Lecture panel 3 Flashcards
Vapour pressure (e) and equilibrium
The partial pressure of air that is exerted by water vapour
equilibrium when the evaporation rate is equal to the condensation rate
how does temperature affect saturation vapour pressure?
increasing temp means increasing saturation vapour pressure
below freezing, is the saturation vapour pressure greater over water or ice?
water
Dew point
temperature to which air must be cooled (at const pressure and vapour content) for saturation to occur
relative humidity (RH)
Ratio of vapour pressure to saturation vapour pressure. amount of water vapour compared with the amount required for saturation.
RH = e/es*100
Specific humidity
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
What happens when an air parcel rises? falls?
- it cools internally and expands as it rises
- compresses and warms as it falls
adiabatic process
temperature change that does not involve the removal or input of heat
lapse rate
rate at which an atmospheric variable (esp temp) changes with altitude
dry and wet adiabatic lapse rate
dry: -10C/1000m
wet: -6C/1000m
wet rate starts after dew point (from environmental lapse rate temp change of surrounding air)
Absolute stability
rising air packet always cooler than environmental rate
Absolute instability
air packet temp always higher than the surrounding air
conditional stability
rising air starts out cooler than environment, but then becomes warmer
What is the typical cloud-condensation nuclei diameter and what is the typical raindrop diameter?
nuclei are 2microns, raindrops are 2000 microns diameter.
moisture droplets in the cloud can be about 20 microns.
What happens if air contains vapour but no condensation nuclei?
air becomes super-saturated (>100% RH) because no droplets can form
condensation nuclei size range
10^-5mm to 10^-1 mm
why is the wet adiabatic rate lower than the dry?
because condensation of water releases energy and warms the surrounding air
What do ships do for clouds?
ship exhaust can be used as condensation nuclei, so clouds form behind the path of ships
are continental or maritime droplets larger?
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
how can precipitation be inhibited or enhanced by condensation nuclei?
too many CCN can hinder precipitation, while very large CCN can enhance precipitation
What droplet formation process dominates in warm clouds (>0C)
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
cold cloud
below -40C, the cloud is glaciated and contains ice crystals
mixed cloud
0 to -40C both liquid and ice crystals included in cloud
how do crystals grow in mixed clouds?
(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
What are the multiple zones that exist within a cloud (profile)
from warmest to coldest:
- diffusional droplet zone (growth by condensation)
- coalescence zone
- rainout zone
- mixed phase zone
- glaciated zone
what occurs in the rainout zone?
growth by coalescence is balanced by loss of larger droplets
four lifting mechanisms
- convergent
- convectional
- orographic
- frontal
What is the ITCZ and its significance?
Inter-tropical convergence zone - trade winds converge here along equator, so there is lifting (convergence) and precipitation
conditions necessary for hurricane development
- 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
orographic precipitation mechanism
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
frontal lifting mechanism
cold fronts -cold air forces warm air aloft - 400 km wide
warm fronts -warm air moves up and over cold air gently
-1000km wide
what type of clouds do warm fronts form?
grey and formless
What type of clouds do cold fronts form?
thunderstorm clouds
Major influences on precipitation distribution
- 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)
