Fogs and Haze

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%).

fog
formation
clearance

Synoptic situations

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.

state
Evaporation and condensation

liquid state
Evaporation and condensation

occurs when ... no. of molecules transferring to vapour state

= no. transferring to liquid state - Saturation

Evaporation and condensation

(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

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.

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)

drops meet a sub-zero surface.
Rime builds up on windward sides

of objects (fence posts).

air reducing visibility at the earth’s surface to:

< 1000m for

met. observations, aviation
and shipping.

< 200m for public service purposes

surface
Ice fog
Tiny suspended ice particles
Usually requires T= -30°C
Very rare in

the UK but often in Siberia!

Fog

we need saturation.

How do we turn unsaturated air into saturated

air?

ways
Evaporation
Condensation

vapour







A2
A1
B2
B1
Vapour
Pressure
Temperature
There are 3 ways
Evaporation
Condensation

vapour

(3) Mix moist

parcels at
different
temperatures

A2

A1

B2

B1

C1b

C1a

C2

Vapour
Pressure

Temperature

There are 3 ways

Evaporation

Condensation

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

in lowest 100m
Moist ground
Light surface wind
Favourable local topography

When are these

conditions likely?

in surface temperature
Cools to dew point
Dew deposition
Air dries, dew point

falls but … mixing maintains condensation

Heat

Temperature

Td

T

Cooling

fog forms

Heat
Temperature




Td
T
Cooling

cooling continues
Temperature falls, fog thickens
Fog now alters radiation balance
Restricts surface

cooling
Soil heat flux also significant

Heat

Temperature

Td

T

Cooling

Cooling

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

from top.

Temperature

Td

T

Cooling

Fog upwind
Air mass change
Upslope motion
Sunrise

Turbulence
Dew evaporating

Pollution
Smog

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

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.

hills.
Air cools adiabatically on ascent
Very common in western and northern

Britain
Tropical maritime airmass

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?

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.

When ants scatter everywhere, the weather is going to be

hot.