Geography 311 – The Atmosphere
Summary notes.
Part II: Moisture, Stability, Precipitation and the Ozone Hole.
Humidity and Saturation
in liquid water, molecules move around (not all at same speed – temperature is a measure
of average speed of molecules). At surface those with enough speed evaporate. Some
water molecules in air will reach liquid and condense. When rate at which molecules
evaporate and rate at which they condense are equal, air is saturated.
If you heat liquid, a greater fraction have enough speed to escape – warmer water
evaporates quicker.
If you cool air, speed of vapor molecules decreases and molecules are more likely to stick
to surface of water and condense.
measures of moisture content:
mass mixing ratio = mass of wv/mass of dry air (eg. in g/kg)
volume mixing ratio = # molecules wv/ # molecules of dry air
= partial pressure of wv/ partial pressure of dry air
vapor pressure (vp) = pressure resulting from water vapor molecules in air (mb)
saturation vapor pressure (svp) = amount of water vapor (measured as a vapor pressure)
needed for saturation to occur (at this point the air can hold no more water and it will
start to condense)
relative humidity = vapor pressure / saturation vapor pressure (measured as %)
dew point = temperature at which the water vapor in the air would be saturated
saturation vapor pressure depends on temperature. The colder it is, the lower the svp.
If air is unsaturated there are 2 ways of bringing it to saturation:
1. cool it (keeping the amount of water vapor the same)
2. add water vapor to it (keeping the temperature the same)
In 1. it would need to be cooled to the dew point.
In 2. the vp would need to be increased to the svp.
Low dew point = dry air
High dew point = moist air
If air’s wv content is constant then humidity is regulated by temperature changes:
If temp. goes down, svp goes down and relative humidity increases
if temp. goes up, svp goes up and relative humidity decreases
At temperatures below 0C svp over ice is less than svp over water
To form ice, vapor must be cooled to frost point (as opposed to dew point)
Adiabatic temperature changes
adiabatic = without gain or loss of heat
when air is compressed it heats up
when air expands it cools
when air rises it passes through regions of successively lower pressure, expands and
cools. It cools at the dry (= unsaturated) adiabatic lapse rate (dry) = 10C/km.
If air rises enough it will cool to the dew point and condensation can occur. This is the
lifting condensation level.
Upon condensation water releases its latent heat, so as this heat is added to the
atmosphere the air will not cool as much when it rises from here. It now cools as it
rises at the moist adiabatic lapse rate (moist or Ts). This depends on the actual
amount of water vapor in the atmosphere, but is ~ 6C/km.
These adiabatic lapse rates are not the same as the actual (measured) temperature changes
as you go up in the atmosphere. The actual temperature falls off at the
environmental lapse rate (env). This rate is useful in determining stability, not in
finding the temperature of a rising air parcel.
Stability:
If a rising parcel of air is warmer than the surroundings it will rise (“unstable air”)
If a rising air parcel is colder than the surroundings it will sink (“stable air”)
For a stable atmosphere the environmental lapse rate is small – ie. temperature falls
slowly with altitude, or increases with altitude (“inversion”). Causes of stable
atmosphere – air aloft is warm compared to air at surface – at sunrise/night
surface air cold (radiational cooling), warm air above.
For an unstable atmosphere, the environmental lapse rate is large – i.e. temperature
falls off rapidly with altitude. Causes of unstable atmosphere – air aloft becomes
colder, surface air warmer – daytime solar heating of surface, air moving over
warm surface, cold advection aloft
Processes that force air to rise:
convective lifting: local surface heating, rising warm air, cool air sinks forming
convective cell circulation
orographic lifting: horizontally moving air forced to rise over a mountain
(topographic barrier)
frontal wedging: formed when masses of warm and cold air meet. warmer, less dense
air forced to rise over colder denser air.
convergence of air: low pressure at surface, collision of oppositely-moving air
masses
Precipitation
typical cloud droplets are too small to produce rain.
typical CCN (cloud condensation nucleus) size ~0.2 m
typical cloud droplet size ~20 m
typical raindrop ~2 mm (2000 m)
droplet growth by vapor condensation is too slow to produce a raindrop (would take a
day or more)
there is a curvature effect which means that it is harder to grow small drops than large
ones
there is a solute effect which means that certain CCN which are hygroscopic (attract
water) provide better nuclei for droplet growth than others.
cloud droplets grow to form raindrops by the process of collision and coalescence (or
coagulation):
some cloud drops are larger than others.
these fall faster and collect smaller drops as they fall
in strong updrafts (e.g. cumulonimbus clouds) drops can actually circulate up and
down inside cloud for several cycles
amount of time drop spends inside cloud is an important factor in determining how
big droplets grow
ice crystals grow in cold clouds (temp < 0C):
droplets generally stay liquid below 0C because they supercool
ice crystals start to form at temperatures of ~ -10C
at temp below –40C there is only ice
between -10C and -40C there is a mixture of ice and liquid drops
ice crystals require ice nuclei to freeze on
ice crystals grow at the expense of water droplets because at temperatures below 0C
the saturation vapor pressure over ice is less than the saturation vapor pressure
over water, so water vapor molecules migrate from the surface of the water drop
to the ice crystal and freeze. Gradually the water droplet evaporates and the ice
crystal grows.
ice can fall as ice crystals, snow or rain – it depends on the temperature at which the
crystal forms and the temperature below the cloud (i.e. whether the ice melts on its
way to the ground)
The Ozone Hole
Ozone formed through series of chemical reactions involving atomic oxygen (O), molecular
oxygen (O2) and ozone (O3). These are the Chapman cycle.
Ozone concentration peaks in the stratosphere at ~ 25 km, and is higher at the poles than at
the equator.
Ozone protects us by absorbing most UV-B radiation before it reaches the ground. UV-B
radiation can cause biological damage, sunburn, skin cancer, premature aging of skin.
Chapman cycle predicts too much ozone. There are additional chemical reactions which
destroy ozone. In the stratosphere these involve nitric acid (NO), chlorine (Cl) and bromine
(Br). These chemicals take part in catalytic cycles (This means that the substance that causes
the ozone destruction is re-generated and can continue to destroy more ozone.)
Chlorine:
Most important ozone destroyer – comes from chlorocarbons/chlorofluorocarbons:
carbon tetrachloride (CCl4): used as a solvent in dry cleaning
CFC-11: used in refrigeration foams and as an aerosol spray
CFC-12: air conditioning, refrigeration foams, aerosol spray
CFCs are man-made. They are potent destroyers of stratospheric ozone because
they do not react with chemicals found in the troposphere. Therefore they get into
the stratosphere. Once in the stratosphere they are broken down by the sun’s UV
radiation and release chlorine. Chlorine reacts with other species and forms:
ClO (chlorine monoxide)
HCl (hydrogen chloride)
ClONO2 (chlorine nitrate)
99% of stratospheric chlorine is in non-ozone destroying forms: HCl and ClONO2
1% is in ozone-destroying form: Cl, ClO
Ozone hole:
found in Spring over Antarctica (discovered in 1985)
Why over Antarctica? Why in Spring? Combination of unique conditions:
1. strong polar vortex in winter isolates v. cold air in stratosphere over Antarctica
2. aerosol particles grow under cold conditions as water and nitric acid condense
on them to form polar stratospheric clouds
3. chemical reactions take place on the surfaces of these particles that would not
normally take place between gas molecules. These reactions involve the chlorine
species above and release chlorine into the atmosphere leaving the nitrogen
species in the particles.
4. When the sun comes up in Spring the chlorine molecules are broken down into
chlorine atoms and take part in the catalytic cycles that destroy ozone.
At some altitudes 90% of ozone is destroyed.
When the polar air warms the vortex breaks down and ozone-rich air from other parts of
the world bring ozone into the region.
The northern polar region is less susceptible to ozone loss because temperatures do
not become as low as the vortex is weaker, so polar stratospheric clouds do not
form as often. However, increasing ozone loss has been seen in the northern
hemisphere over the past decade because of rising levels of chlorine.
Solutions to ozone destruction problem:
phase out/limit production of CFCs with a long lifetime
develop alternative compounds, eg. HCFCs that have a shorter lifetime
phase out use of bromine containing compounds, eg. methyl bromide
International agreements to do these – Montreal Protocol, 1987; London Amendment,
1990; Copenhagen Amendment, 1992.