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3/27/2015
CLOUDS
CLOUD FORMATION
 As a mass of air rises from the surface into the atmosphere the air pressure
decreases and expands as a result
 The air molecules move farther apart, and as a result it will experience a drop in
temperature
 When air rises the amount of heat that is in that mass of air is spread out over a
larger area (due to the expansion of the air molecules) which causes an overall
decrease in temperature
 Therefore, a mass of warm moist air (low pressure system) from the surface rises
and expands. The temperature drops and the air can no longer hold all of the water
vapour
CLOUD FORMATION (CON’T)
TYPES OF CLOUDS:
 As a result the water vapour will condense back into liquid form (water droplets or ice
crystals) which in turn forms clouds, with bad weather usually being the end result
 Note: in order for water vapour to condense it needs a solid surface to condense on or to
form around. In the air these surfaces are provided by particles of dust, dirt, smoke, salt,
pollen. Meteorologists call these particles condensation nuclei.
 After condensation occurs, clouds form and precipitation happens, the result is a mass of
cool or cold dry air (high pressure system). This cool, dry air decends back to the surface
and is warmed up along the way.
The 3 basic types of clouds:
 Cumulus
 Stratus
 Cirrus
 This falling air that is warming up, allows evaporation to occur as the dry air is now capable
of holding water vapour.
 The result of this is an area of clear skies and good weather.
CUMULUS CLOUDS
CUMULUS CLOUDS (CON’T)
 These are either fluffy, clouds or large masses of
 The cold air, which is heavier, forces its way underneath the warm air and causes it to
much darker clouds
 They form at elevations of 500m to 13,000m
 Composed mainly of larger water particles
 They form when a mass of warm, moist air is
forced to rise rapidly
 This happens when a mass of cold, heavy air (high
pressure) moves into an area which is occupied
by a mass of warm moist air.
rise
 If it rises relatively slowly it causes gentle cooling, slow condensation and the
formation of the typical large, white fluffy cumulus clouds
 If it rises rapidly it causes rapid cooling, rapid condensation, and the formation of
large, dark clouds called “cumulonimbus clouds”
 These are the thunderstorm clouds, their formation usually results in storms,
thunder, lightening, heavy rainfall/snowfalls, which do not usually last a long period of
time
 Precipitation falls from the clouds very rapidly
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STRATUS CLOUDS
STRATUS CLOUDS (CON’T)
 From the Latin word stratum meaning layers
 Form at elevations of between 500m and 2000m
 They occur as low sheets or layers of clouds
 They usually cover all or most of the sky
 Composed of either small water droplets or small ice
crystals
 As the low pressure system (warm moist air) moves in, it flows over the top of the
high pressure system (cold, heavy air)
 This usually occurs slowly over a wide area, which results in a slow cooling of the
moist air mass and thus a slow rate of condensation
 These clouds typically result in light precipitation, drizzle, fog, etc…over a large area,
usually for a relatively long period of time.
 They form in an area where a mass of warm, moist air is
moving into an area which is occupied by a mass of
colder, heavier air
CIRRUS CLOUDS
CONTRAILS
 These are high elevation clouds, they form at heights
 Considered to be a type of cloud, the term comes from the
 Due to the very low temperatures at these elevations, cirrus
 These are the vapour trails which are produced by jet planes
 The exhaust from the plane is mainly CO2 and H2O (vapour)
 At -40oC (at least) this vapour freezes to form a white trail of
between 7000m and 13,000m (7-13km)
clouds are always composed of ice crystals rather then
water droplets
 Note: all of the various types of high altitude clouds belong
to the cirrus family
 As sunlight or moonlight passes through these clouds, the
ice crystals split the light into the different colors of the
rainbow. This forms the halos, or rings which sometimes
form circles around the sun or moon
phrase “condensation trails”
ice crystals
 If it lasts for a long time and extends across the sky, the air is
considered to be moist and bad weather can be expected.
 If it only lasts for a short period of time and disappears
quickly then the air is dry and we can expect good weather
FOG
 Fog is a cloud which rests on the ground
 There are 3 basic types of fog that we will discuss in this chapter:
1. Advection fog
2. Radiation fog
3. Upslope fog
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ADVECTION FOG
ADVECTION FOG (CON’T)
 Advection means horizontal movement along the surface
 This type of fog can develop either over the land or the ocean
 This forms when warm air is forced to cool as it passes over, or come into contact
 Usually happens during the spring or fall
with a cold surface
 The cold surface could be a cold ocean surface (such as the Grand Banks) or cold
land surface (such as the Avalon Peninsula)
 The Grand Banks → warm air from the Gulf Stream flows over cold water from
the Labrador Current
of the year.
 The air cools, condensation occurs,
water droplets form, and a cloud
appears at ground level
 The fog will dissipate where/when
there is a change in air flow or as the
land warms up
 Advection Fog is the most common
type of fog here in Newfoundland.
RADIATION FOG
Advection
fog
 Forms on a clear night as Earth’s
surface cools off with no clouds to
prevent heat from escaping.The surface
releases heat which escapes (radiates)
into space.
 As the land cools the air close to the
ground will cool as well
 This cool and heavy air will then flow
into valleys (or low lying areas) where
condensation occurs and fog forms
RADIATION FOG (CON’T)
 In Newfoundland, these valleys usually
contain a pond
 The cold air displaces the warm air
 This cools the warm air mass and fog
forms in the valley
 It forms overnight and will still be
present in the early morning
 It dissipates as the sun rises and the
ground and air warms up
Radiation fog
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 When a mass of warm moist air hits
the mountain range the air will be
forced to rise (as in the Rockies)
 As it rises it cools, condenses and
forms a cloud (fog) toward the upper
part of the slope
Upslope fog
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