Atmospheric Moisture
I. The Hydrological Cycle:
The Hydrological Cycle refers to the global circulation of moisture (and heat)
between the land and sea surface and the atmosphere. The hydrological cycle as shown in the
picture below, is composed of a series of stores or compartments in which moisture is held in
various forms and amounts, and a sequence of transfers and transformations of moisture between
and within the different stores.
Source of energy from the sun pushes forward the hydrological cycle, through
evaporation from water bodies and land surface. The most important water storage is the ocean,
then the sea, lakes and rivers. Water vapour also comes from transpiration of green plants and
perspiration of living animals, including man.
Table of Hydrosphere
Water storage
Amount (Units)
Ocean and sea
2690
Surface water
247 (Ice 90 %)
Underground water
200
Water vapour in atmosphere
25
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Vapour is temporarily locked up as gigantic atmospheric storage. The droplets move
up, following light uprising air. As temperature drops, they condense around hygroscopic nuclei
(condensation nuclei). Through various meteorological processes, water vapour condenses into
different forms of precipitation, such as in solid state (snow), liquid state (rain, drizzles, showers)
and semi-gaseous state (fog and mist). The meteorological processes include frontal activities
and convection.
Some precipitation falls as snow on mountain tops or in high latitudinal zones. It is
stored temporarily until spring thaw. With the rising temperature and stronger insolation, it melts
into water. Much precipitation falls as rain of various intensity and duration.
Rain falls through leaves of various vegetation types at different rates through
interception and through fall. Droplets may stay on leaf blades and stem surface to be evaporated
later. Underground water will be absorbed by the roots of vegetation and through transpiration
process to go back into atmosphere.
When soil is dry or composed of permeable particles, water percolates down the soil
as infiltration, and is stored below the water table as underground water. The discharge of this
subsurface water is delayed as return flow which reaches the river as surface water once more.
Owing to heavy rain, or the lack of protective vegetable cover or saturated soil
condition, infiltration rate is checked, and much water flows on land surface as surface runoff.
This quick flow finally reaches the river and sea.
Part of the water will be absorbed by vegetation and used in urban areas for domestic
and industrial consumption, or for farming. Here, not only much water is channelled to human
use, but also the quality of water is much affected, eg. pollution of water.
The process of evapotranspiration brings water from various water storage back to
the atmospheric reservoir again in the form of water vapour.
II. The Three States of Water:
Water occurs in three states: Crystalline solid, liquid as water, gaseous as water vapour.
Form the gaseous vapour state, molecules may pass into the liquid state by condensation, or
if temperatures are below the freezing point, they can pass by sublimation directly into the solid
state to form ice crystals.
By evaporation, molecules can leave a water surface to become gas molecules in water
vapour.
Water can pass from liquid to solid state by freezing, and from solid state to liquid state by
melting.
These exchanges of heat energy accompanying changes of state are of great importance in
weather science. When water evaporates, sensible heat passes into a hidden form held by the
water vapour - latent heat of vaporization. This results in a drop in temperature of the remaining
liquid. For every gram of water that is evaporated, about 600 calories change into the latent form.
In the process of condensation, an equal amount of energy is release to become sensible heat
and the air temperature rises correspondingly.
The freezing process releases heat energy in the amount of about 80 calories per gram of
water, whereas melting absorbs an equal quantity of heat - latent heat of fusion.
III. Humidity:
1. Introduction:
Although the atmosphere represents a relatively small store, its moisture content is
highly variable in space and time. It is also quite different from the other main stores, which
tend to be dominated by liquid water and ice. Moisture exists in the air largely in gaseous
form as water vapour. Only about 4% is in the form of liquid water droplets and ice crystals,
found mostly in clouds.
The ability of the air to hold water vapour depends solely on temperature. A simple
rule states that the warmer the air, the more moisture it can hold. When a mass of air is
holding the maximum amount of water vapour possible at a given temperature (dew point
temperature), the air is said to be saturated. When it is retaining less than the saturated
amount, the air is referred to as unsaturated and when holding more, it is known as
supersaturated. The moisture content of an air parcel, whether at saturation or not, can be
described in terms of its absolute, relative and specific humidity.
a. Absolute Humidity:
Absolute Humidity refers to the actual quantity of moisture present in the air. It is
defined as the weight of water vapour contained in a given volume of air (gram/m3).
For any specified air temperature, there is a maximum weight of water vapour that a
cubic metre of air can hold - the saturation quantity /satureation Point. At that moment,
the air is saturated and condensation will occur.
Absolute humidity increases with an increase in air temperature since there is an
increase in volume (Holding Capacity) of the air.
b. Relative Humidity:
For any specified temperature there is a definite limit to the quantity of moisture that
can be held by the air (saturation point). The proportion of water vapour present relative to
the maximum quantity is the relative humidity, expressed as a percentage.
A change in relative humidity of the atmosphere can be caused by:
i. diffusion of water vapour from an exposed water surface through the process of
evaporation;
ii. a change of temperature - a fall of temperature results in a rise of relative
humidity; a rise of temperature results in decreased relative humidity.
When the relative humidity reaches 100%, the air is saturated. Any further cooling
will cause condensation of the excess vapour into liquid form. The temperature at which
condensation takes place is known as dew point temperature.
c. Specific Humidity:
It refers to the ratio of the weight of water vapour (gram) to the weight of the moist air
(kg). The average is 10 grams per kg.
It will not be affected by the volume change when air moves vertically.
IV. Adiabatic Processes:
The process of
adiabatic or expansion
cooling depends on
parcels of air rising
through the atmosphere to
higher elevations. The
consequent cooling of the
air is responsible for the
initiation of much of the
condensation at all levels.
Air may be induced to rise by convection, orographic uplift, turbulence in the air flow,
and uplift at frontal surfaces.
The decrease of pressure with height allows the rising air parcel to expand. In
accomplishing the expansion, energy is used up, and this has to be provide for within the air
parcel. Thus the temperature of the air parcel decreases.
When a mass of air is moving to a lower level, it gains heat by contraction.
Adiabatic means that there is no heat exchange between the air parcel and its
surroundings.
Adiabatic cooling and warming in dry air (Relative Humidity below 100%) takes
place at a fixed rate of 10oC/1000m and it is known as the Dry Adiabatic Lapse Rate
(D.A.L.R.)
For air in which condensation is occurring, the Saturated/ Wet Adiabatic Lapse Rate
(S.A.L.R.) pertains (5-6oC/ 1000m). This has a lower value because latent heat was released
into the air in the condensation process offsets the adiabatic temperature loss.
The rate varies because the amount of latent heat released will be much greater for
warm saturated air than for cold saturated air. The rate varies from 4oC/1000m to
9oC/1000m.
Adiabatic Lapse Rates are the temperature changes with height when air is in the
process of moving up and down whereas environmental lapse rate is the temperature changes
with height as might be recorded by an observer ascending in a balloon (6.5oC/ 1000m).
V. Condensation:
Condensation is the direct cause of all the various forms of precipitation.
1. Conditions for Condensation:
a. When the temperature of the air is reduced but its volume remains constant and the air is
cooled to dew point;
b. if the volume of the air is increased without addition of heat; this cooling takes place
because adiabatic expansion causes energy to be consumed through work;
c. when a joint change of temperature and volume reduces the moisture-holding capacity of
the air below its existing moisture content.
Condensation will be much accelerated by the presence of dust particles, salt, etc.
(Hygroscopic/ Condensation Nuclei)
2. Forms of Condensation:
a. Dew and Frost
Dew consists of relatively large water droplets which collects or deposits on cold or
cool ground surfaces at night under clam conditions when the lower layer of the atmosphere
is saturated because of cooling to dew point at night.
Frost consists of ice crystals deposited on a cold surface. The formation of frost is the
same of dew. The different is that the dew point of frost is below 0oC, and water vapour
transform to ice crystals directly through sublimation process. It is formed at night under
calm conditions when the temperature is below freezing point.
b. Mist and Fog
Mist consists of very fine, condensed water droplets. These are held in suspension in
the air because of their smaller size. A mist often forms at very low level of ground level. The
droplets form a thin, fine veil which hinders the visibility. The visibility is obscure, but still
exceeds 1000 m.
Fog contain small condensed water droplets which are larger than those in a mist.
Thus, it forms a thick veil which causes poor visibility below 1000 m.
Fog is usually formed by advection of warm, moist air and by intense radiation at
night, namely advection fog and radiation fog respectively.
i. Advection Fog
An advection fog is formed by condensation under the following conditions.
- when warm, moist air passes over a cooler or cold land or sea surface
horizontally;
- where cold and warm ocean currents meet each other;
- when warm moist air from the ocean merges with cool dry air from the land.
The lower layer of warm air is cooled below dew point by contact with cooler
air or surface, and condensation results in the formation of advection fog.
Condensation is assisted by the presence of hygroscopic particles which act as nuclei
for condensation.
Advection fog is common in spring in Hong Kong. It often disperses during
the day when the sun appears and temperature rises.
ii. Radiation Fog
A radiation fog is formed by condensation when moist air comes into contact
with the cool ground and becomes cooled due to the radiation of heat from the ground.
It occurs in cold weather when the sky is clear (which permits maximum radiation)
and there is calm, stable condition.
Hygroscopic particles in the air act as nuclei for the moisture to condense on.
Radiation fog is common in winter and in industrial regions.
c. Clouds
Clouds consist of extremely tiny droplets of water (0.02 to 0.06 mm in diameter), or
minute crystals of ice. Generally, the formation of cloud is the same of the formation of fog.
i. Favourable Conditions for Cloud Formation
- In order for cloud droplets to form, it is necessary that microscopic dust particles
serve as condensation nuclei.
- There should be a fall of temperature down to dew-point. Where the air
temperature is well below freezing, clouds may form of tiny ice crystals.
However, water in such minute quantities can remain liquid far below normal
freezing temperatures, the liquid is said to be supercooled. Water droplets may
exist at temperatures down to -12oC to -30oC; ice crystals below -30oC.
ii. Classification of Cloud Types
Cloud types can be classified on the basis of two characteristics: Form and
Altitude.
There are two major groups: Stratiform/ layered types, and cumuliform types.
a. Stratiform Clouds
They are blanket like, often covering vast areas, but are fairly thin
comparison to horizontal dimensions. They are subdivided according to the
level of elevation at which they lie.
Stratus
High Clouds (6000 - 12000 m, composed of ice crystals)
- Cirrus is a delicate, wispy (small bundle) cloud, often forming streaks (long,
thin lines) or stringers across the sky.
- Cirrostratus is a more complete layer of cloud, producing a halo about the
sun or moon.
- Cirrocumulus is a layer of closely packed globular pieces of cloud,
arranged in groups or lines.
Middle Clouds (2000 - 6000 m)
- Altostratus is a blanket layer, often smoothly distributed over the entire sky.
It is commonly associated with the development of bad weather.
- Altocumulus is a layer of individual cloud masses. it is usually characteristic
of generally fair conditions.
Low Clouds (ground level to 2000 m)
- Stratus is a dense, low-lying dark grey layer. If rain or snow is falling from
this cloud, it is termed nimbostratus.
- Stratocumulus is a low-lying cloud layer consisting of distinct grayish
masses of cloud between which is open sky. It is usually associated with fair
weather.
b. Cumuliform Clouds
They tend to display a height as great as, or greater than, their horizontal
dimensions.
Cumulus is a white, wool pack cloud mass, often showing a flat base and a
bumpy surface somewhat resembling a head of cauliflower. Small cumulus clouds
are associated with fair weather.
Cumulonimbus is the thunderstorm cloud mass of enormous size which brings
heavy rainfall, thunder and lightning, and gusty winds. A large cumulonimbus
cloud may extend from a height of 300 to 600 m at the base up to 9000 to 12000
m.
Cumulonimbus with anvil
Cumulonimbus with anvil
VI. Precipitation:
1. Air Stability:
The relationship between the environmental lapse rate (ELR) and the dry and
saturated adiabatic lapse (DALR, SALR) determines the stability of the atmosphere at any
particular place. Air stability is a very important meteorological phenomenon, because it
influences the amount and the type of condensation (Clouds, Fog) which take place, together
with other related weather phenomena, such as rain and hail.
a. Unstable Air (Absolute Stability)
A condition of instability exists when uplifted air is encouraged to rise still further and
descending air to sink. As shown in the figure below, instability occurs when the
environmental lapse rate is greater than that of either the wet or dry adiabatic lapse rates.
Under such circumstances, a rising air parcel will become progressively warmer and a
sinking air parcel progressively cooler than the surrounding air. Both situation will
promote further displacement of air, upwards and downwards respectively, from its
original position.
b. Conditionally Stable Air (Conditional Unstable)
Conditionally stability occurs when the environmental lapse rate is less than the dry ,
but greater than the saturated adiabatic lapse rate figure above shows that this type of
stability occurs when moist air is forced upwards (ABCD) and is at first cooler than its
surroundings. At some point during the ascent (at B), condensation will take place and
latent heat will be released into the rising air parcel. Cooling less rapidly at the SALR, it
will ultimately become warmer than the surrounding air (at C). At this stage, it will
become unstable and will continue to rise under its own buoyancy, until once again it
reaches the temperature of its surrounding environment.
c. Stable Air (Absolute Stability)
A state of stability is said to exist in the atmosphere when a vertically displaced parcel
of air tends to return to its original position. This condition is shown in previous diagram
and occurs when the environmental lapse rate is less than the dry and saturated adiabatic
lapse rates.
In this case, the rising, adiabatically cooling parcel of air will be at a lower
temperature than its surrounding, whereas a descending adiabatically warmed parcel will
be warmer, In each case, the parcel of air will tend to return to its original position by
moving downwards in the first case and upwards in the second.
Under stable air conditions, a parcel of air forced to rise is unlikely to go on rising. It
may, however, cool to dew point and, before descending, give rise to thin Stratus or fair
weather cumulus clouds.
d. Highly Stable Air (Inversion)
When air temperature increases with altitude, a condition known as a temperature
inversion exists. Temperature inversions produce the vertical ascent of air below them.
They effectively put a cap on the atmosphere.
For instance, if an air parcel at sea level is forced to rise either at the DALR or the
SALR, it will always be colder than the surrounding air which is increasing in
temperature with height. Being colder than its environment, the air parcel will sink back
to ground level and little cloud development will take place.
2. Formation of Precipitation:
There are four conditions necessary for the formation of major precipitation.
i. air cooling
ii. condensation and cloud formation,
iii. an accumulation of moisture and
iv. the growth of cloud droplets.
When clouds form in the atmosphere, however, they are non- precipitating in 99% of
cases. Clearly stages (c) and (d) are fundamental in precipitation production. Stage (d) is
perhaps the most critical one in precipitation formation. The water droplets and ice
crystals of clouds have to be transformed into heavier particles if they are to fall out of
clouds as precipitation.
There are two main mechanisms by which cloud particles (average diameter about 0.1
mm) increase to a size suitable for precipitation (average diameter 0.5 - 2.0 mm)
a. Collision Mechanism
These collision processes depend on the sweeping up of a mass of tiny cloud particles
by a smaller number of larger particles. Rising and sinking air motions within cloud carry
with them cloud particles of different sizes. Because of friction, particles of different sizes
move at differnet speeds, and this leads to collisions between particles. The larger
droplets tend to catch more of the smaller cloud particles and grow at the latter's expense.
The growth of cloud particles by collision takes different paths. When two liquid
water droplets collide and join together the process is called coalescence. The conjoining
of two ice crystals is known as aggregation, whereas if an ice crystal collects a water
droplet the process is known as accretion. Of the three main categories of precipitation
formation, rainfall results largely from coalescence, snowfall from aggregation and hail
from accretion.
b. Ice-crystal Method
Although supercooled water droplets (i.e. those below 0oC) and ice crystals can
coexist within a cloud, they are unstable with respect to each other. What tends to happen
is that liquid water droplets evaporate. This water vapour then condenses and freezes onto
the surface of the moving and growing ice crystals. These ice crystals then combine by
aggregation into large snowflakes. When falling to lower and warmer levels, these
snowflakes may melt and continue their descent as large raindrops. if the freezing level is
at the surface, or if the air throughout remains very cold, the snowflakes will not melt,
producing snowfall.
In the tropics, rain frequently falls from clouds devoid of ice crystals. In these cases,
raindrops grow by collision processes. Most clouds in extra-tropical latitudes, though,
especially those in the upper, colder layers of the atmosphere, contain both ice crystals
and water droplets. It has therefore been suggested that the ice-crystal method is
responsible for such of the heavy rainfall of mid-latitude areas.
3. Forms of Precipitation
- Precipitation results when condenstaion is occurring rapidly within a cloud.
- Rain is formed when cloud droplets in large numbers are caused to coalesce into drops
too large to remain suspended in the air.
- Falling droplets less than 0.5 mm in diameter make a drizzle.
- Sleet
rain.
(a mixture of rain and snow) consists of pellets of ice produced from freezing of
- Snow consists of masses of crystals of ice, grown directly from the water vapour of the
air, where air temperature is below freezing.
- Hail consists of rounded lumps of ice, having an internal structure of concentric layers.
4. Types of Rainfall
a. Convectional Precipitation
Convectional precipitation results from a convection cell, which is simply an updraft
of warmer air, seeking higher altitude because it is lighter than surrounding air.
On a clear, warm summer morning the sun is shining upon the land. Air over a
warmer patch is warmed more than adjacent air and begins to rise as a bubble. As the air
rises, it cooled adiabatically so that eventually it will reach the same temperature as the
surrounding air and come to rest.
Before this happens, it may be cooled below the dew point. At once condensation
begins, and the rising air column appears as a cumulus cloud whose flat base shows the
critical level above which condensation is occurring. The bulging 'cauliflower' top of the
cloud represents the top of the rising warm air bubble, pushing into higher levels of the
atmosphere. When the convection continues to develop, the cloud may grow to a
cumulonimbus cloud.
The unequal heating of the ground serves only as a trigger effect to release a
spontaneous updraft which is later fed by latent heat energy liberated from the condensing
water vapour.
b. Orographic Precipitation
Orographic means 'related to mountains'. Prevailing winds may be forced to flow over
mountain ranges. As the air rises on the windward side of the range, it is cooled at the
adiabatic rate. If cooling is sufficient, precipitation will result. after passing over the
mountain summit, the air will begin to descend the leeside of the range. Now it will
undergo a warming through the same adiabatic process and, having no source from which
to draw up moisture, will become very dry. A belt of dry climate - rainshadow - may exist
on the lee side of the range.
c. Cyclonic Precipitation
In extra tropical areas, between about 40o and 65o latitude, there is an important belt
of low-pressure systems, moving form west to east. Such mid-latitude depressions give
rise to moderate and generally continuous precipitation over very extensive areas.
When warm air comes into contact with cold air along the air mass boundary at the
polar front, the warmer, less dense air is forced to rise over the colder, denser air. In the
advancing or forward sector of the depression, at the warm front, warm air that is forced
to upraise and cool over the colder air gives rise to a multi-layered cloud of the
nimbostratus type. This may give continuous light-to-moderate precipitation over large
areas and may last 6-12 hours or more, depending on the speed of the depression. In the
rear sector, at the cold front, cold air tends to undercut the warmer air. Precipitation at the
cold front is associated often with cumiliform cloud and is characterised by a shorter
period of heavy showers, sometimes accompanied by thunder. The nature of this more
intense precipitation is related to the steeper frontal zone and hence to the faster ascent of
the warmer air.
VII. World distribution of precipitation
Lines joining places having equal rainfall amounts are termed isohyets.
Generally speaking, equatorial areas have the most precipitation. This is mainly
related to high temperatures and the consequent large moisture-holding capacity of the air,
and also to the presence of large oceanic water surfaces to supply the moisture. Most of the
rainfall is convectional.
Conversely, polar areas can only have small absolute amounts of precipitation
because of low air temperatures and less evaporation.
Middle latitudes have a complicated distributional and total pattern. High rainfall
totals related particularly to the westerlies in both hemispheres and notably to their cyclone
tracks.
Regions of the lowest rainfall coincide with regions of subsiding air, which is warmed
adiabatically and made dry. This occurs mainly in the subtropics on the eastern sides of the
oceans.
Large mountain ranges affect rainfall totals markedly where they lie athwart moist
prevailing winds.
Altitude also plays a part on a more local scale: there is a general increase of
precipitation with height up to about 2 km. Beyond this, totals diminish because of the
coolness of the air.