Chapter 1 Summary and Review

The earth’s first atmosphere from about 4.5 billion years ago was substantially different than today, with hydrogen and helium being the primary gases. Over time, forces such as comet and asteroid impacts, volcanic outgassing, bacterial processes, and escaping gases changed the composition of the atmosphere into what it is today. In today’s atmosphere the lighter gases such as hydrogen and helium reside in the very top part of the atmosphere, called the heterosphere since its composition varies. In the lower 80 km, the homosphere, the atmosphere’s composition is fairly constant. Permanent gases make up the majority of the atmosphere, including nitrogen (78%), oxygen (21%), and argon (1%). Other gases vary, but are important for the atmosphere’s behavior. Water vapor occupies 0.25% to 4% of the atmosphere, varying due to evaporation from oceans and precipitation. Water vapor is extremely important for day to day weather and can even affect temperature with its ability to trap energy (greenhouse effect). Like water vapor, carbon dioxide is a greenhouse gas. It is released by exhalation, decay, volcanoes, and burning. Plants absorb CO₂ for photosynthesis, so concentrations vary seasonally, although the total has been increasing over time. Ozone in the upper atmosphere protects us from ultraviolet radiation, but near the surface it can be an irritating pollutant. Chlorofluorocarbons (CFC’s) can have a detrimental effect on the chemical reactions that produce ozone. The variable gas methane is produced by human mining and agriculture and like fellow greenhouse gas CO₂, has been rising in concentration. Along with gases in the atmosphere, there are also tiny particles of solids and liquids suspended in the air, called aerosols.

The atmosphere varies in density (mass/volume) with height. More molecules and thus greater density are concentrated at the earth’s surface, decreasing with height until there are nothing but a few molecules left. Temperature also varies with height, and the atmosphere can be divided into layers accordingly. In the lowest layer, the troposphere, temperature decreases with height. Most weather occurs in this layer. The stratosphere has increasing temperature with height, which prevents air from rising upwards, and thus only the tops of thunderstorms can penetrate this layer. In the mesosphere, temperature again decreases with height. The temperatures in the thermosphere increase dramatically, but the sparse molecules at this level prevent it from feeling warm. The earth also has a layer called the ionosphere that has an effect on radio waves. In comparison to the other planets in the solar system, the earth is very hospitable. Venus has an extremely dense atmosphere of CO₂, nitrogen, and sulfuric acid clouds. Mars’s atmosphere is much less dense, consisting mostly of CO₂. The outer planets are mostly atmospheres of hydrogen and helium, with small solid cores.

Know

• The theory on earth’s early atmosphere and how it changed.
• Residence time and the homosphere and heterosphere.
• What the most common permanent gases are.
• Which gases are variable – and what causes them to vary.
• What aerosols are and what they can do to the atmosphere.
• The concept of density, and how it varies with height in the atmosphere.
• The four temperature layers in the atmosphere, and what occurs in them.
• How earth’s atmosphere compares to the other planets.

Chapter 2 Summary and Review

Any process requires energy. Energy can be divided into two different categories; potential energy which is stored energy, and kinetic energy which is the energy of movement. Energy cannot be created or destroyed, but it can be transferred in one of three ways: conduction, convection, and radiation. Radiation is the only one that can work in outer space. Electromagnetic radiation travels in waves, with the wavelength determining the properties of the energy. EM radiation varies in wavelength across a spectrum, from gamma, X-rays, ultraviolet, visible light, infrared, microwave, and radio.

A blackbody is a hypothetical object that radiates the maximum possible radiation at every wavelength. The Stefan-Boltzmann Law states that temperature determines how much a blackbody emits. Since the formulation of the Stefan-Boltzmann Law states I=sT⁴, the fourth power indicates that a small increase in temperature causes a large increase in radiation emitted. Since blackbodies don’t really exist in nature, a factor called emissivity must be added to the law. Wien’s Law states that the peak wavelength emitted by a blackbody depends on temperature. So the sun, which is much hotter that the earth, emits radiation at a shorter wavelength. Not all radiation emitted by the sun reaches the earth due to the inverse square law, which states that radiation intensity decreases in intensity in proportion to the distance squared. Only about 1367 W/m² reaches the earth, an amount called the solar constant.

The earth’s orbit is not a perfect circle, it is closes to the sun (perihelion) on January 3 and is farthest (aphelion) on July 3. This has little affect on our seasonal temperature variation. The earth tilts on its axis toward the North Star, a tilt of 23.5^o. On June 21 (or so) the Northern Hemisphere is tilted towards the sun. The sun is directly above the latitude 23.5^o N at noon (solar declination). Therefore the Northern Hemisphere receives more direct (intense) sunlight, and remains in the sun’s illumination for longer periods of time. Around December 21, the opposite occurs. The Southern Hemisphere is tilted toward the sun and receives the more direct sunlight for longer periods. In between the solstices, are the equinoxes, when the earth’s axis is tilted neither toward nor away from the sun, and both hemispheres receive the same amount of daylight.

Know:

• The differences in the types of energy.
• The methods of energy transfer.
• The electromagnetic spectrum and which types have the longer wavelengths.
• The temperature scales (F, C, and K)
• The physical principles of the Stefan-Boltzmann Law, Wien’s Law, and the inverse square law.
• What a blackbody is.
• The cause of the seasons, what the solstices and equinoxes are.
• What solar declination angle is and how it changes with the seasons.
• Why length of daylight and angle of sunlight have an effect on seasons.

Chapter 3 Summary and Review

When the sun’s radiation enters the earth’s atmosphere it is either absorbed, reflected, scattered or transmitted. Ultraviolet and infrared radiation are mostly absorbed by the atmosphere, while much of the visible light is not. Visible light is often reflected, particularly by surfaces with a high albedo, such as snow. The short wavelengths of visible light are scattered by atmospheric gas molecules, a process called Rayleigh scattering. Rayleigh scattering makes daytime skies blue. Mie scattering is produced by aerosols and is not dependant on wavelength. Thus a sky with a lot of aerosols would appear with a white or gray tint. The nonselective scattering of cloud droplets, like Mie, scatter all visible wavelengths

Besides incoming solar radiation, the earth is also emitting longwave radiation. Some escapes into space, but some is absorbed by greenhouse gases such as carbon dioxide and water vapor, trapping the infrared radiation and keeping the earth warmer. The greenhouse gases let some wavelengths escape through an “atmospheric window”, which is “closed” when clouds are present, trapping the extra heat. Considering net radiation, the ground has a surplus and the atmosphere a deficit. Energy is transferred through conduction and convection to try and even the balance. There is also the transfer of sensible heat from the ground. The amount of sensible heat produced by a substance depends on its specific heat, or the amount of energy needed for a certain temperature change. Water has a higher sensible heat than land, and thus takes longer to heat and cool. There is also latent heat, which is released by condensation and absorbed by evaporation. Adding sensible and latent heat to net radiation evens out the energy balance into equilibrium.

The radiation balance only considers the entire earth averaged over a period of time. Looking at energy and temperature over one day, solar radiation has to exceed the outgoing radiation for a while, producing a lag in temperature, peaking 2-4 hours after noon. Similarly, the highest temperatures during the year occur in July or August, after the solstice. Different factors can affect temperature. Locations at a higher latitude receive less radiation and are cooler. Stations in the mountains often are cooler than the valleys, and cool off very rapidly at night. Since the specific heat of water is high, stations near the oceans often are less variable in temperature than inland locations. Atmospheric circulation and ocean currents try to redistribute heat, while local effects can also influence temperature. Temperature variations can cause layers of different densities to form, producing optical effects such as mirages.

Know:

· What radiation does as it enters the atmosphere.

· The different types of scattering.

· What albedo is.

· Understand the radiation balance.

· The difference between latent and sensible heat.

· What factors influence temperature variations.

· A bit about atmospheric optics.

Chapter 4 (thus far) Summary and Review

Water is extremely important to the weather, and can exist in all three states in the atmosphere. Water changes from liquid to gas by evaporation, and from gas to liquid by condensation. When evaporation and condensation are in equilibrium in a hypothetical sealed container of water, the air in the container is said to be saturated. At higher temperatures, it takes more water vapor to reach saturation. Water vapor content can be expressed in numerous ways, including vapor pressure, specific humidity, and mixing ratio. Saturation vapor pressure, saturation specific humidity, and saturation mixing ratio give the theoretical amount of water vapor possible at a certain temperature. When vapor pressure equals saturation vapor pressure, for example, the air is saturated. Relative humidity does not give the amount of water vapor, but instead is a ratio of how close the air is to saturation. Since higher temperatures take more water vapor, a temperature change can cause a change in relative humidity, even if there is no change is water vapor. The dew point gives the temperature to which air would have to be cooled for saturation. Similarly, wet bulb temperature, measured with a sling or aspirating psychrometer, gives a cooled temperature after water has evaporated off a wick. The difference between the wet bulb and the dry bulb temperature is called the wet bulb depression, and can be used to calculate dew point or relative humidity.

There are three different ways to achieve saturation: adding more water vapor, mixing cold air with warm moist air, and cooling the air down to the dew point. Air can be cooled by a diabatic process, in which heat is added or removed, or an adiabatic process, in which the temperature changes but no heat is added or removed. Instead, as a gas expands, it cools, and as it contracts, it warms. Thus a dry parcel rising from the ground would expand and cool off at the dry adiabatic lapse rate (about 1^oC/100 m). Eventually the temperature of the parcel cools off to the dew point and condensation starts, this altitude is called the lifted condensation level. After condensation starts, the parcel continues to rise and cool, but at a slower rate due to the release of latent heat. This rate is called the saturated adiabatic lapse rate, and varies a bit with temperature. The air surrounding the parcel also cools off with increasing height, a rate called the environmental lapse rate.

Know:

· What saturation is.

· The relationship between temperature and saturation.

· The different ways of looking at humidity: vapor pressure, absolute, specific and relative humidity, dew point, and mixing ratio.

· The different ways to achieve saturation.

· The laws of thermodynamics and how they relate to temperature.

· The three different lapse rates and what the lifted condensation level is.