RASAKUMAR .R

1. Lecture. 10
WIND – ROLE ON CROP PRODUCTION - WIND SYSTEMS
OF THE WORLD – INTER TROPICAL CONVERGENCE
ZONES (ITCZ) – WIND SPEED IN DIFFERENT SEASONS-
CLOUDS AND THEIR CLASSIFICATION – CONCEPT OF
CLOUD SEEDING _ PRESENT STATUS

2. Wind: Air in horizontal motion is known as wind
Vertical movement is noticed but negligibly small -
but causes significant weather changes in cloud
formation and rain

3. Effect of wind on crop plants
 Transports heat from lower to higher altitudes
(Sensible/latent)
 Increase transpiration & intake of CO2 and causes
mechanical damage
 Wind helps in pollination and dispersal of seeds
 Light and gentle winds are helpful for cleaning the
agricultural produce
 Hot dry winds damage vegetation in crops by excessive
water loss
 Wind has powerful effect on humidity
 Long dry winds injured blossoms by evaporating
secretion of stigma
 Provides moisture which is necessary for precipitation
 Wind prevents frost by disrupting atmospheric inversion
 Causes soil erosion

4. World Wind systems
 Doldrums
 Trade wind belt
 Prevailing westerlies
 Polar easterlies

6. Doldrums - ITCZ
 Equatorial belts of calms and variable
winds
 5°S and 5°N latitudes
 Wind is light due to negligible
pressure gradient
 Mostly, there are vertical movements
in the atmosphere
 The atmosphere is hot and sticky

7. Trade winds (Tropical Easterlies)
 High temperature results high pressure in upper
levels of the equator
 Then, air is transferred to the northward and
southward directions until 35° North and South
 High pressure at 35°N and 35°S - horse latitude
 Winds deflected by Coriolis force to the right in
Northern hemisphere and left in SH
 Winds flow to equator in NE direction in the NH
and SE in SH. Known as ‘Trade winds’ or ‘Tropical
easterlies’.
 Most constant winds in force over nearly half the
globe.

8. Anti-trade winds
 Supplementary wind system of earth
effective at higher levels
 This system works in opposite direction
to the surface winds
 Anti-trade winds mostly flow from land
to ocean and brings no rain

9. Prevailing Westerlies
 Winds flow from sub-tropical high to low-
pressure area at 60-70° latitudes in both
hemispheres are known as ‘Prevailing
westerlies’.
 Direction of Prevailing westerlies at NH is
SW and SH is NW.
 Winds forceful, irregular as compare to
trade winds in tropics
 High precipitation zone

10. Polar Easterlies / Polar winds
 A permanent high pressure exists on the
poles
 From these high pressure polar regions,
cold winds flow to areas at about 60-65°
latitudes in both the hemispheres.
 The winds flow in NE direction in the NH
and in SE direction SH

11. Mountain winds Valley wind
Blows from mountain up
slope to base
Blow from valley base to
up slope
Occurs during night time Occurs during day time
Cooling of air close to
slope takes place
Over heating of air
adjacent to slope takes
place
Adiabatic heating
decreases this
phenomenon
Adiabatic cooling
decreases this
phenomenon
‘Katabatic winds’ ‘Anabatic winds’

12. Valley wind Mountain winds

13. Sea breeze Land breeze
•During the daytime, land
is heated more than the
adjacent body of water
• At night because of
nocturnal radiation land
is colder than adjacent
sea
•Warmed air over land
expands producing low
pressure
• Pressure gradient is
directed from land to sea
•Isobaric surfaces bend
upward result in cool air
moving sea to land
• There is a gentle flow of
wind from land to sea.
•‘Sea breeze; or ‘On
shore breeze’
• ‘Off-share’ wind is called
‘Land breeze

15. Wind speed in different season
 Primary cause of all winds is regional differences
in temperature
 Rotation of the earth modifies the direction of
motion, till the winds blow along lines of equal
pressure.
 Wind direction & speed are modified frequently
due to seasonal variation in solar radiation and
differential heating of earth’s surface

16. Wind Direction
 Winds are always named after the
direction they come from
 Wind from the south, blowing towards
north is called south wind
 Windward - direction wind comes from,
leeward - direction it blows to
 When a wind blows more frequently from
one direction than from any other, it is
called a prevailing wind.

17.  Aggregation of minute drops of water suspended in
air at higher altitudes
 Rising air currents tend to keep the clouds from
falling to the ground.
Cloud

18. WMO classification
WMO classified clouds according to their height and
appearance into 10 categories under 4 families
 Family A - 3
 Family B - 2
 Family C - 3
 Family D - 2

19. Family A
 Clouds are high
 Mean lower level - 7 km and
 Mean upper level- 12 km
1. Cirrus (Ci)
2. Cirrocumulus (Cc)
3. Cirrostraturs (Cs)

20.  Ice crystals - present
 Looks like wispy and feathery. Delicate, desist, white
fibrous, silky
 Sun rays pass through these clouds and sunshine
without shadow.
 Does not produce precipitation
Cirrus

21. Cirrocumulus
 Like cirrus clouds ice crystals -present
 Looks like rippled sand or waves of the sea shore
 White globular masses, transparent with no shading effect

22. Cirrostratus
 Ice crystals - present
 Whitish veil and covers - sky - milky white
appearance
 Produces “Halo”

23. Family B
 Middle clouds
 Mean lower level is 2.5 km and
 Mean upper level - 7 km
1. Altocumulus (Ac)
2. Altostratus (As)

24. Altocumulus
 Ice water is present
 Greyish or bluish globular masses
 Sheep back - known as flock clouds or wool packed
clouds

25. Altostratus
 Water and ice are present separately
 Fibrous veil or sheet & grey or bluish in colour
 Cast shadow
 Rain occurs in middle and high latitudes

26. Family C
 Lower clouds
 Height of these clouds extends from
ground to upper level of 2.5 km
1. Strato cumulus (Sc)
2. Stratus (St)
3. Nimbostratus (Ns)

27. Stratocumulus
 Clouds are composed of water
 Looks soft & grey, large globular masses & darker
 Long parallel rolls pushed together or broken masses
 Air is smooth above these clouds - strong updrafts
occur below

28. Stratus
 Composed of water
 Clouds resemble grayish white sheet covering the
entire portion of the sky (cloud near the ground)
 Mainly in winter season and occasional drizzle occurs

29.  Composed of water or ice crystals
 Looks thick dark, grey and uniform layer –
reduces day light effectively
 Gives steady precipitation
 Looks like irregular, broken and shapeless
sheet like
Nimbostratus

30. Family D
 Clouds form due to vertical development i.e., due to
convection.
 The mean low level is 0.5 and means upper level
goes up to 16 km.
1.Cumulus (Cu)
2. Cumulonimbus (Cb)

31. Cumulus
 Clouds - water with white majestic appearance - flat base
 Irregular dome shaped and looks like cauliflower with
wool pack and dark appearance below due to shadow
Usually develop into cumulo-nimbus clouds with flat base

32. Cumulonimbus
 Upper levels of these clouds -
ice and water - lower levels
 Thunder head with towering
envil top and develop vertically
 Produces violent winds, thunder
storms, hails and
lightening, during summer.

33. Principles of rainmaking
• Clouds are classified as warm and cold based on cloud
top temperature
• If the cloud temperature is positive - warm clouds
Negative - cold clouds
• Nucleus needed for precipitation differs
• Hygroscopic materials - warm clouds

34. Cloud seeding
• Tools to mitigate
effects of drought.
• Process -
precipitation is
encouraged by
injecting artificial
condensation nuclei -
induce rain from rain
bearing cloud.
• Rain drops are
several times heavier
than cloud droplets.
•
• Mechanisms are
different for cold and
warm clouds.

35. Achieved by two ways
1. Dry ice seeding
2. Silver Iodide seeding
Dry ice seeding
o Dry ice (solid carbon-dioxide) remains as it is at –80°C and
evaporates, but does not melt
o Dry ice is heavy and falls rapidly and has no persistent
effects due to cloud seeding
• Aircrafts are commonly used for cloud seeding
• Aircraft flies across the top and 0.5 – 1.0 cm dry ice
pellets are released
• Falling through - ice crystals is formed
• From these ice crystals rain occurs
• Not economical as 250 kg of dry ice is required for seeding
one cloud
Seeding of cold clouds

36. Aircraft cloud seeding

37. Silver Iodide seeding
• Minute crystals of silver iodide produced in the form of
smoke - efficient ice-farming nuclei at TO below –5°C
• Nuclei are produced from the ground generators, fine
enough to diffuse with air currents
• Silver iodide is the most effective nucleating substance
• Time for silver iodide - some hours- draft a long way and
decay under the sun light
• Appropriate procedure - to release silver iodide smoke
into super cooled cloud from an aircraft
• In seeding cold clouds silver iodide technique is more
useful than dry ice techniques
• No necessity to fly to the top of the cloud

39. Seeding of warm clouds
Water drop Technique
• Coalescence process is mainly responsible for growth of
rain drops in warm cloud.
• Presence of comparatively large water droplets is
necessary to initiate the coalescence process.
• Water droplets or large hygroscopic nuclei are
introduced in to the cloud.
• Water drops of 25 mm are sprayed from aircraft @ 30
gallons per seeding on warm clouds
Common salt technique
• Common salt is a suitable seeding material
• 10 per cent solution or solid.
• Mixture of salt and soap avoid practical problems.
• Spraying is done by power sprayers and air compressors
or even from ground generators.
• Balloon burst technique is also beneficial.
• Gun powder and sodium chloride are arranged to
explode near cloud base dispersing salt particles.