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1

4 factors that control the evolution of clouds

Water Vapour
condensation and ice nuclei
stability
lift

2

Natural Condensation

Process by which molecules of water vapour are brought together in sufficient numbers to form liquid water

3

Hygroscopic nuclei

Condensation nuclei which do not require saturation to be impregnated with water
-water soluble substances -sea salt
-products of combustion

4

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

5

Saturation mixing ratio

the theoretical maximum amount of water vapour that the air at a specific temperature and pressure can hold

6

4 types of vertical motion that lead to the formation of clouds

Convective lift
Mechanical turbulence
Orographic lift
Large scale lift

7

2 main processes that initiate convection in the atmosphere

Daytime Heating
Cold Air advection

8

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

9

5 reasons clouds are important in meteorology

Forms of weather
Precipitation formation
Effects on the heat budget
Atmospheric processes
Actual conditions

10

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

11

3 types of diabatic cooling

Radiation
Advection
Mixing

12

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

13

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

14

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

15

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

16

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.

17

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.

18

What is the dominant process for cloud development in the mid to high latitudes

Large scale dynamic lift

19

Large scale upward vertical velocity is a result of:

A combination of low level convergence and high level divergence.
Usually in a baroclinic zone.

20

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

21

What is the main cause of cloud in the boundary layer

Low level convergence generating upward vertical motion

22

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

23

Low level Convergence

position of fronts troughs and lows etc...that might produce low level convergence.
highs and troughs may indicate dissipation

24

upslope/onshore flows

significant geographic areas of upslope/downslope and onshore/offshore

25

Surface winds greater than 15 kts

over hilly areas strong pressure gradient indicates high potential for at least SC due turbulent mixing

26

Large areas with moist surface conditions

potential for stratus or daytime SC or deeper convection

27

Evaporation of precip

Synoptic scale processes can produce boundary layer cloud through precip and upward vertical motion

28

Advection/conduction

Warm air advection can give stratus
Cold air advection can give SC or convection

29

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.

30

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

31

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

32

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

33

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

34

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

35

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

36

upslope cloud formation

flow of air will be lifted adiabatically
-condensation is dependant on moisture content

37

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.

38

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

39

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

40

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

41

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