301 Flashcards
4 factors that control the evolution of clouds
Water Vapour
condensation and ice nuclei
stability
lift
Natural Condensation
Process by which molecules of water vapour are brought together in sufficient numbers to form liquid water
Hygroscopic nuclei
Condensation nuclei which do not require saturation to be impregnated with water
- water soluble substances -sea salt
- products of combustion
Dimensions of hygroscopic nuclei
Aitken nuclei -less than a tenth of a micrometer
Large Nuclei -from 0.1 up to 1 micrometer
Giant nuclei - greater than 1 micrometer
Saturation mixing ratio
the theoretical maximum amount of water vapour that the air at a specific temperature and pressure can hold
4 types of vertical motion that lead to the formation of clouds
Convective lift
Mechanical turbulence
Orographic lift
Large scale lift
2 main processes that initiate convection in the atmosphere
Daytime Heating
Cold Air advection
The intensity and height of mechanical turbulence relies on what three things
the roughness of the underlying surface
the strength of the wind
the instability of the air
5 reasons clouds are important in meteorology
Forms of weather Precipitation formation Effects on the heat budget Atmospheric processes Actual conditions
2 main processes that cloud is produced in the atmosphere
Cooling - adiabatic cooling
-diabetic cooling
Evaporation - addition of moisture to the atmosphere by evaporation into cooler air
3 types of diabatic cooling
Radiation
Advection
Mixing
5 synoptic situations where large scale dynamic lift produce clouds and weather
In the vicinity of low pressure centres and troughs Regions of baroclinic development Warm fronts and trowals Upper short wave troughs Jet stream maximums
6 broad scale processes used to assess presence of boundary layer cloud and weather
Low level convergence Upslope/onshore flows Surface winds (>15kts) Large area with moist surface conditions Evaporation of precipitation cooling by advection or convection
Orographic lift: the extent and rate of ascent of air undergoing orographic lift depend on the:
Slope and height of terrain
the strength of the wind
Orographic lift:
The extent of the cloud that forms depends on:
the type of cloud depends on:
the moisture of the air
the stability of the air
The difference between radiation fog and advection fog
- development
- dissipation -radiation fog dissipates from the bottom up outside in.
- advection will persist until a change in wind speed or direction - advection fog is not fixed to a daily cycle.
2 ways a parcel of air is heated or cooled in an adiabatic process.
Through cooling of expansion and heating of contraction.
Through the the storage and release of latent heat.
What is the dominant process for cloud development in the mid to high latitudes
Large scale dynamic lift
Large scale upward vertical velocity is a result of:
A combination of low level convergence and high level divergence.
Usually in a baroclinic zone.
5 synoptic situations where large scale dynamic lift will often produce clouds and weather
In the vicinity to of low pressure centres and troughs. Regions of baroclinic development Warm fronts and trowals Upper short wave troughs Jet stream maximums
What is the main cause of cloud in the boundary layer
Low level convergence generating upward vertical motion
6 broad scale processes used to assess the presence of boundary layer clouds and precipitation
(Using surface analysis and short range surface prognosis)
Low level convergence Upslope/onshore flow Surface winds >15 it's Large area with moist surface conditions Evaporation of precipitation Cooling by advection/convection
Low level Convergence
position of fronts troughs and lows etc…that might produce low level convergence.
highs and troughs may indicate dissipation
upslope/onshore flows
significant geographic areas of upslope/downslope and onshore/offshore
Surface winds greater than 15 kts
over hilly areas strong pressure gradient indicates high potential for at least SC due turbulent mixing
Large areas with moist surface conditions
potential for stratus or daytime SC or deeper convection
Evaporation of precip
Synoptic scale processes can produce boundary layer cloud through precip and upward vertical motion
Advection/conduction
Warm air advection can give stratus
Cold air advection can give SC or convection
The 2 factors diurnal variations that are of primary importance
Temperature temps are generally higher during the day due to net increase of radiation at the surface.
Humidity is increased during the day due to warmer air being able to hold more water vapour.
Cumulus evolution
- Moisture increases (due to daytime heating)
- short lived cf appear
- increasing moisture and temps give fair weather CU
- Temps increase faster than moisture, increasing T-Td spread, rising bases
- air parcel rises above level of free convection
- vertical development becomes apparent with TCU tops
- if there is enough kinetic energy from buoyancy, CBs may form
cumulus dissipation
- Surface temperatures decrease
- convective lift diminishes and ceases, no new cumuliform clouds are formed
- existing clouds will erode due to evaporation and mixing at the edges
- tops will be cooled by radiation, will start to colapse and subside
- horizontal winds spread out and break up the cloud
SC/ST daytime formation
- turbulent mixing
- wnds nil to lgt give FG/ST
- winds stronger than 10 kts will give SC
- windspeed will determine thickness
SC/ST nighttime formation
- moist lower levels
- colder air is lifted to mix with warmer air aloft
- wnds nil to lgt give FG/ST
- winds stronger than 10 kts will give SC
- windspeed will determine thickness
SC/ST daytime dissipation
- Initiated by solar heating
- lateral mixing of moist air with dry air
- solar heating dissipates stable cloud best
- the thicker the cloud the more mixing required, longer the dissipation
- the stronger the capping inversion, the greater the solar input required to mix the air
SC/ST nighttime dissipation
top is destabilized from radiative cooling
- cloud sheet is bounded at it’s top by a dry type inversion
- surface RH is low
- the cloud is initially thin
upslope cloud formation
flow of air will be lifted adiabatically
-condensation is dependant on moisture content
Boundary cloud layers are lowered in precipitation by
- Precipitation increases moisture in atmosphere due to evaporation, giving a higher dew point.
- evaporation cools the air by storing latent heat
- higher dewpoint + lower temp = decreased temp/ dewpoint spread giving lower ceilings.
Determining presence of clouds using climatology
Statistics give instant experience to new specialists
helpful is describing persistent meso scale effects such as:
-mills
-nearby bodies of water
surrounding terrain and topography
Determining presence of clouds using correlators
Synoptic features usually have a consistent history
Timing of weather events can be deduced
a good assessment uses all tools available
Potential correlators (pt 1) 700hPa trough 500hPa ridge 700hPa ridge Surface features
700 hPa trough - trailing edge of synoptic cloud or precip, usually for a weak system
500 hPa ridge - leading edge of cloud or precip
700 hPa ridge - leading edge of cloud or precip
Surface features - orientation and pattern of cloud and precip
Potential correlators (pt 2) Short wave troughs or Vort centres deformation zones jet streams positive vorticity advection (PVA) or warm air advectio (WAA)
Short wave troughs or Vort centres - Trailing edges or wrap around shape with vort centre
deformation zones - leading edges of cloud and precip
Jetstreams - Edges of cloud and precip, dry surges
positive vorticity advection (PVA) or warm air advectio (WAA) - edges and shapes of cloud and precip
