Transcript Met Air in vertical motion.pps

CGS Ground School
Meteorology
Air in vertical motion
© Crown copyright 2012.
No part of this presentation may be reproduced without the permission of the issuing authority.
The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.
Air In Vertical Motion
A novice glider pilot can remain airborne with
very little knowledge of the structure of
thermals or their formation.
Likewise he can soar on hills or in lee waves,
but the risk of a premature landing is greater if
he does not understand the principles
involved.
Humidity
When a dry airstream passes over the sea, a
lake or even a damp area of country side, it
absorbs water much like a piece of blotting
paper.
This moisture creeps higher and higher within
the airstream which, in turn, becomes damper
and damper.
The water is held in the form of invisible water
vapour.
The airmass can only hold a given amount of
water vapour (in the same way as blotting
paper can only hold so much water).
When it can hold no more water vapour it is
referred to as being SATURATED.
Humidity
The water
vapour
willheld
CONDENSE
on suitable
amount
of water
by the airmass
nuclei
(eg dust
andof
form
cloud.
compared
with or
thesoot)
amount
water
held if the
The
temperature
at which
this occurs
is called
airmass
was saturated,
is called
the Relative
the
Dew Point.
Humidity
(RH) and is expressed as a percentage
(%).
If an airmass is cooled the amount of water vapour it
can hold is reduced, so the RH increases.
If a saturated airmass is cooled it will no longer be
able to hold the moisture as invisible water vapour.
100%
50% RH
75%
Adiabatic temperature
changes
When a gas is compressed its temperature rises;
that is why the lower end of a bicycle pump soon
gets warm when you inflate a tyre.
The opposite effect occurs if the pressure is
reduced, because the gas expands and cools.
If no heat is allowed to enter or leave a gas; the
temperature changes produced in it, solely through
compression or expansion, are known as Adiabatic
Temperature Changes.
Adiabatic temperature
changes
The simplest way for air to rise occurs when air
blows up and over a hill.
Whilst the air is rising it cools at 3° C per 1000ft,
this is called the DRY ADIABATIC LAPSE RATE
(DALR).
However, when the rising air reaches the dew
point,
cloud forms and the rate of cooling
reduces to approx 1.5°C per 1000ft.
This is called the SATURATED
ADIABATIC LAPSE RATE
(SALR).
DEW
POINT
Latent heat
• Solids, liquids and gases are known as
different energy states.
• When a solid is heated, it absorbs heat
energy and turns into a liquid.
• Likewise, when a liquid is heated, it also
absorbs heat energy and evaporates.
• So when cooling takes place, the heat
energy has to go somewhere.
• This heat energy is lost to the surrounding
air in a form of energy known as LATENT
HEAT.
• The difference between the DALR and the
SALR is due to the latent heat warming up
the surrounding air slightly and reducing
the rate of cooling.
The Föhn effect
The
that
is airmass
forced tonow
rise
Oncecloud
below
theforms
cloudwhen
base,air
the
over
a hill
OROGRAPHIC
CLOUD.
warms
up is
at called
the DALR
(3°C per 1000ft).
If
themeans
cloud isthat
lowthe
enough
to cover
the top
of the
This
airmass
downwind
of the
hill,
it isbe
called
HILLand
FOG.
hill will
warmer
drier. This is known as
The
cloud/fog
will leave a large portion of the
the FÖHN
EFFECT.
moisture on the upwind side of the mountain as
rain or drizzle.
As the airmass descends behind the
hill, it starts to warm.
Warm air can hold more water and
a lot of the moisture is left behind
on the upwind slope, therefore
the airmass quickly becomes
unsaturated.
DEW
POINT
The Föhn effect
Consider
For the remaining
this example:
7000ft of its descent, it
The
air at
temperature
of the
warms
the DALR. upwind
Therefore
the hill is 20°C.
The
hill is 8000ft
high, and
thehill
dew
temperature
downwind
of the
willpoint
be is at
4000ft.
24.5°C. 4.5°C warmer than the initial
During
the first 4000ft of the ascent, the air cools
airmass.
at the DALR. So it cools from 20°C to 8°C.
During the next 4000ft of the ascent, the air cools
at the SALR. So it cools from 8°C to 2°C.
As the air starts to descend, it is initially
in cloud for 1000ft, therefore it
warms at the SALR.
2°C
3.5°C
1.5°C
4000ft
per 1000ft
DEW
8°C
3°C per 4000ft
1000ft
20°C
POINT
24.5°C
Lee waves
When the air flows over a mountain range or
range of hills a wave-like oscillation may form
downwind.
This is best likened to the effect of a stone on a
river bed.
Layers of water above are affected and a number
of ripples may be seen downstream.
The amplitude of the ripples decreases with each
subsequent wave.
Lee waves
Exactly the same effect is seen when air flows
over the mountain range.
Areas of strong lift and strong sink may be
encountered downwind.
Lee waves
Where the peaks of the waves pass through the
dew point of the airmass the peaks will be
marked by lenticular cloud.
The passage of a moister airmass will lower the
height of the dew point and result in the cloud
lowering and the gaps closing.
Dew
point
Lee waves
Beneath the wave a turbulent air flow (rotor) may
develop.
A rotor can make the air extremely turbulent,
possibly sufficient to cause structural damage to
aircraft.
A rotor over the airfield can cause the wind to flow
in different directions at each end of the field.
Lee waves
A small change in wave length can cause a
dramatic change in wind speed and direction.
Lee waves
To produce Lee Waves the following
meteorological conditions must apply:
A wind of 15 kts or more.
Wind speed increasing with height.
Wind direction close to right angle to the hill (max
30° off).
Wind direction remaining almost constant with
height.
Stable air layer above the mountain tops with a
less stable airstream above.
Solar radiation
The sun gives out a tremendous amount of
energy in the form of solar radiation, which
travels across space before arriving at the
earth's atmosphere.
On average:
35% is reflected back
into space.
2 % is absorbed by
the ozone layer.
20 % is absorbed by
dust and cloud.
Only 43 % of the
solar energy reaches
the earth's surface.
Solar radiation
The solar radiation that reaches the Earth's
surface will either be reflected back into the
atmosphere or absorbed. Different surfaces
absorb different amounts of energy:
Surface Type
Energy Absorbed
Energy Reflected
Snow
10 - 30 %
70 - 90 %
Rock
85 - 90 %
10 - 15 %
Grass
70 - 85 %
15 - 30 %
90 %
10 %
Built-up areas
Solar radiation
The amount of heat absorbed also depends on
the angle at which the sun's rays strike the
ground.
A slope perpendicular to the sun's rays will
receive more solar radiation per unit area than
other surfaces.
Some surfaces may be
in the shade and
receive no solar
radiation.
Solar radiation
The net result of the solar radiation is to cause
some areas of the earth's surface to heat at a
different rate to others.
The heat is then conducted to the air above the
ground which, in turn, gets warmer.
Hot air expands and therefore becomes less
dense, the consequences of this is that hot air
rises, whilst cool air sinks.
This leads to the formation of both
Anabatic/Katabatic winds (as described in the
presentation “wind”) and thermals.
Solar radiation
Thermals are formed in just this way, with
parcels of air in contact with the ground heating
more than the surrounding air.
This warmer, less dense air then breaks away
from the ground but what happens next will
depend on the stability of the airmass.
The stability of the airmass is dependent on the
Environmental Lapse Rate (ELR) - the change
of temperature with height for that particular
airmass.
Thermals
Stable environment (ELR less than DALR)
In this example, the ELR is 2°C per 1000ft.
A hot spot forms on the ground, heating the air in contact
with it. It is warmer than the surrounding air and therefore
rises, cooling at the DALR (3°C per 1000ft).
At 1000 ft it is still
warmer than the
surrounding air and
3000ft
14°C
therefore continues to
rise.
At 2000 ft it is the same
2000ft
16°C
temperature as the
surrounding air and
stops rising. The
16°C
1000ft
18°C
thermal stops at 2000 ft.
Surface
20°C
19°C
22°C
Thermals
Unstable environment (ELR more than DALR)
In this example, the ELR is 4°C per 1000ft.
A hot spot forms on the ground, heating the air in contact
with it. It is warmer than the surrounding air and therefore
rises, cooling at the DALR (3°C per 1000ft).
At 1000 ft it is still
warmer than the
surrounding air and
3000ft
8°C
therefore continues to
rise.
As the bubble continues
13°C
2000ft
12°C
to rise the temperature
differential increases.
The bubble accelerates
16°C
1000ft
16°C
and the thermal gets
stronger.
Surface
20°C
19°C
22°C
Convective cloud
If a thermal continues up to the dew point then the water
vapour will condense and cloud will form. The cloud will be
cumulus due to the convection.
During a summer's day you can often see a sequence of
clouds forming over a given spot, before they drift slowly
downwind.
An individual cloud may last 30 minutes or so before the air
cools and the cloud dissipates.
Wind
Dew
point
Convective cloud
A series of clouds in line with the wind is called a cloud
street.
On days when the upper air is moist the cumulus cloud
may slowly spread over the entire sky, cutting out the sun
and preventing further thermals from forming.
With no thermals forming the clouds eventually dissipate
and the whole sequence starts again.
Wind
Dew
point
Convective cloud
A Cumulonimbus cloud is an
extreme version of a convective
cloud.
It starts life as a normal cumulus
cloud. An extremely unstable
atmosphere makes the cloud grow
into a towering cumulus.
Strong vertical currents develop in
the core of the cloud, which help it
develop even further.
Cumulonimbus clouds often have
an anvil like appearance due to
the cloud tops extending into the
strong upper wind.
Wind
Dew
point
Convective cloud
A Cumulonimbus cloud is an
extreme version of a convective
cloud.
It starts life as a normal cumulus
cloud. An extremely unstable
atmosphere makes the cloud grow
into a towering cumulus.
Strong vertical currents develop in
the core of the cloud, which help it
develop even further.
Cumulonimbus clouds often have
an anvil like appearance due to
the cloud tops extending into the
strong upper wind.
The strong currents in the core
become so vigorous that the cloud
can become self sustaining even
after the supply of warm air from
below has been cut off.
Wind
Dew
point
Convective cloud
Inside the cloud, rain drops and
hail stones collide as they are
lifted and descended in the
vigorous currents.
All of these tiny collisions produce
static electricity, which builds up
over time.
When sufficient charge has built,
the cloud discharges the electricity
as lightning.
The thunderstorms can be isolated
events but sometimes they may
organise themselves into lines and
sweep across the countryside.
Convective cloud
Severe downdrafts from a
cumulonimbus cloud can cause a
sudden change in surface wind
direction.
This is called a squall and is often
accompanied by torrential rain,
destructive winds, severe icing
conditions and hail.
THE END
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