METO 637

Lesson 1

Fig. 1.14

1. Troposphere- literally means region where air “turns over”-temperature usually decreases (on average ~6.5°C/km) with altitude

2. Stratosphere- layer above the tropopause, little mixing occurs in the stratosphere, unlike the troposphere, where “turbulent mixing” is common

3. Mesosphere- defined as the region where temperature again decreases with height.

4. Thermosphere- region with very little of the atmosphere’s mass. high energy radiation received by the thermosphere (high temperatures experienced). A small density of molecules (not much “heat” would be felt).

Tropopause

Stratopause

Mesopause

HYDROSTATIC EQUATION

Where H is called the atmospheric scale height

HYDROSTATIC EQUATION-2

HYDROSTATIC EQUATION-3If we assume that g, T, and M* are constant then we get the equations

H is called the scale height

Atmospheric Pressure

• Pressure at a point is the weight of air above that point

• A column of air at the surface weighs about 1 kilogram per square cm.

• Ideal gas law PV = nRT• However the atmosphere also contains water

vapor which can condense at certain temperatures. In this case the ideal gas law does not hold

Fig. 1.12

Adiabatic Lapse Rate

• The First Law of Thermodynamics can be expressed as:

dU = dq + dw where dU is the change in internal energy, dq is

the heat supplied to the system, and dw is the is the work done on the system.

• dH, the change in enthalpy, can be written as dH = dU + pdV + Vdp

• When we raise a parcel of air there is no heat input, hence dq=0 (adiabatic) and dw=pdV

• Therefore dH = -Vdp

Adiabatic Lapse Rate

• The heat capacity of a gas at constant pressure, Cp, is defined as (dH/dT) so that

Cp dT= Vdp

• From the hydrostatic equation we get

dp = -g σ dz

• Hence Cp dT = -V g σ dz

• For a unit mass of gas V=1/σ and we get

dpC

g

dz

dT

Adiabatic Lapse Rate

• For Venus, Earth, Mars and Jupiter the calculated values of Γd are 10.7, 9.8, 4.5 and 20.2 K per kilometer.

• The dry adiabatic lapse rate plays an important role in atmospheric stability.

Fig. 3.17

Lapse Rates and Stability

• Lapse rate is the rate at which the real atmosphere falls off with altitude – the environmental lapse rate

• An average value is 6.5 ºC per kilometer• This should be compared with the adiabatic

lapse rate of 10 ºC.• If the environmental lapse rate is less than 10

ºC, then the atmosphere is absolutely stable• If greater than 10 ºC, it is absolutely unstable

Wet adiabatic lapse rate

• The presence of condensable vapors, such as water vapor, complicates the process.

• As the parcel of air ascends it cools at the dry adiabatic lapse rate until the water vapor reaches saturation – then condensation takes place.

• This releases latent heat – which can raise the temperature of the air parcel.

• Now the lapse rate depends on the amount of water vapor – wet adiabatic lapse rate.

Temperature inversions produce very stable atmospheric conditions in which mixing is greatly reduced. There are two general types of inversions: surface inversions and inversions aloft.

Surface inversions are the result of differential radiative properties of the Earth’s surface and the air above. The Earthis a much better absorber and radiator of energy than air; thus,in the late morning and afternoon hours the lower atmosphere isunstable. The opposite is true in the evening; a stable atmosphere with little vertical mixing prevails.

Role of atmospheric stability

The Nocturnal Inversion

• On clear nights, a temperature inversion develops near the surface.

- Air temperature usually decreases with height. An inversion is a layer of air where temperature increases with height.- Because the layer of air in the inversion is warmer

than the air below it, the cooler air below the inversion cannot rise above it. Pollutants near the surface are therefore trapped below the inversion in the overnight hours.

Fig. 3.18

Inversions aloft are associated with prolonged, severe pollutionepisodes. These types of inversions are caused by the sinking airassociated with the center of high pressure systems (subsidence).As the air sinks it is warmed adiabatically. Turbulence at the verylowest part of the atmosphere prevents subsidence from warmingthat portion of the atmosphere.

Los Angles pollution episodes as well as those over the Mid-Atlantic region are the result of inversions aloft associated with stronghigh pressure systems.

Role of Atmospheric Stability

Temperature Inversions

N2

O2

Ar

O3

Inert gasesCO2

H2

←SO2, NO2,CFC’s, etc

PM

COCH4

N2O

Composition of the Earth’s Troposphere

Atmospheric composition