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