Download Moisture in the Atmosphere

Weather & Climate
LECTURE 2
Moisture in the Atmosphere
Evaporation and Condensation:
• accompanied by absorption/liberation of
heat
• evaporation: energy absorbed when water
increases in temp, and when it changes
from a solid to liquid, and from liquid to a
gaseous state
• condensation: energy lost when water
decreases in temp, and when changes from
a gaseous to a liquid state, and from a liquid
state to a solid state
Moisture in the Atmosphere
Today’s lecture emphasis:
- cooling and condensation
- dependent on the amount of moisture in
cooling air
- saturated vs unsaturated
Water Content Indices
Exam questions based on this are common.
Be sure to familiarise yourself with these
indices
1. Vapour Pressure
2. Humidity:
- absolute
- specific
- relative
3. Dew point/Condensation Level
Water Content Indices
Vapour Pressure
- that part of the total atmospheric pressure
due to water vapour
- max amt of water vapour air can hold at a
specific temp = saturation vapour pressure
- S.V.P is dependent on temperature
- higher temp, more moisture, therefore
higher S.V.P
Water Content Indices
Humidity:
Absolute Humidity
- density of water vapour (weight per unit
volume of air - g/m³
- changes when air expands or contracts
- all things remaining constant, absolute
humidity falls when an air parcel expands
Water Content Indices
Humidity:
Specific Humidity
- weight of water per unit mass of air (g/kg)
- does not change as air expands or
contracts
- therefore not temperature dependent
(holding all things constant)
Water Content Indices
Humidity:
Relative Humidity
- ratio of water vapour to max possible at
the current temperature
- (specific humidity/saturation specific
humidity) x 100%
Water Content Indices
Relative Humidity Short-coming:
- confusing to compare RH of air of different
temps because:
- air in area X with with temp of 30 deg C
with a RH of 50% may contain 16g of water
- air in area Y with a temp of 4 deg C with a
RH of 50% may contain only 2g of water
- Therefore, RH not a good measure to
compare absolute quantities of moisture in
the air between 2 areas
- Better way is to use vapour pressure
Water Content Indices
Dew Point
- temp to which air must be cooled to reach
saturation
- saturation: point where condensation
occurs ie Condensation point/level
- if saturation occurs below 0 deg C, it is
known as the frost point
Lapse Rates
Lapse Rate: Rate at which temperatures
decrease with increasing altitude
Before moving on to lapse rates, we have to
understand 2 concepts:
1) Diabatic Process
2) Adiabatic Process
Lapse Rates
Diabatic Process:
- involves addition/removal of energy from a
system
- boiling water
- air cooling as it moves over a cold surface
Lapse Rates
Adiabatic Process:
- where temp changes without addition or
removal of heat
- according to the gas laws
- air cools when it expands, heats up when
compressed
Lapse Rates
This leads us to
1) Dry Adiabatic Lapse Rate [DALR]
2) Saturated (Wet) Adiabatic Lapse Rate
[SALR]
Lapse Rates
1) Dry Adiabatic Lapse Rate [DALR]
- Rate at which a RISING parcel of
unsaturated air cools
- about 10 deg C for every 1000m of ascent
Lapse Rates
2) Saturated Adiabatic Lapse Rate [SALR]
- when air reaches the condensation level, it
becomes saturated
- continues to cool at a slower rate. Why?
- some of the heat loss is used to convert
water vapour into condensation (clouds/ice)
- SALR about 5 deg C per 1000m of ascent
Lapse Rates
Altitude
SALR
Condensation
Level
DALR
Temperature
Lapse Rates
Introducing…
The Environmental Lapse Rate
- vertical change in temperature through still
air
- 6.5 deg C per increase in 1000m
- it is variable: changes from day to day, place
to place, altitude to altitude
Lapse Rates
The ELR determines a parcel of air’s
stability
If a parcel of air within an air mass is heated
locally (eg forest fire), its static stability is
determined by the ELR
Static stability: the parcel of air’s
susceptibility to uplift
Lapse Rates
Static Stability
Statically unstable air: continues to rise
given an initial upward push
- occurs when density of a parcel of air is
less than the surrounding environment
(imagine a helium-filled balloon)
Statically Stable: resists upward
displacement, sinks back to original
position once heating stops
- when density of air parcel is more than
that of the surrounding
Lapse Rates and Adiabatic Lapse Rates
When looking at rising parcels of air, we need
to consider:
1) Dry
2) or Saturated ?
- determines the lapse rate at which it will rise
3) The ELR
Lapse Rates and Adiabatic Lapse Rates
These combinations will determine air parcels
of:
1) Absolute Instability/Unstable Air
2) Absolute Stability/Stable Air
3) Conditional Instability/Conditionally
unstable Air
Instability/Absolutely Unstable
For Instability to occur,
ELR> DALR & SALR
Height
SALR
ELR
DALR
Temp
Instability/Absolutely unstable
When ELR>DALR,
Ht
SALR
ELR
C
DALR
Temp
- rising parcel cools at a
slower rate than
surrounding
- hence gets progressively
warmer in comparison to
surrounding
- unstable because:
- even when heating of the parcel stops, it continues
to rise (due to difference in density)
- rise at an increasing rate as temp difference between
the air parcel and surrounding increases
Instability/Absolutely Unstable
When ELR>SALR
Ht
SALR
ELR
C
DALR
Temp
Therefore, when
ELR>DALR, SALR,
instability occurs and
the air parcel
continues to rise
- air parcel cools even
more slowly (energy used
due to condensation)
- temperature differences
even greater
- rate of rise therefore
increases at an increasing
rate
- instability increases
Stability/Absolutely Stable
When ELR<DALR, SALR
Height
SALR
ELR
DALR
Temp
Stability/Absolutely Stable
Ht
When ELR<DALR
ELR
SALR
C
- rising parcel of
unsaturated air cools more
rapidly than surrounding air
- becomes relatively denser
DALR
Temp
- once heating stops, will
sink to original position
When ELR<SALR,
- saturated air cools at SALR
- still remains colder than surrounding
- tends to sink to original position when heating stops
Conditionally Unstable
When ELR is between DALR and SALR
- and dependent on whether there is heating beyond the
level of free convection
Height
Qn: What happens as air rises at
the ELR depicted?
SALR
Condensation Level
ELR
DALR
Temp
Conditionally Unstable
Qn: What happens within the yellow section as the air
parcel cools at the SALR?
- will remain stable, sink to
original position
- if heating continues will
eventually rise to equal ELR
Height
SALR
Level of free convection
Condensation Level
ELR
DALR
Temp
Conditionally Unstable
What happens when air parcel continues to rise above
LFC at SALR?
- cools slower (hence warmer)
than atmosphere
- less dense than atmosphere, so rises
readily
Height
- easily forms clouds
SALR
Level of free convection
Condensation Level
ELR
DALR
Temp
Factors Affecting ELR
The ELR is not constant, but can vary according
to:
- Time of Day/Amt of Insolation
- Advection (Lateral Movement) of Cold/Warm air
at different levels
- Advection of an air mass with a different ELR
Limits to Rising Unstable Air
Does unstable air ever stop rising
YES. Otherwise the earth’s atmosphere will be
replaced by a vacuum.
- unstable air will usually eventually rise to a
layer of stable air
- if not, mechanism of entrainment will limit the
rise
Air Inversions
In general, temperatures decrease with elevation
in the troposphere
- reverse can happen: temperatures can
increase with height in troposphere
- situation known as ‘inversion’
- extremely stable, rising air experiences
negative buoyancy, resists vertical mixing
Air Inversions
Ht
Less Warm Air
Warm air
Inversion Layer
Cool air
Ground
Temp
Conditions:
- calm, clear, anti-cyclonic conditions
- rapid terrestrial radiation at/near ground level
Air Inversions
Air inversions set up conditions for the formation of
- dew
- frost
- frost dew
And of special interest, Mists and Fog
- Radiation Fog
- Advection Fog
NB: The third type of fog in your notes, Upslope Fog, is not
a result of temp inversion, but more so due to the adiabatic
process due to a decrease in pressure
Condensation and Cloud Formation
3 main mechanisms of cloud formation:
- Orographic Uplift
- Frontal Lifting
- Localised Convection
Form different types of clouds
- high clouds
- middle clouds
- low clouds
- clouds with vertical development
Condensation and Cloud Formation
HOMEWORK:
Produce a set of notes on the mechanisms of
cloud development and cloud types.
Hand up during first lecture in Term 2 for
checking
- Those who fail to do so will stay back on Fri
afternoon to complete it
END