5. Triggers and Inversions

Question 1

Orographic triggers with conditionally unstable air

Answer 1

If cloud forms depends on the size of the mountain, if high enough to lift air above the instability level, air will rise and clouds will form

Question 2

The more moisture…

Answer 2

…the greater the instability
…the lower the cloudbase
…the higher the cloud tops

Question 3

Name the convection triggers

Answer 3

Orographic
Thermal
Frontal
Non frontal convergence

Question 4

Throughs

Answer 4

Non frontal convergence (low pressure) -> Convection

Question 5

Turbulence cloud

Answer 5

Mixing of air in mountain lee at high winds, turbulence causes temperature structure to change (redistribution) and cloud to form

Question 6

Three main reasons for clouds dissipating

Answer 6

• Air temperature increase
• Mixing with drier air (non-adiabatic)
• Sinking of air

Question 7

Four ways for inversions to form

Answer 7

1. Frontal inversion (warm air sliding over cold air)
2. Surface cooling (a few hundred to max 1,000 feet)
3. Subsidence of stable air masses (inversion stops at 2-3000 ft as air spreads out, typical summer day)
4. Turbulence inversion

Question 8

Formula to calculate the temperature difference at the top of a turbulence inversion

Answer 8

((DALR - ELR) x Height of turbulence layer (in ‘000ft)) / 2

Question 9

Effect of inversions on aircraft in flight

Answer 9

Reduced visibility (smoke, fog, dust) can’t dissipate quickly)
Reduced performance as aircraft climbs from colder air beneath the inversion into warmer air