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).
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
Fog
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!
Why does fog form?
Water vapour condenses onto atmospheric particles.
For condensation we need saturation.
How do we turn unsaturated air into saturated air?
There are 3 ways
Vapour |
Pressure |
Condensation |
Evaporation |
Temperature |
There are 3 ways
Vapour |
|
Pressure |
(1) Cool air to |
A |
dew-point |
A1 |
|
2 |
|
Condensation |
|
Evaporation |
|
Temperature |
|
There are 3 ways
Vapour |
|
|
Pressure |
|
|
|
A |
A1 |
|
2 |
|
Condensation |
|
|
B2 |
|
|
B1 |
Evaporation |
|
|
Temperature |
|
(1)Cool air to dewpoint
(2)Add water vapour
There are 3 ways
Vapour |
|
|
|
Pressure |
|
|
|
|
|
A |
A1 |
|
|
2 |
|
Condensation |
|
C1a |
|
B2 |
C2 |
|
|
|
|
|
|
B1 |
C1b |
Evaporation |
|
|
|||
|
Temperature |
|
|
(1)Cool air to dewpoint
(2)Add water vapour
(3)Mix moist parcels at different temperatures
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
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?