Precipitation Processes 1

Question 1

What are the implications for DALR, SALR and LCL for rain and cloud formation?

Answer 1

Cloud level is determined by the pressure of the lifting condensation level. The greater the pressure, the higher the LCL will be.
The top of the cloud is determined by the intersection between the ELR and the SALR . At this intersection, there is no difference in temperatures of air, and so condensation, and therefore cloud, cannot occur. If there is no intersection, the cloud top is determined by the change in temperature and pressure of the atmosphere (assuming water source volume is not changed).

• High cloud levels form above 6km, are primarily composed of ice and are typically thin and white, but can appear in a array of colours when the sun is low on the horizon. Includes cirrus clouds.
• Medium cloud levels form between 2 and 6km and are primarily composed of moisture droplets but can be composed of ice crystals when temperatures are cold enough. Includes alto-cumulus and alto-stratus clouds
• Low cloud levels form below 2km and are mostly composed of water droplets.

Storm clouds are comprised of all three cloud layers, e.g. cumulonimbus. Absolute and conditional instability allows for condensation to occur over a large vertical distance, as the ELR and SALR do not intersect. This causes thick clouds, which have very heavy rain and form an anvil shape in above and in front of the heavy rain. Due to the circular cell in the deep thundercloud, water droplets can rise, freeze, descend and melt continuously. This form layers of ice around a water droplet or cloud condensation nuclei, resulting in hail stones that have ‘rings’ when cut.

The mixing of air masses is a very common cause of precipitation and cloud condensation. This air mixing can be highly localised due to localised heating and local topography and so cause localised rain.

Question 2

What are the differences between stable, unstable and conditionally stable atmospheres?

Answer 2

Absolute Instability
The air parcel is always warmer than the surrounding environment. The air parcel rises.
ELR < DALR/SALR

Results in thunder clouds or hail formation

Absolute Stability
The air parcel is always cooler than the surrounding environment. The air parcel descends as low as possible.
DALR/SALR < ELR

Results in clear skies, very hot temperatures in summer and very cool temperatures in winter.

Conditional stability
The air parcel is cooler than the surrounding environment below the condensation level and warmer than the surrounding environment above the condensation level.
ELR  <   DALR/SALR (warmer)
 [CONDENSATION LEVEL]
DALR/SALR (cooler)  <  ELR

Question 3

Describe and explain the cloud formation and precipitation processes associated with forced (mechanical) uplift

Answer 3

1. Dry air rises at the adiabatic lapse rate until condensation occurs.
2. At this point, clouds form and the air looses moisture due to precipitation. The air continues to rise and so cools at the wet adiabatic lapse rate, and continues to dry as more water is lost. Cloud and precipitation occur until the top of the hill is reached.
3. Dry air descends at heats at the dry adiabatic lapse rate on the fair side of the hill. As the air heats at the dry adiabatic lapse rate for the entirety of its descent, its final temperature at ground level is greater than that on the other side of the hill.

If forced mechanical occurs repeatedly (there are multiple mountains/ranges), each uplift reduces the moisture in the air, reduces the intensity of rainfall and increases the condensation level’s altitude.

Cloud formation can occur on the hill’s fair side as air can flow in waves. The air flows above (= clouds) and below (= no clouds) the condensation level, forming dotted clouds called rotor clouds.

Question 4

Saturated adiabatic lapse rate (SALR) is . . .

Answer 4

The rate of temperature change (6℃/km) of an saturated air mass due solely to changes in pressure associated with rise of falling through the atmosphere

Question 5

Unsaturated or dry adiabatic lapse rate (DALR) is . . .

Answer 5

The rate of temperature change (10℃/km) of an unsaturated air mass due solely to changes in pressures associated with rising or falling through the atmosphere

Question 6

Lifting condensation level (LCL) is . . .

Answer 6

The height at which condensation and clouds are formed. It is the same level as the base of the cloud.

Question 7

Latent heat (Qe) is . . .

Answer 7

Energy released by water changing from a gas to a liquid or a liquid to a solid

Question 8

Environmental Lapse Rate (ELR)

Answer 8

The rate of temperature change of atmosphere temperature due to altitude