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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 - PowerPoint PPT Presentation
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Weather & ClimateLECTURE 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
Temperature
Altitude
DALR
Condensation Level
SALR
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
DALR
SALR
Temp
Height
ELR
Instability/Absolutely unstable
DALR
SALR
Temp
Ht
ELRC
When ELR>DALR,
- 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
DALR
SALR
Temp
Ht
ELRC
When ELR>SALR
- 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
Therefore, when ELR>DALR, SALR,
instability occurs and the air parcel continues to rise
Stability/Absolutely Stable
DALR
SALR
Temp
Height
When ELR<DALR, SALR
ELR
Stability/Absolutely Stable
DALR
SALR
Temp
Ht
ELR
C
When ELR<DALR
- rising parcel of unsaturated air cools more rapidly than surrounding air
- becomes relatively denser
- 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
DALR
SALR
ELR
Condensation Level
Temp
HeightQn: What happens as air rises at the ELR depicted?
Conditionally Unstable
DALR
SALR
ELR
Condensation Level
Temp
Height
Qn: What happens within the yellow section as the airparcel cools at the SALR?
- will remain stable, sink to original position- if heating continues will eventually rise to equal ELR
Level of free convection
Conditionally Unstable
DALR
SALR
ELR
Condensation Level
Temp
Height
Level of free convection
What happens when air parcel continues to rise above LFC at SALR?
- cools slower (hence warmer) than atmosphere- less dense than atmosphere, so risesreadily- easily forms clouds
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
Inversion Layer
Ground Temp
Ht
Conditions:
- calm, clear, anti-cyclonic conditions
- rapid terrestrial radiation at/near ground level
Cool air
Warm air
Less Warm Air
Tro
po
sp
her
e
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