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Chapter 15 AIR, CLIMATE AND WEATHER
WEATHER is a description of the physical conditions of the atmosphere, e. g. humidity, temperature, pressure, wind velocity. CLIMATE is a description of the long-term pattern of weather in a particular area. It is based on the average measurement of several weather conditions, e. g. temperature, precipitation.
ATMOSPHERE
The earth's atmosphere is about 500 km (300 mi.). The composition of the atmosphere has changed over the millions of years of the earth existence. Original atmosphere lacked oxygen and contained gases like ammonia, methane, sulfur oxides, nitrogen oxides, and water. The main components of our atmosphere are nitrogen (~78%) and oxygen (~21%). Water vapor concentrations vary from 0 to 4% depending on air temperature and available moisture. CO 2 makes about 0.03% of the atmosphere. Aerosols are small droplets suspended in the air.
ATMOSPHERIC LAYERS
TROPOSPHERE is the layer immediately adjacent to the earth's surface. 18 km thick at the equator to about 8 km thick at the poles. It is here where most of the weather occurs. Air circulates in great vertical and horizontal convection currents. Redistributes heat and moisture around the planet; uniform composition due to wind flow. More dense than other layers; it contains about 75% of the atmospheric gases. STRATOSPHERE extends from the troposphere up to about 50 km (31 mi). Tropopause is the imaginary line that separates the troposphere from the stratosphere. Temperature increases with altitude. It is more diluted and calm than the troposphere. 1/1000 less water. Ozone layer; it absorbs ultraviolet radiation (UV rays).
MESOSPHERE extends from the stratopause up to 80 km above sea level. Stratopause is the imaginary line that separates the stratosphere from the mesosphere. Temperature drops in the mesosphere down to -80oC. THERMOSPHERE extends above the mesopause up to about 1,600 km. Mesopause is the imaginary line that separates the mesosphere from the thermosphere A region of highly ionized gases. Temperature increases dramatically due to solar and cosmic radiation (above 150oC). There is no sharp boundary marking the end of the atmosphere.
THE WEATHER ENGINE
The flow of energy from the sun causes the cycling of air and water. This cycling creates climate and distributes the energy and materials through the environment. Solar energy can be absorbed by the earth or reflected. Visible light passes through the atmosphere until it reaches the ground. Ultraviolet (UV) rays are mostly absorbed by the ozone layer in the stratosphere. Infrared radiation is mostly absorbed by CO 2 and H 2 O in the troposphere. Albedo is the name given to the reflective properties of the surface. Clouds, snow and ice have a high albedo. Forest canopy, oceans and dark surfaces have a low albedo. The net average global albedo is 30%. Eventually all of the energy absorbed is emitted back into the atmosphere in the form of infrared energy. Infrared radiation has long wavelength and low energy Infrared radiation is reabsorbed by gases (CO 2 and H 2 O) in the atmosphere. These gases release the energy very slowly providing most of the heat in the lower atmosphere. The rising warm air is replaced by colder air from above creating convection currents. CONVECTION AND ATMOSPHERIC PRESSURE Most of the solar energy is used to evaporate water. Every gram of evaporating water absorbs 580 calories of energy as it changes from liquid to gas.
See: http://www.jracademy.com/~wotawaj/about.html OCEAN CURRENTS Surface ocean currents are the result of wind pushing the surface water and of the Coriolis effect. As surface water moves, deeper water moves up to replace it creating deep ocean currents. Cold water at high latitudes sinks where the temperatures are near 4ºC, the temperature at which water is the densest. See this site for the causes of ocean currents and other explanations: http://earth.usc.edu/~stott/Catalina/Oceans.html http://www.oceansonline.com/ocean_currents.htm Cycling currents bring water north and south and redistribute heat. SEASONAL WINDS AND MONSOONS Monsoons are seasonal winds especially in the Indian ocean and SE Asia. Monsoon is a seasonal reversal of wind patterns due to the change of seasons causing different warming and cooling of the oceans. Monsoons are created by the earth’s tilt in relation to the sun. In December and January, the sun is most intense just south of the equator; In June and July, during the northern summer, the sun is most intense just north of the equator. The land reaches higher temperatures than oceans. There is more land in the northern hemispheres. The air warms and rises and cool air from the south flows north to replace it bringing with it moisture and rains. This is called the summer monsoon. Where the sun shines most intensely, evaporation and convection currents are very strong. FRONTAL WEATHER Solar energy hits the equator directly perpendicular and the poles at an oblique angle. Air pressure is the greatest and the air the warmest at sea level. This is due to the pull of gravity and the weight of the air above. Cold air moves closer to the ground and lifts the warm air up. The boundary between two air masses with different temperature and density is referred to as a cold front. A moving cold front displaces warm air with cold air.
Air pressure and air density decrease with increasing altitude. Air becomes cooler and less dense as it rises, and its water vapor condenses and precipitates. Approaching cold fronts cause towering clouds called thunderheads, which reach the stratosphere where the jet stream pushes the cloud top into a characteristic anvil shape. If the advancing mass of air is warmer than the local air, a warm front results. Warm fronts often have many layers of clouds at different levels. The warm air slides over the cold air mass and bring drizzle and cloudy skies. See animation: http://www.classzone.com/books/earth_science/terc/content/visualizations/ es2002/es2002page01.cfm?chapter_no=visualization CYCLONIC STORMS Low pressure areas are caused by rising warm air. Winds swirl into these low-pressure areas, turning counterclockwise in the Northern Hemisphere due to the Coriolis effect. Hurricanes or typhoons are caused by large differences in temperature between air and ground and a large supply of moisture. A low pressure area sucks in wind laden with moisture from the adjacent areas. The Coriolis Effect causes spiraling of the winds. http://www.weatherquestions.com/What_causes_hurricanes.htm See animation and explanation of how hurricanes form: http://news.bbc.co.uk/cbbcnews/hi/newsid_4020000/newsid_4022100/4022129.stm Hurricanes, typhoons and cyclones are different names for the same phenomenon. Tornadoes Tornadoes are cyclonic storms formed over land but the rotation is not caused by the Coriolis Effect of the earth. Tornadoes formed where dry, cold air from the north collides with humid, warm air moving north from the Gulf of Mexico. A thunderstorm is the first step in the creation of a tornado. A thunderstorm happens when there is moisture in the atmosphere and warm air rises rapidly over dense, cold air. The intense vertical currents generate a thunderhead.
This pulls in moist air from the ocean and brings rain to the Sahel of Africa. This trough gives rise to tropical depressions that follow one another in regular succession across the Atlantic and some become the hurricanes in the Caribbean and Gulf of Mexico. Weather modification Seeding clouds with ionized particles can initiate precipitation if water vapor is present and air temperatures are near condensation point. Hail suppression by cloud seeding also can be effective.
CLIMATE
Climate is the pattern of weather over a large region of the globe over a long period of time. There have been abrupt climatic changes in the past, e. g. ice ages. Theories about the causes for climatic changes:
- Interaction of unrelated events, e. g. radiation from supernovas, volcanic activity and seafloor spreading, solar radiation.
- Cyclic changes in the sunspots and magnetism.
- Milankovitch cycles: Orbital variations (every 100,000 year) of the earth rotation around the sun; shifts in the tilt of the earth's axis (40,000 years) and wobbling of the axis (26,000 years).
EL NIÑO
El Niño is also called El Niño Southern Oscillation or ENSO. Warm surface water is kept near Indonesia in the Pacific Ocean by prevailing trade winds. There is an upwelling of cold water near the coast of South America. Every few years (3-5 years) there is a reversal of the winds that keep the surface warm water on the western Pacific. During El Niño years there is shift of the jet stream to a more southerly position creating more rain and storms over California and the mid-west of U.S. La Niña is the name given to the intervening years. http://library.thinkquest.org/20901/overview_5.htm The Pacific Decadal Oscillation is a pattern similar to El Niño that occurs in the north Pacific.
A very large pool of warm water moves back and forth every 30 years or so. Ocean surface temperatures are warmer in western North America and cooler in the western Pacific; then the reverse occurs. This shift affects the salmon run, which is significantly affected by the warm sea temperatures.
ANTHROPOGENIC IMPACT (HUMAN-CAUSED)
Intergovernmental Panel on Climate Change ( IPCC ) is a UN sponsored working group that studied human impact on climate. Scientists disagree about the causes, timing and consequences of global climate change, the overwhelming majority agree that there are unmistakable signals of human impacts on the world’s climate. Conclusions of IPCC published in 1955:
- World climate has changed significantly in the last century.
- There is evidence of human influence on the climate.
- Models suggest that the trend is to increase the global mean temperatures between 1oC and 3.5oC by 2100. In the third climate report released by the IPCC in 2001, it is stated that “recent changes in the world’ climate have had discernible impacts on physical and biological systems.” There are many skeptics in the scientific community about climate changes. Faulty models and selective data have been used in making predictions. There is skepticism on the part of a number of scientists: We don’t know enough to make predictions. Current computer models are still inadequate. Politicians, lobbying groups and industry have unduly influenced the IPCC reports. The troposphere is not warming like the surface of earth.
GREENHOUSE GASES
Burning fossil fuels, agriculture, deforestation and industrial processes contribute to the production of greenhouse gases. CO 2 has increased steadily this century. Increase is attributed to the burning of fossil fuels This trend could bring the concentration up to 500 ppm by the year 2100, twice the pre-industrial level. CO 2 traps infrared radiation emitted from the earth's surface.
China is second. Japan and the European Union produce about half as much as the US per capita. Little progress has been made in this area. The Kyoto Protocol was an agreement made 1997 by 160 nations to cut back on the emissions of CO 2 , N 2 O and CH 4. An average of 5.2% below the 1990 level by the year 2012. Individual nations have different limits. In 2002, all nations of the European Union and Japan ratified the agreement; United States did not ratify the agreement. Many are opposed to limits on greenhouse gas emissions. It has not been proven scientifically. Cost of making the changes. Fear of job loss.
