Moisture, Stability, Precipitation and the Ozone Hole summary and notes




Moisture, Stability, Precipitation and the Ozone Hole summary and notes


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Moisture, Stability, Precipitation and the Ozone Hole summary and notes


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 (Tdry) = 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 (Tmoist 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 (Tenv). This rate is useful in determining stability, not in finding the temperature of a rising air parcel.


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



typical cloud droplets are too small to produce rain.

typical CCN (cloud condensation nucleus) size ~0.2 mm

typical cloud droplet size ~20 mm

typical raindrop ~2 mm (2000 mm)

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.)


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.



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