Overview of the Earth s Atmosphere Composition 99% of the atmosphere is within 30km of the Earth s surface. N 2 78% and O 2 21% The percentages represent a constant amount of gas but cycles of destruction and production are constantly maintaining this amount. Fig. 1 7, p. 10 Chapter 2 Energy: Warming the earth and Atmosphere Chapter 3 air Fig. 1 9, p. 11 1
Comparison of Kelvin, Celsius, and Fahrenheit scales, along with some world extremes. Air is a measure of the average speed of the molecules (kinetic energy). In the cold volume of air the molecules move more slowly and crowd closer together (left). In the warm volume, they move faster and farther apart (right). Conversions: C = 5/9 x ( F 32) K = C + 273 Latent Heat = the energy required to change a substance (like water) from one phase to another (eg. Solid to liquid). eg. Because it takes energy to evaporate water (typically taken from the water itself), the water left behind has less energy and is cooler. Every time a cloud forms, it warms the atmosphere. Inside this developing thunderstorm, a vast amount of stored heat energy (latent heat) is given up to the air, as invisible water vapor becomes countless billions of water droplets and ice crystals. 2
The transfer of heat from the hot end of the metal pin to the cool end by molecular contact is called conduction. The rising of hot air and the sinking of cool air sets up a convective circulation. Normally, the vertical part of the circulation is called convection, whereas the horizontal part is called wind. Near the surface the wind is advecting smoke from one region to another. Rising air expands and cools; sinking air is compressed and warms. Why? As the air parcel rises, the pressure decreases, allowing the parcel to expand. This takes energy (to expand) and the air molecules slow down, so the parcel cools. As the parcel sinks, the parcel volume shrinks (due to increased pressure). Because the air molecules have less space to bounce around, their velocity increases, causing a rise in. Radiation characterized according to wavelength. As the wavelength decreases, the energy carried per wave increases. The sun s electromagnetic spectrum and some of the descriptive names of each region. The numbers underneath the curve approximate the percent of energy the sun radiates in various regions. 0.4 μm = 400 nm 0.7 μm = 700 nm 3
Absorption of radiation by gases in the atmosphere. The shaded area represents the percent of radiation absorbed by each gas. The strongest absorbers of infrared radiation are water vapor and carbon dioxide. The bottom figure represents the percent of radiation absorbed by all of the atmospheric gases. The hotter sun not only radiates more energy than that of the cooler earth (the area under the curve), but it also radiates the majority of its energy at much shorter wavelengths. (The area under the curves is equal to the total energy emitted, and the scales for the two curves differ by a factor of 100,000.) (a) Near the surface in an atmosphere with little or no greenhouse gases, the earth s surface would constantly emit infrared (IR) radiation upward, both during the day and at night. Incoming energy from the sun would equal outgoing energy from the surface, but the surface would receive virtually no IR radiation from its lower atmosphere. (No atmospheric greenhouse effect.) The earth s surface air would be quite low, and small amounts of water found on the planet would be in the form of ice. (b) In an atmosphere with greenhouse gases, the earth s surface not only receives energy from the sun but also infrared energy from the atmosphere. Incoming energy still equals outgoing energy, but the added IR energy from the greenhouse gases raises the earth s average surface to a more habitable level. Air in the lower atmosphere is heated from the ground upward. Sunlight warms the ground, and the air above is warmed by conduction, convection, and infrared radiation. Further warming occurs during condensation as latent heat is given up to the air inside the cloud. 4
On the average, of all the solar energy that reaches the earth s atmosphere annually, about 30 percent (30/100) is reflected and scattered back to space, giving the earth and its atmosphere an albedo of 30 percent. Of the remaining solar energy, about 19 percent is absorbed by the atmosphere and clouds, and 51 percent is absorbed at the surface. The earth atmosphere energy balance. Numbers represent approximations based on surface observations and satellite data. While the actual value of each process may vary by several percent, it is the relative size of the numbers that is important. Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays. The average annual incoming solar radiation (yellow line) absorbed by the earth and the atmosphere along with the average annual infrared radiation (red line) emitted by the earth and the atmosphere. 5
The apparent path of the sun across the sky as observed at different latitudes on the June solstice (June 21), the December solstice (December 21), and the equinox (March 20 and September 22). On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface. The daily variation in air is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth s surface. Where incoming energy exceeds outgoing energy (orange shade), the air rises. Where outgoing energy exceeds incoming energy (gray shade), the air falls. 6
On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air with increasing height above the surface is called a radiation inversion. An idealized distribution of air above the ground during a 24 hour day. The curves represent the variations in average air above a grassy surface for a midlatitude city during the summer under clear, calm conditions. (a) Clouds tend to keep daytime s lower and nighttime s higher, producing a small daily range in. (b) In the absence of clouds, days tend to be warmer and nights cooler, producing a larger daily range in. Monthly data and annual range for (a) St. Louis, Missouri, a city located near the middle of a continent and (b) Ponta Delgada, a city located in the Azores in the Atlantic Ocean. Monthly data and annual range for (a) St. Louis, Missouri, a city located near the middle of a continent and (b) Ponta Delgada, a city located in the Azores in the Atlantic Ocean. 7
Temperature data for (a) San Francisco, California (38 N) and (b) Richmond, Virginia (38 N) two cities with the same mean annual. Temperature data for (a) San Francisco, California (38 N) and (b) Richmond, Virginia (38 N) two cities with the same mean annual. Mean annual total heating degree days across the United States (base 65 F). 8