Surface Based Layers Flashcards
What is the most common surface based layer?
Fog, which is simply a cloud that is in contact with the ground. Most commonly it is a stratus type cloud
How is fog and other obscurations (mist, blowing snow, fog, freezing fog, smoke) to vision depicted on a GFA?
Within the dashed orange line
What are the key ingredients for the formation of fog?
- Moisture (high relative humidity)
- Condensation Nuclei (small particles upon which water condenses. This can be dust, smoke, sea salt, etc. The more nuclei, the denser the fog)
- Very** light surface winds** (to set up mixing action. Without wind, only dew will appear instead of fog
- A cooling process OR a process that adds moisture (evaporation) for the condensation
For fog to form, you need moisture, condensation nuclei, light surface winds, and _______
Either a cooling process OR a process that adds moisture to the air for the condensation to happen.
It is these last two processes that determin which type of fog will form
On a METAR how much of a spread does the dew point need to have for us to anticipate fog?
3° or less between temp and dew point is when you can expect fog. Especially if the temp is dropping or dew point is rising
If the dew point and temp are the same, is fog formation guarenteed?
No, relative humidity is only one ingredient for the formation of fog
What are all the different kinds of fog? (7)
- Radiation fog
- Advection fog
- Upslope fog
- Frontal fog
- Steam fog
- Ice fog
- Industrial fog
What is radiation fog?
- Formed by radiation cooling on clear nights when relative humidity is high and light winds are present
- Most likely shortly after sunrise
- Usually patchy and only a few hundred feet thick
- As the winds pick up or the run rises and warms the groun the fog dissipates
After the sun rises, how does the fog get burned off?
Fog actually dissipates from the ground up. Air doesnt absorb the short wave radiation from the sun very well, so once the earth takes it in and emits it in long wave raditation, that heats the fog closest to the ground first and then ‘eats away at it’ from the ground up
What is advection fog?
- Formed by the advective cooling as a result of horizontal movement of warm, moist air over land or sea surface that is colder than itself.
- Common in coastal areas
- Thickest advection fog will form at night with winds up to 15kts
- Very common during winter warming periode or early spring thaws.
How long does advection fog typically last?
Advection fog is usually dense and covers a wide area that can last for several days.
It will tend to persist until wind direction changes or increases beyond 15kts. At which point it will become a low stratus cloud
What is upslope fog?
As air moves as a light to moderate wind up the side of a slope, it cools and condenses (if its moist enough).
What is frontal fog?
- Also called precipitation fog.
- Caused by continuous precipitation falling ahead of a warm front and into the colder air below
- The rain evaporates in the cold air, adding moisture to the cold air and saturating it, causing the fog to form
- Frontal fog happens at a warm front or trowal
*
What is steam fog?
- Also known as sea smoke
- Forms when extremely dry, cold air blows over water (lake, ocean, ponds).
- The cold air mixes with the warm moist air over the water. The moist air cools and condenses out the water vapour as fog or mist
- Steam fog is formed via evaporation, not cooling
What is ice fog?
- Forms when cold air (-30° or colder) suddenly has water vapour added to it
- Often created by the exhaust from engines from large industrial centers on a cold winter ay
- Can produce visibilities close to zero and last for several days
What is the definition of haze?
Particles of dust, smoke, salt, etc that are so small they cannot be felt or seen with the naked eye. (These particles are called lithometers)
Haze often forms on days with high temperatures and gives the sky a milky appearance but can also have a yellow or blue hue against dark backgrounds
Can bring visibility to zero in very stable air, especially if flying into the sun.
Can be caused by smoke or pollution or ash
