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1 st Law of Thermodynamics Heat Transfer Lecture 4 February 15, 2010 1020 1016 1016 1020 To convert from Z time to CST, subtract 6 hours. 05Z = 11 PM CST 1020 1020 1024 1016 1020 1016 1024 1020 1016 Review from last week

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1st Law of ThermodynamicsHeat Transfer

Lecture 4

February 15, 2010


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1020

1016

1016

1020

To convert from Z time to CST, subtract 6 hours. 05Z = 11 PM CST

1020

1020


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1024

1016

1020

1016

1024

1020

1016


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Review from last week

In a cold cloud, all precipitation begins in the form of snow (ice crystals)

  • 5 Main Precipitation Types

    1. Rain drops of liquid water

    2. Snow  ice crystals

    3. Sleet frozen rain drops

    4. Freezing Rain  rain the freezes on contact with a cold surface

    5. Hail  large pieces of ice

    How do we get this variety if the origin of the precipitation is the same?


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Snow

  • The surface temperature is 25°F (-4°C) and increases with height before decreasing.

  • However, since the temperature remains below freezing at every height, any precipitation that falls will remain as snow.

http://www.srh.noaa.gov/jetstream//synoptic/precip.htm


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  • Flurries -  Light snow falling for short durations. No accumulation or light dusting

  • Showers - Snow falling at varying intensities for brief periods of time. Some accumulation is possible.

  • Squalls -  Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant. Snow squalls are best known in the Great Lakes region.

  • Blowing Snow - Wind-driven snow that reduces visibility and causes significant drifting.

  • Blizzard - Winds over 35 mph with snow and blowing snow reducing visibility to less than ¼ mile for more than 3 hours. 


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Sleet

  • Surface is below freezing

  • As snow falls into the layer of air where the temperature is above freezing, the snow flakes partially melt.

  • As the precipitation reenters the air that is below freezing, the precipitation will re-freeze into ice pellets that bounce off the ground, commonly called sleet.

  • The most likely place for freezing rain and sleet is to the north of warm fronts. The cause of the wintertime mess is a layer of air above freezing aloft.

http://www.srh.noaa.gov/jetstream//synoptic/precip.htm


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Freezing Rain

  • Freezing rain will occur if the warm layer in the atmosphere is deep with only a shallow layer of below freezing air at the surface.

  • The precipitation can begin as either rain and/or snow but becomes all rain in the warm layer.

  • The rain falls back into the air that is below freezing but since the depth is shallow, the rain does not have time to freeze into sleet.

  • Upon hitting the ground or objects such as bridges and vehicles, the rain freezes on contact.

http://www.srh.noaa.gov/jetstream//synoptic/precip.htm


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Energy

  • Energy is the ability or capacity to do work on some form of matter

  • Work is done on matter when matter is either pushed, pulled, or lifted over some distance

  • Potential energy – how much work that an object is capable of doing

    PE = mgh

  • Kinetic energy – the energy an object possesses as a result of its motion

    KE = ½ mv2


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Laws of Thermodynamics

  • 1st Law of Thermodynamics – Energy cannot be created or destroyed.

    • Energy lost during one process must equal the energy gained during another

  • 2nd Law of Thermodynamics – Heat can spontaneously flow from a hotter object to a cooler object, but not the other way around. 

  • The amount of heat lost by the warm object is equivalent to the heat gained by the cooler object


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First Law of Thermodynamics

  • Conservation of energy:

    • q = Δe + w

  • The amount of heat (q) added to a system is equal to the change in internal energy (Δe) of the system plus any work (w) done by the system


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Heat

  • Heat is a form of energy and is the total internal energy of a substance

  • Therefore the 1st law states that heat is really energy in the process of being transferred from a high temperature object to a lower temperature object.

  • Heat transfer changes the internal energy of both systems involved

  • Heat can be transferred by:

    • Conduction

    • Convection

    • Advection

    • Radiation


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Specific Heat

  • Heat capacity of a substance is the ratio of heat absorbed (or released) by that substance to the corresponding temperature rise (or fall)

  • The heat capacity of a substance per unit mass is called specific heat.

  • Can be thought of a measure of the heat energy needed to heat 1 g of an object by 1ºC

  • Different objects have different specific heat values


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  • 1 g of water must absorb about 4 times as much heat as the same quantity of air to raise its temperature by 1º C

  • This is why the water temperature of a lake or ocean stays fairly constant during the day, while the temperature air might change more

  • Because of this, water has a strong effect on weather and climate


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Lowest energy same quantity of air to raise its temperature by 1º C

LIQUID

Highest energy

Latent Heat

  • Latent heat is the amount of energy released or absorbed by a substance during a phase change

FOR WATER:

334 J/g

released

2260 J/g

released

SOLID

LIQUID

GAS

334 J/g

absorbed

2260 J/g

absorbed

SOLID

LIQUID

GAS


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  • Example 1 same quantity of air to raise its temperature by 1º C: Getting out of a swimming pool

  • In the summer, upon exiting a swimming pool you feel cool. Why?

  • Drops of liquid water are still on your skin after getting out.

  • These drops evaporate into water vapor. This liquid to gas phase change causes energy to be absorbed from your skin.


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  • Example 2 same quantity of air to raise its temperature by 1º C: Citrus farmers

  • An orange crop is destroyed if temperatures drop below freezing for a few hours.

  • To prevent this, farmers spray water on the orange trees. Why?

  • When the temperature drops below 32oF, liquid water freezes into ice.

  • This liquid to solid phase change causes energy to be released to the fruit.

  • Thus, the temperature of the orange remains warm enough to prevent ruin.


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  • Example 3: Cumulus clouds

  • Clouds form when water vapor condenses into tiny liquid water drops.

  • This gas to liquid phase change causes energy to be released to the atmosphere.


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Types of Heat Transfer many atmospheric processes.

  • Heat can be transferred by:

    • Conduction

    • Convection

    • Advection

    • Radiation


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Conduction many atmospheric processes.

  • Conduction is the transfer of heat from molecule to molecule within a substance

  • Molecules must be in direct contact with each other

  • If you put one end of a metal rod over a fire, that end will absorb the energy from the flame.

  • Molecules at this end of the road will gain energy and begin to vibrate faster

  • As they do, their temperature increases and they begin to bump into the molecules next to them.

  • The heat is being transferred from the warmer end to the colder end, and eventually to your finger.


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Conduction many atmospheric processes.

  • The measure of how well a substance can conduct heat depends on its molecular structure.

  • Air does not conduct heat very well

  • This is why, in calm weather, the hot ground only warms the air near the surface a few centimeters thick by conduction!


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Convection many atmospheric processes.

  • Convection is the transfer of heat by the mass movement of a fluid (such as water and air) in the vertical direction (up and down)

  • Convection occurs naturally in the atmosphere

  • On a sunny day, the Earth’s surface is heated by radiation from the Sun.

  • The warmed air expands and becomes less dense than the surrounding cold air.

  • Because the warmed air is less dense (weighs less) than cold air, the heated air rises.


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Convection many atmospheric processes.

  • As the warm air rises, the heavier cold air flows toward the surface to replace the rising air.

  • This cooler air becomes heated in turn and rises.

  • The cycle is repeated.

  • This vertical exchange of heat is called convection and the rising air parcels are known as thermals


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Convection many atmospheric processes.

  • The warm thermals cool as they rise.

  • In fact, the cooling rate as a parcel rises can be calculated

    • If the thermal consists of dry air, it cools at a rate of ~10°C/km as it rises. This is called the lapse rate.

  • Convection is one process by which clouds can form.

  • As the temperature of the thermal cools, it may reach a point where it reaches saturation (the temp. and dewpoint are the close to the same)

  • Thermals condense and form a cloud.


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Advection many atmospheric processes.

  • Advection is the transfer of heat in the horizontal direction.

  • The wind transfers heat by advection

  • Happens frequently on Earth

  • Two types:

    • Warm air advection (WAA): wind blows warm air toward a region of colder air

    • Cold air advection (CAA): wind blows cold air toward a region of warmer air


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“Cold Air Advection” many atmospheric processes.

“Warm Air Advection”


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Radiation many atmospheric processes.

  • All things with a temperature above absolute zero emit radiation

  • Radiation allows heat to be transferred through wave energy

  • These waves are called electromagnetic waves

  • The wavelengths of the radiation emitted by an object depends on the temperature of that object (i.e., the sun mainly emits radiative energy in the visible spectrum, and the earth emits radiative energy in the infrared spectrum).

  • Shorter wavelengths carry more energy than longer wavelengths


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Radiation a photon of infrared radiation.

Emitted radiation can be:

  • Absorbed

    Increasing the internal energy of the gas molecules.

  • Reflected

    Radiation is not absorbed or emitted from an object but it reaches the object and is reflected back. The Albedo represents the reflectivity of an object and describes the percentage of light that is sent back.

  • Scattered

    Scattered light is deflected in all directions, forward, backward, sideways. It is also called diffused light.

  • Transmitted

    Radiation not absorbed, reflected, or scattered by a gas. The radiation passes through the gas unchanged.


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Examples of Heat Transfer a photon of infrared radiation.

  • http://www.wisc-online.com/objects/ViewObject.aspx?ID=SCE304


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Kirchoff’s Law a photon of infrared radiation.

  • Good absorbers of a particular wavelength are good emitters at that wavelength and vice versa

  • Our atmosphere has many selective absorbers Carbon Dioxide, Water Vapor, etc…

  • These gases are good at absorbing IR radiation but not solar radiation

  • Thus these gases are called greenhouse gases due to the fact they help to absorb and reemit IR radiation back toward the Earth’s surface thus keeping us warmer then we would otherwise be


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Solar Radiation Budget a photon of infrared radiation.


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Earth-Atmosphere Energy Balance a photon of infrared radiation.


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