Temperature Scales

Transcription

1 TEMPERATURE is a measure of the internal heat energy of a substance. The molecules that make up all matter are in constant motion. By internal heat energy, we really mean this random molecular motion. Molecular motion is therefore the reason any substance has a temperature. The more the molecules that make up a substance move, the the higher its temperature.

2 Temperature Scales

3 HEAT TRANSFER can be accomplished through four means: (1) Conduction: fast-moving molecules of warmer substance 1 collide with neighboring molecules of colder substance 2, which are moving more slowly. This forces the molecules of the colder substance to speed up. Substance 2 becomes hotter as a result of its physical contact with substance 1. This form of heat transfer often occurs between the atmosphere and the earth s surface and is also known as sensible heat flux.

4 HEAT TRANSFER can be accomplished through four means: (2) Phase changes: A liquid evaporates into an overlying gas, a process which requires energy and therefore removes heat from the liquid. This often occurs between the atmosphere and earth s surface. It is known as a latent heat flux. The dryness of the desert surface means it can t cool much through latent heat flux and therefore must cool almost exclusively through sensible heat flux. Inefficient evaporation of the desert surface is one reason deserts are so hot.

5 HEAT TRANSFER can be accomplished through four means: (3) Convection: Typically occurs when a liquid or gas is heated from below. The heated portion becomes lighter and rises, being replaced by heavier, and cooler liquid or gas. This redistribution of heat occurs in both the atmosphere and the ocean.

6 HEAT TRANSFER can be accomplished through four means: (4) Radiation: The radiation emanating from substance 1 encounters substance 2, which absorbs the radiation. The absorbed radiation heats substance 2.

7 RADIATION is a wave that moves through space at a constant speed: 300,000,000 m/s in a vacuum This wave is analogous to the ripples on a pond that propagate when the pond s surface is disturbed by a rock. The difference is that instead of waves of water propagating through space, radiation involves waves of an electromagnetic field. Radiation comes in many forms... radio waves microwaves heat from a fire light Ultraviolet rays X-rays Gamma rays

8 The various forms of radiation are distinguished by their wavelength, the distance between successive crests of the wave. (a) has a long wavelength (b) has a short wavelength The longer the wavelength, the less energetic, so that (a) is less energetic than (b).

9 n=c/l n=frequency l=wavelength c=speed of light Energy is proportional to frequency

10 The various forms of radiation are organized according to their wavelengths (and hence energy levels), creating the electromagnetic spectrum. Visible light

11 Objects constantly emit radiation according to their temperature. Objects that emit with 100% efficiency are called blackbodies, and have a distribution of wavelengths of emitted radiation is given by the Planck function, which has a characteristic shape: This curve is for an object with a temperature of about 5800K, the approximate temperature of the sun.

12 the distribution s peak wavelength... is inversely proportional to the temperature of the object l max =2898/T (Wien s Law) T must be in Kelvin! The hotter the object, the shorter the typical emission wavelengths

13 The total energy emitted by the object is the area under the curve... and is proportional to the fourth power of the object s temperature (=s T 4 ). This relationship is known as the Stefan-Boltzmann law. T must be in Kelvin!

14 The wavelength distributions of the radiation emitted by the sun and the earth are very different, because the sun is so much hotter than the earth. Here the curves are normalized! The Planck functions for temperatures characteristic of the sun and the earth. The peak wavelength of the sun s distribution is at about 0.5 microns (green light), while the peak wavelength for the earth s distribution is at about 10 microns (infrared radiation).

15 Because there is little overlap in wavelength between the radiation emitted by the earth and the radiation that reaches Earth from the sun, radiation on Earth may be separated in two wavelength bands: One known as Solar radiation or shortwave radiation. and another called terrestrial radiation or longwave radiation. The separation point is 3 microns

16 A poker placed in a hot fire glows different colors depending on its temperature. Objects that emit terrestrial radiation may still appear to have color, but in this case the color is normally unrelated to temperature. Here color is determined by how visible light is absorbed, scattered, and/or reflected. Turn on a red lamp in an otherwise dark room and blue objects appear black.

17 A blackbody is something that emits and absorbs all the radiation that falls on it with 100% efficiency. Because at typical Earth temperatures an object s color is unrelated to the radiation it emits, a good blackbody can have any color. Snow emits and absorbs radiation with nearly 100% efficiency.

18 The total flux of energy transferred from one object to another varies according to the distance between the two objects. This relationship is known as the inverse-square law. Flux is proportional to 1/d 2 We expect the sunshine a planet receives to decrease as the distance from the sun increases.

19 The radiation flux can also vary because of the angle between the surface intercepting the radiation and the direction of the radiation s propagation. The more oblique the angle, the less energy is received. This is why the poles are cooler than the tropics (not because its dark a lot).

20 The earth also reflects solar radiation. The reflectivity of the planet or planetary albedo of the earth is about 0.3, meaning that about 30% of the incoming solar flux is reflected back to space. Certain regions are typically much more reflective than others.

21 Absorbed solar flux by planet =(1-A)S/4 S=Solar constant, the solar flux at the top of a planet s atmosphere (S=1370 Wm 2 on Earth) A=Planetary albedo Disc of Earth exposed to sunlight pr 2 is shared by area of Earth s surface 4pr 2 So the average solar flux is S/4

22 Planetary Energy Balance st e 4 = (1-A)S/4 Outgoing Terrestrial Radiation must balance Net Solar Radiation at the top of the atmosphere when the climate is in equilibrium T e is the effective radiating temperature (if planet were a blackbody from space or had no atmosphere)

23 The greenhouse effect of Venus The average solar flux over the surface of Venus is approximately 661 W/m 2. Venus is very reflective of sunshine. In fact, it has a reflectivity (or albedo) of 0.8, so the planet absorbs approximately 661 X 0.2 = 132 W/m 2. By assuming that the incoming radiation equals the outgoing radiation (energy balance), we can convert this into an effective radiating temperature by invoking the Stefan-Boltzmann law (total energy = s T 4 ). We find that T e =220K. But Venus surface has a temperature of T s =730K!!! The explanation for this huge discrepancy is the planet s greenhouse effect.

24 The greenhouse effect of Earth The average solar flux over the surface of Earth is approximately 343 W/m 2. The earth has a much lower albedo than Venus (0.3), so the planet absorbs approximately 343 X 0.7 = 240 W/m 2. By assuming that the incoming radiation equals the outgoing radiation, we can convert this into an effective radiating temperature by invoking the Stefan-Boltzmann law (total energy = s T e4 ). We find that T=255K. Earth s surface has a temperature of T s =288K While much smaller than Venus greenhouse effect, earth s is crucial for the planet s habitability. Without the greenhouse effect, the temperature today in Seattle would be about 0 degrees Fahrenheit.

25 S/4 (S/4)A st e 4 1-layer Atmosphere st s 4 =2sTe 4 (S/4)(1-A) st s 4 st e 4 Earth st s 4 =(S/4)(1-A)+sTe 4 T s =2 1/4 T e

26 Two reasons why this one-layer atmosphere is wrong Greenhouse gases are not blackbodies The real atmosphere has a vertical temperature gradient

27 Main Constituents of the Earth s Atmosphere Nitrogen 78% Oxygen 21% Argon 1% Water Vapor 0-4% Carbon Dioxide 0.036% (increasing) Nitrogen, Oxygen, and Argon contribute little to the greenhouse effect. Water vapor and carbon dioxide contribute a lot even though their concentration is low.

28 A greenhouse gas is defined as a gas that absorbs significantly the radiation emitted by the earth and its atmosphere. Important Greenhouse Gases (concentrations in parts per million volume) water vapor ,000 carbon dioxide 360 methane 1.7 nitrous oxide 0.3 ozone 0.01 chlorofluorocarbons ~0.0007

29 Why do certain gases interact with radiation? When radiation impinges on a molecule, it can excite the molecule, either by vibrating or rotating it. Molecules of a particular kind of gas have a different shape from molecules of another type of gas, and so are excited by radiation in different ways. CH 4 H 2 O CO 2

30 Because of their varying geometries and sizes, different molecules absorb radiation of different wavelengths. For example, CO 2 tends to absorb radiation of a wavelength of 15 microns (this wavelength excites bending vibration of the CO 2 molecule), whereas H 2 O tends to absorb at wavelengths around 12 microns (rotation of the H 2 O molecule). CH 4 H 2 O CO 2

31 Molecules with more than two atoms tend to absorb radiation more effectively than diatomic molecules such as N 2 and O 2. These molecules are too symmetric - they don t bend well. This is why diatomic nitrogen and oxygen are not greenhouse gases. CH 4 O 2 H 2 O CO 2

32 Note that the separation between the solar and terrestrial spectra occurs at about 3 microns

33 Interaction of the atmosphere with radiation

34 So how does this create a greenhouse effect? The greenhouse effect occurs because the atmosphere is relatively transparent to the wavelengths of solar radiation, while it absorbs infrared radiation. So a large chunk of the sun s radiation makes it to the earth s surface. At the same time, the atmosphere containing greenhouse gases absorbs the radiation emitted by the earth s surface, and re-emits it back to the surface, increasing the total energy the surface receives. This forces the earth s surface to become warmer than it would be otherwise.

35 Thermal structure of the atmosphere Keep in mind: 90% of the atmosphere s mass is in the troposphere.

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37 The greenhouse effect is a naturallyoccurring phenomenon on the earth as it is on Venus. The enhancement of this effect by increasing greenhouse gases is the reason for concern about climate change.