CLOUD FORMATION

Question 1

What is adiabatics?

Answer 1

Temp change within a system but there’s no exchange of energy with the surroundings.
As a bubble of air rises, the pressure around it decreases and the bubble expands and temp within the bubble decreases.

Question 2

What are the values of:
DALR
SALR
ELR

Answer 2

DALR = 3 deg/1'000ft
SALR = 1,5 deg/1'000ft
ELR = Variable. In ISA it's 1,98 deg/1'000ft

Question 3

Why is SALR lower than DALR?

Answer 3

Lower because latent heat is released during condensation

Question 4

What’s the relationship between DALR and SALR with increasing height?

Answer 4

SALR is nearing DALR.
In cold temps the when air is almost saturated, the actual amount of water vapour is low so very little latent heat is released. SALR is then nearly as high as DALR
In hot temps, saturated air contains large amount of water vapour and condensation releases large amounts of latent heat. SALR is then lower then DALR

Question 5

Describe:
Absolute stability
Absolute instability
Conditional stability

Answer 5

Stability: ELR higher than both SALR and DALR
Instability: ELR lower than both SALR and DALR
Conditional: ELR is in between SALR and DALR

Question 6

What happens if ELR is negative?

Answer 6

Inversions.

Are extremely stable

Question 7

What is the dewpoints lapse rate?

Answer 7

0,5 degrees/1’000ft

Question 8

Describe cloud formation?

Answer 8

It’s hotter surrounding a parcel of air. It starts to rise. If ELR is 5 deg/1’000ft and the parcel decreases by 3 deg/1’000ft the surrounding air is always warmer and the parcel rises. Eventually the DALR meets the Dew point and that’s the base of the cloud.
SALR is increasing with height.
In warm air with higher moisture content the SALR is lower and it takes longer time to intersect the ELR, which leads to higher cloud tops.
When SALR and ELR intersects that’s the cloud top

Question 9

Define and name some low level clouds?

Answer 9

Exists from 0 - 6’500ft
NS
SC
ST

Question 10

Define and name some middle level clouds?

Answer 10

Exists from 6'500 - 23'000ft
AS
AC
CU
NS up to 16'000ft

Question 11

Define and name some high level clouds?

Answer 11

Exists from 16'000ft - 45'000ft
CI
CC
CS
CB

Question 12

Describe the Fohn wind?

Answer 12

Air cools at DALR until it hits dewpoint and then cools at SALR.
As air comes over the top there’s no lifting and the air flows down the opposite side
Initially the air warms at SALR but since most moisture is lost, condensated out as clouds, it becomes unsaturated at a lower temperature than the original dewpoint.
Air then warms at DALR, this warming wind is called the Fohn wind

Question 13

What’s needed for Hail to form?

Answer 13

Updraughts more than 10m/s are needed

Question 14

Formula for calculating cloudbase?

Answer 14

Cloudbase = Temp-Dewpoint x 400

Question 15

How can air be lifted?

Answer 15

Convection
Convergence
Frontal uplift
Orographic uplift

Question 16

What are the requirements for Thunderstorms to form?

Answer 16

1. Lapse rate greater than SALR extending through at least a layer of 10’000ft
2. Sufficient watervapour to provide early saturation to form and maintain clouds
3. Trigger action

Question 17

What are the 3 stages of a Thunderstorm?

Answer 17

1. Initial. Last for about 20 min
2. Mature. Starts with the onset of precipitation. Both up-and down draft occurs. Cloud can rise quickly at 5’000ft/min. Gustfronts up to 20nm ahead of the storm.
   Last for about 30 min
3. Dissipating. Mostly downdraughts. Lasts from 30 min up to 3h. An anvil starts to form at the top when the troposphere is reached
   Down draughts are removing all the moisture.

Question 18

How does lightning form?

Answer 18

As drops fall they gain speed. At about 9m/s they start to split. Larger parts becomes + and smaller becomes -
Negative parts are lifted while larger falls. Once the correct potential is reached a discharge takes place.

Question 19

What is S:t Elmos fire an indication of?

Answer 19

That the air is charged and that lightning is possible

Question 20

Avoidance criteria for Thunderstorms?

Answer 20

20 nm around and 5’000ft above the biggest ones

Question 21

Where does lightning usually occurs?

Answer 21

Within 5’000ft of the freezing level. +10 to -10 degrees

Question 22

Where is icing likely in a Thunderstorm?

Answer 22

From 0 to -40 degrees

Worst is between -3 to -10 when there’s large supercooled droplets. Smaller droplets are higher up where it’s colder

Question 23

Define the following:
Mist
Fog
Haze

Answer 23

Mist = Vis 1'000 - 5'000m
Fog = Vis less than 1'000m
Haze = Vis less than 5'000m but not due to moisture

Question 24

Describe RVR

Answer 24

Horizontal distance a pilot in the threshold area can see marker boards at 15ft above the ground. Mesured by transmissometers
Is used when MET VIS falls below 1’500m
Measured at 3 points:
Touchdown-Midpoint-Endpoint
Last two may be omitted if they are equal or greater to the touchdown value or if they are above 400m

Question 25

Describe how radiation fog forms?

Answer 25

Forms on clear nights with relative humidity and light winds (2-8 kts)
Ground loses heat by radiation and becomes cooler and cools the air in contact with it. If this lowers the temp below dewpoint watervapour condenses out as fog if there are light winds. If there’s no wind frost is formed

Question 26

How is advection fog formed?

Answer 26

Forms when moist air moves over a cold surface. Can occur over land or over sea. Need winds up to 15 kts, high humidity so little cooling is needed for saturation.
The cold surface must must have a temp lower than the dewpoint of the warm moist air. Disperses with change of airmasses or higher winds

Question 27

When does ice form on an aircraft?

Answer 27

1. Water is present in liquid state
2. Ambient temp is below 0 deg C
3. Airframe is below 0 deg C

Question 28

What do you know about supercooled droplets?

Answer 28

Droplets that are below freezing temp but doesn’t have a freezing nuclei to freeze onto.
Can exist down to -40 deg
Freezes when they collide with an aircraft
Below -20 deg, large supercooled drops are lkely to freeze without a nuclei. Smaller ones may still exist

Question 29

Describe formation of Clear Ice?

Answer 29

Forms when large supercooled drops freeze on impact
They don’t freeze completely as latent heat is released which warms the drop and delaying the freezing process so that the ice can form back on the wing
Occurs in CB, NS and CU between 0 to -20 deg C knetic heating may worsen the icing

Question 30

Describe formation of Rime Ice?

Answer 30

Forms when smaller drops which freezes instantly with no flowback.
Can form in any cloud below 0 deg C
Also forms in CU below -20 deg

Question 31

Describe rain ice?

Answer 31

At a warm front rain falls from a NS, first into warm air, then into the cold air below and is supercooled. If aircraft is above freezing level and aircarft is below 0 deg C and drops are striking the plane ice can form i much the same way as clear ice is formed.

Question 32

How is frost formed?

Answer 32

Forms when air is cooled down to saturation and airframe is below 0 deg C. Forms by sublimation, water vapor turns into ice. Occurs on ground at night.
Can also happen on take-off at sub-zero temp when an aircraft climbs through an inversion into warm moist air. Forms much like Rime Ice

Question 33

Why should engine anti-ice be on at +10 deg or less?

Answer 33

There’s a pressure drop in the early stages of the engine that can cause the temp to drop by 5-10 deg
Also the temp probe shows a temp warmer than actual due to kinetic heating