1. AMS Weather Studies Introduction to Atmospheric Science, 4th Edition
Chapter 4
Heat, Temperature, and Atmospheric Circulation

2. • Case-in-Point - Death Valley
Death Valley has the hottest and driest climate in North America.
The average daily high temperature ranges from 650F in December and January to 1150F in July.
The average daily low temperature ranges from 390F in December and January to 880F in July.
Death Valley continues (temperatures) next

3. Death Valley holds many North American high temperature records; the highest was on 10 July 1913 when the temperature reached 1340F.
The highest temperature ever recorded was at Aziza, Libya on 13 September 1922 when the temperature rose to 1360F.
More on temperatures next

4. Air temperatures are recorded by standard thermometers mounted within instrument shelters. Shelters are located about 4.5 ft above the surface and shield instruments from direct sunshine and precipitation.
Temperatures at ground level are often considerably higher than shelter temperatures. For example, on 15 July 1972 the ground temperatures in Death Valley reached 2010F while the shelter temperature was 1280F.
What causes the extreme temperatures in Death Valley next

5. What causes the extreme heat in Death Valley?
A combination of topographic setting, atmospheric circulation, and intense solar radiation is responsible for the extreme heat and desert conditions of Death Valley.
Topography of Death Valley next

6. More on the topography next

7. The valley is 130 mi long and 6 to 14 mi wide
The valley is 130 mi long and 6 to 14 mi wide. It is surrounded by high steep mountains. At the lowest point, Badwater, the valley is 282 ft below sea level.
The prevailing west to east moist air flow inland from the Pacific Ocean encounters four major mountain ranges before reaching Death Valley.
More on Death Valley next

8. Average annual precipitation is only 1.92 in.
The moisture is rung out of the atmosphere by the time it reaches the valley. Clouds and precipitation form where winds blow upslope and dry conditions prevail where winds blow down slope. After four trips up and down there is little or no moisture left by the time it reaches Death Valley.
Average annual precipitation is only 1.92 in.
Intense solar radiation heats the dry desert surface which consists of rock, soil, and sparse vegetation. The lack of moisture means that most of the heat energy is used to raise the temperature of the ground surface rather than evaporate water.
Blooms in the Valley next

9. On rare occasions when significant rain does fall in Death Valley, a spectacular bloom of colorful wild flowers spreads over the valley floor.
Driving Question: Causes and Consequences of heat transfer in the Earth - Atmosphere next
However, if rainfall is locally heavy, flash flooding is likely. With little protective vegetative cover to anchor the soil, even a brief downpour can cause considerable erosion and property damage.

10. 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 is heat transferred?
How does heat affect atmospheric circulation?
This chapter will answer these questions

11. 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

12. 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

13. 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

14. 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

15. Crickets next
Some electronic thermometer are designed to give remote temperature readings by mounting the sensor at the end of a long cable joined to the instrument. This system has replaced standard liquid-in-glass thermometers at NWS facilities nationwide.

16. Satellite sensor (radiometers) monitor surface temperatures remotely by measuring the intensity of emitted infrared radiation.
Regardless of the type of thermometer they all have to be accurate and have a good response time.
Crickets: One surprisingly accurate method to tell the temperature is to count the chirps of a cricket. Crickets are cold-blooded organisms so their activity depends on air temperature. For temperatures above 54o F (12o C) the number of cricket chirps heard in an 8 second period plus 4 approximate the air temp in degrees C.
Heat Transfer next

17. Heating the Atmosphere
They are still here!
Heating the Atmosphere

18. The deviation from 65 is measured.
Heating Degree Days used to calculate the household energy consumption for heating their homes and offices.
The deviation from 65 is measured.
65 – 35 = 30 degree-days for the day where 35 is the average temperature.
Other measures for temperatures next

19. 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)

20. Radiation
Radiation – is both a form of energy and a means of energy transfer.
It requires no intervening physical medium; that is, it can travel through a vacuum.
Radiation is the principal means whereby the Earth-atmosphere system gains heat from the sun. Radiation is also the principal means whereby heat escapes from the planet to space to maintain a habitable environment.
More on Radiation next

21. Absorption of radiation involves the conversion of electromagnetic energy to heat. All objects both absorb and emit electromagnetic radiation.
If an object absorbs more radiation than it emits its temperature will rise. This is radiational heating.
Radiational cooling is the opposite. If an object emits more radiation than it absorbs it’s temperature will fall.
Conduction and Convection next

22. Conduction Conduction
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
Convection next

23. Convection 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

24. 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

25. Water Phase Changes
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

26. • Thermal Response and Specific Heat
Whatever the method, the transfer of heat from one place to another is accompanied by changes in temperature.
Heat gain - temperature rises
Heat loss - temperature falls
Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1 degree C.
More on specific heat next

27. Water has the greatest specific heat of any naturally occurring substance.
Therefore, the specific heat of all substances is measured relative to that of liquid water. The variation in specific heat between substances implies that different materials have different capacities for storing internal energy.
If you add heat energy to a substance with a low specific heat it will undergo a greater temperature rise than a substance with a high specific heat.
1 calorie of heat energy raises the water 10 C while it raises sand 50 C.
Specific Heat chart next

28. Water’s capacity to store heat next

29. Water‘s exceptional capacity to store heat has important implications for weather and climate, at middle and high latitudes.
A large body of water does not warm as much during the day (or in summer) and does not cool as much at night (or in winter). That is, a large body of water exhibits a greater resistance to temperature change, called thermal inertia, than does a landmass.
Air temperature is regulated to a considerable extent by the temperature of the surface over which the air resides or travels.
Air over a large body of water tends to take on similar temperature characteristics as the surface water.
Difference between maritime and continental climates next

30. 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)

31. • Heat Imbalance: Atmosphere versus Earth’s Surface
Weather is a response to unequal rates of radiational heating and radiational cooling within the Earth-atmosphere system.
Imbalances in rates of radiational heating and cooling from one place to another produce temperature gradients. In response to temperature gradients the atmosphere circulates and thereby redistributes heat.
Chart on incoming and outgoing heat transfer next

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

34. Latent Heating
Some of the absorbed solar radiation is used to vaporize(evaporate) water at Earth’s surface
This energy is released to the atmosphere when clouds form(condensation)
Large amounts of heat are needed for phase changes of water compared to other substances

36. Sensible heat next

37. 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
© AMS

38. Convection transports heat from the Earth's surface into the
troposphere.
Because air is a relatively poor conductor of heat,
convection is much more important than conduction as a heat transfer
mechanism within the troposphere.
More on conduction and convection next

39. fair-weather cumulus).
An example of sensible heat combining with latent heat channeling heat from Earth's surface happens during thunderstorm development.
Updrafts (ascending branches) of vapor-laden air in convection currents often produce cumulus clouds- (puffy, white cotton floating in the sky -
fair-weather cumulus).
If conditions are favorable convection currents can surge to great altitudes with cumulus clouds merging and billowing upward to form towering
cumulonimbus clouds, also known as thunderhead clouds.
Heat transfer at night next

40. Heat transport from the atmosphere to Earth’s surface is the usual situation at night (especially with clear skies) when radiational cooling causes Earth's land surface to become cooler than the overlying air.
Remember-nearly all weather is confined to the troposphere implying that heat transport by sensible and latent heating operates primarily within the lower atmosphere.
Radiational processes dominate heat and temperature distribution above the troposphere.
Heat imbalances Tropics versus mid latitudes next

41. 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
© AMS

42. 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 an 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 than middle latitude cyclones
Heat transport by ocean circulation
Contributes via wind-driven 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

43. The Gulf Stream flows along the East Coast from Florida to the Delaware coast

44. 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
© AMS

45. The rate of heat distribution varies seasonally; when steep temperature gradients prevail the weather is more dynamic. Storm systems are large and intense, winds are stronger, and weather is changeable. Such weather is typical of winter.
When air temperature varies little the weather tends to be more tranquil, and large-scale weather systems are generally weak and ill-defined. This is the case in summer.
Summer may have intense systems but they are usually shorter lived and more localized.
Variations of air temperature next

46. 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
© AMS

47. 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 available to warm the Earth, and consequently, the atmosphere
Relative humidity also affects the ability of evaporation to occur

48. Variation of Air Temperature
Annual Temperature Cycle
Daily Temperature Cycle

49. In the tropics the temperature difference between night and day often is greater than the winter to summer temperature contrast.
At mid and high latitudes, the march of mean monthly temperature lags behind the monthly variation in solar radiation so that the warmest and coldest months of the year typically do not coincide with the times of maximum and minimum solar radiation, respectively.
Seasonally temperature lags next

50. The troposphere's temperature profile takes time to adjust to seasonal change in solar energy input.
Warmest portion of the year occurs 1 month after the summer solstice.
Coldest portion occurs 1 month after the winter solstice.
In the US, the temperature cycle lags behind the solar cycle by about 27 days but in coastal climates it may be up to 36 days.
The winter-to-summer temperature contrast is less in maritime climates.
Ground cover makes a difference next

51. Ground characteristics. Influence heat transfer movement
Over the course of 24 hours there are variations in temperature with the lowest temperatures usually occurring shortly after sunrise.
The day's highest temperature is usually recorded in early to mid afternoon, even though solar radiation peaks around the local solar noon.
Ground characteristics. Influence heat transfer movement
Air over a dry surface warms more than air over a moist or
vegetated surface.
When the surface is dry, absorbed radiation is used primarily for
sensible heating of the air hence the air temperature is higher.
When the surface is moist, much of the absorbed radiation is used to evaporate water, the air temperature is lower.
Urban Heat Island next

52. 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

53. What has happened here?

54. 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 new air mass
The degree of modification the air mass receives as it travels over the Earth’s surface

55. Variation of Air Temperature
Cold Air Advection
Warm Air Advection

56. If cold air advection is extreme, air temperatures may drop precipitously throughout the day, in spite of bright sunny skies.
Air temperatures may climb through the evening hours as a consequence of strong warm air advection, so that the day's high temperature occur at night.
Don't forget about air that moves vertically - air cools as it rises but warms as it descends.
Conclusions next

57. 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
© AMS

58. • Conclusions
Unequal rates of radiational heating and cooling give rise to temperature gradients.
Heat is transferred from warmer to colder localities via radiation, conduction and convection, and latent heating.
Imbalances in radiational heating and radiational cooling within the Earth-atmosphere system are ultimately responsible for the circulation of the atmosphere. The imbalances can either be vertical or horizontal.
Basic Understandings next

59. • Basic Understandings
Temperature is directly proportional to the average kinetic energy of the atoms or molecules composing a substance.
Heat is the name given to energy transferred from a warmer object to a colder object.
Heat flows from locations of higher temperature to locations of lower temperature.
Heat transfer occurs via radiation, conduction and convection, as well as by phase changes of water (latent heating).
More Basic Understandings next

60. All objects absorb and emit radiation
All objects absorb and emit radiation. If absorption exceeds emission the temperature of the object rises, and if emission exceeds absorption the temperature of the object falls.
Convection is the transport of heat within a fluid via motion of the fluid itself and is much more important than conduction in transporting heat within the troposphere.
When water changes phase, heat is either absorbed from or released into the environment.
The winter-to-summer temperature contrast is greater in continental climates than in maritime climates.
Heat is transported from the warmer Earth’s surface to the cooler troposphere via latent heating (vaporization of water followed by cloud development) and sensible heating (conduction plus convection).
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