1st Law of Thermodynamics Heat Transfer

1st Law of Thermodynamics

Heat Transfer

Lecture 6

October 14, 2009

Review

GOES:  Geostationary Operational Environmental Satellite

Maintain constant altitude (~36,000 km) over a single point, always over the equator  

Imagery is obtained approximately every 15 minutes 

Generally has poor spatial resolution but good temporal resolution

POES: Polar Operational Environmental
Satellites

circular orbit moving from pole to pole closer to the Earth (879 km) than GOES

Sees the entire planet twice in a 24 hour period.

Good Spatial Resolution: Lower altitude results in higher resolution images

Poor Temporal Resolution: Over any point on Earth, the satellite only captures two images per day.

Review

Visible

Measures visible light (solar radiation, 0.6 mm) which is reflected back to the satellite by cloud tops, land, and sea surfaces.

Thus, visible images can only be seen during daylight hours!

Infrared (IR)

Displays infrared radiation (10 to 12 mm) emitted directly by cloud tops, land, or ocean surfaces. 

Wavelength of IR depends solely on the temperature of the object emitting the radiation

Advantage: You can always see the IR satellite image

Water Vapor (WV)

Displays infrared radiation emitted by the water vapor (6.5 to 6.7 mm) in the atmosphere

Can determine dry layers from moist layers in the atmosphere

Review

RADAR

§Radar uses electromagnetic radiation to sense precipitation.

§Sends out a microwave pulse (wavelength of 4-10 cm) and listens for a return echo.

§If the radiation pulse hits precipitation particles, the energy is scattered in all directions

§The intensity of precipitation is measured by the strength of the echo, in units of decibels

§Doppler Radar: can determine velocity as well as reflectivity

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

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

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.

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

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!

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.

Convection

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

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

Radiation

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

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.

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

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