AMS Weather Studies Introduction to Atmospheric Science 4
- Slides: 34
AMS Weather Studies Introduction to Atmospheric Science, 4 th Edition Chapter 4 Heat, Temperature, and Atmospheric Circulation © AMS 1
Case-in-Point § Death Valley – Hottest and driest place in North America – 134°F in 1913 § 2 nd highest temperature ever recorded on Earth – Summer 1996 § 40 successive days over 120°F § 105 successive days over 110°F – Causes: § § § © AMS Topographic setting Atmospheric circulation Intense solar radiation 2
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 © AMS 3
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 © AMS 4
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 © AMS 5
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 © AMS 6
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 © AMS 7
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 2 nd law of thermodynamics § Heat flows toward lower temperature so as to eliminate the gradient § Heat flows/transfers in the atmosphere – – – © AMS – Radiation Conduction Convection Phase changes in water (latent heat) 8
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 © AMS 9
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 © AMS § Examples – fiberglass insulation and thick layer of fresh snow 10
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 © AMS 11
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 © AMS 12
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 © AMS 13
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 © AMS 14
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 © AMS 15
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) © AMS 16
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 © AMS 17
Heat Imbalance: Atmosphere vs. Earth’s Surface © AMS 18
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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 © AMS 20
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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 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 © AMS 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: © AMS § § § Air mass exchange Storms Ocean currents 24
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) © AMS § At high latitudes, surface waters cool, sink and flow southward as cold 25 bottom water
The Gulf Stream flows along the East Coast from Florida to the Delaware coast © AMS 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 © AMS 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 © AMS – 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 © AMS 29
Variation of Air Temperature Annual Temperature Cycle © AMS Daily Temperature Cycle 30
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 © AMS 31
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: © AMS – The initial temperature of the air new mass § The degree of modification the air mass receives as it travels over the Earth’s surface 32
Variation of Air Temperature A. Cold Air Advection B. Warm Air Advection © AMS 33
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 34
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