1. The Earth is an oblate spheroid.
    
2. The three major layers of the Earth are:
    
   • The core is approximately 7000 kilometers in diameter (3500 kilometers in radius) and is located at the Earth's center.

     The core is a layer rich in iron and nickel that is composed of two layers:
      

     • The inner core is theorized to be solid with a density of about 13 grams per cubic centimeter and a radius of about 1220 kilometers.
        
     • The outer core is liquid and has a density of about 11 grams per cubic centimeter. It surrounds the inner core and has an average thickness of about 2250 kilometers.

   • The mantle surrounds the core and has a thickness of 2900 kilometers.

     The mantle is almost 2900 kilometers thick and comprises about 83% of the Earth's volume. It is composed of two main layers.
      

     • The upper mantle exists from the base of the crust downward to a depth of about 670 kilometers. This region of the Earth's interior is thought to be composed of peridotite, an ultramafic rock made up of the minerals olivine and pyroxene.
        
       • The top layer of the upper mantle, 100 to 200 kilometers below surface, is called the asthenosphere. Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle. The rocks in this upper portion of the mantle are more rigid and brittle because of cooler temperatures and lower pressures.
        
     • The lower mantle that extends from 670 to 2900 kilometers below the Earth's surface. This layer is hot and plastic. The higher pressure in this layer causes the formation of minerals that are different from those of the upper mantle.

   • The crust is less dense than, and floats on top of the mantle. This material is cool, rigid, and brittle.
      
     • Oceanic crust is thin and measures between 5 to 10 kilometers thick. It is also composed of basalt and has a density of about 3.0 grams per cubic centimeter.
        
     • Continental crust is 20 to 70 kilometers thick and composed mainly of lighter granite. The density of continental crust is about 2.7 grams per cubic centimeter.
        
       • It is thinnest in areas like the Rift Valleys of East Africa and in an area known as the Basin and Range Province in the western United States (centered in Nevada this area is about 1500 kilometers wide and runs about 4000 kilometers North/South).
          
       • It is thickest beneath mountain ranges and extends into the mantle.
    
3. Isostacy
    
   • The tectonic plates of the continental and oceanic crust have the ability to rise and sink. This phenomenon, known as isostacy, occurs because the crust floats on top of the mantle like ice cubes in water.
      
   • When the Earth's crust gains weight due to mountain building or glaciation, it deforms and sinks deeper into the mantle. If the weight is removed, the crust becomes more buoyant and floats higher in the mantle.
      
   • This process explains recent changes in the height of sea-level in coastal areas of eastern and northern Canada and Scandinavia.
      
     • Some locations in these regions of the world have seen sea-level rise by as much as one meter over the last one hundred years. This rise is caused by isostatic rebound.
        
     • Both of these areas where covered by massive glacial ice sheets about 10,000 years ago. The weight of the ice sheets pushed the crust deeper into the mantle. Now that the ice is gone, these areas are slowly increasing in height to some new equilibrium level.

1. The Continental Drift Theory

   In the 19th and early 20th centuries, several scientists suggested that the continental masses had the ability to move across the Earth's surface.

   These theories were based on the following evidence:

   • Some continents seem to fit together like a jigsaw puzzle.
      
   • Geologic deposits of rocks on the East coast of North and South America are similar to deposits found on the West coast of Africa and Europe.
      
   • Locations of fossil occurrences suggested that some of the continental masses may have been connected in the geological past.
      
   • Paleoclimatic evidence indicates that now tropical regions on some continents had polar climates in the past. This may indicate that these regions were located at different latitudes.

    
2. The Modern Theory of Plate Tectonics
    
   • Both oceanic and continental crust are composed of numerous tectonic plates that float on top of the mantle.
      
   • Convection currents within the mantle cause these plates to move slowly across the asthenosphere.
      
   • The lithosphere is a layer that includes the crust and the upper most portion of the asthenosphere. This layer is about 100 kilometers thick and has the ability to glide over the rest of the upper mantle. Because of increasing temperature and pressure, deeper portions of the lithosphere are capable of plastic flow over geologic time.
      
   • The lithosphere is the zone of earthquakes, mountain building, volcanoes, and continental drift.
      
     • Earthquakes - a sudden movement of the Earth's plates.

       Most earthquakes are produced along faults, tectonic plate boundary zones, or along the mid-oceanic ridges. At these areas, large masses of rock that are moving past each other can become locked due to friction. Friction is overcome when the accumulating stress has enough force to cause a sudden slippage of the rock masses.

       The magnitude of the shock wave released into the surrounding rocks is controlled by the quantity of stress built up because of friction and the distance the rock moved when the slippage occurred.

     • Mountain Building - one plate pushing against another plate, causing the crust to deform.
          
     • Sea-floor spreading, occuring at volcanic rift zones along the mid-oceanic ridge (Atlantic), represent areas of crustal creation.
          
     • Volcano - generally a conical shaped hill or mountain built by accumulations of lava flows produced by hot plumes of magma that rise to the Earth's surface.

       The most active earthquakes and volcanoes are found along the plate boundaries making up an area called the "Ring of Fire". This area rims the Pacific Ocean.
        

     • Plate Tectonics at geology.com
        
     • Mount Saint Helens VolcanoCams
        
     • New Zealand VolcanoCams
        
     • Old Faithful WebCam at Yellowstone National Park
        
     • USGS Volcano Hazards Program Webcams
        
     • San Andreas Fault Map  

(climate, 9 min)

• All of the Earth's energy comes ultimately from the Sun.
   
  • Sunlight reaching the Earth's surface unmodified by atmospheric processes is termed direct solar radiation.
     
  • Solar radiation that reaches the Earth's surface after it was altered by atmospheric scattering is called diffused solar radiation.
     
  • Not all of the direct and diffused radiation available at the Earth's surface is used to do work (photosynthesis, creation of sensible heat, evaporation, etc.). Some of the radiation received at the Earth's surface is redirected back to space by reflection.
     
  • Why Is The Sky Blue?
     
    • Specifically, it is the scattering of light by the atmosphere that causes the blue color of the sky. When the sun is directly over head, scattering is at its least. When the sun moves toward the horizon, the increasing angle at which the light is hitting the atmosphere causes scattering to increases.
       
    • Skylight is the light we perceive when looking away from the sun at the sky.
       
    • Sunlight is the light observed when looking directly at the sun.
       
    • Daylight may be thought of as a combination of skylight and sunlight.
       
    • Sunlight and skylight differ in appearance.
       
      • The word scatter literally means "to cause to separate widely". Small particles of matter, such as air molecules and dust, cause the individual photons of sunlight beams to deflect (scatter) in all directions.
         
      • As sunlight enters the earth's atmosphere, violet and blue wavelengths are scattered the most, followed by green, yellow, orange and red, in that order. Shorter wavelengths (violet, blue) are scattered about ten times more than the longer red wavelengths
         
      • If our observer looks directly at the sun (sunlight), he perceives it as one color (whitish, yellow, orange or red, relative to atmospheric conditions and the sun's position in the sky), though the sky itself appears blue because of the scattering effect.
         
      • When the sun is overhead and the weather is clear, the entire sky (skylight) appears light blue.
         
      • As the sun continues on its westward path, increased scattering takes place till, shortly before sundown, most of the blue and violet wavelengths are "scattered out". Skylight is now composed of the longer wavelengths of light, orange and red.
         
      • Skylight ranges from very pale blue to deep blue. The purity and saturation of the blue is influenced by atmospheric moisture, dust and pollution. Generally speaking, the sky is palest when the atmosphere is humid or laden with dust.
   
• Climate includes the temperatures, humidity, rainfall, and numerous other meteorogical factors in a given region over long periods of time.
   
  • Sometimes less than 50% of the solar radiation arriving at the outer atmosphere actually reaches the Earth's surface.
     
  • The reflectivity or albedo of the Earth's surface varies with the type of material that covers it. For example, fresh snow can reflect up to 95% of the sunlight that reaches it surface.

    Some other surface reflectivities are:
    

    • Dry sand - 35 to 45%
    • Broadleaf deciduous forest - 5 to 10%
    • Needleleaf confierous forest - 10 to 20%
    • Grass type vegetation - 15 to 25%
    • The Earth's average albedo, including both the atmosphere and the surface, is about 30%.
     
  • The most widely used system for classifying the world's climates uses categories based on the annual and monthly averages of temperature and precipitation.
     
    • Tropical Moist Climates: all months have average temperatures above 18^o Celsius.

      Tropical moist climates extend northward and southward from the equator to about 15^o to 25^o of latitude. In these climates all months have average temperatures greater than 18^o Celsius. Annual precipitation is greater than 1500 mm.

    • Dry Climates: with deficient precipitation during most of the year.

      Dry climates extend from 20^o to 35^o North and South of the equator and in large continental regions of the mid-latitudes often surrounded by mountains.

      The most obvious feature of this climate is that evaporation exceeds precipitation.

    • Moist Mid-latitude Climates with Mild Winters.

      This climate generally has warm and humid summers with mild winters. Its extent is from 30^o to 50^o of latitude mainly on the eastern and western borders of most continents.

    • Moist Mid-Latitude Climates with Cold Winters.

      This climate has warm to cool summers and cold winters. The location is between 50^o and the Polar circle. The average temperature of the warmest month is greater than 10^o Celsius, while the coldest month is less than −3^o Celsius. Winters are severe with snowstorms, strong winds, and bitter cold from Continental Polar or Arctic air masses.

    • Polar Climates: with extremely cold winters and summers.

      Polar climates have year-round cold temperatures with the warmest month less than 10^o Celsius. They are found on the northern coastal areas of North America, Europe, Asia, and on the landmasses of Greenland and Antarctica.

      The surface of the polar tundra is permanently frozen to depths of several meters, a condition known as permafrost. Vegetation is dominated by mosses, lichens, dwarf trees and scattered woody shrubs.

      The surface of the polar ice caps is permanently covered with snow and ice.

     
  • Interactive Climate Map
     
  • Climate Zone
   
   
• Weather is a set of all the phenomena occurring in a given atmosphere at a given time.
   
  • Air masses
     
    • An air mass is a large areas of air that has a similar temperature and humidity level.
       
    • The type of air mass is determined by where it comes from and what is passes over. This is because the air takes on properties from the surfaces it travels over.
       
    • The boundary between two different air masses is called a front.
     
  • Air pressure
     
    • Air pressure is the weight of air pressing down on the earth's surface. Pressure varies with air masses.
       
    • Low pressure air is warm air rising and rotating in a counter-clockwise direaction. As the air rises, it cools, condenses and forms clouds.
       
    • High pressure air is cool air sinking and rotating in a clockwise direction. There are usually few clouds in the air.
       
    • Wind can be defined simply as air in motion. Wind develops as a result of spatial differences in atmospheric pressure. Generally, these differences occur because of uneven absorption of solar radiation at the Earth's surface. Wind speed tends to be at its greatest during the daytime when the greatest spatial extremes in atmospheric temperature and pressure exist.
       
    • Winds are named according to the compass direction of their source. A wind from the north blowing toward the south is called a northerly wind. There are sixteen principal bearings of wind direction. Most meterological observations report wind direction using one of these sixteen bearings.
       
    • Horizontally, at the Earth's surface wind always blows from areas of high pressure to areas of low pressure (vertically, winds move from areas of low pressure to areas of high pressure), usually at speeds determined by the difference in air pressure between pressure centers.
     

    

     
    

  • Precipitation
     
    • Precipitation is any form of moisture which falls to the earth. This includes rain, snow, hail and sleet.
       
    • Precipitation occurs when water vapor cools. When the air reaches its saturation point (dew point) the water vapor condenses on dust particles in the air, forming tiny droplets of water - clouds.
       
    • Clouds take the shape of the wind that supports them. When the drops of condensation become too large for the wind to support, rain falls.
     
  • Thunderstorms
     
    • The most common type of thunderstorm is the air mass storm, which normally develops in late afternoon hours when surface heating produces the maximum number of convection currents in the atmosphere.
       
      • The life cycle of an air mass storm has three distinct stages:
         
        
        • The first stage of air mass thunderstorm development is called the cumulus stage. In this stage, masses of warm humid air rise and cool to form clusters of puffy white cumulus clouds. The clouds are the result of condensation and deposition which releases large quantities of latent heat. The added heat energy keeps the air inside the cloud warmer than the air around it. The cloud continues to develop as long as more humid air is added to it from below. Updrafts dominate the circulation patterns within the cloud.
           
        • When the updrafts reach their maximum altitude in the developing cloud, usually 12 to 14 kilometers, they change their direction 180^o and become downdrafts. This marks the mature stage.

          The storm is at its most intense stage of development and is now a cumulonimbus cloud. The top of the cloud takes on the familiar anvil shape, as strong stratospheric upper-level winds spread ice crystals in the top of the cloud horizontally. At its base, the thunderstorm is several kilometers in diameter. The mature air mass thunderstorm contains heavy rain, thunder, lightning, and produces wind gusts at the surface.
           

        • The mature thunderstorm begins to decrease in intensity and enters the dissipating stage after about half an hour. Air currents within the convective storm are now mainly downdrafts as the supply of warm moist air from the lower atmosphere is depleted. Within about 1 hour, the storm is finished and precipitation has stopped.
       
      

       
      

    • Severe thunderstorms form if the conditions exist to enhance and prolong the mature stage of development. Severe thunderstorms are defined as convective storms with frequent lighting, accompanied by local wind gusts over 50 miles per hour, or hail that is 1 inch in diameter or larger.
       
      • In most severe thunderstorms, the movement of the storm, in roughly an easterly direction, can refresh the storm's supply of warm humid air. With a continual supply of latent heat energy, the updrafts and downdrafts within the storm become balanced and the storm maintains itself indefinitely.
         
      • Movement of the severe storm is usually caused by the presence of a mid-latitude cyclone cold front or a dry line some 50 to 150 miles ahead of a cold front.
         
      • The diagram above shows the features associated with a severe thunderstorm.
         
        • This storm would be moving from left to right because of the motion associated with a mid-latitude cyclone.
        • The upper-level dry air wind is generated from the mid-latitude cyclone. It causes the tilting of vertical air currents within the storm so that the updrafts move up and over the downdrafts.
        • The green arrows represent the updrafts which are created as warm moist air is forced into the front of the storm.
        • At the back end of the cloud, the updrafts swing around and become downdrafts (blue arrows). The leading edge of the downdrafts produces a gust front near the surface.
        • As the gust front passes, the wind on the surface shifts and becomes strong with gusts exceeding 50 miles per hour, temperatures become cold, and the surface pressure rises.
        • Warm moist air that rises over the gust front may form a roll cloud. These clouds are especially prevalent when an inversion exists near the base of the thunderstorm.
         
      • A tornado is a vortex of rapidly moving air associated with some severe thunderstorms.
         
        
        • Winds within the tornado funnel may exceed 250 miles per hour. High velocity winds cause most of the damage associated with these weather events.
           
        • Tornadoes also cause damage through air pressure reductions. The air pressure at the tornado center is approximately 800 millibars (average sea-level pressure is 1013 millibars) and many human made structures collapse outward when subject to pressure drops of this magnitude.
           
        • The destructive path of a tornado is usually about half a kilometer wide, and usually no more than 25 kilometers long - but may be much larger.
           
         
        

         
        

      • Hurricanes are intense cyclonic storms that develop over the warm oceans of the tropics. They begin as a mass of thunderstorms. When these storms develop a cyclonic airflow between them, a tropical storm has formed.
         
        • These tropical storms go by other names in the various parts of the world: India/Australia - cyclones; western North Pacific - typhoons; and the Philippines - baguio.
           
        • Surface atmospheric pressure in the center of a hurricane tends to be extremely low. The lowest pressure reading ever recorded for a hurricane (typhoon Tip, 1979) is 870 millibars (mb). However, most storms have an average pressure of 950 millibars.
           
        • To be classified as a hurricane, sustained wind speeds must be greater than 118 kilometers per hour at the storm's center. Wind speed in a hurricane is directly related to the surface pressure of the storm.

1. The hydrologic cycle (diagram above) is a conceptual model that describes the storage and movement of water between the biosphere (where life is found), atmosphere (from the surface of the earth to the edge of space), lithosphere (the earth's crust and upper mantle), and the hydrosphere (where water is found).
    
2. Water on this planet can be stored in any one of the following reservoirs:
    
   • Atmosphere - the vast gaseous envelope of air that surrounds the Earth.
      
   • Oceans - a body of saline water found occupying all or part of the Earth's ocean basins. There are five recognized oceans: the Atlantic, the Southern Ocean, the Pacific Ocean, the Indian Ocean, and the Arctic Ocean.
      
   • Lakes - bodies of standing water found on the Earth's continental land masses. The water in a lake is normally fresh.
      
   • Rivers - long narrow channels of water that flow as a function of gravity and elevation across the Earth's surface. Many rivers empty into lakes, seas, or oceans.
      
   • Soils - layers of unconsolidated material found at the Earth's surface. Soil normally consists of weathered mineral particles, dead and living organic matter, air space, and the soil solution.
      
   • Glaciers - large long lasting accumulations of snow and ice that develop on land. Most glaciers flow along topographic gradients because of their weight and gravity.
      
   • Snowfields - areas of permanent snow accumulation. Usually at high altitudes or latitudes.
      
   • Groundwater - water that occupies the pore spaces found in some types of bedrock.
    

3. Water moves from one reservoir to another by way of processes like
    
   • Evaporation - the process by which liquid water is converted into a gaseous state.
      
   • Condensation - the change from vapor to liquid that occurs with cooling.
      
   • Precipitation - any aqueous deposit, in liquid or solid form, that develops in a saturated atmosphere (relative humidity equals 100%) and falls to the ground generally from clouds.
      
   • Deposition - the change from gas to solid that occurs with cooling.
      
   • Runoff - the flow of precipitation water to stream channels located at lower elevations. Occurs when the infiltration capacity of an area's soil has been exceeded.
      
   • Infiltration - the absorption and downward movement of water into the soil layer.
      
   • Sublimation - the process where ice changes into water vapor without first becoming liquid.
      
   • Transpiration - the process of water loss from plants through stomata, small openings found on the underside of leaves.
      
   • Melting - the physical process of a solid becoming a liquid.
      
   • Groundwater flow - flow of groundwater because of gravity.
    

4. The oceans supply most of the evaporated water found in the atmosphere.
    
   • 97% of all the water on the Earth is in the oceans.
      
   • The other 3% is held as freshwater in:
      
     • glaciers and icecaps - 2.05%
        
     • groundwater - 0.68%
        
     • lakes - 0.01%
        
     • soil moisture - 0.005%
        
     • the atmosphere - 0.001%
        
     • streams and rivers - 0.0001%
        
     • within living things - 0.00004%

• Aeolian landforms are produced by either the erosive or constructive action of the wind.
• Coastal and oceanic landforms
• Fluvial landforms are associated with the action of running water.
• Mountain and glacial landforms
• Slope landforms relate to the "lay of the land"
• Volcanic landforms - the shape of the volcano depends on the type of material ejected.

 
• Vegetation
• Ecosystems
• Biomes

• Earthquakes
• Floods
• Hurricanes
• Landslides
• Tsunamis
• Volcanoes
• Wildfires

• Wastewater
• Pollution
• Ozone Depletion
• Acid Rain
• Oil Spills
• Pesticides
• Global Warming
• Environmental Protection Agency
• Greenpeace
• DEFRA (UK)