Moisture and Stability

Question 1

Bouyancy force

Answer 1

Vector sum of upthrust and weight of parcel

Drives convection of convective clouds

Question 2

Adiabatic process

Answer 2

No energy or mass enters or leaves the system. Many processes in the atmosphere are adiabatic since air is a poor conductor of heat and mixing is usually slow.

Question 3

Adiabatic processes examples

Answer 3

Often in vertical motions
Ascent of dry convective plumes
Large scale lifting/subsidence

Question 4

Non-adiabatic processes examples

Answer 4

Often near surface
Radiative heating/cooling
Surface heating/cooling
Removal of water from atmosphere by precipitation
Water added from evaporation of precipitation falling from above
Condensation or evaporation in an undilute airmass is ‘pseudoadiabatic’.

Question 5

Lapse rate

Answer 5

-1 X vertical gradient of temperature.

Question 6

Dry adiabatic lapse rate

Answer 6

Decrease of temperature with height due to decrease in pressure

Question 7

Potential temperature

Answer 7

temperature a parcel would have if compressed adiabatically to 100 kPa
Vertical mixing within PBL is driven by solar heating of surface and makes potential temperature vertically uniform.

Question 8

Atmospheric water

Answer 8

Troposphere contains nearly all water in the atmosphere, mostly in vapor form but some as droplets and ice-crystals.
mixing ratio in middle troposphere: 1 g /kg
mixing ratio in lower troposphere: 10 g/kg

Question 9

Effects of atmospheric water on dynamics in troposphere

Answer 9

Convective processes partly driven by bouyancy from latent heat of phase changes. warming-ascent. cooling-decent.
Outside clouds moist air is less dense than dry air. This boosts the bouyancy of a parcel.

Question 10

Latent heat

Answer 10

Released when water condensates or freezes and removed if water evaporates or melts. same amount of energy both ways.

Question 11

Importance of water vapour

Answer 11

the most important greenhouse gas

Affect emission/absorbtion of radiation which affect climate.

Question 12

Sources of water vapour

Answer 12

Evaporation from surface (requires sunlight). Evaporation of precipitation.

Question 13

Sinks of water vapour

Answer 13

Precipitation. Condensation at surface (dew, frost)

Question 14

Saturation

Answer 14

Equal rates of gain and loss of water molecules between liquid and vapour. When the actual vapour pressure is equal to the saturation vapour pressure. Condensation can start at this point. At surface of drop air is always saturated.

Question 15

Saturated water vapour pressure

Answer 15

Hypothetical value of e at which saturation would occur.

Question 16

Relative humidity

Answer 16

RH=e/es. In ambient air any value of RH is possible.

Question 17

(saturation) Mixing ratio

Answer 17

(kg/kg) ratio of mass of water to original mass of dry air

Question 18

Vapour density

Answer 18

(kg/m3) mass of water vapour per unit volume of moist air

Question 19

Dew point depression

Answer 19

difference between temperature and dew point temperature

Question 20

Specific humidity

Answer 20

(kg/kg) ratio of mass of mater vapour to total mass of moist air

Question 21

Supersaturation

Answer 21

percentage excess of vapour pressure beyond saturated value

Question 22

Factors constant during dry adiabatic ascent:

Answer 22

vapour mixing ratio and potential temperature

Question 23

Factors decreasing during dry adiabatic ascent

Answer 23

temperature, saturated vapor pressure and saturated mixing ratio.

Question 24

Factors constant during saturated adiabatic ascent

Answer 24

total water mixing ratio and equivalent potential temperature

Question 25

Absolute stability

Answer 25

A parcel will always be colder and more dense than the environment and will therefore always return to it’s original position. The slope of the environmental lapse rate is steeper than both the dry and saturated adiabatic lapse rates.

Question 26

Absolute instability

Answer 26

Parcel is always warmer and less dense than the environment and will be lifted. The dry adiabatic lapse rate and saturated lapse rate are steeper than that of the environment.

Question 27

Conditional instability

Answer 27

A parcel is forces to lift above the LCL and LFC eventhough this is a stable part of the atmosphere. Above the LFC it is unstable so the parcel is lifted.

Question 28

Level of free convection (LFC)

Answer 28

Where the temperature of the environment decreases faster than the moist adiabatic lapse rate of a parcel at the same level.

Question 29

Lifted condensation level (LCL)

Answer 29

Where the dry lapse rate meets the mixing ratio from the surface. Here condensation can start. RH=100%

Question 30

Convective instability

Answer 30

Unstable until parcel is lifted to LCL when it becomes unstable. This is because the environmental lapse rate is steeper than the dry lapse rate but not the wet.

Question 31

CAPE (convective available potential energy)

Answer 31

Measure of intensity of vertical motions inside convective clouds. Work done by bouyancy force on parcel to lift it from one neutral bouyancy level (LFC) to another (EL). Area between environmental and moist lapse rate between LFC and EL is proportional to the potential energy.

Question 32

Triggering CAPE

Answer 32

Parcel needs to be lifted to LFC for the CAPE to converted into kinetic energy.

Question 33

CIN (convective inhibition)

Answer 33

Barrier which opposes the forced lifting needed to trigger the onset of any convection. Work needed to lift parcel to LFC. Area below LFC where environment is warmer than the parcel.