MET 10: Chapter 2 Warming the Earth and Atmosphere Dr. Craig Clements San José State University

Temperature Is the measure of the average speed or thermal energy of the particles in a substance. (How fast the atoms/molecules are moving)

Three temperature scales: KelvinKelvin CelsiusCelsius FahrenheitFahrenheit What does 0° K mean?What does 0° K mean? Absolute zero, molecules would posses minimum amount of energy and no thermal motion. °K= °C+273

Heat is energy in the process of being transferred from one object to another because of the temperature difference between them. Latent Heat (Hidden Warmth) Water vapor is an invisible gas that becomes visible when it changes into larger liquid or solid particles (such as ice). This process of transformation is known as a change of state or phase change. The heat energy required to change a substance such as water from one state to another is called latent heat.

Latent Heat (Hidden Warmth) Example: cooling produced by evaporating water Examine a small drop of water. At drop’s surface water molecules are constantly escaping (evaporating). Because the more energetic, fast-moving molecules escape most easily, the average motion of all the molecules left behind decreases. Since temperature is a measure of the average molecular motion, the slower motion suggests a lower water temperature. Evaporation is a cooling process

Evaporation is a cooling process because the energy needed to evaporate the water– to change its phase from liquid to a gas– comes from the water. The energy lost during evaporation can be thought of as carried away by the water vapor molecule. The energy is thus “stored” or “hidden” and we call this latent heat. Latent Heat (Hidden Warmth)

Condensation Opposite of evaporation is condensation. Condensation is a process where a gas changes into a liquid. Is condensation a cooling process or warming process? Warming.

Phase changes

Latent Heat Latent heat is an important source of atmospheric energy. Once the vapor molecules become separated from the earth’s surface, they are transported by the wind. Rising to high altitudes where the air is cold, the vapor changes into liquid and ice cloud particles. During these processes, a tremendous amount of heat energy is released into the environment.

There are three modes of energy transmission in the atmosphere.  Conduction: the transfer of energy in a substance by means of molecular excitation without any net external motion.  Convection: the transfer of energy by mass motions within a fluid or gas, resulting in actual transport of energy.  Radiation: the propagation of electromagnetic waves through space. Energy transmission

Conduction

Convection

During the process of convection, bubbles of air form at the surface and rise. These vertical currents are known as. During the process of convection, bubbles of air form at the surface and rise. These vertical currents are known as thermals. The horizontal motion of air (wind) carries properties of the air (temperature, moisture, etc.) with it. This is called advection. In meteorology, the vertical exchange of heat is called convection

Photographer unknown The sport of paragliding is dependent on thermals

The Rising and Sinking Air Game Take an invisible “blob” called an air parcel. This air parcel is wrapped with imaginary elastic, plastic like a balloon. The parcel can expand and contract freely. But…neither external air nor heat is able to mix with the air inside. Also, as the air parcel moves, it does not break apart, but remains as a single unit. At the earth’s surface the parcel has the same temperature and pressure as the surrounding air.

The Rising and Sinking Air Game Suppose we lift the parcel. Remember air pressure always decreases as we move up into the atmosphere. As the parcel rises, it enters a region where the surrounding air pressure is lower. To equalize the pressure, the parcel molecules inside push the parcel walls outward, expanding it. The molecules use some of their own energy to expand the parcel. This energy loss shows up as slower molecular speeds = lower parcel temperature. Air that rises always expands and cools!

The Rising and Sinking Air Game

If the parcel is lowered, it returns to a region where the air pressure is higher. The higher outside pressure squeezes (compresses) the parcel back to its original (smaller) size. Because the air molecules have a faster rebound velocity after striking the sides of the a collapsing parcel, the average speed of the molecules goes up. This increase in molecular speed represents a warmer parcel temperature. Therefore, Air that sinks always warms by compression!

Electromagnetic radiation  Radiation is the transfer of energy by rapid oscillations of electromagnetic fields.  The most important general characteristic is its wavelength ( ), ______________________________.  Radiation travels through space at the speed of light (3 x 10 8 m s -1 ) or 670,616,630 MPH. Defined as the crest-to-crest distance

Radiation  What emits radiation? –All objects with a temperature greater than 0°K emit some type of radiation (energy)  Examples:  Radiation laws: –Warmer objects emit more intensely than cold objects. (Stefan-Boltzmann Law) –Warmer objects emit a higher proportion of their energy at short wavelengths than cold objects. (Wien’s Law)

Wien’s Law: E = σT 4 λ = w / T λ = maximum wavelength (μm) w = constant = (μm K) T= temperature of the object (K) Stefan-Boltzmann Law: E = radiation emitted (W m -2 ) σ = Stefan-Boltzmann constant= 5.67 x (W m -2 K -4 ) T= temperature of the object (K)

Review questions  Considering the previous discussion –Which object would emit more (intensity) radiation: Earth or Sun? –If you were examining the radiation emitted by both the Sun and Earth, which would have a longer wavelength? –What wavelength radiation are you emitting right now? Sun Earth infrared

Solar Radiation (Sunlight)  Sunlight is primarily made up of the following: –Visible Light (44%) –Infrared Radiation (48%) –Ultraviolet Radiation (7%) Unit: 1  m = m

Terrestrial or Longwave Radiation  Planets mainly emit infrared radiation  Radiation emitted by planets occurs mainly at wavelengths _____ than those contained in solar radiation Solar Radiation (“Shortwave”) Terrestrial Radiation (“Longwave”) longer

Solar vs. Terrestrial Radiation  The sun is much hotter than planets; therefore, sunlight consists of shorter wavelengths than planetary radiation ;  Thus …

Energy from the Sun  Obviously, the Sun provides the Earth with it’s energy. The question is, how much of the Sun’s energy does the Earth get?  Sun’s energy is either –Scattered (reflected away) or –Absorbed  Scattering happens by bouncing off –Particles in the atmosphere –Earth’s surface  Absorption happens when certain gases absorb the energy –The reality is the only certain gases absorb certain wavelengths.

Absorption of radiation  Absorption of shortwave radiation by atmospheric gas molecules is fairly weak; –most absorption of shortwave radiation occurs at the Earth’s surface.  Most gases do not interact strongly with longwave radiation, however –Greenhouse gas molecules absorb certain wavelengths of longwave radiation.

Absorption of Radiation in the Earth’s Atmosphere

Fig 2.11

Incoming solar radiation  Each ‘beam’ of incoming sunlight can be either: –Reflected back to space:  Clouds  Atmosphere  Surface –Or absorbed; either by atmosphere (e.g. clouds or ozone) or Earth’s surface. Albedo

Recap  ______________ radiation comes from the sun and is composed of both ultraviolet and visible radiation  __________________ radiation comes from the Earth and is composed of infrared radiation  Recall that everything (above a temperature of 0K) emits some type of radiation (energy) with a particular wavelength. Shortwave or solar Longwave, terrestrial or infrared

Review - sensors that measure radiation  A _________________ measures solar radiation.  A__________________ measures infrared radiation (terrestrial) that comes from the Earth. Pyranometer Pyrgeometer