Weather Studies Introduction to Atmospheric Science American Meteorological
- Slides: 33
Weather Studies Introduction to Atmospheric Science American 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
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: § 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 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 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 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 6
§ Thermometer Measuring Air Temperature – 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 7
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 2 nd 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) 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 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 § 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 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 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 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 Note – Water has a higher specific heat than Earth substances. This is an important aspect of weather – The amount of heat required to raise 1 gram of a substance 1 Celsius degree 14
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 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) 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 17
Heat Imbalance: Atmosphere vs. Earth’s Surface 18
Heat Imbalance: Atmosphere vs. Earth’s Surface 19
Latent Heating Latent heat of vaporization Latent heat of fusion § 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 20
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 21
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 22
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 23
G ul f St r ea m 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 24
The Ocean Conveyor Belt System Contributes to Heat Transfer from Low Latitudes to High Latitudes 25
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 26
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 27
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 28
Variation of Air Temperature Annual Temperature Cycle Daily Temperature Cycle 29
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 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 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: – 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 33
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