Chapter 1 met theory

Meteorology - Chapter 1:

Met Theory

Topics:

1. The Atmosphere
2. Clouds
3. Pressure
4. Wind
5. Humidity
6. Temperature
7. Stability

Chapter 1 – 1: The Atmosphere

Atmospheric Composition

The Earth’s ATMOSPHERE is made up of:

-78% nitrogen
-21% oxygen
-1% rare gases (argon, carbon dioxide,
water vapour)


Water Vapour

Of all of the ingredients that make up the
atmosphere, the most important
component for pilots is WATER VAPOUR.

It is the water vapour that is responsible
for the formation of clouds, fog, and
precipitation. In other words, it is the
water vapour that produces weather.


Properties of the Atmosphere

A parcel of atmospheric air processes 3
properties, namely mobility, capacity for
expansion, and capacity for compression:

MOBILITY – like a body of water over a river
bed, a body of air can move over the Earth’s
surface.



CAPACITY FOR EXPANSION – like a
balloon, a parcel of air can expand. When
it does, the parcel of air cools.



CAPACITY FOR COMPRESSION – like a
deflating balloon, a parcel of air can
compress. When it does, the temperature
rises.


Divisions of the Atmosphere

The layer of atmosphere closest to the Earth is
called the TROPOSPHERE.

The Troposphere extends to a height of 28,000 feet
at the Poles, and 54,000 feet at the Equator.

In the Troposphere, air pressure, density, and
temperature all decrease with increasing altitude.
In other words, with increasing altitude, the air
becomes lighter, thinner, and colder.

Because almost all water vapour is found in the
Troposphere, all active weather occurs in this layer.



Above the Troposphere is a layer called
the STRATOSPHERE. In the
Stratosphere, the temperature ceases
to drop, and remains at -56oC.



The boundary layer separating the
Troposphere from the Stratosphere is
called the TROPOPAUSE.



In an earlier chapter on Flight Instruments, we
talked about “ICAO Standard Atmosphere”. (Recall
that ICAO stands for “International Civil Aviation
Organization”).
This “standard” atmosphere is based on averages
of atmospheric conditions at 49o of Latitude. By
definition, ISA (ICAO Standard Atmosphere) is:

-Pressure = 29.92” Hg (inches of Mercury)
-Temperature = 15oC at sea level
-Temperature Decrease (or Lapse Rate) =
2oC per 1,000 feet above sea level

Chapter 1 – 2: Clouds

High Clouds

Clouds can be classified according to their
altitude.

HIGH clouds have bases starting from
16,500 feet to 45,000 feet.

The name of these clouds are prefixed with
“Cirro”.


Mid Level Clouds

MID LEVEL clouds have bases starting
from 6,500 feet to 23,000 feet.

The name of mid level clouds are
prefixed with “Alto”.


Low Level Clouds

LOW LEVEL clouds have bases starting at the
Earth’s surface, or up to 6,500 feet.

The name of low level clouds have no prefix
attached to them.


Cumulus Clouds

Types of clouds can be further classified
according to their characteristics.

Clouds of vertical development (i.e. puffy or
cotton ball type clouds) are called CUMULUS
clouds.


Stratus Clouds

Clouds that form in horizontal layers or sheet
are called STRATUS clouds.


Fractus Clouds

Clouds that are windblown or broken are
referred to as FRACTUS clouds.


Nimbus Clouds

And finally, clouds from which precipitation
falls are called NIMBUS clouds.


Cloud Names

Now, if we combine any of the above
classifications of clouds with a cloud’s
characteristics, we can determine the full
name of a cloud.

For example, a high level cloud that is cotton-
like is called CIRROCUMULUS cloud.

Cirro = high
Cumulus = cotton-like


Cloud Names

A high level cloud that is made up of sheets
or layers of clouds is called CIRROSTRATUS
cloud.

Cirro = high
Stratus = horizontal layers


Cloud Names

A mid-level cloud that is rounded and puffy is
called ALTOCUMULUS cloud.

Alto = middle level
Cumulus = puffy


Cloud Names

A mid-level grey cloud that covers the whole
sky is called ALTOSTRATUS cloud.

Alto = middle level
Stratus = horizontal formation


Cloud Names

Low level cloud that resembles a series of
patches or rounded masses is called
CUMULUS cloud.

no prefix = low level
Cumulus = rounded masses


Cloud Names

A uniform layer of low level cloud is called
STRATUS cloud.

no prefix = low level
Stratus = layer cloud


Cloud Names

Low level layer cloud that is windbroken is
called STRATUSFRACTUS cloud.

Stratus = layer cloud (horizontal
formation)

Fractus = windblown


Cloud Names

Heavy masses of vertically developed cloud
from which precipitation is falling is called
CUMULONIMBUS cloud.

Cumulo (from Cumulus) = mass cloud
(vertical development)

Nimbus = precipitation


Cloud Names

Review your Environment Canada Clouds Poster
included with your Groundschool kit for further
cloud names and descriptions.


Sky Condition

The SKY CONDITION refers to the amount of sky
that is covered by cloud, as observed from the
surface up.

The sky condition can be any one of the following:

-SKC = ‘sky clear’ = no cloud
-FEW = ‘few’ = >0/8 to 2/8 cloud coverage
-SCT = ‘scattered’ = 3/8 to 4/8 cloud coverage
-BKN = ‘broken’ = 5/8 to <8/8 cloud coverage
-OVC = ‘overcast’ = 8/8 cloud coverage

Chapter 1 – 3: Pressure

Atmospheric Pressure

ATMOSPHERIC PRESSURE is the weight of the air
above us.

The greater the amount of air above us, or the
greater the density of the air above us, the greater
the downward pressure the air will apply on us.

Atmospheric pressure changes from location to
location. If there is dense, heavy air over an
area, the pressure will be higher than under an area
of less dense air.


Atmospheric Pressure

The pressure (whether it be “high” pressure or
“low” pressure) is important to pilots because it:

-affects our altimeters (as discussed in the
chapter on Flight Instruments)

-controls the wind (as we will learn in this
chapter)


Mercury Barometer

For aviation purposes, pressure is measured
with a Mercury Barometer.

A simplified Mercury Barometer would be a
dish filled with liquid mercury and an
inverted test-tube held in the dish of
mercury.


Mercury Barometer

As the weight of the atmosphere
increases (i.e. an increase in atmospheric
pressure), it will push down on the
surface of the mercury, thereby forcing it
to rise up in the tube.


Mercury Barometer

If we now measure how many inches the
mercury rises in the tube, (e.g. 29.92
inches), then we can determine the
altimeter setting. In this case, we would
call the altimeter setting 29.92” Hg.


Mercury Barometer

In aviation, we use inches of mercury (”Hg) to express atmospheric
pressure.

Other units used to measure pressure are millibars and kilopascals.


Isobars

ISOBARS are lines drawn on a Weather Map
that join places of equal atmospheric
pressure.

Isobars never cross one another, but tend to
form circular patterns.

Although we commonly use inches of mercury
to express pressure in aviation, the Isobars on
Weather Maps are presented in millibars.


Low Pressure Area

If we examine the pattern that the
Isobars form on this Weather Map, we
notice that as we move from the center
of the map to the upper right corner, the
pressure continually drops.

We therefore conclude that there is a
“low” pressure area in the top right
corner.


High Pressure Area

Likewise, if we look at the pattern as we
move from the center of the map to the
bottom right corner, we notice that the
pressure continually rises.

This would indicate that there is a “high”
pressure area in the bottom right corner.


High Pressure, Low Pressure

Likewise, the pattern of Isobars would
indicate a “high” pressure area in the
upper left corner, and a “low” pressure
area in the lower left corner of this
Weather Map.


Trough

A TROUGH is an elongated u-shaped area
of low pressure.

A trough is like a “valley” of low pressure.


Ridge

A RIDGE is a protruding neck of high
pressure.

A ridge is like a “mountain range” of
high pressure.


Col

A COL is a “neutral” area between two
high pressure areas and two low
pressure areas.


Pressure Gradient

PRESSURE GRADIENT is the rate of
change of pressure over a given distance.

The pressure gradient can be a shallow
gradient (i.e. a small rate of change), or a
steep gradient (i.e. a large rate of
change).


Pressure Gradient

The pressure gradient (or the nearness of
the Isobars) is an indication of the
strength of the wind.

Where there is a shallow gradient (i.e.
where the Isobars are far apart), there
will be light winds.

Where there is a steep gradient (i.e.
where the Isobars are close together), the
wind will be strong.


Wind

WIND is simply air trying to move (as it
wants to) from an area of high pressure
to an area of lower pressure.

Just like an inflated balloon, the air
inside the balloon is under high
pressure. The air outside the balloon is
at a much lower pressure. The air wants
to escape from the balloon. In other
words, the air wants to move from the
high pressure area to the low pressure
area.


Wind

So, if we had the above Weather
Map, the air would want to move
from the high pressure area to the
low pressure area. The wind would
tend to blow from the high to the
low.

However, as we are about to see, it
gets a little more complicated than
this…


Wind

Because the Earth is not stationary, but is
rotating beneath the atmosphere, the wind
does not move in a straight line (relative to
the Earth’s surface) as it attempts to move
from a high pressure area to a low pressure
area. It becomes influenced by a force
called CORIOLIS FORCE.

In the Northern Hemisphere, Coriolis Force
causes the air movement to be deflected to
the right (in relation to the Earth’s
surface), causing it to flow parallel to the
Isobars.


Wind

If we had a high pressure area on either
side of a low pressure area, we know
that the wind would want to blow into
the low (from high to low).


Wind

However, Coriolis Force says that in its
movement, the wind gets deflected to the
right.


Wind

This pattern shows how the wind
tends to blow clockwise around a
HIGH.


Wind

It also shows how the wind tends to
blow counter-clockwise around a
LOW.


Wind

Remember this picture to help you recall
whether the wind blows clockwise or
counter-clockwise around a high or low…

It is a picture of a “high clock over top of a
low counter”.

high = clockwise
low = counter-clockwise

The wind blows clockwise around a high, and
counter-clockwise around a low.


Wind

Here’s another trick…

When outdoors, you can always tell
where the low pressure area is if you
stand with your back to the wind…


Wind

In this position, the low will be to your left.


Wind

There is one more element that affects
the precise direction of the wind.

In fact, the wind does not blow exactly
parallel to the Isobars. SURFACE
FRICTION between the moving air and
the Earth’s surface tends to slow down
its motion and retards the effect of the
Coriolis Force.

Therefore, the air tends to move across
the isobars at an angle inward toward a
low, and outward from a high.

Chapter 1 – 4: Wind

Wind

As you can imagine, the wind is a very important factor for
pilots. The wind can have a negative or positive effect for us:
- on takeoff - wind affects takeoff distance
- wind affects takeoff safety (gusts, crosswind)
- in cruise - wind affects groundspeed (time, fuel, money)
- on landing - wind affects landing distance
- wind affects landing safety (gusts, crosswind)

We will now look at different types of wind…


Sea Breeze

A SEA BREEZE is a wind that blows from
the sea (or a large body of water) to the
land.

Note: When referring to wind direction, we
always refer to the direction from which it
is blowing (e.g. a north wind blows from
the north).


Sea Breeze

A Sea Breeze blows during the day.

The Earth’s surface is a better conductor of
heat than water. During the day, the sun
heats the Earth (more-so than the water),
which in turn heats the air above it. This
warmed air (over the land) rises.

Note: Warm air, which is less dense, tends
to rise. Cool are, which is more dense,
tends to sink.


Sea Breeze

This rising air (over the land) creates a
low pressure area over the land.
(Because the air is rising, there is less
downward pressure created by the
atmosphere, resulting in a lower
pressure).

In contrast, the air over the water (sea)
will be of a higher pressure.


Sea Breeze

We know that the air tends to move from
a high pressure area to a low pressure
area. So, during the day, the wind will
blow from the sea to the land, creating a
Sea Breeze.


Land Breeze

A LAND BREEZE is a wind that blows
from the land to the sea (or a large body
of water).


Land Breeze

A Land Breeze works opposite to a Sea
Breeze, and blows at night.

At night, all the sun’s warmth radiates
from the Earth’s surface into the upper
atmosphere, and the air over the land
becomes cool. Water retains heat
better, so the air over the water remains
warmer.

The warmer air over the water will
rise, creating a low pressure area over the
water. In contrast, the air over the land will
be of a higher pressure.


Land Breeze

With the high pressure over the land at
night, the wind will blow from the land to
the sea, creating a Land Breeze.


Mountain Wind

Wind in the vicinity of mountains can be
extremely challenging for a pilot. In fact, it
is recommended that you seek the advise
(or perhaps even training) of a pilot with
mountain flying experience before flying
in the mountains.

When the wind blows through a mountain
valley, the valley creates a “funnel
effect”, whereby the wind velocity
increases substantially. This strong wind
can also lead to pronounced turbulence.
We’ll now look at some specific types of
mountain winds…


Mountain Wind

An ANABATIC WIND blows up a mountain
slope during the day.

As the sun heats the dark surface of the
mountain slope, the warmed surface radiates
its heat to warm the air above it. This warm
air rises, creating a wind that blows up the
mountain slope.


Mountain Wind

A KATABATIC WIND blows down a
mountain slope.

If the mountain tops are snow covered,
the air at the caps will be cooled. This cold
dense air will sink, causing the wind to
blow down the slope.

An Anabatic Wind can turn into a
Katabatic Wind at night. The removal of
the sun’s heat causes the mountain slope
to cool, thereby cooling the air above it.
Again, this cool dense air will flow down
the mountain slope.


Mountain Wind

A MOUNTAIN WAVE forms when the wind blows over
the top of a mountain peak.
Just like the airflow over the top of a wing, the wind
blowing over a mountain top will have:

-increased speed
-decreased pressure
-decreased temperature

In the chapter on Flight Instruments, we learned how
this effect can cause the Altimeter to read in error by
as much as 3,000 feet!
The decreased temperature can lead to airframe icing
(ice accumulation on the airplane).


Mountain Wind

Turbulence associated with a
mountain wave is most frequent and
most severe just beneath the wave
crest at or below mountaintop level.


Mountain Wind

On the leeward side of the mountain
there can be strong downdrafts (as
much as 2000 to 5000 feet per
minute) and very turbulent eddies.


Wind Gust

A wind GUST is a rapid change of wind
speed or direction, that is of brief
duration (seconds).

Gusts are usually caused by obstacles
being in the way of the wind’s path (e.g.
hangars, buildings, irregular terrain, etc.)


Wind Squall

A wind SQUALL is a rapid change of
wind speed or direction, that is of
prolonged duration.

A squall is usually caused by the passage
of a fast moving cold front.


Eddies/Mechanical Turbulence

EDDIES, also known as MECHANICAL
TURBULENCE, is disturbed airflow
(similar to eddies of water in a river
or stream). They are caused by
irregular surfaces in the wind’s path
(like rocks in a shallow river) such as
hills, buildings, etc.

Mechanical Turbulence only occurs in
the lower levels of the atmosphere
(usually below 3,000 feet), and
depends on the strength of the wind
being disturbed.


Wind Shear

A wind SHEAR is a sudden or violent
change in wind speed or direction.

Wind shears are most commonly
associated with thunderstorms.
They can be extremely dangerous
because the wind can change much
faster than an airplane’s ability to
accelerate or decelerate. They are
especially dangerous near the
ground during takeoff and landing.


Jet Stream

A JET STREAM is a tube-like band of high
speed wind at high altitudes (20,000 to
40,000 feet). This band can be from 3,000
to 7,000 feet thick, with a core wind of
100 to 150 knots. This wind flows from
west to east.

There are two Jet Streams across North
America: one lies approximately across
Canada and the other across the USA. The
Jet Streams migrate south in the summer,
and move back north in the winter.


Clear Air Turbulence

CLEAR AIR TURBULENCE (CAT) is a
very turbulent condition that occurs
in a cloudless sky, usually associated
with a Jet Stream or Mountain Wave.
Because it occurs in a clear sky, CAT is
almost impossible to forecast.


Wind Speed and Direction

In aviation, wind speed is
expressed in knots (nautical miles
per hour).

Wind direction is the direction
from which it is blowing. Using the
compass rose to express precise
direction, a wind blowing from the
south would be a wind of 180o.



A wind of 040o would be blowing from
the north-east (NE).



A VEER is a clockwise change in wind
direction.

For example, if the wind changed from
270o to 300o, we would say that the
wind veered.



A BACK is a counter-clockwise
change in wind direction.

For example, if the wind changed
from 270o to 240o, we would say
that the wind backed.


Diurnal (Daily) Wind Variations

We all know that the wind tends to
increase during a hot afternoon, and
then calms at night. This is due to
Diurnal Variation…



During the hot afternoon, the sun
heats the Earth’s surface. The Earth
then heats the air above it by
radiation. This warming air rises. As it
rises, it expands, cools, and begins to
fall again.

As it falls, it transfers the higher level
wind (from about 3,000 feet) to the
surface. The higher level wind in
unaffected by surface friction and is
therefore stronger, and flows more
parallel to the Isobars.



As a result, during the daytime, the
wind veers and increases in strength.

At night, the wind resumes its normal
direction and speed: it backs and
decreases.

Chapter 1 – 5: Humidity

Humidity

HUMIDITY is the amount of moisture in
the air.

This moisture can be one of 2 forms:

- invisible form (which is water
vapour)

- visible form (which is water
droplets or ice crystals,
making up clouds or fog)


Condensation

CONDENSATION is when water
vapour changes into water droplets. In
other words, the moisture changes
from a gas to a liquid, or from its
invisible form to its visible form.

Condensation can be seen as moisture
on the inside of a window on a cold
winter day.


Sublimation

SUBLIMATION is when water vapour
changes into ice crystals. In other
words, the moisture changes from a
gas to a solid. Again, it changes from
its invisible form to its visible form,
but in this case, the liquid stage is
bypassed.

Sublimation can be seen as frost on a
car window on a cold winter morning.


Sublimation

Here is an important point to remember
about humidity:

Warm air can hold more moisture
than cold air

A parcel of warm air has the ability to hold
more water molecules than a similar parcel of
cold air.


Saturated Air

SATURATED AIR is when a parcel of air
contains the maximum amount of water
vapour that it can hold at a given
temperature.


Saturated Air

If the air is saturated (i.e. it contains all the
moisture it can hold), and then the temperature
drops, that parcel of air will have more moisture
than it can hold. (Remember: warm air can hold
more moisture than cold air).

This excess moisture (or vapour) will be forced into
condensation or sublimation. The excess moisture
will change from its invisible form to its visible
form, creating either cloud, fog, dew, or frost.

You’ve noticed that fog, dew and frost tend to
form at night, when the temperature drops. Clouds
form in the higher altitudes. Remember… the
temperature decreases with increasing altitude.


Saturated Air

So, saturated air can be forced into
condensation or sublimation be decreasing
the temperature.

Another way for this to happen is to increase
the moisture content of the air. If the air is
already saturated, then adding more
moisture will also force the excess vapour
into condensation or sublimation.

An example of this is when you see your
breath on a cold day (since the air you
breathe out has a lot of moisture in it from
your lings)


Super-Cooled Water Droplets

SUPER-COOLED WATER DROPLETS are
liquid water droplets that exist in the
liquid form at temperatures well below
0oC. This is a condition that does not
normally happen, and requires specific
atmospheric conditions to exist. They are
sometimes associated with thunderstorms
cells. They can exist at temperatures as
low as -40oC.
Super-cooled water droplets are a hazard
because, when they are disturbed (e.g. by
a wing), they turn into ice instantaneously.
They create a rapid accumulation of
airframe icing.


Dewpoint

The DEWPOINT is the temperature to
which unsaturated air must be cooled to
become saturated.

The dewpoint is the temperature at
which invisible moisture changes into
visible moisture. It is the temperature at
which fog, dew, frost, or clouds form.


Relative Humidity

RELATIVE HUMIDITY is the ratio of the
amount of water vapour present in the
air to the amount it would hold if it
were saturated (at the same pressure
and temperature).

For example, if the air is holding 80% of
the moisture that it can hold, then we
say that the Relative Humidity is 80%.

Saturated air has a Relative Humidity of
100%.


Relative Humidity

If a parcel of air is heated, then its Relative
Humidity decreases. (Remember: warm air
can hold more moisture than cold air).

If a parcel of air is cooled, then its Relative
Humidity increases.


Relative Humidity

The smaller the spread between the
temperature and the dewpoint, the higher
the Relative Humidity.

Chapter 1 – 6: Temperature

Temperature

As we’ve already stated, the sun heats
the Earth, and the Earth heats the
atmosphere above it by radiation.

This is an important point to remember.
The atmosphere is heated from
below, not from above.


Seasonal Variation

So, why is the atmosphere’s temperature
different at different places?

One reason is due to SEASONAL VARIATION.
The Earth’s axis of rotation is not
perpendicular to the Earth’s path of
travel, but is at a “tilt”. Hence, during North
America’s summer months, the sun’s rays are
more perpendicular to the continent’s
surface (shine from overhead).
But in the winter, the sun is lower on the
horizon, so the sun’s rays are at more of an
angle to the continent’s surface.


Seasonal Variation

Like a beam of light shining directly onto
a surface, the light’s rays are
concentrated in a small area. But if we
shine the light at an angle to the surface,
then that same beam covers a larger
surface area.

If both beams are producing the same
amount of energy (heat), then the beam
of light shining from directly above will
concentrate its heat over a smaller are.
Therefore, this surface will be warmer.
The sun has the same effect on the
Earth’s surface in summer vs. winter.


Latitudinal Variation

This same principle explains LATITUDINAL
VARIATION of the Earth’s temperature.

Locations near the Equator have the sun
more directly overhead than locations
further north or south of the Equator. Hence,
near the Equator the temperatures are
warmer.


Topography

TOPOGRAPHY (the makeup of the Earth’s
surface) also has an effect on temperature.
Since dark colours absorb more light than
light colours do (this is Physics!), dark colours
get warmer when the sun shines on them.

The same holds true for the Earth’s surface.
Dark coloured terrain (dark soil, asphalt, etc.)
gets hotter than does light coloured terrain
(water, snow, etc.).

Therefore, the atmosphere above a dark
surface will be warmer.


Cloud Cover

CLOUD COVER can have an effect on
temperature. During the day, the absence of
cloud cover allows for maximum heat from
the sun to heat the Earth’s surface, creating
warmer air (by radiation).

However, at night, a clear sky allows all the
Earth’s heat (gained during the daytime) to
radiate into the upper atmosphere, creating
cool temperatures at the surface.

A cloudy night produces a sort of blanket,
keeping the heat near the surface, creating a
warmer night.


How the Atmosphere is Heated

The atmosphere can be heated by any one
of 4 methods:

-Convection
-Advection
-Turbulence
-Compression

We will look at each of these…


How the Atmosphere is
Heated/Convection

CONVECTION works much like bubbles that form
in a pot of boiling water.
The air nearest the Earth’s surface is warmed.
Because warm air is less dense than cold air, it
begins to rise. As it rises, it cools, by expansion.
(Remember… at higher altitudes the pressure
decreases, allowing the air to expand. When it
expands, it cools).
As the air cools, it becomes more dense
(heavier), and begins to fall again, replacing the
rising warm air below.

It is the rising air that warms the air aloft.


Heated/Advection

ADVECTION refers to the horizontal
movement of air from one place to
another. Advection heating occurs when
cool air moves over a warm surface.

The warm surface warms the air above
it.


Heated/Turbulence

When an obstruction in the path of
the air’s movement (such as a hill or
irregular terrain) disturbs
it, TURBULENCE is created.

This turbulence can push the warm
air aloft.


Heated/Compression

When a parcel of air is COMPRESSED, it
warms.

This can occur on the leeward side of a
mountain range. As the air flows down
the mountain, it is compressed at the
mountain’s base. The compressed air
becomes warmer.


How the Atmosphere is Cooled

The atmosphere can be cooled by any one
of 3 methods:

-Radiation
-Advection
-Expansion



Cooled/Radiation

At night, solar RADIATION ceases. All the
heat absorbed by the Earth’s surface from
the previous day radiates, or
transfers, into the upper atmosphere. As
a result, the lower levels of the
atmosphere cool.


Cooled/Advection

Remember, ADVECTION refers to the
horizontal movement of air from one place
to another. Advection cooling occurs when
warm air moves over a cool surface.

The cool surface cools the air above it.


Cooled/Expansion

When a parcel of air EXPANDS, it cools.
(This is the opposite of compression).

If you’ve ever used a can of spray paint,
then you’ve experienced this. The paint
inside the can is under compression. As it
comes out of the nozzle, it expands, and
cools. You may have noticed that the tip of
your finger on the nozzle gets cold!


Cooled/Expansion

The same thing happens when air is
forced to rise… the pressure
decreases, so the air expands. When it
expands, the temperature decreases.


Isotherms

ISOTHERMS are lines drawn on a
Weather Map that join places of equal
temperature.


Temperature Scales

The international aeronautical unit used to express
temperature is Degrees Celsius.

In Degrees Celsius:
- the freezing point of water = 0oC
- the boiling point of water = 100oC

However, you may come across some airplane
manuals (especially for airplanes built in the USA) that
express temperature in Degrees Fahrenheit.

In Degrees Fahrenheit:
- the freezing point of water = 32oF
- the boiling point of water = 212oF


Temperature Scales

To convert from oC to oF:

oF = 9/5 oC + 32
oC = 5/9 (oF – 32)

Or, simply use your E6B Flight Computer!


Density vs. Temperature

Cold air is more dense than warm air.
Therefore, it is heavier and tends to
sink.

Warm air is less dense, or lighter, and
tends to rise.


Lapse Rate

LAPSE RATE is the rate of decrease in
temperature with height.

There are 3 different Lapse Rates:

- ICAO Standard Lapse Rate
- Dry Adiabatic Lapse Rate
- Saturated Adiabatic Lapse Rate



ICAO Standard Lapse Rate

The ICAO STANDARD LAPSE RATE is an
average lapse rate, as derived by ICAO.
(Remember, ICAO = International Civil
Aviation Organization).

This standard lapse rate is 1.98oC/1000
feet

For simplicity, we commonly say that it
is 2oC/1000 feet.

This ICAO Standard Lapse Rate is an
assumption used for the calibration of
aircraft Altimeters.


Dry Adiabatic Lapse Rate

The DRY ADIABATIC LAPSE RATE is
the actual lapse rate in air that is
not saturated. This is the lapse rate
when the temperature is greater
that the dewpoint.

By definition, this lapse rate is
3oC/1000 feet.


Standard Adiabatic Lapse Rate

The SATURATED ADIABATIC LAPSE RATE (or
sometimes called the WET ADIABATIC LAPSE RATE) is
the lapse rate in air that is saturated. This is the lapse
rate when the temperature meets the dewpoint.

(Remember that at the dewpoint, moisture changes
into its visible form. Clouds form at the dewpoint. The
base of clouds, then, represents the altitude at which
the temperature meets the dewpoint).

By definition, the Saturated Adiabatic Lapse Rate is
1.5oC/1000 feet.

So, at or above the base of clouds, the lapse rate
becomes 1.5oC/1000 feet.


Standard Adiabatic Lapse Rate

Here is a sample problem:

Question: If the surface temperature is 10oC and the
dewpoint is 1oC, what is the altitude of the base of the
clouds?

Solution: We know that below the cloud base, the air is
unsaturated, so the lapse rate is 3oC/1000 feet (we use the
Dry Adiabatic Lapse Rate below the cloud base).

So, at: -1000 feet above ground, the temperature = 7oC
-2000 feet above ground, the temperature = 4oC
-3000 feet above ground, the temperature = 1oC
Therefore, at 3000 feet we’ve reached the dewpoint of
1oC, and cloud will begin to form.


Inversion

An INVERSION is when the temperature
increases with height.

Inversions are not the norm. They are
usually associated with a frontal surface.
(We will talk more about fronts soon).


Isothermal Layer

An ISOTHERMAL LAYER is when the
temperature remains constant (neither
decreases nor increases) throughout a
layer for some depth.

Chapter 1 – 7: Stability

Stability

STABLE air is air that resists upward
of downward displacement.

UNSTABLE air is air that tends to
move further away when displaced.


Unstable Air

If a parcel of air is warmer than the
surrounding air, it will tend to rise.

(Remember: warm air is less
dense, or lighter, then cool air)

This parcel of air is therefore
unstable.


Stable Air

Air that is cooler than the
surrounding air will resist upward
motion.

(Remember: cool air is more
dense and therefore will not want
to rise)

This parcel of air is therefore
stable.


Lapse Rate vs. Stability

The steeper the lapse rate, the
more unstable the air.


Flight Characteristics in Stable Air

Flight through STABLE air will provide the following
flight characteristics:
- poor low level visibility (because stable air tends
not to rise, so the pollutants get trapped
near the surface)
- stratus type cloud (layer cloud)
- steady precipitation (e.g. that “all day” type of
rain, which is characteristic of stratus type
cloud)
- steady (constant) winds
- smooth flying conditions


Flight Characteristics in Unstable Air

Flight through UNSTABLE air will provide the following
flight characteristics:

- good visibility (except in precipitation)
- cumulus type cloud (heap type cloud of vertical
development, built from unstable, rising air)
- showery precipitation (e.g. “bursts” of rain,
which are characteristic of cumulus type
cloud)
- gusty winds
- turbulent flying conditions (produced by rising
columns of unstable air)
- smooth flying conditions


Lifting Agents

LIFTING AGENTS are the forces or conditions
that provide the lift to initiate rising currents of
air.

If a lifting agent provides a force onto a parcel
of unstable air, then this air will experience
significant lift. Stable air will have substantially
less lift.

When air is lifted to a higher altitude, it expands
and cools. If it expands and cools
sufficiently, then cloud formation occurs, and
hence weather is produced.


Lifting Agents

There are 5 different Lifting Agents:

-Convection
-Orographic Lift
-Frontal Lift
-Mechanical Lift
-Convergence



Lifting Agents/Convection

CONVECTION works much like the
bubbles that form in a pot of boiling
water. (We talked about this earlier
when we looked at how the
atmosphere is heated).

The warm surface heats the air above
it. This warm, less dense air rises,
expands, cools, and falls again,
replacing the rising warm air below.


Lifting Agents/Orographic Lift

OROGRAPHIC LIFT occurs when air
that is moving horizontally meets
uneven terrain. It gets disturbed and
is pushed upward.


Lifting Agents/Frontal Lift

FRONTAL LIFT occurs when a wedge
of cold dense air moves horizontally
and pushes under a mass of warm air
(much like a snow plow). The warm
air will be forced aloft.

This is an example of a “Cold Front”
(hence the term “Frontal Lift”).


Lifting Agents/Mechanical Turbulence

MECHANICAL TURBULENCE occurs
when air that is moving horizontally
meets an obstruction (e.g. a building
or hangar). It is disturbed and
becomes turbulent. This turbulence
can cause the air to be pushed
upward.


Lifting Agents/Convergence

When two horizontally opposing air
masses meet, they will be forced to
rise by CONVERGENCE.

Chapter 1 met theory

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