AtmosphereAtmosphere

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) in

to 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

temporarily until spring thaw.– With temperature rising, it melts into water.– Rain falls through leaves of various vegetation types at different rat

es 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 thr

ough 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 waterExchanges 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 v

apour 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 th

e 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. F

urther 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 thr

ough 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, whi

ch 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 w

hich (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 dro

plets 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 f

orm 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 (St

ratiform 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 fl

at base and a head of cauliflower.– Cumulonimbus is the thunderstorm cloud mass of enor

mous 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

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 crysta

ls 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 extra-tropical 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 vap

our 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 amount

s 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 hemisphe

re 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.