ESCI 129 Intro to the Earth Dr. Steven B. Newman Exam 1 Study Guide Use this as an outline to focus your study on topics we covered in class. Chapter 1 The Earth and Its Atmosphere Weather vs. Climate: Weather is the state of the atmosphere at a given place and time. Climate is the “average” weather for a location (avg. temperature, precipitation, etc.) along with the extremes of summer and winter. It takes at least 30 years of data to determine the climate of a given location. Composition of the Atmosphere: The “air” contains approx. 78% Nitrogen, 20.9% Oxygen, 0.9% Argon and 0.037% CO2. These gasses are present in all samples below 80 km from any location. There are a number of “variable components,” that are in different amounts depending on location. The two most important are water vapor (up to 4% by volume) and ozone (only 0.00007% by volume). Water vapor absorbs both solar and earth (terrestrial) radiation, while ozone (present mostly in the stratosphere) filters harmful ultraviolet (UV) radiation from sunlight. There are also aerosols—particulates that are found in higher concentrations near the ground (as their source is the surface of the earth). The most common of the naturally occurring ones are forest fire smoke, volcanic ash, desert sand particles and sea salt particles. Human-made particles are usually not so good for our health and often are considered pollutants. Transportation vehicles account for around half of the air pollution on earth. Atmospheric Evolution: Earth is approx. 4.5 billion years old. Original (primary) atmosphere of hydrogen, helium, methane and ammonia was driven off by heat of earth and sun, and lack of strong gravity to hold them in. Original secondary atmosphere (caused by volcanic outgassing as earth cooled) was mostly nitrogen, water vapor and CO2. As planet cooled, water vapor condensed and precipitated out, filling the oceans and absorbing most of the CO2. Free oxygen appeared about 3-3.5 billion years ago with the evolution of green plants (photosynthesis). Vertical Structure: Air pressure is the force exerted per unit area of the overlying atmosphere. As there is less air as one goes up, pressure ALWAYS decreases with altitude. At sea level, normal atmospheric pressure is 14.7 lbs/in2. The National Weather Service (NWS) uses millibars as the unit of pressure on maps. Normal sea level pressure in these units is 1013.2 mb. Pressure decreases faster in the lower atmosphere and more slowly at higher altitudes. The atmosphere also divides itself into four layers by temperature profile (determined by sending up an instrument package attached to a balloon—a”rawinsonde”). The layers in order from ground up are troposphere, stratosphere, mesosphere and thermosphere. See Fig. 1.10, page 11 for details on temperature profiles in each layer. Chapter 2 Energy: Warming the Earth and the Atmosphere Heat vs. Temperature: Heat is defined as the total energy of motion of molecules in a system. It is a form of energy. At ground level, molecules of air don’t move that fast, but there are so many of them, that there is a substantial amount of heat. Temperature is defined as the “average” energy of motion of molecules in a system. At the top of the atmosphere, individual molecules can move very quickly (thus they have a very high temperature) but there are so few of them that the total energy (the heat) is almost non-existent. At the surface, we can consider heat and temperature to be interchangeable terms. Temperature scales: In the U.S., we use the Fahrenheit scale. Water freezes at 32oF and boils at 212oF. The rest of the world uses the Celsius scale, in which water freezes at 0oC and boils at 100oC. The 9-5 ratio of Fahrenheit to Celsius give rise to conversion equations: C=5/9 (F-32) and F-(9/5 C) +32 A final scale, called the Kelvin scale is used by physicists. It uses Celsius degrees, but is shifted from the Celsius scale by 273. Water freezes at 273K and boils at 373K. Latent vs. Sensible Heat: Latent heat cannot be measured by a thermometer. Adding latent heat to a substance does not change its temperature. Sensible heat can be measured by a thermometer. When you heat a pot of water, the rising temperature is a measurement of the sensible heat being added to the water. Latent heats are usually associated with phase changes (ice to water, water to vapor, ice to vapor) in either direction. When water evaporates (goes from liquid to vapor) the heat required (latent heat of vaporization) the energy put into the water is used to break chemical bonds between oxygen and nitrogen. None of that energy is used to heat the water, so a thermometer won’t show a temperature change during the phase change. Going from a lower to a higher state requires energy. Going from a higher to a lower state releases heat into the surrounding atmosphere. The latent heat of vaporization of water is very high (600 cal/g). Latent heat of melting is only 80 cal/g. And latent heat of sublimation (ice to vapor) is 680 cal/g. Specific heat: The energy required to raise the temperature of a gram of a substance one degree Celsius. Water has a relatively high specific heat (1 cal/g). Other substances require less heat to raise their temperature. Heat Transfer: Energy (heat) can be transferred from one substance to another by various mechanisms. The direction of transfer is always from the hotter to the colder substance. Conduction is the direct transfer by physical conduct. Some substances are good conductors of heat (gold, copper, etc.). Air is a very poor conductor of heat. Radiation is the transfer of heat by electromagnetic waves. This is how the energy from the sun reaches the Earth. Radiation works in a vacuum as well as in the atmosphere. The farther you get from the source, the less you feel the radiated heat. Convection is a very powerful method of transferring heat vertically in gasses or liquids. The gas/liquid is heated by conduction and begins to rise (warm fluids are less dense—lighter—than cold fluids) and carries the heat upward. All three processes are at work transferring heat from the earth’s surface to the atmosphere. Earth’s Energy Budget: Sunlight contains all the wavelengths of light, from Gamma Rays to X-Rays to UV Rays to Visible Light to Infrared (IR) Rays to Microwaves and Radio/TV Waves. However, about 95% of sunlight is visible light. Visible light penetrates our atmosphere and about 50% reaches the ground and warms the ground. Of the remaining 50%, some is absorbed by atmospheric gasses and the rest (about 30-32%) is reflected by clouds and the surface. That portion of incoming sunlight that is reflected is called the Earth’s albedo. Different substances have different albedoes. See the table in the text. As the ground heats up, it radiates IR radiation back into space. Some of that IR radiation is absorbed by CO2, water vapor and ozone and is re-emitted back down to the surface, warming the earth even more. This is essentially what happens in a greenhouse (glass lets visible light through but doesn’t let IR through), so this is called the “greenhouse effect.” Climatologists are concerned that increasing amounts of CO2 in the atmosphere will raise the earth’s temperature to dangerous levels (global warming). In spite of what you might read in the news, the human contribution to global warming is still being debated. Chapter 3 Seasonal and Daily Temperatures Motions of Earth: The earth spins on its axis once per 24 hours. This is called rotation. The earth also orbits the sun once every 365 ¼ days. This motion is called revolution. One full revolution is called a year. The earth’s orbit is not perfectly circular, but rather is just slightly elliptical, with the sun at one focus of the ellipse (see the text for a diagram). That means that we are not always the same distance from the sun. The earth makes its closest approach to the sun on January 4th each year. That point in our orbit is called perihelion. We are farthest from the sun six months later on July 4th (aphelion). At perihelion we are 148 million km away, and at aphelion we are 152 million km away from the sun. The seasons: The earth’s axis is NOT vertical to its orbital plane, but is inclined at an angle of 23 ½ degrees from vertical. This changes the angle at which the sun’s rays strike the ground from one location to another, and from one month to another. When the sun is higher in the sky, the rays are more intense and heat the ground more effectively. When the sun is lower in the sky, the rays come in at a shallow angle and are not very effective in heating the ground (Fig. 3.2, page 56). During the months of June, July and August, the northern hemisphere is tilted in toward the sun, resulting in more intense rays and warmer temperatures (our summer). Also, you can see in Fig. 3.8, page 61 that the higher angle results in more hours of daylight in the northern hemisphere. On June 21-22 each year, the direct ray of the sun strikes the Tropic of Cancer at 23 ½ degrees North. Daylight is 12 hours long at the equator and increases as you go north, reaching 24 hours at The Arctic Circle, 66 ½ degrees North. This is the first day of summer in the northern hemisphere. Six months later, the direct rays of the sun are striking the Tropic of Capricorn (Dec. 21-22)—the first day of our winter. Daylight increases in the southern hemisphere, reaching 24 hours at the Antarctic Circle. There is no daylight on that day at the Arctic Circle. At the Equator, the days and nights are always 12 hours long. During the intervening three-month periods (on March 21 and Sept. 21) all locations receive 12 hours of daylight. These are known as equinox days. The Vernal Equinox occurs on March 21-22 and is the first day of spring. The autumnal equinox occurs on Sept. 21-22 and is the first day of fall. Lag of Seasons: It takes time for the earth to heat up and cool down in response to sunlight. Because the surface is 71% water, and water has a high specific heat, the oceans heat and cool slowly. It usually takes about 4-5 weeks for the atmosphere (which, you recall, is heated from below) to reach its maximum temperature after June 21. So our hottest days of summer are in late July and early August. In winter, minimum sunlight occurs on Dec. 21, but the coldest days of winter are usually in late January and early February. This delay in hot/cold vs. max/min solar radiation is called the lag of seasons. This lag also works on a daily basis. Max sunlight is at solar noon, but the daily high is usually between 2 and 3 PM. At night, under clear skies, the Earth loses heat to space by radiational cooling. As the ground gets cold, the air touching the ground gets cold (conduction). But the air higher up does not get as cold (air is a poor conductor of heat), resulting in a low level radiation inversion. Temperature inversions often form in valleys as cold air (which is heavier and denser) drains downhill into the bottom of the valley. The hillsides above are left with warmer air, resulting in an inversion at the floor of the valley. Measuring temperature: Temperature is measured using a thermometer. Older thermometers use mercury or alcohol sealed in an evacuated glass tube. As air molecules strike the bulb, they cause the liquid to expand up the tube. Alcohol must be used when temperatures drop below -30oC, as mercury would freeze at those temperatures. More recently, electrically operated thermometers are becoming more common. The most frequently used type is the thermistor, which sends and electrical current through a wire whose resistance changes as the temperature changes. These are much more sensitive and reactive than mercury/alcohol thermometers. The most sensitive thermometers are called thermocouples. They are often used in fever thermometers that are placed in one’s ear.