Lecture 2: Composition

1
Lecture 2 Composition Structure of the
Atmosphere (Ch1)

• Composition (continued)
• Vertical structure of the atmosphere

Hurricane Florence, a problem for Bermuda on
Monday, was expected to bring strong winds and
heavy rain to eastern Newfoundland on Wednesday.
(NOAA) by that time, Florence will probably be
gearing down from a hurricane to a "very intense
post-tropical storm," Environment Canada's
Canadian Hurricane Centre said in a statement
issued early Monday.
2
O3 (ozone) and the ozone hole

• radiatively/climatologically active (accounts
  for
• peak T in mid-atmosphere)
• inactive relative to weather
• essential relative to life absorbs ultraviolet
  radiation
• peak concentration of up to about 15 ppm in
  mid-stratosphere

• depletion in spring over poles (esp. Antarctica)
• reactions on surface of polar stratospheric ice
  clouds
• over long term, size of ozone hole governed by
  emissions of CFCs, lifetime order 100 years
• year-to-year variability determined mostly by
  temperature variations (NASA)

3
Aerosols

• size 0.1 ?m ? 100 ?m or larger (smallest formed
  from sulphate gases)
• influence visibility (VV see Appendix C)
• increase shortwave reflectivity
• trap outgoing longwave radiation
• form cloud condensation nuclei

UA farm Ellerslie 28 May 2001
An uncertain feedback in climatic modelling DMS
(dimethyl sulphide) gas released by decay of
ocean biota generates aerosol with radiative
impact as well as acting as cloud condensation
nucleii (CCN) 1oC reduction in N. hemisphere sfc
temp a year after Pinatubo eruption 1991
4
Vertical Structure
Fig 4-3
Fig 1-8
Density (?) and pressure (p) both decrease with
increasing elevation
5
Relates to Fig. 4-2
p0 weight of air divided by area A M g
/ A Pascals
6
Vertical Structure
We shall neglect these layers
Fig. 1-9
7
Ideal Gas Law (equation of state)
(p96, 4th edition)
p , pressure Pa r , density kg m-3 R
287 J kg-1 K-1 , specific gas const. for
dry air T , temperature K
8
Question these paragliders are flying at a
height of 1000 m above sea-level. A pilots
instrument reports
so the air density at flight level is?
9
(p104, 4th edition)
The Hydrostatic Law
Gives the change in pressure (?p) associated with
an increase (?z) in height

• air density kg m-3
  (approximately 1, near ground)
• g grav. acceln 9.81 m s-2
  (approximately 10)

Thus near ground, pressure increases by an
amount ?p - 10 Pa for each 1 m increase in
height
100 Pa ( 1 mb) per 10m 100 mb per 1 km
10
Question if those paragliders descend 100 m,
estimate the pressure at their new flight level
To find p2
11
Question if those paragliders descend 100 m,
estimate the pressure at their new flight level
To find p2 Need to use with ?z - 100 m. But
we need the density ?
12
Approximate the density as
(note the unit conversions we work in MKS units)
?
(implies)
A negative step in height z gives us a positive
step in pressure p