ATMOSPHERIC MOISTURE AND THE HYDROLOGICAL CYCLE
BASIC CONCEPTS:
1.
MEAN WATER CONTENT OF THE ATMOSPHERE
(in cm of rainfall equivalent)
NORTHERN
HEMISPHERE
JAN
FEB
1.9
3.4
SOUTHERN
WORLD
HEMISPHERE
2.5
2.0
2.2
2.7
MOST OF THE MOISTURE CONTENT OF THE ATMOSPHERE IS
FOUND WITHIN THE FIRST 10 km FROM THE EARTH'S SURFACE.
THE DISTRIBUTION OF MOISTURE IS UNEVEN AS THE ABOVE
TABLE INDICATES, AS WELL BETWEEN OCEANS AND CONTINENTS
BUT THE AMOUNT OF MOISTURE IN THE ATMOSPHERE AT ANY
ONE TIME IS ABOUT 14 x 1012 TONS, THE EQUIVALENT OF 25
mm OF RAIN OVER THE ENTIRE EARTH. MOISTURE EXISTS IN
THE ATMOSPHERE IN ALL THREE STATES: GASEOUS, AS
INVISIBLE VAPOUR; LIQUID AS CLOUDS, FOG AND RAIN
DROPLETS; AND SOLID, AS ICE CRYSTALS AND HAILSTONES.
2.
HUMIDITY:
ABSOLUTE HUMIDITY - ACTUAL AMOUNT (gm) OF WATER
VAPOUR/m3 IN THE AIR EXPRESSED AS WEIGHT PER UNIT
VOLUME (g/m3). THE MAXIMUM A UNIT VOLUME OF AIR CAN
CONTAIN AT THAT TEMPERATURE IS THE SATURATION ABSOLUTE
HUMIDITY
SPECIFIC HUMIDITY - MASS OF VAPOUR PER
MASS
(KILOGRAM) OF AIR INCLUDING ITS MOISTURE AT THAT
TEMPERATURE. IT STAYS CONSTANT DESPITE CHANGES IN
TEMPERATURE
RELATIVE
HUMIDITY
-
ACTUAL
MOISTURE
CONTENT
OF
A
SAMPLE OF AIR AS A PERCENTAGE OF THAT CONTAINED IN THE
SAME VOLUME OF SATURATED AIR AT THE SAME TEMPERATURE.
RH IS A FUNCTION OF TEMPERATURE OF THE AIR. THE WARMER
THE AIR THE MORE WATER VAPOUR IT CAN HOLD.
3.
VAPOUR IN THE AIR HAS ITS OWN PRESSURE KNOWN AS
VAPOUR PRESSURE (mb). TWO TYPES:
a.
SATURATION VAPOUR PRESSURE - PRESSURE AT THAT
POINT WHEN THE WATER MOLECULES MOVING BETWEEN AIR
AND WATER SURFACE IS IN EQUILIBRIUM. GOVERNED BY
TEMPERATURE.
b.
ACTUAL VAPOUR PRESSURE
4.
DEW POINT - TEMPERATURE AT WHICH VAPOUR CONDENSES.
FOR CLOUDS, THE DEW POINT IS AT THE LEVEL OF THE
CLOUD BASE OR CONDENSATION LEVEL
PRECIPITATION
1.
DIFFERENT FORMS OF PRECIPITATION
ALL PRECIPITATION ORIGINATES FROM CLOUDS. HUMIDITY
IS
ADDED
TO
THE
AIR
BY
EVAPORATION
AND
TRANSPIRATION. IT IS TURNED TO CLOUD DROPLETS IN
THE AIR UPON CONDENSATION AND COALESCE. CLOUD
DROPLETS WHICH ARE OF SIGNIFICANT SIZE ARE
RETURNED TO LAND AND SEA BY PRECIPITATION.
RAIN, SNOW, SLEET, HAIL, DEW, FOG
2.
CONDITIONS FOR PRECIPITATION GENERATION
A.
AVAILABILITY OF MOISTURE
i.
EVAPORATION AND TRANSPIRATION
ii. ADVECTION
iii. MIXING
iv. PREVAILING WINDS
B.
COOLING
i.
CONVECTIONAL UPLIFT
ii. OROGRAPHIC UPLIFT
VERTICAL VELOCITY
iii. FRONTAL UPLIFT
C.
CONDENSATION
CONDENSATION OR HYGROSCOPIC NUCLEI
D.
DROPLET GROWTH
i.
COLLISION-COALESCENCE THEORY
COLLISSION, COALESCENCE AND 'SWEEPING' OF SMALLER
DROPLETS BY THE LARGER ONES AS THEY FALL
CRITICISM: BIGGER PARTICLES CAN EASILY BREAK UP
PARTICULARLY AS THERE IS AN UPWARD CURRENT. THIS
THEORY APPLIES TO WARM CLOUDS
LANGMUIR OFFERED A VARIATION OF THIS THEORY
FALLING DROPS HAVE TERMINAL VELOCITIES DIRECTLY
RELATED TO THEIR DIAMETERS SUCH THAT THE LARGER
DROPS MIGHT OVERTAKE AND ABSORB SMALL DROPLETS AND
THAT THE LATTER MIGHT BE SWEPT IN THE PATH OF THE
FORMER AND BE ABSORBED BY THEM
LARGE DROPS WOULD BE PRESENT IF CLOUD TOP DOES NOT
REACH ABOVE FREEZING LEVEL. IN THIS SITUATION,
GIANT NUCLEI MUST BE PRESENT
FOR FREEZING ICE CRYSTALS, FALLING THROUGH THE
CLOUD AS LARGE WATER DROPLETS INITIATE THIS
MECHANISM INTO ACTION
TURBULENCE IN CUMULIFORM
ENCOURAGE COLLISSIONS
CLOUDS
MAY
SERVE
TO
ii. BERGERON-FINDEISEN THEORY
THE RELATIVE HUMIDITY OF AIR IS GREATER WITH
RESPECT TO AN ICE SURFACE THAN WITH RESPECT TO A
WATER SURFACE. AS THE AIR TEMPERATURE FALLS BELOW
0OC THE ATMOSPHERIC VAPOUR PRESSURE DECREASES MORE
RAPIDLY OVER AN ICE SURFACE THAN OVER WATER
SURFACE. THIS RESULTS IN THE SATURATION VAPOUR
PRESSURE OVER WATER BECOMING GREATER THAN THAT
OVER ICE, ESPECIALLY BETWEEN TEMPERATURES OF -5OC
AND -25OC WHERE THE DIFFERENCE EXCEEDS 0.2 mb. IF
ICE CRYSTALS AND SUPERCOOLED WATER DROPLETS EXIST
TOGETHER IN A CLOUD THE LATTER TEND TO EVAPORATE
AND DIRECT DEPOSITION TAKES PLACE FROM THE VAPOUR
ON TO THE ICE CRYSTALS. THIS PROCESS IS CALLED
SUBLIMATION. THE GROWTH OF CRYSTALS IS RAPID
ENOUGH TO GENERATE CRYSTALS WHICH ARE SUFFICIENTLY
LARGE FOR THEM TO FALL TO THE GROUND.
THE PRESENCE OF FREEZING NUCLEI IS NECESSARY. ONCE
MINUTE ICE CRYSTALS HAVE FORMED THEY GROW READILY
BY DEPOSITION FROM VAPOUR. SPLINTERS ARE EASILY
FORMED AS CRYSTALS BREAK AND THESE EASILY
AGGREGATE UPON COLLISION.
THIS THEORY RECEIVES GREATER ACCEPTANCE THAN THE
EARLIER ONES.
IN CONNECTION WITH COOLING (B) ALL THE THREE WAYS
ABOVE INVOLVE ADIABATIC COOLING, ALTHOUGH COOLING CAN
TAKE PLACE IN OTHER WAYS eg RADIATION COOLING, MIXING,
ADVECTION. THESE OTHER WAYS DO NOT PRODUCE SUFFICIENT
LAYER OF CLOUDS TO GENERATE HEAVY RAINFALL, BUT DEW,
FOG, OR SLIGHT DRIZZLE.
IN ORDER FOR US TO UNDERSTAND ADIABATIC COOLING
(TEMPERATURE DROPS AS A RESULT OF EXPANSION OF VOLUME
NOT AS A RESULT OF LOSS OF ENERGY TO THE SURROUNDING)
WE MUST REFER TO A WARM PARCEL OF MOIST AIR.
AS A PARCEL OF WARM AIR RISES, IT COOLS AT THE DRY
ADIABATIC LAPSE RATE (10OC/KM). AS THE RISING AIR
COOLS, ITS CAPACITY FOR HOLDING MOISTURE DECREASES AND
ITS RELATIVE HUMIDITY INCREASES. IF IT RISES HIGH
ENOUGH AND COOLS SUFFICIENTLY, IT WILL REACH 100 % RH
AT THE DEW POINT TEMPERATURE. THIS MARKS THE
CONDENSATION LEVEL OR THE LOCATION OF THE CLOUD BASE.
BEYOND THIS POINT CONDENSATION TAKES PLACE.
DURING CONDENSATION LATENT HEAT IS RELEASED AND IF THE
AIR IS RISING AND COOLING THE RELEASE OF LH WILL
SLACKEN THE RATE OF COOLING. THIS RATE IS THE WET OR
SATURATED ADIABATIC LAPSE RATE (5OC/KM).
THE ABOVE TWO RATES MUST BE SEEN IN THE CONTEXT OF THE
ENVIRONMENTAL LAPSE RATE WHICH IS THE RATE OF COOLING
OF AIR WITH HEIGHT AND PERTAINS TO AIR THAT IS NOT IN
MOTION.
STABILITY AND INSTABILITY
A PARCEL OF WARM AIR IS BUOYANT AND THEREFORE IT RISES
BECAUSE IT IS WARMER THAN THE SUROUNDING AIR. IN OTHER
WORDS, THE PARCEL OF AIR HAS TEMPERATURES ABOVE THAT
OF ITS SURROUNDING AND IF THIS CONDITION PERSISTS AT
ALL LEVELS, EVEN AFTER CONDENSATION, THEN THIS
SITUATION IS KNOWN AS A STATE OF ABSOLUTE INSTABILITY.
A PARCEL OF COLDER AIR (COLDER THAN ITS SURROUNDING)
CAN BECOME UNSTABLE IF IT IS FORCED UP A SLOPE AND
CONDENSATION TRIGGERS THE RELEASE OF LH. FROM THIS
POSITION ONWARDS IT IS CHARACTERISED BY INSTABILITY
AND THEREFORE KNOWN AS CONDITIONAL INSTABILITY.
3.
PRECIPITATION DISTRIBUTION
THE AREAL DISTRIBUTION OF RAINFALL IS GOVERNED VERY
MUCH BY RELIEF OF THE GROUND SURFACE ESPECIALLY FOR
STORMS OF THE FRONTAL TYPES WITHIN THE WESTERLY
PREVAILING WINDS. THIS IS EVIDENT IN PLACES LIKE
ENGLAND,
WHERE
THE
RELATIONSHIP
BETWEEN
ANNUAL
RAINFALL DISTRIBUTION IS CLOSELY RELATED TO THE RELIEF
OF THE PENNINES, PARTICULARLY ON THE WINDWARD SLOPES.
IN THE HUMID TROPICS THE RAINFALL TENDS TO BE SMALL
SCALE, USUALLY OF THE CONVECTIONAL TYPE WHICH IS NOT
GOVERNED BY RELIEF. IN THIS SITUATION, THE ANNUAL
RAINFALL DISTRIBUTION SHOWS GREATER 'SPOTTINESS' AND
THE INFLUENCE OF RELIEF IS LESS EVIDENT. IN JAVA,
ANNUAL RAINFALL INCREASES WITH HEIGHT BUT DECREASES
AFTER A CERTAIN POINT, INCREASES TO A SECOND MAXIMUM
BEYOND WHICH IT DECREASES TO THE SUMMIT.
SUMMARY
1.
CERTAIN
CONCEPTS
ARE
IMPORTANT
IN
UNDERSTANDING MOISTURE CONDITIONS OF THE AIR
THE
2.
MOISTURE IN THE AIR CAN BE TRANSFORMED TO ANY OF
THE THREE STATES
3.
HEAVY, SUSTAINED RAINFALL CANNOT FORM FROM ANY
PROCESS OF COOLING OF AIR. ONLY COOLING DUE TO
ADIABATIC PROCESS CAN GUARANTEE HEAVY RAINFALL
4.
OTHER CONDITIONS MUST ALSO PREVAIL FOR RAIN TO
FORM
AVAILABILITY
OF
MOISTURE
SUPPLY,
CONDENSATION & MECHANISM FOR DROPLET GROWTH
5.
TWO THEORIES HAVE BEEN PUT FORWARD TO EXPLAIN
DROPLET GROWTH IN CLOUDS TO FORM PRECIPITATION
AREAL PRECIPITATION DISTRIBUTION ON THE GROUND IS VERY
MUCH GOVERNED BY RELIEF AS WELL AS THE NATURE OF
STORMS THEMSELVES
DISCUSSION
1.
LOOK AT THE RIANFALL DATA OF CHANGI (REFER TO
MONTHLY WEATHER REPORTS FROM THE METEOROLOGICAL
OFFICE) AND DRAW A HISTOGRAM OF THE MONTHLY
RAINFALL FOR ANY ONE YEAR. COMMENT ON THE MONTHLY
DISTRIBUTION.
2.
REFER TO ANY RAINFALL DISTRIBUTION MAP OF THE
WORLD AND ACCOUNT FOR THE RAINFALL PATTERN THAT
YOU SEE.
3.
SHOW HOW SMOKE HAZE IS DIFFERENT FROM SMOG?
INCLWEBL4