Lecture panel 3

Question 1

Vapour pressure (e) and equilibrium

Answer 1

The partial pressure of air that is exerted by water vapour

equilibrium when the evaporation rate is equal to the condensation rate

Question 2

how does temperature affect saturation vapour pressure?

Answer 2

increasing temp means increasing saturation vapour pressure

Question 3

below freezing, is the saturation vapour pressure greater over water or ice?

Answer 3

water

Question 4

Dew point

Answer 4

temperature to which air must be cooled (at const pressure and vapour content) for saturation to occur

Question 5

relative humidity (RH)

Answer 5

Ratio of vapour pressure to saturation vapour pressure. amount of water vapour compared with the amount required for saturation.
RH = e/es*100

Question 6

Specific humidity

Answer 6

Mass of water vapour per unit mass of air (qv)
qv = 0.622e/P
where e is vapour pressure and P is air pressure
0.622 = ratio of the molecular weight of water vapour to the average molecular weight of dry air

Question 7

What happens when an air parcel rises? falls?

Answer 7

• it cools internally and expands as it rises

- compresses and warms as it falls

Question 8

adiabatic process

Answer 8

temperature change that does not involve the removal or input of heat

Question 9

lapse rate

Answer 9

rate at which an atmospheric variable (esp temp) changes with altitude

Question 10

dry and wet adiabatic lapse rate

Answer 10

dry: -10C/1000m
wet: -6C/1000m
wet rate starts after dew point (from environmental lapse rate temp change of surrounding air)

Question 11

Absolute stability

Answer 11

rising air packet always cooler than environmental rate

Question 12

Absolute instability

Answer 12

air packet temp always higher than the surrounding air

Question 13

conditional stability

Answer 13

rising air starts out cooler than environment, but then becomes warmer

Question 14

What is the typical cloud-condensation nuclei diameter and what is the typical raindrop diameter?

Answer 14

nuclei are 2microns, raindrops are 2000 microns diameter.

moisture droplets in the cloud can be about 20 microns.

Question 15

What happens if air contains vapour but no condensation nuclei?

Answer 15

air becomes super-saturated (>100% RH) because no droplets can form

Question 16

condensation nuclei size range

Answer 16

10^-5mm to 10^-1 mm

Question 17

why is the wet adiabatic rate lower than the dry?

Answer 17

because condensation of water releases energy and warms the surrounding air

Question 18

What do ships do for clouds?

Answer 18

ship exhaust can be used as condensation nuclei, so clouds form behind the path of ships

Question 19

are continental or maritime droplets larger?

Answer 19

maritime are larger - there are more condensation nuclei over continents, and therefore more (and smaller) droplets. Droplets in maritime clouds are more likely to grow by collision and coalescence

Question 20

how can precipitation be inhibited or enhanced by condensation nuclei?

Answer 20

too many CCN can hinder precipitation, while very large CCN can enhance precipitation

Question 21

What droplet formation process dominates in warm clouds (>0C)

Answer 21

coalescence and collision are the primary mechanism for drop growth. Condensation dominates in the very initial stages of droplet formation.
-as the droplet falls, it creates a zone of negative pressure that pulls in other droplets

Question 22

cold cloud

Answer 22

below -40C, the cloud is glaciated and contains ice crystals

Question 23

mixed cloud

Answer 23

0 to -40C both liquid and ice crystals included in cloud

Question 24

how do crystals grow in mixed clouds?

Answer 24

(0 to -40C) crystals grow due to Bergeron-Findeisen process. Lower saturated vapour pressures over ice means that ice crystals grow at the expense of water droplets

Question 25

What are the multiple zones that exist within a cloud (profile)

Answer 25

from warmest to coldest:

1. diffusional droplet zone (growth by condensation)
2. coalescence zone
3. rainout zone
4. mixed phase zone
5. glaciated zone

Question 26

what occurs in the rainout zone?

Answer 26

growth by coalescence is balanced by loss of larger droplets

Question 27

four lifting mechanisms

Answer 27

1. convergent
2. convectional
3. orographic
4. frontal

Question 28

What is the ITCZ and its significance?

Answer 28

Inter-tropical convergence zone - trade winds converge here along equator, so there is lifting (convergence) and precipitation

Question 29

conditions necessary for hurricane development

Answer 29

• warm ocean waters to depths of about 50m
• thunderstorm development
• light upper level winds
• no hurricanes exactly at the equator due to the lack of Coriolis
• hurricanes are convergent systems, so require air rising in low pressure zone

Question 30

orographic precipitation mechanism

Answer 30

winds push air mass up mountain (from ocean), where it condenses and rains (on ocean side)
-mechanism also works within continent but less moisture is in the water

Question 31

frontal lifting mechanism

Answer 31

cold fronts -cold air forces warm air aloft - 400 km wide
warm fronts -warm air moves up and over cold air gently
-1000km wide

Question 32

what type of clouds do warm fronts form?

Answer 32

grey and formless

Question 33

What type of clouds do cold fronts form?

Answer 33

thunderstorm clouds

Question 34

Major influences on precipitation distribution

Answer 34

• water vapour content (over oceans vs continents/ high elevation)
• mechanisms for uplifting air
• orographic effects
• amount and type of cloud condensation nuclei (maritime aerosols enhance precipitation, and abundant microscopic aerosols over continents suppress precipitation)