MET 10: Chapter 2

Warming the Earth and AtmosphereDr. Craig Clements

San José State University

TemperatureTemperature 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 Three temperature scales:scales:•KelvinKelvin•CelsiusCelsius•FahrenheitFahrenheit

•What does 0What does 0°° K mean? K mean?

Absolute zero, Absolute zero, molecules would molecules would posses minimum posses minimum amount of energy and amount of energy and no thermal motion.no thermal motion.

°K= °C+273°K= °C+273

HeatHeat is energy in the process of being transferred from one object to another because of the temperature difference between them.

Latent Heat (Hidden Warmth)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)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)Latent Heat (Hidden Warmth)

CondensationCondensation

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 changesPhase changes

Latent HeatLatent 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 There are three modes of energy transmission in the atmosphere.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 transmissionEnergy transmission

ConductioConductionn

ConvectionConvection

During the process of convection, During the process of convection, bubbles of air form at the surface bubbles of air form at the surface and rise. These vertical currents are and rise. These vertical currents are known as known as thermals..

The horizontal motion of air (wind) The horizontal motion of air (wind) carries properties of the air carries properties of the air (temperature, moisture, etc.) with (temperature, moisture, etc.) with it. This is called it. This is called advectionadvection..

In meteorology, the vertical In meteorology, the vertical exchange of heat is called exchange of heat is called convectionconvection

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

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 108 m s-1) or 670,616,630 MPH.

Defined as the crest-to-crest distanceDefined 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 = σT4

λ = w / Tλ = maximum wavelength (μm)w = constant = 0.2897 (μm K)T= temperature of the object (K)

Stefan-Boltzmann Law:

E = radiation emitted (W m-2)σ = Stefan-Boltzmann constant= 5.67 x 10-8 (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 =

0.000001 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

Absorption of Radiation in the Earth’s Atmosphere

Fig 2.11

Incoming solar radiationIncoming 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.

AlbedoAlbedo

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