AMS Weather Studies Introduction to Atmospheric Science, 5 th Edition

1. AMS Weather StudiesIntroduction to Atmospheric Science, 5th Edition Chapter 4 Heat, Temperature, & Atmospheric Circulation

2. Driving Question • What are the causes and consequences of heat transfer within the Earth-atmosphere system? • This chapter covers: • Distinguishing temperature and heat • Heat transfer processes • Thermal response and specific heat • Heat imbalances • How does heat affect atmospheric circulation?

3. Case-in-PointExtreme Heat of Death Valley, CA • 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 Cooperative Weather observing station at Furnace Creek Ranch

4. Distinguishing Temperature & Heat • All matter is composed of molecules or particles in continual vibrational, rotational, and/or translational motion. • 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 • 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 warmer object to colder

5. Distinguishing Temperature & Heat • Temperature Scales • Absolute zero • Temperature at which theoretically all molecular motion ceases • No electromagnetic radiation is emitted • Absolute zero = -459.67°F = 273.15°C = 0 K

6. Distinguishing Temperature & Heat • Temperature scales measure 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 = 4.1868 joules • British Thermal Units (BTU) • Amount of energy required to raise 1 pound of water 1 Fahrenheit degree • 1 BTU = 252 cal = 1055 J

7. Distinguishing Temperature & Heat Liquid-in-glass thermometer • Thermometer • Liquid-in-glass thermometer • Uses mercury or alcohol • Bimetallic thermometer • Two strips of metal with different expansion/contraction rates • Electrical resistance thermometer • Thermograph • Measures and records temperature The change of temperature during the passage of a cold front as determined by an electronic thermometer. Bilmetallic thermometer

8. Distinguishing Temperature & Heat • Shielding temperature sensors • Important properties • Accuracy • Response time • Location is important • Ventilated • Shielded from weather Enclosure for the NWS electronic temperature sensor

9. Heat Transfer Processes • Temperature gradient • 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 • Latent heat – phase changes in water

10. Heat Transfer Processes • Radiation • Both a form of energy and a means of energy transfer • Occurs even in a vacuum, such as space • Absorption of radiation by an object causes the temperature of object to rise • Converts electromagnetic energy to heat • Radiational heating • Absorption at greater rate than emission • Radiational cooling • Emission at greater rate than absorption

11. Heat Transfer Processes • Conduction • Transfer of kinetic energy of atoms or molecules by collision between neighboring atoms or molecules • Heat conductivity • Rate of heat transport across an area to a temperature gradient • Some materials have a higher heat conductivity than others • Solids (metal) are better conductors than liquids • Liquids are better than gases (air) • Conductivity impaired by trapped air • Examples: fiberglass insulation, thick layer of fresh snow

12. 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.

13. Heat Transfer Processes • Convection • Consequence of differences in air density • Transport of heat within a substance via movement of substance itself • Substance must liquid or gas • Very important process for transferring heat in atmosphere • The convection cycle • Ascending warm air expands, cools and eventually sinks back to ground

14. Heat Transfer Processes • Latent heating • 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

15. Thermal Response and Specific Heat • Temperature change caused by input/output of a quantity of heat varies among substances • Specific heat • 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.

16. Thermal Response and Specific Heat • Thermal inertia • Resistance to a change in temperature • Large body of water exhibits greater resistance to temperature change than land because of difference in specific heat • Maritime climate • Immediately downwind of the ocean experience much less annual temperature change • Continental climate • Locations well inland experience greater annual temperature change San Francisco, CA, has a maritime climate while St. Louis, MO, has a continental climate.

17. Heat Imbalance: Atmosphere vs. Earth’s Surface • At Earth’s surface • Absorption of solar radiation is greater than emission of IR • In atmosphere • Emission of IR radiation to space is greater than absorption of solar radiation • Therefore, • Earth’s surface has net radiational heating • Atmosphere has net radiational cooling. • So, Earth’s surface transfers heat to the atmosphere, making up difference

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

20. Heat Imbalance: Atmosphere vs. Earth’s Surface • Latent Heating • Some absorbed solar radiation used to vaporize water at Earth’s surface. • Energy released to the atmosphere when clouds form • Comparatively, large amounts of heat needed for phase changes of water • Sensible Heating • Heat transfer via conduction and convection that can be sensed by temperature change and measured by a thermometer

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

22. 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 Produces cumulus clouds If it continues vertically, cumulonimbus clouds form Heat Imbalance: Atmosphere vs. Earth’s Surface

23. Heat Imbalance: Atmosphere vs. Earth’s Surface • Bowen Ratio • Describes how 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

24. Surface energy budget through the course of a year at Yuma, AZ and Madison, WI. • R = net radiation absorbed • H = sensible heating • LE = latent heating • G = storage

25. Heat Imbalance: Tropics vs. Middle and High Latitudes • Earth’s surface unevenly heated due to higher solar altitudes in the tropics than higher latitudes • Causes a temperature gradient, resulting in heat transfer • Poleward heat transport • Air mass exchange • Storms • Ocean currents

26. 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 middle and high latitudes • Air mass properties of depend on source region • Modify as they move • Heat transport by storms • Tropical storms and hurricanes are greater contributors to poleward heat transport than middle latitude cyclones

27. Heat Imbalance: Tropics vs. Middle and High Latitudes • Heat transport by ocean circulation • Contributes via wind-driven surface currents and thermohaline circulation • Thermohaline circulation is density-driven movement of water masses • Transports heat energy, salt, and dissolved gases over great distances and depths • Meridional overturning circulation (MOC) • At high latitudes, surface waters cool and sink, then flow southward as cold bottom water The Gulf Stream flows along the East Coast from Florida to the Delaware coast.

28. Why Weather? • Imbalances in radiational heating/cooling create temperature gradients • Earth’s surface the troposphere • Low and high latitudes • Heat transported in the Earth-atmosphere system to reduce temperature differences • Cause-and-effect chain starts with the Sun, ends with weather • Some solar radiation is absorbed (converted to heat), some to converted to kinetic energy • Causes winds, convection currents, and north-south exchange of air masses • Rate of heat redistribution varies by season • Causes seasonal weather and air circulation changes

29. Variation of Air Temperature • Radiational controls • Factors that affect local radiation budget and air temperature • Time of day and time of the year • Solar altitude and duration of radiation • Cloud cover • Surface characteristics • Annual temperature cycle represents these variations • Annual temperature maximums and minimums do not occur at exact max/min of solar radiation, especially in middle and high latitudes • The atmosphere takes time to heat and cool • Average lag time in US = 27 days • Up to 36 days with the maritime influence

30. Variation of Air Temperature

31. 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, imbalance in favor of incoming vs. outgoing radiation continues so the atmosphere also continues to warm

32. Variation of Air Temperature Daily Temperature Cycle

33. Variation of Air Temperature • Surface cover • Dry soil heats more rapidly than moist • Less energy used to evaporate water • Especially in drought, energy used only to heat soil, soil becomes hotter • Relative humidity also affects evaporation • Snow • High albedo • Less energy absorbed by the surface or 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 • When skies are clear, or light winds or calm conditions

34. Variation of Air Temperature • Air mass advection • Horizontal movement of an air mass from one location to another • Cold air advection (A) • Horizontal movement of colder air into a warmer area • Warm air advection (B) • Horizontal movement of warmer air into a colder area • Significance of air mass advection to local temperature • Initial temperature of the air new mass • Degree of modification the air mass as travels over the Earth’s surface

35. Variation of Air Temperature • Urban heat island effect • City of warmth surrounded by cooler air • In a city: • Relative lack of moisture • Absorbed heat raises temperature (not for evaporation) • Greater concentration of heat sources (cars, air conditioners, etc) • Multiple reflections and lower albedo • Building materials conduct heat more readily than soil and vegetation • Develops best on nights when air is calm and sky is clear

36. Variation of Air Temperature Providence, RI 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. Buffalo, NY