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 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 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 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: by cooling the air by evaporation of water into the air 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. The formation of a fog layer occurs when a moist air mass is cooled to its saturation point (dew point). This cooling can be the result of radiative processes (radiation fog), advection of warm air over cold surfaces (advection fog), evaporation of precipitation (precipitation or frontal fog), or air being adiabatically cooled while being forced up a mountain (upslope fog). 

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. Advection fog, unlike radiation fog always involves the movement of air. Fog, while a traffic hazard, is very important water source for some plants and animals - particularly along the coast of California

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 rotor or Lenticular clouds Lee waves may exist without the obvious signs of clouds. lenticular clouds remain almost stationary whilst the air moves through them.

As species: lenticularis

STEAM FOG - forms when cold air moves over warm water (e. g 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. Observed over unfrozen lakes in autumn or winter. Steam fog also forms above a wet surface on a sunny day - over a road after a summer rain shower.

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: clear (cloud-free) nights (cold), 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