EO 302.1 Explain five reasons why clouds are important in meteorology.
FIVE REASONS WHY CLOUDS ARE IMPORTANT IN METEOROLOGY
Forms of weather: The presence of clouds constitutes a form of weather.
Precipitation formation: Precipitation is formed in clouds.
Effects on the heat balance: Clouds affect the heat balance of the Earth
through the absorption and reflection of radiation.
Atmospheric processes: Because of their forms and types, clouds can
provide useful clues as to the processes that are taking place in the
atmosphere (for example, relative stability, convection or mixing).
Actual conditions: Identifying a cloud type can provide a clue as to the
amount of moisture in the area as well as the probability of icing or
Turbulence – GOOD TO KNOW
EO 302.2 State the two main processes in the atmosphere that produce clouds.
TWO MAIN PROCESSES IN THE ATMOSPHERE THAT PRODUCE CLOUDS
For clouds to form, the air must be at or near saturation. There are two
processes in which unsaturated air can become saturated.
Cooling: Clouds may be formed by the cooling of moist air through diabatic
and adiabatic cooling.
Evaporation: Clouds may be formed by the evaporation of liquid water into
cooler air – KNOW THIS
EO 302.3 Explain the diabatic processes that lead to
the formation of clouds
A diabatic process is one in which there is an exchange or transfer of heat
across the boundaries of a parcel; in other words, something warm heats up
something cold, or vice versa.
THREE COMMON TYPES OF DIABATIC COOLING:
- Radiation
-There are two types of radiation cooling that lead to the formation or
modification of cloud. Radiation cooling at the surface can, through
conduction, result in the formation of fog or stratus. Radiation cooling aloft may cause cloud to form and to develop in the vertical - Advection
- Mixing - good to know
EO 302.4 Explain how the adiabatic cooling process
causes the formation of clouds in the
atmosphere.
Describe the ADIABATIC PROCESS:
An adiabatic process is one in which there is no exchange of heat across the
boundaries of a parcel, and heating or cooling of the parcel occurs through
compression or expansion of the parcel itself due to pressure change.e. The
other way to warm or cool an air parcel is by storage (or absorption) of
latent heat via the evaporation process or release of latent heat through
Condensation. - know
Adiabatic cooling
If we lift a parcel of moist air into the atmosphere, it also undergoes a
decrease in density due to expansion, since there is a decreasing weight of
atmosphere bearing down on it. The lifted parcel cools as it expands; it does not readily exchange heat with its surroundings – Know
- lower surrounding pressure, expansion causes cooling
EO 302.5 List the five synoptic situations where large
scale dynamic lift will often produce clouds
and weather.
THE FIVE SYNOPTIC SITUATIONS are:
Large scale dynamic lift is the dominant process of cloud production in the
mid to high troposphere. Large scale upward vertical velocity results from a
combination of low level convergence and upper level divergence.
THE FIVE SYNOPTIC SITUATIONS
- In the vicinity of low pressure centres and troughs
- In regions of baroclinic development
- In advance of warm fronts and TROWALs
- Ahead of upper short wave troughs (positive vorticity advection – PVA*)
- In the right entrance and left exit of jet stream maximums –KNOW
EO 302.6 State the main cause of cloud formation in
the boundary layer.
What is the main cause of CLOUD FORMATION IN THE BOUNDARY LAYER
The main cause of cloud formation in the boundary layer is low level
convergence generating upward vertical motion. - KNOW THIS
EO 302.7 List the six broad scale processes used to
assess the presence of boundary layer clouds
and precipitation
SIX BROAD SCALE PROCESSES USED TO ASSESS THE PRESENCE OF
BOUNDARY LAYER CLOUDS AND PRECIPITATION:
Low level convergence: Positions of fronts, troughs, lows, trowals and
others alike might produce low level convergence. Alternately, consider
ridges and highs for the role they play in the dissipation process.
Upslope/Onshore flows: Geographic areas displaying significant upslope
or downslope features as well as areas with onshore or offshore flows may
prove significant.
Surface winds (> 15 knots): Areas of mechanical turbulence mixing
featuring surface winds of more than 15 knots may be significant. A strong
pressure gradient over a hilly area indicates a high potential for at least
some stratocumulus due to turbulent mixing.
Large area with moist land surface conditions: Large areas with moist
land surface conditions give the potential for:
- Stratus
- Cumulus
- Stratocumulus
- Deeper convection
Evaporation of precipitation: Synoptic scale processes can produce
boundary layer cloud through evaporation of precipitation and upward
vertical motion.
Cooling by advection or conduction: Areas of low level cooling can lead
to stratus, especially when the low level flow advects over cooler water or a cool land surface. On the other hand, a flow of cooler air over warm water
can increase the moisture content of the air and may lead to the
development of stratocumulus or convective cloud.
Once the broad scale boundary processes have been identified, it is useful to
evaluate other parameters (such as stability and historical data) to
determine what types of clouds and weather are most likely to occur - KNOW
EO 302.8 Explain the role of diurnal variations in the
formation and dissipation of boundary layer
clouds and weather.(two factors)
The two factors of primary importance – which, in fact, drive the other
factors - are temperature and humidity.
Temperature
During the day, surface temperatures will generally increase due to the net
increase of radiation at the surface. Conversely, at night, surface
temperatures will generally decrease as the amount of radiation reaching the
surface is reduced.
Humidity
Higher surface temperatures during the day allow the atmosphere to hold
greater amounts of moisture, while lower temperatures at night may trigger
condensation as the temperature approaches the dew point.
EFFECT OF DIURNAL VARIATION ON THE FORMATION AND
DISSIPATION PROCESSES OF CUMULUS, STRATUS, AND
STRATOCUMULUS:
CUMULUS
Evolution
- Short-lived cumulus fractus
- Fair weather cumulus
- Rising cumulus bases
- Temperature-dew point spread increases
- Towering cumulus
- Possible cumulonimbus
Dissipation
- Surface temperature decreases due to sun setting
- Convective lift diminishes and ceases
- Clouds erode due to evaporation and mixing
- Cloud tops may subside or collapse
- Horizontal winds spread out or break up the cloud
STratus or STRATACUMULUS
Turbulent mixing in the lower layers
- Fog and stratus formed with winds under 10 knots
- Stratocumulus formed with winds stronger than 10 knots
Nighttime formation
- Nocturnal cooling
- Clear skies
- Stable air
- Wind speed
› Fog and stratus formed with winds under 10 knots
› Stratocumulus formed with winds stronger than 10 knots
Daytime dissipation (involves solar heating)
- The thicker the cloud, the greater the amount of mixing required
for dissipation
- The stronger the capping inversion, the greater the solar input
required to mix the air
Nighttime dissipation (involves mixing)
- Dry inversion at cloud top
- Low surface humidity
- Thin cloud (initially
EO 302.9 Explain how upslope clouds and precipitation
are formed in boundary layer.
A flow of air moving over an increasingly higher ground elevation will be lifted adiabatically. Moisture content of the air will determine whether or not condensation will occur due to the upslope terrain
EO 302.10 Explain how boundary layer clouds are
lowered in precipitation.
Precipitation increases moisture concentration in the lower levels of the atmosphere due to evaporation, resulting in a higher dew point.
The evaporation process cools the air by storing latent heat, resulting in
lower temperatures. An increasing dew point combined with a decreasing
temperature means the temperature-dew point spread decreases and the
cloud ceiling lowers.
This can be of paramount importance to aviation. Height of cloud bases and
lowering ceilings in upstream locations can be useful in determining the
timing of the arrival of lower ceilings at your station
EO 302.11 State the use of climatology and correlators
to determine the presence of clouds and
Weather
POTENTIAL
CORRELATORS
CORRELATES WITH SOURCE FOR
ASSESSMENT
700 hPa trough Trailing edge of synoptic
cloud or precipitation,
usually for weak systems
- 700 hPa chart
- Satellite imagery
500 hPa ridge Leading edge of cloud or
precipitation
- 500 hPa chart
- Satellite imagery
700 hPa ridge Leading edge of cloud or
precipitation
- 700 hPa chart
- Satellite imagery
Surface features (fronts,
troughs)
Orientation and pattern of
cloud and precipitation
- Surface analysis
- Satellite imagery
Short-wave troughs or
vorticity centers
Trailing edges or
wrap-around shape with
- 500 hPa chart
vorticity centre - Satellite imagery
Deformation zones Leading edges of cloud and
precipitation
- 500 hPa chart
- Satellite imagery
Jet streams Edges of cloud and
precipitation, dry surges
- 250 hPa chart
- Satellite imagery
Positive vorticity advection
(PVA), warm air advection
(WAA)
Edges and shapes of cloud
and precipitation
- 500 hPa chart
- Satellite imager
