Lesson Eight: Meteorology II
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Don’t read:
o You have to read everything covered in this lesson.
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Humidity
o Water vapour most important gas in the atmosphere in regards to weather.
o Moisture in atmosphere comes from evaporation of surface water bodies and
from transpiration from Earth’s vegetation.
 Exists as either visible moisture (droplets or ice crystals) or invisible
moisture (vapour)
o Recall the water cycle and phase changes from science.
o Any given mass of air can only hold so much moisture; warmer parcels of air can
hold more moisture than colder parcels of air.
 When a mass of air contains the maximum amount of w.v. it can hold at a
given temperature, it is said to be saturated.
 If the temperature falls lower after saturation point, droplets will form via
condensation (that’s how clouds, fog and dew are formed)
 If the temperature is below freezing when the saturation occurs, the w.v.
changes directly into ice crystals via deposition (high-level clouds)
o Any change of state (no change in temperature during a phase change) is
associated with heat transfer.
 Condensation: releases latent heat of vaporization
 Evaporation: absorbs latent heat of vaporization
 Freezing: releases latent heat of fusion
 Melting: absorbs latent heat of fusion
 Latent heat shown visibly in thunderstorms and hurricanes; their
tremendous energy is derived from releases of latent heat.
o Condensation and deposition can’t take place unless there’s condensation nuclei
(dust/dirt particles) for droplets to cling onto.
o Supercooled water droplets
 Sometimes water stays in the liquid phase even below 0oC
 Affected by the chemical and physical properties (like size) of the nuclei;
some don’t allow freezing until well below 0oC
 Often found between 0oC and -15oC, can exist down to -40oC
o Dewpoint
 Temperature to which unsaturated air must be cooled at constant
pressure to become saturated.
 When the spread between temp. and dewpoint are small, the air is
considered nearly saturated.
o Relative humidity
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Ratio between actual w.v. present in the air to the amount which the
same volume of air would hold if it were saturated (0 to 100%)
 When a mass of air is heated (at constant moisture), its relative humidity
decreases.
 When a mass of air has moisture added to it (at constant temperature),
its relative humidity increases
 The smaller the spread between temp and dewpoint, the higher the
relative humidity.
o Dew and Frost
 Vegetation cools by radiation at night to dewpoint
 Dew collects when temperature is above 0, frost forms when it’s below
freezing
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Temperature
o Earth’s surface is warmed by the short-wave radiation from the Sun (solar
radiation).
o The air above the Earth’s surface is warmed by the long-wave radiation from the
Earth (terrestrial radiation).
o Temperature varies across the globe based on several different factors.
 Diurnal variation
 Day-night cycle
 Solar radiation warms the ground during the day (solar radiation
in > terrestrial radiation out)
 Solar radiation ceases during the night (terrestrial radiation > 0) so
the surface cools
 Seasonal variation
 Tilt of the Earth affects angle of incidence of Sun’s rays during a
day
 Summer = greater solar radiation than winter
 Latitude
 Sun is more directly overhead equatorial regions than at higher
latitudes; greater solar insolation at lower latitudes.
 Tropics get more radiant energy than poles.
 Topography (terrain)
 Land surfaces absorb more solar radiation than water surfaces
AND radiate radiation out more than water
 Vegetation acts as insulation
 Some solar radiation is reflected (affected by the albedo of the
surface in question)
 Clouds
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Clouds can reflect incoming solar radiation, and trap outgoing
terrestrial radiation
How the atmosphere is heated
 Convection
 “Warm air rises, cool air descends”
 Air over warm surface becomes buoyant, rises; compensating
flow of cold air descends to take its place.
 Mixes heat throughout the atmosphere.
 Advection
 Horizontal movement of an air parcel.
 Cold air moves over a warm surface, heated in its lowest layers by
the warm earth.
 Turbulence
 Turbulence due to friction.
 Mixes heat throughout the atmosphere.
 Compression
 Descending air increases in atmospheric pressure, causing
temperature to rise.
 Phenomenon called subsidence, common at the center of high
pressure systems.
Isotherms
 Lines on a weather map joining places of equal temperature.
Density and Temperature
 Density defined as mass per unit volume.
 Warm air is less dense than cold air; can be explained by the Kinetic
Molecular Theory from science
 Less dense fluids tend to rise above denser fluids, until its density
matches that of the surrounding fluid.
How the atmosphere is cooled
 Lapse rate defined as the rate of decrease of temperature with height.
 Radiation
 Nighttime release of heat towards space
 Affects no more than the lower few thousand feet of the
atmosphere.
 Reduced if clouds are present.
 Advection
 Warm air moving over a colder surface.
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Adiabatic expansion
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Parcel of air rises due to some external forcing, expands due to
lower pressure surrounding the parcel
 Expansion causes it to cool without a transfer of heat.
o Adiabatic lapse rates
 Dry adiabatic lapse rate: 3oC/1000’
 Saturated lapse rate: 1.5oC/1000’
 Because moist air is more insulating, it cools at a slower rate with
increase in height.
 Average lapse rate: 2oC/1000’
 ICAO lapse rate: 1.98oC/1000’
o Inversions and Isothermal Layers
 Normally temperature decreases with height
 Sometimes, warmer air may be found at a higher altitude; such a reversal
is called an inversion
 An isothermal layer is one where the temperature remains constant
throughout a layer of some depth.
 Ground based inversions favour poor visibility by trapping fog, some and
other obstructions in the lower levels of the atmosphere.
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Stability
o Normal flow of air is generally horizontal.
o But external forcings can force air to move vertically.
o Air that resists upward or downward displacement and tends to return to its
original horizontal level is said to be stable.
o Vertical currents associated with an unstable condition may cause turbulence
and thunderstorms.
o If a mass of rising air is cooler than the air around it, it is considered stable. If
disturbed, it will go back to its original position.
o Unstable air is indicated by a steep lapse rate.
o Flight characteristics of stable air
 Poor low-level visibility
 Stratus type cloud
 Steady precip
 Steady winds which change markedly with height
 Smooth flying conditions
o Flight characteristics of unstable air
 Good visibility
 Heap type cloud (cumuliform)
 Showery precip
 Gusty winds
 Turbulence may be moderate to severe
o Lifting agents
 Convection
 Orographic lift (i.e. mountain lift)
 Frontal lift (when different masses meet, warm air forced aloft the by
advancing/receding wedge of cold air)
 Mechanical turbulence
 Convergence (low pressure area, winds blow across isobars into the
centre of the low, where the buildup of air forces it to rise)
o Read Vapour Trails
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Air Masses
o Defined as a section of the troposphere with uniform properties of temperature
and pressure in the horizontal.
o Takes on original properties from the surface over which it has formed.
o Air mass formed over a large body of water is moist, called maritime air
o Air mass formed over land is dry, called continental air
o Three main sources of air masses of North America
 The Arctic Region (North Pole to permafrost line)
 The Polar Region (Permafrost line to where mean temperature is 10oC)
 Tropical Region (lies below 30oN)
o Air masses of North America
 Continental Arctic (cA) and Continental Polar (cP)
 Cold dry air masses which originate over the intensely cold ice and
snow covered surface of the north; brings a cold wave southwards
 Maritime Arctic (mA) and Maritime Polar (mP)
 Cold air masses formed over the Arctic, acquire moisture as they
move south over the cold waters of the North Pacific/North
Atlantic
 mP warmer and moister than mA
 Maritime Tropical (mT)
 Warm, moist air from the tropical seas/oceans moving
northwards.
 In winter, mP, mA and cA are most common
 In summer, mA, mP and mT are common.
o Weather in an air mass
 Moisture content, cooling process and stability of the air affect the
weather in an air mass
 Maritime air has a high moisture content and cloud/precip is common
o Read Modification of Air Masses
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Fronts
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In general
 Cold Air Mass
o Associated with unstable air/instability, has all the
properties of unstable air
 Warm Air Mass
o Associated with stable air/stability, has all the properties
of stable air.
Transition zone between two different air masses called a front
Responsible for a lot of weather changes
Read Polar Front
Sloping side of the cold air is called a frontal surface
Development of a Frontal Depression
 Air on northern side termed arctic/polar air, normally cold and dry
 Air on southern side termed tropical air, normally warm and moist
 Difference in properties of these two air masses results in a surface of
discontinuity
 Essentially, the warm air extends into the cold air, the cold air extends
into the warm air, and a frontal wave forms between the cold and warm
air, resulting in a depression (low pressure area)
Cold front is that part of a frontal system along which cold air is advancing and is
shown as blue triangles on the weather map
Warm front is the part of a frontal system along which cold air is retreating and
is shown as red semi-circles on the weather map
Occluded front is when a cold front moves faster than the warm front and
catches up with it; shown as alternating blue triangles and red circles
Stationary front is where the frontal surface doesn’t move; shown as alternating
blue triangles/red circles facing in opposite directions
Warm front
 Warm air advances on retreating cold air
 Warm air rises over the cold air in a long, gentle slope; cloud formation
typically 500 nm or more in advance of it
 Moves at relatively slow speeds, affects a vast area for a considerable
length of time
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If warm air is moist and stable, CCANS indicates the passing of a warm
front
 Cirrus
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 Cirrostratus
 Altostratus
 Nimbostratus
 Stratus
 If warm air is moist and somewhat unstable, thunderstorms may be
embedded in the stratiform layers
 Passing of a warm front marked by a rise of temp, and the sky becomes
relatively clear
Cold front
 When a mass of cold air overtakes a mass of warm air, the cold dense air
forces the warm air up (steep slope)
 Width of cloud cover only 50 nm ahead of the front
 Showery precipitation, usually associated with cumuliform development
 Passing of cold front marked by a sharp fall in temp and rapid clearing
Stationary front
 Some part of a front along which cold air is neither advancing nor
retreating
 No motion to cause front to move because the opposing air masses of are
of equal pressure
 Surface wind tends to blow parallel to front, weather conditions similar
to those associated with a warm front though generally less intense and
extensive
Occluded front
 Cold front overtakes a warm front and lifts the warm sector off the
ground
 Cool air catching up to colder air: warm occlusion
 Characteristics of a warm front, where cool air rides atop the cold
air, both lifting the trapped warm air in between up
 Cold air catching up to cool air: cold occlusion
 Characteristics of a cold front, where cold air forces the cooler air
and warmer air up
 Warm air caught in between called a trowal (trough of warm air aloft)
Read Upper Front
Frontal Weather
 Actual weather associated with a front may extend over an area of many
miles; don’t think that the line drawn on the map is where all the
weather happens!
 Frontal zone aloft called frontal surface, frontal zone on the ground
called the front
 Frontogenesis means a front which is increasing in intensity
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Frontolysis means a front which is decreasing in intensity
Cold front
 Long line of cumulus clouds on western horizon usually an
indication of an approaching cold front.
 Surface wind
o Will always veer as the front passes. Gustiness may be
associated with windshift
 Temperature
o Temperature drops
 Visibility
o Usually improves after passage of a cold front
 Pressure
o Approach marked by a decrease in pressure, passing
marked by a rise in pressure
 Turbulence
o Associated with a cold front, although thunderstorms are
not always present
 Precipitation
o Frontal rain/snow usually narrow, showery in character
 Icing in turbulent cumulus clouds may be severe
 Squall line
o Long line of squalls and TS which sometimes accompanies
the passage of a cold front is called a squall line
o Usually associated with a fast moving cold front that is
undercutting an unstable warm air mass
o Form anywhere from 50 nm to 300 nm in advance of front
itself
Warm front
 CCANS
 Windshift
o Wind will veer with passing of a warm front, change will be
much more gradual than cold front
 Temperature
o Gradual rise in temp
 Visibility
o Low ceilings and restricted vis associated with warm fronts
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Turbulence
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o Thunderstorms can be embedded in the main cloud deck,
and those storms would be responsible for turbulence, if
any
 Precipitation
o Begins in As layer of cloud, precip gets heavier as front
gets closer
Read Weather at Trowals and Upper Fronts
Precipitation and Fog
o Precip occurs when water droplets grow sufficiently in size and weight to fall due
to gravity
o Vertical air currents can cause droplets to move about, collide with other drops
and grow in size until they fall as rain (process called coalescence)
o In stratiform clouds, lack of vertical air currents means that drops can’t coalesce
into bigger drops, and thus fall as drizzle
o Steady precipitation falls from a layer of stratiform cloud
o A shower (sudden burst of heavy precip) falls from cumuliform clouds
o Large clouds like thunderstorms can be divided into an ice crystal region (top), a
snow and supercooled water drop region (middle) and a water region (bottom)
o Drizzle
 Very small drops which seem to float
 At or below freezing level, drizzle will be supercooled and freeze on
impact (freezing drizzle)
o Rain
 Large water droplets
 Freezing rain is composed of supercooled water droplets that freeze after
striking an object
o Hail
 When a supercooled water drop collides with an ice crystal, it at once
freezes on the latter, imprisoning a little air which causes it to freeze in
the form of soft ice
 Water freezes on it in the form of hard, transparent ice
 Finally the ball falls out of the cloud as a hailstone, a hard, transparent
layer of ice covering a soft, white core
 Often encountered in a thunderstorm
o Snow pellets (soft hail)
 If water region lying below supercooled region of a cloud is not deep,
hailstone doesn’t acquire that hard, transparent outer layer.
 Soft, white ice.
o Snow
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W.v. in air deposits directly into ice crystals
Formed of an agglomeration of ice crystals and usually have a
hexagonal/star-like shape
 Snow grains are tiny snow crystals that have acquired a coating of rime
 Fall from non-turbulent clouds
Ice prisms
 Tiny ice crystals in the form of needles
 Can fall from a cloudless sky
 Exist in stable air masses and at very low temps
Ice pellets
 Formed by the freezing of raindrops
 Generally rebound when striking the ground
Read Precipitation and Cloud type
Fog
 Stratus in contact with the ground
 Formed when air is cooled below dewpoint, or if moisture is added to the
air
 Most likely to occur in coastal areas where moisture is abundant
 Types of Fog (RAUSIP)
 RAU – Formed by cooling
 SIP – Formed by addition of moisture
 Radiation Fog
o Formed on clear nights with light winds
o Ground cools losing heat through radiation
o Air in direct contact with ground cools, and if air is moist
and temperature is lowered below dew point, fog will
form
o Forms typically only over ground (ground fog)
o Dissipates within a few hours after sunrise as the sun
warms the earth and radiation heating causes the temp to
rise
 Advection Fog
o Caused by drifting of warm damp air over a colder land or
sea surface
o Can persist for days and covers a wide area
 Upslope Fog
o Caused by cooling of air due to expansion as it moves up a
slope; light winds necessary
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Steam Fog
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o Forms when cold air passes over a warm water surface;
warm surface evaporates into the cold air, causing the
colder air to become saturated
 Ice Fog
o Forms in moist air during extremely cold, calm conditions.
o Formed by deposition, typically when w.v. is added to the
air through fuel combustion.
 Precipitation-Induced Fog
o Addition of moisture to the air through evaporation of rain
or drizzle
o Known as frontal fog as well
Read Haze and Visibility (focus on the visibility definitions, as well as the
difference between VMC and IMC)