Transcript Weather phenomena associated with local energy budgets (mist, fog

Weather phenomena associated
with local energy budgets (mist,
fog, dew, temperature
inversions, land and sea
breezes).
At the end of our third lesson this week
1. I will know the three states in which moisture can
be found in the atmosphere and understand the
terms condensation, evaporation, sublimation,
freezing and melting
2. I can explain the formation of weather
phenomena associated with local energy
budgets (mist, fog, dew, temperature inversions,
land and sea breezes)
Fog and Cloud formation:
How does
air become
saturated?
North Sea Fog – a coloured visible satellite image
Dew, Frost, Haze and Fog
Dew: Common on vegetation early morning after a clear night.
(a) How? Air is cooled to dew point and becomes saturated.
(b) Happens on clear nights because surface radiates heat
and this is not trapped by a cloud cover.
(c) Dew forms on vegetation because it cools more quickly
than (say) a road surface; vegetation has a lower heat capacity
per unit area.
Dew formation releases latent heat, and this reduces the rate at which
the surface cools. Therefore the dew point temperature is often the
minimum night-time temperature.
Frost 1: Forms if dew point is below 0°C. This is now called the frost point.
(a) White frost (or hoar frost) forms by ice deposition directly from the vapour, in the
form of 6-sided crystals.
Frost 2:
Black frost (or black ice) forms when the dew point is just above 0°C so the dew
forms first and subsequently freezes as temperature drops below 0°C. A layer of ice is
formed without the creation of small crystallites.
(iii) Haze: Dry haze is caused by particulate nuclei, which scatter light like the blue of
the sky, and when there is some water deposition, the haze thickens and becomes
wet haze.
FOG – defined when visibility < 1000m
Fog is equivalent to a cloud at the surface.
Fog forms in one of three ways:
1. by cooling the air
2. by evaporation of water into the air
3. by mixing of two air masses such that when combined, the mixture is saturated
The types of fog are named with reference to the method by which the air becomes
saturated.
ADVECTION FOG - air is cooled to saturation by having
warm moist air moving over a cold surface..
San Francisco has a lot
of advection fog since
the surface water near
the coast is much colder
than the water farther off
shore. Warm moist air
from the Pacific Ocean is
advected over the cold
coastal waters, chilling
the advected air from
below.
UPSLOPE FOG
- forms as moist air flows up an elevated plain, hill or mountain. As the air ascends,
it expands, cools adiabatically (same as any ascending air), and saturates.
Orographic uplift
Lee Wave clouds
or Lenticular clouds
rotor
As species: lenticularis
STEAM FOG - forms when cold
air moves over warm water (e.g.
heated outside swimming pool)
water evaporates from the pool
into the air, increasing the dew
point and if mixing is sufficient
the air above becomes saturated.
The colder air directly above the
water is heated from below and
rises forming what appears to be
steam.
RADIATION (GROUND) FOG - produced by the radiative cooling at
the surface
- common over land in late autumn and winter.
- also form in low-lying areas.
Favourable conditions:
1. clear (cloud-free) nights (cold),
2. shallow layer of moist air near the ground (e.g. recent rainfall, or near water body),
3. long nights,
4. light winds
Radiation cooling takes place
from the top of the fog layer;
mixing continues in the fog
layer, further cooling the ground
surface.
VALLEY FOG. Cold air and high moisture content in
river valleys make them susceptible to radiation fog.
Radiation fogs form at the ground and are deepest
around sunrise - sometimes an increase in thickness
at sunrise due to the evaporation of dew supplying
moisture to the fog
3 methods of cloud formation:
1. Cool air directly
2. Mix with colder air
3. Reduce pressure (adiabatic expansion and cooling)
The rapid drop in pressure above
an aircraft wing can result in sufficient
adiabatic cooling to cause saturation
Eddies - Big and Small
• Eddy – a circulation formed downwind from an object (examples?)
• Rotor – rotation formed downwind from a mountain wave
• Wind shear –
change of wind
speed or
direction with
height
Sea and Land Breezes
• Types of thermal circulations
• Sea breeze (scale of motion?)
• Land breeze
• Sea breeze front – leading
edge of the sea breeze
Florida sea breezes can have huge walls
of clouds. Make for great gliding due to
vertical movement

Sea and Land Breezes
• http://oceanservice.noaa.gov/education/pd/oceans_weather_climate
/media/sea_and_land_breeze.swf