The atmosphere Hlfdn gstsson, with contributions from Gurn Nna Petersen, Einar Sveinbjrnsson and Halldr Bjrnsson Weather happens here Some facts Weather happens in the troposphere, where roughly 90% of the mass of the atmosphere is located The tropopause is at roughly at km above the surface, higher at the equator than at the poles The temperature on average decreases with elevation in the troposphere but increases in the stratosphere Atmospheric pressure is a measure of the weight of the above air column Atmospheric pressure decreases with height Gas% of volume N2N2 78, ,95 Ar0,93 +trace amounts of some gasses Gas% of volume H2OH2O0-4 CO 2 0,038 CH 4 (metan)0,00003 O 3 (ozon)0,01 Atmospheric composition Aerosols Pollution: Natural and anthropogenic Cloud condensation nuclei Radiation The atmosphere absorbs radiation Gas, particles, water drops Ultraviolet radiation and ozon Visible radiation absorbed Infrared radiation absorbed by GHG The atmosphere reflects and scatters Raileigh scattering Mie scattering Raileigh scattering Gasses Short wavelengths Mostly visible light Blue sky Red sunsets and sunrises Mie scattering Particles, pollution, dust All wavelengths Mostly visible light Redder sunsets and sunrises Dublin 16. aprl 2010 Solar radiation Solar constant 1372 W/m2 Rotation axis tilt 23.5 Heats the surface and the atmosphere Einar Sveinbjrnsson Solar radiation Incoming and outgoing radiation P = T x x C P pressure (hPa) T temperature (K) density (kg/m 3) C - constant P ~ T x If the temperature is the same then the pressure is only dependant on the density Equation of state for an ideal gas AB What is the pressure at the surface under two columns of identical height and temperature? P = T x x C PP Equation of state for an ideal gas AB What if A gets warmer B - gets colder Not so simple as an increase in temperature will lead to an expansion of the gas P = T x x C Equation of state for an ideal gas Distances between pressure levels Cold Atmospheric forces and airflow 0. Gravity 1. Pressure gradient force 2. Coriolis force 3. Frictional force at surface (4. centripetal force) ma = F = F g + F pg + C + F f +... 0. Gravity Hydrostatic equilibrium Height gravity Pressure gradient Pressure gradient = gravititional force HL 1020 hPa980 hPa Pressure gradient 1000 km 40 hPa / 1000 km 1. Pressure gradient forces tvr myndir + ME Because of the rotation of the earth, all moving particles in the atmosphere experience the Coriolis effect / force. 2. Coriolis force 2 f sin , where is the latitude and is the angular speed of the rotation. Geostrophic wind Equilibrium of the Coriolis and pressure gradient forces Denser isobars -> stronger winds Geostrophic wind at 300 hPa - No surface friction Surface friction Surface winds, with surface friction Differential versions of equation describing atmospheric motion Momentum Equation of state Continuity Equation of state Thermodynamic Differential versions of equation describing atmospheric motion Momentum Equation of state Continuity Equation of state Thermodynamic Atmospheric water Absolute humidity vs. specific humidity Atmospheric water Specific humidity Absolute humitity Relative humidity How close to saturation? Warmer air can hold more water vapour than colder air When saturated we will have clouds forming Dew point Vapor pressure Saturation vapor pressure Temp. (C) Max and min temperature and precip. Precip (mm) Phoenix, Arizona Temp. (C) RH (%) Phoenix, Arizona Max and min temperature and relative humidity. Relative humidity Saturation vapor pressure Cloud condensation nuclei P>E P