Weather & Climate LECTURE 2
description
Transcript of Weather & Climate LECTURE 2
![Page 1: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/1.jpg)
Weather & ClimateLECTURE 2
![Page 2: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/2.jpg)
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
![Page 3: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/3.jpg)
Moisture in the Atmosphere
Today’s lecture emphasis:
- cooling and condensation
- dependent on the amount of moisture in cooling air
- saturated vs unsaturated
![Page 4: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/4.jpg)
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
![Page 5: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/5.jpg)
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
![Page 6: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/6.jpg)
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
![Page 7: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/7.jpg)
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)
![Page 8: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/8.jpg)
Water Content Indices
Humidity:
Relative Humidity
- ratio of water vapour to max possible at the current temperature
- (specific humidity/saturation specific humidity) x 100%
![Page 9: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/9.jpg)
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
![Page 10: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/10.jpg)
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
![Page 11: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/11.jpg)
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
![Page 12: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/12.jpg)
Lapse Rates
Diabatic Process:
- involves addition/removal of energy from a system
- boiling water
- air cooling as it moves over a cold surface
![Page 13: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/13.jpg)
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
![Page 14: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/14.jpg)
Lapse Rates
This leads us to
1) Dry Adiabatic Lapse Rate [DALR]
2) Saturated (Wet) Adiabatic Lapse Rate [SALR]
![Page 15: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/15.jpg)
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
![Page 16: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/16.jpg)
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
![Page 17: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/17.jpg)
Lapse Rates
Temperature
Altitude
DALR
Condensation Level
SALR
![Page 18: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/18.jpg)
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
![Page 19: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/19.jpg)
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
![Page 20: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/20.jpg)
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
![Page 21: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/21.jpg)
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
![Page 22: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/22.jpg)
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
![Page 23: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/23.jpg)
Instability/Absolutely Unstable
For Instability to occur,
ELR> DALR & SALR
DALR
SALR
Temp
Height
ELR
![Page 24: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/24.jpg)
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
![Page 25: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/25.jpg)
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
![Page 26: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/26.jpg)
Stability/Absolutely Stable
DALR
SALR
Temp
Height
When ELR<DALR, SALR
ELR
![Page 27: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/27.jpg)
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
![Page 28: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/28.jpg)
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?
![Page 29: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/29.jpg)
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
![Page 30: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/30.jpg)
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
![Page 31: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/31.jpg)
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
![Page 32: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/32.jpg)
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
![Page 33: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/33.jpg)
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
![Page 34: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/34.jpg)
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
![Page 35: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/35.jpg)
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
![Page 36: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/36.jpg)
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
![Page 37: Weather & Climate LECTURE 2](https://reader033.fdocuments.in/reader033/viewer/2022051401/56814ab6550346895db7ca18/html5/thumbnails/37.jpg)
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