Fogs and Haze

Сентябрь 3, 2022

Содержание

2. Mist – a suspension of small water droplets or moist hygroscopic particles that slightly obscures visibility.
3. Condensation and fog Properties of water Formation of dew, frost, rime Types of fog formation clearance
4. Water Three phases … solid, liquid, gas (vapour) Latent heat heat absorbed or emitted to change
5. Evaporation = transfer of liquid water molecules to the vapour state Evaporation and condensation
6. water vapour molecules condensing Condensation = transfer of water vapour molecules to the liquid state Evaporation
7. water vapour molecules condensing = Dynamic equilibrium for a plane surface of pure water occurs when
8. Dew What do you need? Moisture, cooling, condensation surface Clear skies at night (loss of long
9. Dew Ground cools Air near ground is cooled to dew point Water condenses onto ground Latent
10. For sub-zero temperatures Frost occurs when T ground frost for ground temp air frost for screen
11. Hoar frost or rime? Super cooled droplets required Deposits of ice when drops meet a sub-zero
12. Fog What is it? A suspension of microscopic water droplets in the air reducing visibility at
13. Freezing fog Super-cooled fog drops which freeze on impact with a surface Ice fog Tiny suspended
14. Why does fog form? Water vapour condenses onto atmospheric particles. For condensation we need saturation. How
15. Condensation Vapour Pressure Temperature There are 3 ways Evaporation
16. (1) Cool air to dew-point A2 A1 Vapour Pressure Temperature There are 3 ways Evaporation Condensation
17. (1) Cool air to dewpoint (2) Add water vapour A2 A1 B2 B1 Vapour Pressure Temperature
18. (1) Cool air to dewpoint (2) Add water vapour (3) Mix moist parcels at different temperatures
19. Fog Classification There are four main types: Radiation - cooling of ground by radiation Advection -
20. 1. Radiation fog Favourable conditions Clear sky or thin, high cloud Moist air in lowest 100m
21. Stage 1. Dew deposition Clear night, strong radiative cooling Light winds Rapid fall in surface temperature
22. Initial formation Surface cooling 2 metre wind Air becomes saturated Shallow fog forms Heat Temperature Td
23. Stage 3. Mature fog - sky visible Sky visible so radiative cooling continues Temperature falls, fog
24. Stage 4. Mature fog - sky obscured Sky obscured Surface cooling stops Fog top becomes radiating
25. Additional processes... Lowering of stratus to the ground Advective Effects Fog upwind Air mass change Upslope
26. Clearance mechanisms Solar radiation – raises air temperature above dew point. Increase in gradient wind –
27. 2. Advection Fog Warm advection fog: Warm moist air moving over cold land or sea. Cooling
28. 3. Upslope fog Formed by Warm, moist air forced to rise over hills. Air cools adiabatically
29. 4. Frontal fog Formed by: Ahead of warm front … rain from warm air falling into
30. Examples
31. A more concrete example St. Petersburg, end of October–beginning of November. Here lakes, rivers, and swamps
32. A more concrete example H Heat island Fog
33. Types of fogs. Radiation fog Advection fog Evaporation fog Mixing fog
34. Could you name more fog types? Upslope Fog Ice Fog Freezing Fog
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Mist – a suspension of small water droplets or moist hygroscopic

particles that slightly obscures visibility. Mist is reported when visibility exceeds 1 km; below that limit the obscuration would be classed as fog.
Fog – a visible suspension of water droplets in the atmosphere near the surface, and defined by international agreement as reducing visibility to less than 1 km. There is no physical distinction between fog and cloud, other than the fact that the base of clouds is above the surface of the ground.
Haze is reported when visibility decreases to values from 1 to 10 km (dry haze: relative humidity is less 100%).

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Condensation and fog
Properties of water
Formation of dew, frost, rime
Types of fog
formation
clearance
Synoptic

situations

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Water
Three phases … solid, liquid, gas (vapour)
Latent heat
heat absorbed or emitted

to change state.
Relative humidity
amount of water vapour the air holds as a percentage of amount it could hold when saturated.
Dew-point temperature
temperature at which air just becomes saturated w.r.t water when cooled at constant pressure.

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Evaporation = transfer of liquid water molecules to the vapour state
Evaporation

and condensation

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water
vapour
molecules
condensing
Condensation = transfer of water vapour molecules to the liquid

state

Evaporation and condensation

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water
vapour
molecules
condensing
=
Dynamic equilibrium for a plane surface of pure water occurs

when ... no. of molecules transferring to vapour state = no. transferring to liquid state - Saturation

Evaporation and condensation

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Dew
What do you need?
Moisture, cooling, condensation surface
Clear skies at night (loss

of long wave radiation)
Light/calm winds to prevent mixing
Moisture source

What is it?
Condensation of water vapour onto surface whose temperature is < Td
small water drops D < 1mm

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Dew
Ground cools
Air near ground is cooled to dew point
Water condenses onto

ground
Latent heat given out during condensation
slows temperature fall
Air near the ground becomes drier
∴ Dew point falls
Temperature must fall further for condensation to continue.

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For sub-zero temperatures
Frost occurs when T < 0°C
ground frost for

ground temp < 0°C
air frost for screen temp < 0°C
Classified as slight, moderate, severe, very severe (temperature and wind)
Ice deposits onto cooled surfaces are hoar frost (terminology: deposition/sublimation)

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Hoar frost or rime?
Super cooled droplets required
Deposits of ice when drops

meet a sub-zero surface.
Rime builds up on windward sides of objects (fence posts).

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Fog
What is it?
A suspension of microscopic water droplets in the air

reducing visibility at the earth’s surface to:
< 1000m for met. observations, aviation
and shipping.
< 200m for public service purposes

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Freezing fog
Super-cooled fog drops which freeze on impact with a surface
Ice

fog
Tiny suspended ice particles
Usually requires T= -30°C
Very rare in the UK but often in Siberia!

Fog

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Why does fog form?
Water vapour condenses onto atmospheric particles.
For condensation we

need saturation.
How do we turn unsaturated air into saturated air?

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Condensation
Vapour
Pressure
Temperature
There are 3 ways
Evaporation

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(1) Cool air to dew-point
A2
A1
Vapour
Pressure
Temperature
There are 3 ways
Evaporation
Condensation

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(1) Cool air to dewpoint
(2) Add water
vapour
A2
A1
B2
B1
Vapour
Pressure
Temperature
There are

3 ways

Evaporation

Condensation

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(1) Cool air to dewpoint
(2) Add water
vapour
(3)

Mix moist
parcels at
different
temperatures

C1b

C1a

Vapour
Pressure

Temperature

There are 3 ways

Evaporation

Condensation

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Fog Classification
There are four main types:
Radiation - cooling of ground by

radiation
Advection - (warm advection) cooling of air by conduction
Upslope - cooling of air by adiabatic expansion
Frontal - increasing moisture by evaporation
Also:
Steam - (cold advection) increasing moisture by evaporation

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1. Radiation fog
Favourable conditions
Clear sky or thin, high cloud
Moist air in

lowest 100m
Moist ground
Light surface wind
Favourable local topography
When are these conditions likely?

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Stage 1. Dew deposition
Clear night, strong radiative cooling
Light winds
Rapid fall in

surface temperature
Cools to dew point
Dew deposition
Air dries, dew point falls but … mixing maintains condensation

Heat

Temperature

Cooling

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Initial formation
Surface cooling
2 metre wind < 1kn
Air becomes saturated
Shallow fog

forms

Heat

Temperature

Cooling

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Stage 3. Mature fog - sky visible
Sky visible so radiative cooling

continues
Temperature falls, fog thickens
Fog now alters radiation balance
Restricts surface cooling
Soil heat flux also significant

Heat

Temperature

Cooling

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Stage 4. Mature fog - sky obscured
Sky obscured
Surface cooling stops
Fog

top becomes radiating surface.
Fog deepens due to radiative cooling from top.

Temperature

Cooling

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Additional processes...
Lowering of stratus to the ground
Advective Effects
Fog upwind

Air mass change
Upslope motion

Sunrise
Turbulence
Dew evaporating

Pollution
Smog

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Clearance mechanisms
Solar radiation – raises air temperature above dew point.
Increase in

gradient wind – turbulence
Advection of cloud over the fog – radiation warms the fog top (most efficient method)
Advection of drier air – lowers surface dew point

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2. Advection Fog
Warm advection fog:
Warm moist air moving over cold land

or sea.
Cooling to dew point from below.
Mostly sea fog
Over land, stratus
Cold advection fog:
Cold air moving over warm water.
Moisture evaporates then condenses again.
Arctic Sea Smoke.

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3. Upslope fog
Formed by
Warm, moist air forced to rise over hills.
Air

cools adiabatically on ascent
Very common in western and northern Britain
Tropical maritime airmass

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4. Frontal fog
Formed by:
Ahead of warm front … rain from warm

air falling into very cold, stable air.
Rain evaporates
Layer eventually becomes saturated and fog develops
Often produces stratus not fog.
How do you clear it?

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Examples

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A more concrete example
St. Petersburg, end of October–beginning of November.
Here

lakes, rivers, and swamps that make the air to be very humid cover for a large area.
A huge anticyclone occupies almost whole Europe. Its center is found to SW from St. Petersburg. Short days and long nights make the air mass colder and colder from day to day.
Cold and stable air mass and clear sky at nighttime plus very humid air (due to local condition) facilitates formation of a fog. However, within the town “warm island” (here the air temperature is about 20 C higher) no fog may appear, while over colder suburbs the fog forms.
As result, over airport Pulkovo the fog can be formed, over St. Petersburg cannot. That is the local weather.

Слайд 32

A more concrete example
H
Heat
island
Fog

Слайд 33

Types of fogs.
Radiation fog
Advection fog
Evaporation fog
Mixing fog

Слайд 34

Fogs and Haze

Содержание

Mist – a suspension of small water droplets or moist hygroscopic

Condensation and fogProperties of waterFormation of dew, frost, rimeTypes of fogformationclearanceSynoptic

WaterThree phases … solid, liquid, gas (vapour)Latent heatheat absorbed or emitted

Evaporation = transfer of liquid water molecules to the vapour stateEvaporation

water vapourmoleculescondensingCondensation = transfer of water vapour molecules to the liquid

water vapourmoleculescondensing=Dynamic equilibrium for a plane surface of pure water occurs

DewWhat do you need?Moisture, cooling, condensation surfaceClear skies at night (loss

DewGround coolsAir near ground is cooled to dew pointWater condenses onto

For sub-zero temperaturesFrost occurs when T < 0°C ground frost for

Hoar frost or rime?Super cooled droplets requiredDeposits of ice when drops

FogWhat is it?A suspension of microscopic water droplets in the air

Freezing fogSuper-cooled fog drops which freeze on impact with a surfaceIce

Why does fog form?Water vapour condenses onto atmospheric particles.For condensation we

CondensationVapourPressureTemperatureThere are 3 waysEvaporation

(1) Cool air to dew-point A2A1VapourPressureTemperatureThere are 3 waysEvaporationCondensation

(1) Cool air to dewpoint (2) Add water vapourA2A1B2B1VapourPressureTemperatureThere are

(1) Cool air to dewpoint (2) Add water vapour (3)

Fog ClassificationThere are four main types:Radiation - cooling of ground by

1. Radiation fogFavourable conditionsClear sky or thin, high cloudMoist air in

Stage 1. Dew depositionClear night, strong radiative coolingLight windsRapid fall in

Initial formationSurface cooling 2 metre wind < 1knAir becomes saturatedShallow fog

Stage 3. Mature fog - sky visibleSky visible so radiative cooling

Stage 4. Mature fog - sky obscuredSky obscuredSurface cooling stops Fog

Additional processes...Lowering of stratus to the ground Advective Effects Fog upwind

Clearance mechanismsSolar radiation – raises air temperature above dew point.Increase in

2. Advection FogWarm advection fog:Warm moist air moving over cold land

3. Upslope fogFormed byWarm, moist air forced to rise over hills.Air

4. Frontal fogFormed by:Ahead of warm front … rain from warm

Examples

A more concrete exampleSt. Petersburg, end of October–beginning of November. Here

A more concrete exampleHHeatislandFog

Types of fogs.Radiation fogAdvection fogEvaporation fogMixing fog

Could you name more fog types?Upslope FogIce FogFreezing Fog

Похожие презентации

Condensation and fog
Properties of water
Formation of dew, frost, rime
Types of fog
formation
clearance
Synoptic

Water
Three phases … solid, liquid, gas (vapour)
Latent heat
heat absorbed or emitted

Evaporation = transfer of liquid water molecules to the vapour state
Evaporation

water
vapour
molecules
condensing
Condensation = transfer of water vapour molecules to the liquid

water
vapour
molecules
condensing
=
Dynamic equilibrium for a plane surface of pure water occurs

Dew
What do you need?
Moisture, cooling, condensation surface
Clear skies at night (loss

Dew
Ground cools
Air near ground is cooled to dew point
Water condenses onto

For sub-zero temperatures
Frost occurs when T < 0°C
ground frost for

Hoar frost or rime?
Super cooled droplets required
Deposits of ice when drops

Fog
What is it?
A suspension of microscopic water droplets in the air

Freezing fog
Super-cooled fog drops which freeze on impact with a surface
Ice

Why does fog form?
Water vapour condenses onto atmospheric particles.
For condensation we

Condensation
Vapour
Pressure
Temperature
There are 3 ways
Evaporation

(1) Cool air to dew-point
A2
A1
Vapour
Pressure
Temperature
There are 3 ways
Evaporation
Condensation

(1) Cool air to dewpoint
(2) Add water
vapour
A2
A1
B2
B1
Vapour
Pressure
Temperature
There are

(1) Cool air to dewpoint
(2) Add water
vapour
(3)

Fog Classification
There are four main types:
Radiation - cooling of ground by

1. Radiation fog
Favourable conditions
Clear sky or thin, high cloud
Moist air in

Stage 1. Dew deposition
Clear night, strong radiative cooling
Light winds
Rapid fall in

Initial formation
Surface cooling
2 metre wind < 1kn
Air becomes saturated
Shallow fog

Stage 3. Mature fog - sky visible
Sky visible so radiative cooling

Stage 4. Mature fog - sky obscured
Sky obscured
Surface cooling stops
Fog

Additional processes...
Lowering of stratus to the ground
Advective Effects
Fog upwind

Clearance mechanisms
Solar radiation – raises air temperature above dew point.
Increase in

2. Advection Fog
Warm advection fog:
Warm moist air moving over cold land

3. Upslope fog
Formed by
Warm, moist air forced to rise over hills.
Air

4. Frontal fog
Formed by:
Ahead of warm front … rain from warm

A more concrete example
St. Petersburg, end of October–beginning of November.
Here

A more concrete example
H
Heat
island
Fog

Types of fogs.
Radiation fog
Advection fog
Evaporation fog
Mixing fog

Could you name more fog types?
Upslope Fog
Ice Fog
Freezing Fog