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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)