Chapters 6-11 + Midterm questions Flashcards
What is an adiabatic process?
When an air parcel is forced to rise or sink, it adjusts to the pressure level of the environment by expanding or compressing (i.e. by doing work to the parcel). This changes the temperature of the air parcel but does not exchange heat with the environment. When it rises, it cools (does work on the environment), and when it sinks, it warms (the environment does work on it).
Lapse Rate
The rate of decrease or increase in temperature with increasing or decreasing altitude.
Specific humidity does not change, but relative humidity does. The RH of a rising parcel increases since colder air parcels are able to hold less water.
Dry adiabatic lapse rate
The parcel is unsaturated (RH<100%)
10C/km
Saturated adiabatic lapse rate
The parcel is saturated (RH = 100%) 6C/km
In this case, condensation or evaporation of water is occurring in rising or sinking air parcels
When is air more dense?
In a fixed air pressure, it is when it is colder.
Buoyancy force
This relates the difference between the density of the environment and the density of the air parcel. If the air is warmer/less dense than the environment, it floats. If it is colder/more dense then it sinks.
Absolutely stable environment
This is when the environmental lapse rate is less than the saturated lapse rate - meaning also less than the dry lapse rate (less than 6C/km). This can also happen at inversion lapse rates where the temperature rises with altitude.
- This stability usually occurs over a cool surface where the denser air is below the warmer air
How can stratus or stratiform clouds form?
When air parcels are forced to rise in a absolutely stable environment, causing it to spread horizontally and possible form clouds.
Absolutely unstable environment
This is when the environmental lapse rate is greater than both the dry and saturated lapse rates (greater than 10C/km). In this case, if the air is displaced upwards, it will simply continue to rise.
- It is usually over a warm surface causing the surface air to heat up due to radiation and advection.
- Associated with convection, thunderstorms and severe weather
What type of clouds does an absolutely unstable environment form?
When an air parcel in an unstable environment is saturated, clouds with vertical development are formed.
Conditionally unstable environment
This is when the environmental lapse rate is in between the saturated lapse rate and the dry lapse rate (6<here<10).
- It is stable for unsaturated air parcels but unstable for saturated air parcels
- Troposphere is usually in this state
What affects the environmental lapse rate (how to make larger or smaller)?
- When there is warmer air below and colder air aloft it leads to a less stable environment and a larger environmental lapse rate
- When there is colder air below and warmer air aloft it leads to a more stable environment and a smaller environmental lapse rate
2 factors that affect the air parcel lapse rate
- Higher RH. The parcel is able to saturate more easily and at a lower altitude so then it follows the saturated adiabatic lapse rate. Environmental instability is more likely since the air parcel now has a lower lapse rate.
- Higher temperature. If the parcel is saturated then its lapse rate is reduced causing a highly likelihood for instability.
2 causes of instability
- Cooling air aloft (advection, radiative cooling of cloud tops)
- Warming of surface (insolation, advection, warm surface) The warm air parcel would rise and be warmer than the air surrounding it
Vertical mixing of a stable air layer
This brings the layer closer to a dry adiabatic process causing it to destabilize with respect to saturated parcels. This means that the lapse rate is increased.
- Mixing means that the warmer air (that usually is rising) is brought down and the cooler air is pushed up
What is subsidence of air?
Instead of looking at how small air parcels behave in the environment, subsidence is looking at movements of whole layers of air. More specifically, it is the downward motion of air over a large area as it cools and becomes more dense.
Why does subsidence cause layers to become more stable?
The sinking air aloft causes a warmer middle troposphere which means that warm air lies above cold air. This is associated with temperature inversions at lower levels proving that inversions are very stable.
- If pollution is found below the inversion it is then trapped as inversion suppresses vertical motions
What do we want for layer stability?
Warm air aloft with cold air underneath. This is because warm air rises and cold air sinks.
What layer subsidence movements cause stability?
Decent and squashing of the air. As the air shrinks, it becomes more dense causing it to sink and descend. The shrinking also causes the upper area of the layer to warm faster than the lower area ultimately causing stability.
What layer subsidence movements cause instability?
Lifting and stretching. As the air ascends, its expands. To add to that, the rising causes the upper area of the layer to cool faster than the lower area which means instability.
Convective potential instability
When an air parcel is lifted, certain parts may cool faster than others. This happens when the bottom of the layer is more saturated than the upper layer and so the bottom layer cools at a slower saturated rate while the upper layer cools at a faster dry lapse rate. This causes layer instability as the colder air is found aloft. It also causes the environmental lapse rate of the layer to increase.
Radiative cooling by clouds
There is a strong imbalance on cloud tops where clouds are constantly emitting long wave radiation faster than it absorbs it. This means that the top layers of the clouds are cooled causing steep lapse rates below and instability. This instability causes downdrafts enabling stratus clouds to turn into stratocumulus clouds.
When do cumulus clouds develop?
These develop when air parcels are brought to saturation by some sort of lifting mechanism. The environmental profile needs to be conditionally stable or convectively unstable. (this means that it happens when there is cooling aloft due to large scale lifting, when there is warming below or when there is more humid low-level air so parcels cool at a reduced rate.
4 types of lifting to form clouds
- Convection (surface heating)
- Lifting along topography (like mountains)
- Convergence of air (cause of snow in MTL)
- Lifting along weather fronts (like cold fronts)