Lecture 3 – Mass balances
Water, carbon, nitrogen, oxygen and sulfur are important mass cycles in the Earth-atmosphere system.
The hydrologic or water cycle is the most important of all because survival on earth depends on it.
Another important cycle which we have recently become concerned with is the carbon cycle.
Water continuously moves between oceans, the atmosphere, the cryosphere and the land.
The total amount of water on earth is constant, but it changes forms between liquid, gaseous,
and solid forms.
The movement of water between ocean, atmosphere and land reservoirs is called the hydrologic
Water enters the atmosphere from land and ocean through evaporation, travels great distances
in the form of water vapour and clouds and then returns to the surface through precipitation.
The amount of water moved through the hydrologic cycle each year is roughly equivalent to
about 1m depth of liquid water spread uniformly over the surface of the earth.
To evaporate 1 m of water requires an average of 80 Wm of energy, which comes from the sun
Water affects climate in different ways
Atmospheric water vapour absorbs solar and terrestrial radiation and accounts for about half of
the greenhouse gas effects
Clouds reflect solar radiation and block the escape of longwave radiation
Evaporation from surface and vegetation cools these surfaces
Energy released during condensation in the cloud formation process drives global atmospheric
Water can alter surface albedo of a surface through snow fall, enhanced soil moisture and by
fostering vegetation growth
The water balance:
The water budget of a region may be written as:
ΔS = P + D – E – Δr
Where ΔS is the net storage change in soil moisture content, P is precipitation in liquid or frozen form, D
is surface condensation (dewfall or frost), E is evapotranspiration and Δr is runoff. The annual average longitudinal distribution of the surface hydrological cycle is given in above figure.
Note: most precipitation occurs just north of equator; most evaporation occurs just south of
About 60% of the earth is covered with clouds, but their presence does not necessarily mean
Clouds are important in climatology because they strongly influence and redistribute the surface
input of water and energy (radiant energy fluxes and latent heat transport from the tropics
Cloud formation requires the following three conditions:
o Sufficient moisture in the atmosphere to condense
o A medium on which condensation can occur – cloud nuclei
o A mechanism to produce atmospheric cooling by rising air
Mechanism of Air Lifting
There are different mechanisms that cause air to rise in the atmosphere Convective Lifting: parcels of hot, buoyant air from the surface rise (and cool) until the temperature in
the parcel drops and the air saturates to produce convective clouds. When condensation begins, the
latent heat is released which further warms the air, making it more buoyant.
Topographic Movement: large-scale horizontal pressure forces push bodies of moist air mass against
Air Convergence: warm, moist air is rapidly forced to mix with cooler, dry air in the overhead air
Frontal Lifting: advancing warm air fronts rise over dense cold air masses to form frontal clouds.
Types of Clouds:
Cold clouds -50 to -15 °C ice particles
Mixed clouds -40 to 2 °C water and ice
Warm clouds > 0 °C water droplets
Clouds are also classified according to their temperature (see above), morphology and the height of
Precipitation Formation Mechanisms in Clouds:
Clouds contain water droplets, ice particles or both depending upon cloud type, height and location. It
is very difficult for water and ice particles to reach the ground because of uplift processes in the cloud
and re-evaporation of falling droplets.
Apart from droplet growth by condensation on nuclei, there are other mechanisms of cloud droplet
Collision process: it produces progressively large cloud particles. There are three types of
1. Coalescence of liquid droplets (in warm mixed clouds)
2. Aggregation of ice particles (in cold and mixed clouds)
3. Accretion of water droplets on to ice particles (in mixed clouds)
Bergeron-Findeisen Process: in this process ice particles grow at the expense of water
droplets due to relative difference in vapour pressure at the same temperature. Water evaporates from water droplets and deposits on ice particles, because more energy is
required to release a water molecule from ice particles. This process occurs in mixed
Precipitation and Dewfall
Precipitation is produced when air parcels become supersaturated with water vapour and
condensation and cloud formation occurs
It is highly variable in space and time but it’s average values in a particular region is fairly
constant under current climate conditio