes
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Latent heating
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Movement of heat from one location to another due to phase changes of water
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Example: evaporation of water, movement of vapor by winds, condensation elsewhere
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Temperature change caused by input/output of a quantity of heat varies among substances
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Specific heat
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The amount of heat required to raise 1 gram of a substance
1 Celsius degree
The contrast in specific heat is one reason why the sand is hotter than the water.
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Thermal inertia
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Resistance to a change in temperature
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Large body of water exhibits greater resistance to temperature change than land because of difference in specific heat
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Maritime climate
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Immediately downwind of the ocean experience much less annual temperature change
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Continental climate
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Locations well inland experience greater annual temperature change 16
San Francisco, CA, has a maritime climate while
St. Louis, MO, has a continental climate.
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
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At Earth’s surface
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Absorption of solar radiation is greater than emission of IR
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In atmosphere
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Emission of IR radiation to space is greater than absorption of solar radiation
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Therefore,
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Earth’s surface has net radiational heating
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Atmosphere has net radiational cooling.
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So, Earth’s surface transfers heat to the atmosphere, making up difference
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
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Heat Imbalance:
Atmosphere vs. Earth’s Surface

Latent Heating
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Some absorbed solar radiation used to vaporize water at Earth’s surface.
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Energy released to the atmosphere when clouds form
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Comparatively, large amounts of heat needed for phase changes of water
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Sensible Heating
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Heat transfer via conduction and convection that can be sensed by temperature change and measured by a thermometer
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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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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
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Produces cumulus clouds
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If it continues vertically, cumulonimbus clouds form
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Heat Imbalance:
Atmosphere vs. Earth’s Surface
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Bowen Ratio
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Describes how energy received at the Earth’s surface is partitioned between sensible heating and latent heating
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Bowen ratio =
[(sensible heating)/(latent heating)]
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At the global scale, this is
[(7 units)/(23 units)] = 0.3
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Surface energy budget through the course of a year at
Yuma, AZ and Madison, WI.
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R = net radiation absorbed
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H = sensible heating
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LE = latent heating
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G = storage
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25
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Earth’s surface unevenly heated due to higher solar altitudes in the tropics than higher latitudes
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Causes a temperature gradient, resulting in heat transfer
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Poleward heat transport
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Air mass exchange
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Storms
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Ocean currents
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Heat Imbalance: Tropics vs.
Middle and High Latitudes
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Heat transport by air mass exchange
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North-south exchange of air masses transports sensible heat from the tropics into middle and high latitudes
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Air mass properties of depend on source region
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Modify as they move
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Heat transport by storms
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Tropical storms and hurricanes are greater contributors to poleward heat transport than middle latitude cyclones
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Heat Imbalance: Tropics vs.
Middle and High Latitudes
The Gulf Stream flows along the East Coast from Florida to the Delaware coast.
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Heat transport by ocean circulation
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Contributes via wind-driven surface
27 currents and thermohaline circulation
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Thermohaline circulation is densitydriven movement of water masses
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Transports heat energy, salt, and dissolved gases over great distances and depths
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Meridional overturning circulation
(MOC)
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At high latitudes, surface waters cool and sink, then flow southward as cold bottom water
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Why Weather?
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Imbalances in radiational heating/cooling create temperature gradients
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Earth’s surface the troposphere
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Low and high latitudes
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Heat transported in the Earth-atmosphere system to reduce temperature differences
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Cause-and-effect chain starts with the Sun, ends with weather
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Some solar radiation is absorbed (converted to heat), some to converted to kinetic energy
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Causes winds, convection currents, and north-south exchange of air masses
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Rate of heat redistribution varies by season
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Causes seasonal weather and air circulation changes
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Variation of Air Temperature
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Radiational controls
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Factors that affect local radiation budget and air temperature
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Time of day and time of the year
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Solar altitude and duration of radiation
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Cloud cover
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Surface characteristics
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Annual temperature cycle represents these variations
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Annual temperature maximums and minimums do not occur at exact max/min of solar radiation, especially in middle and high latitudes
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The atmosphere takes time to heat and cool
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Average lag time in US = 27 days
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Up to 36 days with the maritime influence
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Variation of Air Temperature
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Variation of Air Temperature
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Lowest temperature usually occurs just after sunrise
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Based on radiation alone, minimum temperature would occur after sunrise when incoming radiation becomes dominant
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Highest temperature usually occurs in the early to middle afternoon
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Even though peak of solar radiation is around noon, imbalance in favor of incoming vs. outgoing radiation continues so the atmosphere also continues to warm
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Variation of Air Temperature
Daily Temperature Cycle
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Variation of Air Temperature
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Dry soil heats more rapidly than moist
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Less energy used to evaporate water
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Especially in drought, energy used only to heat soil, soil becomes hotter
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Relative humidity also affects evaporation
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Snow
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High albedo
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Less energy absorbed by the surface or converted to heat
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Snow reduces sensible heating of overlying air
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Some of the available heat is used to vaporize snow
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Snow is an excellent infrared radiation emitter
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Nocturnal radiational cooling is extreme
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When skies are clear, or light winds or calm conditions
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Variation of Air Temperature
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Air mass advection
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Horizontal movement of an air mass from one location to another
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Cold air advection (A)
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Horizontal movement of colder air into a warmer area
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Warm air advection (B)
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Horizontal movement of warmer air into a colder area
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Significance of air mass advection to local temperature
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Initial temperature of the air new mass
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Degree of modification the air mass as travels over the Earth’s surface
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Variation of Air Temperature
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City of warmth surrounded by cooler air

In a city:
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Relative lack of moisture

Absorbed heat raises temperature (not for evaporation)

Greater concentration of heat sources (cars, air conditioners, etc)
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Multiple reflections and lower albedo
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Building materials conduct heat more readily than soil and vegetation
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Develops best on nights when air is calm and sky is clear
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Variation of Air Temperature
Providence, RI
Buffalo, NY
Satellite-produced maps of Providence, RI (top) and Buffalo, NY (bott0m) highlighting the role that differences in development patterns/vegetation cover can have on a city’s urban heat island. Providence has a significantly stronger heat island signature.
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