Transcript Atmosphere

Atmosphere
Part II
Moisture
Contents
 Hydrological Cycle
 Three States of water
 Humidity
 Adiabatic Processes
 Condensation
 Precipitation
 World distribution of precipitation
Hydrological Cycle
 It is the global circulation of moisture (and heat) between
the land and sea surface and the atmosphere.
Hydrological Cycle
 It 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.
 The most important storage is the ocean, sea, lakes and
rivers.
Hydrological Cycle - Terms
 Water evaporate from water bodies and land surface.
 Atmosphere
– Vapour is temporarily locked up as gigantic atmospheric storage.
– As temperature drops, it condenses (around condensation nuclei)
into different forms of precipitation – solid state (snow, hail), liquid
state (rain, drizzles, showers) and semi-gaseous state (fog and mist).
 Land surface and vegetation
– Snow on mountain tops or in high latitudinal zones, which is stored
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temporarily until spring thaw.
With temperature rising, it melts into water.
Rain falls through leaves of various vegetation types at different
rates through interception and through fall.
Droplets may stay on leafs and stems surface to be evaporated into
atmos. Later.
Underground water will be absorbed by roots of vegetation and
through transpiration to back into atmos.
Hydrological Cycle - Terms
 Underground
– When soil is dry (permeable), water percolates into soil as
infiltration, and is stored below the water table as underground water.
– The water flow of this subsurface water is called return flow which
reaches rivers as surface water.
– If infiltration is checked (few vegetation or saturated soil), water
flows on land surface as surface runoff.
 Some water will be used for domestic, industrial
consumption and farming. Water quality may be down grade
by pollution.
 Evapotranspiration brings water from various water storage
back to the atmos. again in the form of water vapour.
Hydrological Cycle
Hydrological Cycle
Three States of water
Exchanges of heat energy between 3 states of water
is called latent heat.
Humidity
 The ability of the air to hold water vapour depends solely
on temperature.
 A mass of air is holding the maximum amount of water
vapour possible at a given temperature (dew point
temperature), it is said to be saturated.
 Less than the maximum amount is said to be unsaturated.
 Holding more, it is known as supersaturated.
Absolute Humidity
 It refers to the actual quantity of moisture
present in the air. (gram/m3)
Relative Humidity
 The proportion of water vapour present relative to
the maximum quantity, expressed as percentage (%)
 The change in relative humidity can be caused by:
– Increase water vapour amount (evaporation,
transpiration, sea breeze…..)
– Changes of temperature.
Relative Humidity
 Relative humidity reaches 100%, it is saturated.
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.
Adiabatic Processes
 Adiabatic means that there is no heat exchange between the
air parcel and its surroundings environment.
 The process of adiabatic depends on parcels of air rising
through the atmosphere to higher elevations. (expansion
cooling)
 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. (loss heat – cooling)
 When a air parcel moving to lower level, it gains heat by
contraction.
 Expansion
energy is used up
temp. decrease
Adiabatic Cooling
Dry & Wet Adiabatic Lapse Rate
 Dry Adiabatic Lapse Rate (DALR):
– Relative Humidity below 100%, adiabatic cooling and warming
takes place at a fixed rate (-10oC/1000m)
 Saturated Adiabatic Lapse Rate (SALR):
– It is lower than the DALR for latent heat was released into air,
which offsets the adiabatic temperature loss. -(5~6)oC/1000m)
Adiabatic Lapse Rate and
Environmental Lapse Rate
 Adiabatic Lapse Rates
– The temperature changes with height of a air parcel
which rising or subsiding.
 Environmental Lapse Rate
– The actual environmental temperature changes with
height (-6.5oC/1000m)
Adiabatic Lapse Rate and
Environmental Lapse Rate
Condensation
 Condensation is the direct cause of all the various
forms of precipitation.
 Conditions for condensation:
– Air temperature drops to dew point temp. but its volume
remains constant
– Volume of the air parcel expands. (adiabatic cooling)
– Joint functions of above two causes, which reduces the
moisture-holding capacity of the air.
 Condensation will be accelerated by the presence of
condensation nuclei / hygroscopic nuclei (dust, salt,
smoke,……)
Forms of Condensation
 Dew and Frost
 Mist and Fog
– Advection Fog
– Radiation Fog
 Clouds
Dew and Frost
 Dew consists of relatively large water droplets
which (condense) collect or deposit on cold or
cool surface under clam conditions.
 Frost consists of ice crystals condense on a cold
or cool surface, but the dew point temp. is below
0oC. Water vapour transform to ice crystals
directly through sublimation process.
Mist and Fog
 Mist and fog are very fine condensed water
droplets suspended in lower level of air.
 Visibility
– Mist < 1000m
– Fog > 1000m
 They are usually formed by advection of
warm, moist air (advection fog) and by
intense radiation at night (radiation fog).
 Condensation nuclei are very important for
their formation.
Advection Fog
 The lower layer of warm air is cooled below dew
point temp. by contact with cooler air or surface.
 Formation conditions
– Warm, moist air passes over a cooler land or sea
surface horizontally.
– Cold and warm ocean currents meet each other
– Warm moist air (may be from ocean) merges with cool
dry air (may be from land), which is also called frontal
fog.
 It is very common in spring in HK.
 It will be vanish during the day time when
appears and temperature rises.
Advection Fog
Radiation Fog
 Moist air is cooled for heat loss from ground by
radiation
 It occurs in cold weather when the sky is clear,
clam and stable condition.
 It is common in winter and in the industrial
regions
Clouds
 Clouds consist of extremely tiny water
droplets or minute ice crystals suspended
in upper level air.
 The formation is the same of those of fog.
 Favourable formation conditions
– Air temperature fall down to dew point for
form water droplets or ice crystals.
– Presence of condensation nuclei
• Water in such minute quantities can remain liquid
form far below 0oC without condensation nuclei, it
is said supercooled water (-12oC to -30oC).
Classification of Cloud Types
 It can be classified on two characteristics: Form
(Stratiform and cumuliform) and Altitude.
 Stratiform:
– They are blanket like, often covering vast areas, but are
fairly thin comparison to horizontal dimensions.
 Cumuliform:
– They tend to display a height as great as their horizontal
dimensions.
– Cumulus is a white, wool pack cloud mass, showing a
flat base and a head of cauliflower.
– Cumulonimbus is the thunderstorm cloud mass of
enormous size which brings heavy rainfall, thunder and
lightning, and gusty winds. It extend from a height of
300 to 600m at the base up to 9000 to 12000m.
Altitude
Stratus
Cumulus
Cumulonimbus
Cloud types (low to middle Level)
Cloud types (High Level)
Stability and Precipitation
 Air Stability
– Absolute instability
– Conditionally stable (conditionally unstable)
– Absolute stability
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Highly stable (Inversion)
Formation of precipitation
Forms of precipitation
Types of rainfall
– Convectional precipitation
– Orographic precipitation
– Cyclonic precipitation
Air Stability
 It is determined by the relationship between
the environmental lapse rate (ELR) and the
dry and saturated adiabatic lapse rate
(DALR and SALR).
 It is a very important meteorological
phenomenon because it influences the
amount and the type of condensation (clouds,
fog….), together with other related weather
phenomena, such as rain and hail…...
DALR and SALR
ELR and ALR
Absolute instability
 Definition:
– When uplifted air is encouraged to rise still further and
descending air is continued to sink.
– Environmental lapse rate is greater than that of both dry
and wet adiabatic lapse rates. ELR > DALR >SALR
Conditionally Stable / Unstable
 Definition:
– Environmental lapse rate is less than the dry but greater
than the saturated adiabatic lapse.
– DALR > ELR > SALR
Absolute Stability
 Definition:
– A vertically displaced air parcel tends to return to its
original position.
– DALR > SALR > ELR
Highly Stable Air (Inversion)
 Definition:
– When the environment air temperature increase with
altitude, temperature inversion exist.
– It effectively put a cap on the atmosphere.
Formation of Precipitation
 Four conditions for precipitation
– a) Cooling air
– b) Condensation and cloud formation
– c) Accumulation of moisture
– d) Growth of cloud droplets
 When clouds form, they are 99.9% non-precipitating.
 Stages (c) and (d) are fundamental in precipitation
production.
 Stages (d) is the most critical one in precipitation formation.
It is because the water droplets and ice crystals of clouds
have to be transformed into heavier particles.
 There are 2 main mechanisms to increase size of cloud
droplets for precipitation.
Collision Mechanism
 Rising and sinking air motions within cloud carry
with different size of droplets of ice-crystals.
 The larger droplets tend to catch more of the
smaller cloud particles and grow.
 When two liquid water droplets collide and join
together the process is called coalescence.
 The conjoining of two ice crystals is known as
aggregation.
 An ice crystal collects a water droplet, this process
is known as accretion.
 Rainfall largely results from coalescence, snowfall
from aggregation and hail from accretion.
Collision / Direct capture
Ice-crystal / Bergeron Method
 Although supercooled water droplets and ice
crystals can co-exist within a cloud, they are
unstable and liquid water droplets will evaporate.
 The evaporated vapour then condenses and freezes
onto the ice crystals surface and ice crystals grow
into large snowflakes.
 In the tropics areas, raindrops grow by collision
processes.
 Ice-crystals method is responsible for extratropical latitudes (strong convection) and heavy
rainfall of mid-latitude areas.
Bergeron Method
Forms of Precipitation
 Rain:
– When cloud droplets are caused to coalesce into drops
too large to remain suspended in the air, rain is formed.
 Drizzle:
– The diameter of falling rain drops less than 0.5mm.
 Sleet:
– A mixture of rain and snow
 Snow:
– Falling ice crystals which grows directly from water
vapour to solid form (dew point temp. below 0oC)
 Hail:
– It consists of rounded lumps of ice, having an internal
structure of concentric layers.
Types of rainfall
 Convectional Rain
 Orographic / Relief Rain
 Cyclonic / Frontal Rain
Convectional Rain
Convectional Rain
Orographic / Relief Rain
Orographic Rain
Cyclonic / Frontal Rain
Cyclonic / Frontal Rain
Cyclonic / Frontal Rain
World distribution of precipitation
World distribution of precipitation
 Lines joining places having equal rainfall
amounts are termed isohyets.
 Equatorial areas have the most precipitation.
– High temp. and the consequent large moisture-holding
capacity of air, and presence of large oceanic water
surfaces to supply moisture.
– Convectional rain
 Polar areas
– They have small absolute amounts of precipitation for
low temp. and less evaporation.
 Middle latitudes have a complicated pattern.
– High rainfall related to the westerlies in both
hemisphere and notably to their cyclone tracks.
World distribution of precipitation
 Subtropics (horse latitudes) on the eastern sides of
oceans (western side of continents) are the lowest
rainfall regions for subsiding air (adiabatic
warming process).
 Large mountain ranges
– More rainfall in windward side and few rainfall in
leeward side.
 Altitude also plays a important role in local scale.
– There is general increase of precipitation with height
(relief rain) up to about 2 km.
– Beyond this level, less rainfall for dryness of the air.