condensation on a macro-scale causing phenomenons Flashcards
at constant pressure, saturation can be achieved by either _________ or _________ of air
- evaporation
- cooling
what is the optimal condition for forming dew?
- forming on vegetation early morning after a clear and calm night
- vegetation: cools quickly than road surface
- clear: clouds can trap heat. clear sky means no clouds and cool night (cold air holds less water so more water v. can be released)
- calm: no wind. warm air above doesn’t mix with coolest air near the ground
why is the dew point temp usually the minimum night time temp?
dew forms due to condensation, releases latent heat, so it reduces the rate on temperature cooling.
when does frost form
when dew point is below 0C- frost point
what is white frost?
water vapour changes directly to ice crystals via deposition
what is black frost?
temperature is below 0C but no ice formed due to low humidity. condensation can’t happen –> no latent heat released –> great damage to vegetation as they freezes internally
-black- colour of damaged tissue
what is frozen dew?
- dew point is just above 0C so dew forms first then freezes
- dew point then drops below 0C and dew freezes.
- not via deposition
- ice formed without crystal
list the phenomenons that reduce visibility according to their strength.
haze< mist < fog (reduces visibility the most)
compare dry and wet haze
dry- particles selectively allow some rays of sunlight to be scattered while allowing other rays to be penetrated. (yellowish colour)
wet- some water condensation found on the particles –> haze thickens as particles grow in size–> wet haze (grey white colour)
why does condensation form clouds
at LCL, RH= 100% which means a lot of H2O molecules are found–> greater collision forming bigger lump of H2O. appears white as sizes increase are big enough to scatter visible light.
what is mist?
suspended water droplets (not big enough to form raindrops)
what is fog
like mist with lower visibility; like clouds at the surface
how is fog formed
- cooling of air (cold water holds less water so more water v. releases)
- evaporation of water into the air (water v. added to the air)
- by mixing of two air masses so the mixture is saturated (the graph)
3 types of fog?
- radiation fog
- valley and upslope fog
- advection fog
what are the favourable conditions for forming radiation (ground) fog?
- long night (cold)
- clear sky (cold; no clouds trapping heat)
- shallow layer of moist air near ground (eg. from rain)
- light wind (no mixing of air)
- common over land in late autumn and winter (cold air)
- low lying areas (cold ir sinks downhill)
- forms deepest around sunrise as air is the coolest
what is a brief mechanism of radiation fog?
surface cooling cools the air above, causing the air to release water vapour and fog fog (more moisture–> more favourable)
why does shallow fog dissipate by afternoon?
as sunlight penetrates the fog and warms the ground–> convection causes mixing –> fog droplets evaporates
compare valley fog and upslope fog
valley- cold air sinks downhill (denser)–> collects at valley bottoms –> air cools further due to cold surface (hills can be clear all day); cold humid air are found at river valley
upslope- moist air flowed up the hill/mountains, cools adiabatically, expands and saturates –> form fog if sufficient moisture
what causes advection fog?
when warm moist air (eg. tropical maritime) moves over cold surface (cold coastal water) –> air cools to dew point –> form fogs (like in hot showers)
what are fog and dew important sources to plants and animals?
they bring water sources
what is evaporation fog
- obtained by mixing 2 unsaturated air masses of different temp. to produce a supersaturated one. (eg. moist air coming out form your mouth in winter)
- temp. and mixing ratio of the resulting airmass would be an average of both air masses.
signs used for mixing ratio?
w, r
what is steam fog
- when cold air meets warm water.
- water is evaporated adding moisture to the cold air, making it saturated.
- cold air is heated by warm water –> rises and become steam
- seen in lakes in autumn/early winter, when air is cold and lake is still warm (from summer)
compare advection fog with steam fog
advection-when warm, moist air meets cold surface (coast)
steam- when cold moist air meets warm water
define clouds
- visible aggregation of tiny water droplets or ice crystals suspended in the air.
- can be found at high elevation or low ground
what are the paths to saturation?
- cool air directly (higher RH since water v. releases)
- mix with colder air (to cool air)
- reduce pressure (adiabatic expansion and cooling)
how does cloud form briefly
air rises, expands, cools, saturates (more water v. released) –> more collision –> condensation (droplets aggregate)
what are the 4 mechanisms for uplift?
- surface heating causes convection
- lifting along topography
- convergence of air due to low pressure system
- lifting along weather fronts
what are rising plumes?
- warm, buoyant air
- usually caused by convection
describe convective clouds
- localised, caused by surface heating
- due to localised condensation, clouds are usually heaped, with domed tops and flat bases (Cu, Cb)
- when clouds reaches the stable layer (resist vertical motion), it spreads out horizontally –> stratiform clouds (Sc)
dew point drops by ____C/km. it drops because pressure decreases as air rises, hence easier to reach saturation. (pressure inside the parcel is lower than the environmental pressure = saturation + growth)
2C/km
difference between T and Td reduces by ___C for each km increase.
8
what is the equation used to find cloud base height
125( T-Td)
OG= (T-Td difference)/8 x 1000 (from km to m)
what is the lifting condensation level
when T=Td
what do the clouds dissipate at the leeward side of the mountain
leeward side- air get compressed downslope –> pressure increases in parcel greater than environment –> droplets evaporates –> clouds dissipate
why is the initial temp at the windward side different form the final temp at the leeward side
windward- air temp decreases by both dry and wet adiabatically
leeward- air temp increases dry adiabatically only
lenticular clouds
formed at leeward side under stable flow over mountains.
can extend for 100km
rotor clouds
- at the leeward side
- cause turbulence and shear
- hazard for aircrafts
warm front develops -______-type clouds horizontally
stratus. as its is harder for warm air to replace cold air- less energy inspired - stratus
cold front develops _______-type clouds vertically
cumulus. easier for cold air to replace warm air - more energy - larger clouds formed
why are clouds usually associated with depressions and troughs?
convergence of air are found in low pressure system, forming clouds
why are fine weather associated with high pressure system
high pressure system –> divergence of air at the surface –> sinking of air –> clouds dissipate –> clear sky
