Lecture 3/4 - Instability, Moisture and Clouds Flashcards
In what ways can there be moisture in the atmosphere? (5)
Evaporation Humidity Stability Condensation Lapse rates
What is the Lapse rate?
Related to condensation
(DALR) = 9.8C/Km
(SALR) – rate cannot be constant, different moisture means different cooling speeds. With dry air cooling faster.
Latent heat released on condensation, reduces lapse rate
What are the characteristics of a stable atmosphere?
Stable – cold air below, no buoyancy involved.
No free convection, all forced convection (flat) giving stratiform clouds.
What are the characteristics of an unstable atmosphere?
Unstable - warm air below, naturally buoyant.
Free convection gives cumuliform clouds. Which is limited by atmospheric structure.
What is Conditional Instability?
This is the moisture content. Always staying colder than the environment.
What is Potential Instability?
Saturated on bottom, not on top. Moved upwards, top layer cools as it’s drier, bottom cools less quickly. Air overturns at that point. Example of this is heavy rain in mountain regions.
What is dew?
Air in contact with ground cools as ground cools. When it has dropped in temperature, plane surface of water occurs on Hygroscopic.
What is Radiation Fog?
Successive layers of air particles with vapour carry on cooling with the surface beneath them. ½ m/s of wind to take air parcel that’s saturated, introducing new air beneath it and cools the saturation and condenses.
What is Advection Fog?
As it moves to cooler sea, air parcel cooled by sea. Reaches dew point then condenses.
What is the process of which Frost appears?
Ground cools without condensing and reaches 0oC. Air relatively dry, when the air drops below 0. Plant vegetation act as condensation nuclei. When the plant reaches below 0, ]droplets freezing as pockets of ice on plant. Air will condense ice onto the surface.
What are the paths to saturation?
Free convection - cumuliform clouds, caused by surface heating. Cool air over warm water. Warm layer advected at height. Cold pool of air at height.
Forced convection (adiabatic cooling). Lifting over physical barrier (e.g. Mountain). Or lifting over density boundary (front).
How are clouds formed?
Clouds form when the air is saturated and cannot hold any more water vapour (cooled to condensation level e=es, RH 100%.).
What are the two ways in which clouds form?
- ) Amount of water in the air has increased (e.g. through evaporation) So the air cannot hold any more water.
- ) The air is cooled to its dew point. (the point where condensation occurs) and the air is unable to hold any more water.
In what ways can clouds increase in size?
Growth by Bergeron process (mixed cloud with ice)
Growth by collision/coalescence
Growth by seeder feeder mechanisms (lower clouds fed from above)
What causes clouds to form? (5)
- Surface heating - When the ground is heated by the sun it heats the air causing it to rise. Tends to produce cumulus clouds.
- Topography/orographic forcing - When air is forced to rise over a barrier of mountains or hills it cools as it rises. Layered clouds are often produced this way.
- Frontal - Formed when a mass of warm air rises up over a mass of cold, dense air over large areas along fronts.
- Convergence - Streams of air flowing from different directions are forced to rise where they flow together, or converge.
- Turbulence - A sudden change in wind speed with height creating turbulent eddies in the air.
