CLOUD FORMATION Flashcards
What is adiabatics?
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.
What are the values of:
DALR
SALR
ELR
DALR = 3 deg/1'000ft SALR = 1,5 deg/1'000ft ELR = Variable. In ISA it's 1,98 deg/1'000ft
Why is SALR lower than DALR?
Lower because latent heat is released during condensation
What’s the relationship between DALR and SALR with increasing height?
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
Describe:
Absolute stability
Absolute instability
Conditional stability
Stability: ELR higher than both SALR and DALR
Instability: ELR lower than both SALR and DALR
Conditional: ELR is in between SALR and DALR
What happens if ELR is negative?
Inversions.
Are extremely stable
What is the dewpoints lapse rate?
0,5 degrees/1’000ft
Describe cloud formation?
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
Define and name some low level clouds?
Exists from 0 - 6’500ft
NS
SC
ST
Define and name some middle level clouds?
Exists from 6'500 - 23'000ft AS AC CU NS up to 16'000ft
Define and name some high level clouds?
Exists from 16'000ft - 45'000ft CI CC CS CB
Describe the Fohn wind?
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
What’s needed for Hail to form?
Updraughts more than 10m/s are needed
Formula for calculating cloudbase?
Cloudbase = Temp-Dewpoint x 400
How can air be lifted?
Convection
Convergence
Frontal uplift
Orographic uplift
What are the requirements for Thunderstorms to form?
- Lapse rate greater than SALR extending through at least a layer of 10’000ft
- Sufficient watervapour to provide early saturation to form and maintain clouds
- Trigger action
What are the 3 stages of a Thunderstorm?
- Initial. Last for about 20 min
- 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 - 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.
How does lightning form?
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.
What is S:t Elmos fire an indication of?
That the air is charged and that lightning is possible
Avoidance criteria for Thunderstorms?
20 nm around and 5’000ft above the biggest ones
Where does lightning usually occurs?
Within 5’000ft of the freezing level. +10 to -10 degrees
Where is icing likely in a Thunderstorm?
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
Define the following:
Mist
Fog
Haze
Mist = Vis 1'000 - 5'000m Fog = Vis less than 1'000m Haze = Vis less than 5'000m but not due to moisture
Describe RVR
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
Describe how radiation fog forms?
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
How is advection fog formed?
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
When does ice form on an aircraft?
- Water is present in liquid state
- Ambient temp is below 0 deg C
- Airframe is below 0 deg C
What do you know about supercooled droplets?
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
Describe formation of Clear Ice?
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
Describe formation of Rime Ice?
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
Describe rain ice?
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.
How is frost formed?
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
Why should engine anti-ice be on at +10 deg or less?
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
