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 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. apríl 2010

Solar radiation Solar constant 1372 W/m2 Rotation axis tilt 23.5° Heats the surface and the atmosphere Einar Sveinbjörnsson

Solar radiation

Incoming and outgoing radiation

P = T x ρ x C P – pressure (hPa) T – temperature (K) ρ – density (kg/m 3) C - constant P ~ T x ρ 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 ρ 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 ρ 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 = Σ 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

tvær myndir + MÁE

Because of the rotation of the earth, all moving particles in the atmosphere experience the Coriolis effect / force. 2. Coriolis force 2 f Ω sin φ, where φ is the latitude and Ω 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<E Evaporation and precipitation

Mean evaporation on land in july (mm/month) Evaporation