1. AMS Weather Studies Introduction to Atmospheric Science, 4th Edition
Chapter 4
Heat, Temperature, and Atmospheric Circulation
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2. 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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3. 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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4. 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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5. Temperature Scales
Absolute zero is the temperature at which theoretically all molecular motion ceases and no electromagnetic radiation is emitted
Absolute zero = °F = °C = 0 K
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6. 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 = 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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7. Measuring Air Temperature
Thermometer
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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8. Heat Transfer Processes
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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9. 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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10. Conduction and Convection
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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11. 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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12. Conduction and 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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13. 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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14. Thermal Response and Specific Heat
Temperature change caused by input/output of a specified quantity of heat varies from substance to substance
Specific heat
The amount of heat required to raise 1 gram of a substance 1 Celsius degree
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15. Thermal Inertia
Thermal inertia is a resistance to a change in temperature
A large body of water exhibits a greater resistance to temperature change than land because of difference in specific heat
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16. 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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17. 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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18. Heat Imbalance: Atmosphere vs. Earth’s Surface
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19. © AMS

20. Latent Heating
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 substances
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22. 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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23. 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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24. 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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25. Heat Imbalance: Tropics vs. Middle and High-Latitudes
Heat transport by air mass exchange
North-South exchange of air masses transports sensible heat from the tropics into the middle and high latitudes
The properties of air mass depend on its source region
Air masses modify as they move away from their source region
Heat transport by storm
Tropical storms and hurricanes are greater contributors to poleward heat transport then middle latitude cyclones
Heat transport by ocean circulation
Contributes via wind-drive surface currents and thermohaline circulation
The thermohaline circulation is the density-driven movement of water masses
Transports heat energy, salt, and dissolved gases over great distances and depths
Meridonal overturning circulation (MOC)
At high latitudes, surface waters cool, sink and flow southward as cold bottom water
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26. The Gulf Stream flows along the East Coast from Florida to the Delaware coast
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27. 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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28. 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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29. 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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30. Variation of Air Temperature
Annual Temperature Cycle
Daily Temperature Cycle
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31. 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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32. 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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33. Variation of Air Temperature
Cold Air Advection
Warm Air Advection
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34. Anthropogenic Influence
An urban heat island is an example of anthropogenic influence on the
Earth’s climate
An urban heat island is a city of warmth
surrounded by cooler air
Caused by:
Relative lack of moisture in the city
More available heat from absorbed radiation is used to raise the temperature of city surfaces and less for evaporation of water
Greater concentration of heat sources in a city (cars, air conditioners, etc)
Lower albedo of city surfaces
Building materials conduct heat more readily than soil and vegetation
Develop best on nights when the air is calm and the sky is clear
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