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The atmosphere Hálfdán Ágústsson, with contributions from Guðrún Nína Petersen, Einar Sveinbjörnsson and Halldór Björnsson 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 10-14 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 Atmospheric composition Gas % of volume N2 78,08 02 20,95 Ar 0,93 +trace amounts of some gasses Gas % of volume H2O 0-4 CO2 0,038 CH4 (metan) 0,00003 O3 (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. apríl 2010 Solar radiation Solar constant 1372 W/m2 Rotation axis tilt 23.5° Heats the surface and the atmosphere Einar Sveinbjörnsson Solar radiation Solar radiation Solar radiation Incoming and outgoing radiation Equation of state for an ideal gas P=TxρxC P~Txρ P – pressure (hPa) T – temperature (K) ρ – density (kg/m3) C - constant If the temperature is the same then the pressure is only dependant on the density Equation of state for an ideal gas A B P=TxρxC What is the pressure at the surface under two columns of identical height and temperature? P <, =, > P Equation of state for an ideal gas A B What if A – gets warmer B - gets colder P=TxρxC Not so simple as an increase in temperature will lead to an expansion of the 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 = Σ F = Fg + Fpg + C + Ff + ... 0. Gravity Hydrostatic equilibrium Height Pressure gradient Pressure gradient = gravititional force gravity 1. Pressure gradient forces H Pressure gradient 1000 km 1020 hPa L 980 hPa 40 hPa / 1000 km tvær myndir + MÁE 2. Coriolis force Because of the rotation of the earth, all moving particles in the atmosphere experience the Coriolis effect / force. 2 f Ω sin φ , where φ is the latitude and Ω is the angular speed of the rotation. Equilibrium of the Coriolis and pressure gradient forces Geostrophic wind 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 Thermodynamic Continuity Thermodynamic Equation of state Differential versions of equation describing atmospheric motion Momentum Thermodynamic Continuity Thermodynamic Equation of state 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 Precip (mm) Temp. (°C) Phoenix, Arizona Max and min temperature and precip. Temp. RH (%) (°C) Phoenix, Arizona Max and min temperature and relative humidity. Relative humidity Saturation vapor pressure Cloud condensation nuclei Cloud condensation nuclei Evaporation and precipitation P>E P<E P<E Evaporation and precipitation Evaporation Mean evaporation on land in july 1985-1999 (mm/month)