Lecture #6: Air Pressure, Density, and Temperature Structure Temperature and Processes affecting Temperature: The amount of gas present in the atmosphere, the temperature of the gas, and gravity determine the structure of the Earth’s atmosphere. The temperature of the atmosphere differs from location to location, with time, and with altitude. Temperature is proportional to the average kinetic energy of an air molecule: constant x T = ½ x M x v2 where T = temperature, M = average mass of one air molecule, and v = average speed of an air molecule. So the higher the temperature, the faster air molecules will travel in the atmosphere. Gravity keeps these molecules from flying out into space. In order for an air molecule to escape the Earth, it must be high enough (so it won’t collide with another molecule before escaping), and hot enough (traveling fast enough). Fortunately for us, very few molecules can escape the Earth’s gravitational field. Near the surface of the Earth, molecules cannot travel more than 1/10,000,000 meter before colliding with another molecule, making the lower part of the atmosphere well mixed. The temperature at a given location and time is affected by energy transfer processes: CONDUCTION by molecular motions, the dominant mechanism in solids and also important in the upper layers of the atmosphere (above 100 km) where molecules are relatively far apart. The medium as a whole experiences no molecular movement. CONVECTION by fluid motions, the dominant mechanism in the oceans and important in the lower atmosphere. Convection is the transfer of energy, gases and particles by mass movement of air predominantly in the vertical direction. ADVECTION by wind. Similar to convection, but in the horizontal direction. RADIATION consisting of waves traveling at the speed of light: the only mechanism capable of transferring energy through a vacuum (e.g., between the sun and the rest of the universe). Temperature is measured in degrees centigrade (C), fahrenheit (F), and Kelvin (K), and are related to each other as follows: ºC = (ºF – 32º) x (5/9) ºF = ºC x (9/5) + 32º ºC = K – 273.16 Temperature Profiles:The bottom 100 km of the atmosphere is called the homosphere. The homosphere is broken up into layers according to the temperature profile shown below. The transition from one region to another is determined by the rate of temperature change with height. OPOSPHERE: The lowest 10 – 15 km of the atmosphere. In the troposphere, the ce. m) TRATOSPHERE: The stratosphere is characterized by a temperature inversion, s he Temperature Versus AltitudeFigure 3.30102030405060708090100180 200 220 240 260 280 300101326555122.90.80.220.0520.0110.00180.00032Temperature (K)TropopauseStratopauseMesopauseStratosphereTroposphereMesosphereThermosphereOzonelayerPressure (mb)Altitude (km)Pressure (mb)TRtemperature decreases with increasing altitude. This is because sunlight heats the surfaThe troposphere contains 90% of the mass of the atmosphere. Some of this heat is redistributed through CONVECTION. All weather occurs in the troposphere. The troposphere can be further broken up into the boundary layer (surface to 500- 3000and the free troposphere. The variation of temperature within the boundary layer is shown in Figure 3.4 in your text. Screating stability (low mixing). A temperature inversion is when warm air overliecooler air. Here the air is stable against vertical motions. The stratosphere contains about 9.5% of the mass of the atmosphere, and extends to a height of about 50 km. Tstratosphere contains the ozone (O3) layer, which absorbs UV radiation and re-emits thermal IR radiation (we will cover the stratospheric ozone layer in week 9). The absorbtion of UV radiation by ozone causes heating in the stratosphere.MESOSPHERE: When ozone concentrations decrease, the atmosphere begins to cool again. This region of decreasing temperature is called the mesosphere (~50 – 100 km). The mesosphere holds less than 1% of atmospheric mass. THERMOSPHERE: In the thermosphere (~100 – 400 km), the air (O2 and N2) is heated by X-rays and other dangerous radiation from the sun. Fortunately for us, this prevents this harmful radiation from reaching the surface of the Earth. Air Pressure and Density Structure: The pressure and density of the atmosphere are closely related - both decreasing with altitude. At the surface, the pressure is about 1013 millbars (mb). Altitude (z)Pressure (P) , density(ρ) Air pressure (force per unit area) is the summed weight of all gas molecules between a horizontal plane and the top of the atmosphere divided by the area of the plane: P = F/A where P = pressure [N/m2 or mb], F = force [N], and A = area [m2] Density of air is mass of air per unit volume of air: ρ = M/V where ρ = density [kg/m3], M = mass [kg], and V = volume [m3].Equation of State: Pressure, density, and temperature are related by the equation of state. The relationship between pressure and volume is expressed by Boyle’s law: PiVi = PfVf (at constant temperature) The subscript i = initial, and f = final. The relationship between volume and temperature is expressed by Charles’ law: Vi/T0 = Vf/Tf (at constant pressure) Avogadro’s law expresses the relationship between the total number of molecules of a gas, and the volume it occupies: V ∝ n (read as V is proportional to n) where V = volume [m3], and n = total number of molecules. Combining these three laws (Boyle’s, Charles’, and Avogadros’) gives you the ideal gas law, or equation of state: PV = nRT where R is a constant (the universal gas constant). Examples of the above relationships will be provided in class. Mixing Ratios: Dalton’s law of partial pressures states that the pressure of a gas mixture equals the sum of the pressures exerted by the individual gases. For example, for a mixture of O2 and N2, the total pressure of the gas mixture (Ptotal): Ptotal = PO2 + PN2 The number concentration of a gas (N = number of molecules per unit volume of air) is an absolute quantity (e.g. it doesn’t matter how many other types of gas molecules are present). In the atmospheric sciences, the concentration of gas is usually expressed in relative terms (as opposed to absolute terms such as the number concentration). This relative quantity is called the mixing ratio (χ): χ = Nq/Nair = Pq/Pair (where q = gas q - for example O2, or CO2) The