Weather Studies Introduction to Atmospheric Science American Meteorological Society

1. Weather StudiesIntroduction to Atmospheric ScienceAmerican Meteorological Society Chapter 4 Heat, Temperature, and Atmospheric Circulation Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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

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

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

5. Temperature Scales • Absolute zero is the temperature at which theoretically all molecular motion ceases and no electromagnetic radiation is emitted • Absolute zero = -459.67°F = 273.15°C = 0 K

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 = 4.1868 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

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

8. Heat Transfer • 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)

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

10. Conduction and Convection • 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

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

12. Conduction and 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

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

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 Note – Water has a higher specific heat than Earth substances. This is an important aspect of weather.

15. Specific Heat • Specific heat is the reason the sand is hotter than the water Consider the role specific heat plays In continental vs. maritime climates – see next slide

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)

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

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

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

20. Latent Heating Latent heat of vaporization • 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 Latent heat of fusion

21. 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

22. 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

23. 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

24. Role of Gulf Stream in Poleward Heat Transport • The ocean contributes to poleward heat transport via wind-driven surface currents and deeper conveyor-belt-like currents that traverse the lengths of the ocean basins • Warm surface currents like the Gulf Stream are a heat source for the atmosphere – they flow from the tropics into middle latitudes and supply heat to the cooler mid-latitude troposphere Gulf Stream

25. The Ocean Conveyor Belt SystemContributes to Heat Transfer from Low Latitudes to High Latitudes

26. 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

27. 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

28. 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

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

30. Variation of Air Temperature • The Urban heat island • Lack of moisture and greater concentration of heat sources in cities lead to higher temperatures • Runoff is in sewers • Much soil is built over or paved over • More solar energy is available to heat the air, as less is used for evaporation • City surfaces also generally have a lower albedo • Less reflection yields more absorption and conversion to heat • Heat sources include motor vehicles, space heaters, etc. • Best developed at night when the air is calm and the sky is clear

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

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

33. Variation of Air Temperature