1st Law of ThermodynamicsHeat Transfer

Lecture 4February 18, 2010

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Review from last weekIn 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 crystals3. Sleet frozen rain drops4. Freezing Rain rain the freezes on

contact with a cold surface5. Hail large pieces of ice

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

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

• 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.

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

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

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

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

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

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

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

• 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

Substance Value (J g−1K−1)

Liquid Water 4.183

Ice 2.050

Wood 0.420

Sand 0.835

Air 1.012

Latent Heat• Latent heat is the amount of energy

released or absorbed by a substance during a phase change

LIQUID

2260 J/g

released

334 J/g

released

2260 J/g

absorbed

334 J/g

absorbed

FOR WATER:

Lowest energy

Highest energy

SOLID

SOLID

LIQUID

LIQUID

GAS

GAS

Example 1: 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.

Example 2: 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.

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.

• The release of latent heat during cloud formation drives many atmospheric processes.

Types of Heat Transfer• Heat can be transferred by:

–Conduction

–Convection

–Advection

–Radiation

Conduction• 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.

Conduction• 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!

Substance Heat Conductivity

Still air at 20 °C 0.023

Water at 20 °C 0.60

Ice 2.1

Granite 2.7

Iron 80

Convection• 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.

• 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

Convection

Convection• 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.

Advection• 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

“Cold Air Advection” “Warm Air

Advection”

• 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

Radiation

• A photon of ultra-violet radiation carries more energy than a photon of infrared radiation.

• The shortest wavelengths in the visible spectrum are purple, and the longest wavelengths are red.

RadiationEmitted radiation can be:

• AbsorbedAbsorbedIncreasing the internal energy of the gas molecules.• ReflectedReflected 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.• ScatteredScattered Scattered light is deflected in all directions, forward, backward, sideways. It is also called diffused light. • TransmittedTransmitted Radiation not absorbed, reflected, or scattered by a gas. The radiation passes through the gas unchanged.

Examples of Heat Transfer

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

Kirchoff’s Law

• 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

Solar Radiation Budget

Earth-Atmosphere Energy Balance