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

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 © AMS

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

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

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

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

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

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) © AMS

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

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 © AMS

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

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 © AMS

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

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

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

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

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

Heat Imbalance: Atmosphere vs. Earth’s Surface © AMS

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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

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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

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

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

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 © AMS

The Gulf Stream flows along the East Coast from Florida to the Delaware coast © AMS

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

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

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

Variation of Air Temperature Annual Temperature Cycle Daily Temperature Cycle © AMS

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

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 © AMS

Variation of Air Temperature Cold Air Advection Warm Air Advection © AMS

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