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Heat

Learning Objectives   





 


Movement in the atmosphere, weather systems and storms are all the direct result of differential heating of the atmosphere.  Some parts of the atmosphere are heated more than others.  As air is warmed it becomes less dense and rises, leading to movements in the atmosphere.  An understanding of heat and the differential heat received at the surface is essential to the understanding of Meteorology.

The Sun is the ultimate source of heat energy for the atmosphere.  The energy received on the surface of the Earth is not equal over the Earth.

The Earth is curved.  The bulge of the Earth (the equator) receives more direct solar radiation than the poles.

The Earth is also tilted on its axis.  The Earth is tilted 23.5 degrees.  Solar radiation at the Equator is more direct than solar radiation at the poles.  The hemisphere that is tilted towards the Sun receives greater solar radiation than the hemisphere pointed away from the Sun.  The seasons are the expression of this fact.  The hemisphere that is tilted towards the Sun experiences summer while the hemisphere pointing away from the Sun experiences winter.



Divisions on the Earth that may seem arbitrary are based on this tilt:
 

Tropics: lie between 23.5 degrees North latitude and 23.5 degrees South latitude.  This corresponds to the tilt of the Earth on its axis.  In the tropics the Sun will be positioned directly overhead at some point during the year.  The tropics receive the most direct solar radiation.

The Tropic of Cancer and the Tropic of Capricorn delineate the Tropics Zone.

Arctic and Antarctic: The Arctic lies between 66.5 degrees and 90 degrees North latitude, the Antarctic lies between 66.5 degrees and 90 degrees South latitude.  Because of the 23.5 degree tilt of the Earth on its axis (90-23.5=66.5) all points that lie within these areas will experience at least one 24 hour period during the year when no solar radiation is received at the surface.  Some areas will experience more than 24 hours, the poles (90 degrees) are the extreme; they experience a 6 month period during which no solar radiation is received at the surface.  Conversely, all points within these areas experience at least one 24 hour period when the solar radiation is continuous.  The poles, at the extreme, will receive continuous solar radiation for a 6 month period.

The Arctic Circle and the Antarctic Circle delineate the Arctic and Antarctic.
 



Calendar dates are also based on the variation of sunlight received at the surface because of the tilt of the Earth's axis.

 

Solstice: marks the longest and shortest periods of daily solar radiation received at the surface.

 

 Winter Solstice:  Approximately December 21.  All points within the Northern Hemisphere receive the least amount of solar radiation at the surface for the year.

·         All points within the Arctic Circle experience at least one twenty four hour period when no solar radiation is received.

 

Summer Solstice: Approximately June 21.  All points in the Northern Hemisphere receive the greatest amount of solar radiation at the surface for the year.

·         All points within the Arctic Circle experience at least one twenty four hour period when continuous solar radiation is received at the surface.

 

The opposite occurs in the Southern Hemisphere.

Equinox: Approximately March 21 and September 21.  The length of time when solar radiation is received at the surface is equal at all points on the Earth.  (12 hours of sunlight, 12 hours of darkness)


Because of the curved Earth and the tilt of the Earth's axis the Earth's surface receives differing amounts of solar radiation.


Heat Energy

 

Energy is defined as the ability to do work.  The Sun provides energy in the form of radiant or heat energy.
Kinetic energy is the energy of motion.  Molecules and atoms are in random motion due to kinetic energy.  If heat energy is applied the kinetic energy increases and molecules and atoms increase speed.

 

Temperature is a measurement of the average speed of molecules in the atmosphere.  If heat energy is added, kinetic energy increases and temperature rises.  If heat energy is removed the molecules move more slowly and temperature decreases.

 

Heat is a form of energy, Temperature is a measure of that energy.

 

Absolute Zero: the temperature at which molecular movement stops.

 

Heat can be expressed in many ways.

Heat can be expressed in Joules (J) which is a unit of energy.

More commonly heat is expressed in Calories(cal).  A Calorie is the amount of energy required to raise or lower the temperature of one gram of water one degree C.

 

The heat energy contained within a substance can be measured by its temperature but mass must also be considered.

Which contains more energy: a cup of water at 100 degrees or a gallon of water at 75 degrees?
ANS: The gallon of water at 75 degrees.  THINK:  if energy is the ability to do work then which would melt more ice?

 


Specific Heat: measured in calories it is the amount of heat (energy) required to raise or lower the temperature of one gram of a substance one degree C.  The specific heat of water is 1 calorie.

 

Heat Capacity: is the resistance of a substance to a change in temperature or the amount of energy one gram of a substance must absorb to cause a change in temperature of one degree C.
 

There are three states of matter:  liquid, solid and gas.

Change of state: when a substance changes from a liquid to a gas, gas to a liquid; solid to liquid, liquid to solid etc.

 

Latent heat: the energy required to cause a change of state in a substance.  This is a  VERY important concept in meteorology.  Water has a high latent heat, it absorbs/releases LOTS of energy before it can change state.
 

Heat of vaporization : associated with evaporation/condensation, it is the amount of energy required to change a liquid to a gas or a gas to liquid.  It takes 540 cal for one gram of water to change state from gas to liquid/liquid to gas.

CONSIDER:

  • it takes one cal of energy to heat one gram of water from 99 to 100 degrees
  • BUT it takes 540 cal of energy to change one gram of water at 100 degrees to  steam at 100 degrees


Heat of fusion :  associated with freezing/thawing, it is the amount of energy required to change a liquid to a solid or a solid to a liquid.  It takes 80 cal for one gram of water to change state from liquid to solid/solid to liquid.

CONSIDER:

  • it takes one cal of energy to change the temperature of one gram of water from 1 degree to 0 degrees C
  • BUT it takes 80 cal of energy to change one gram of 0 degree C water to ice at 0 degrees


Therefore....  Energy is constantly released and absorbed by water in the atmosphere as water changes state.


Heat transfer:

 

Conduction: heat energy moves from molecule to molecule through a substance.  Air is NOT a good conductor of heat energy,  heat transfer in atmosphere is through convection.

Convection:  Air near ground is heated by energy emitted from the surface of the Earth.  As the air near the ground is heated it rises, cooler air above sinks.  A convection cell is formed.

  The Convection Process:

Air mass heats
Molecules increase speed and gas expands becoming less dense
Air rises

OR

Air mass cools
Molecules decrease speed and gas contract becoming more dense
Air sinks
 

Air Masses:  an air mass is a "parcel" of air with relatively uniform temperature (and other properties).  Air masses can be VERY LARGE, the size of several states.  An air mass will absorb heat energy and moisture from the Earth's surface from areas over which they "sit."  Through the convection process they will rise and sink causing both vertical and horizontal movements within the atmosphere.  As air masses move horizontally they redistribute heat energy from one part of the Earth to another.