7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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Understanding
lying
Weather
Derek
Piggott
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
3/31
lsoavailable
from A C B
lack
BE
GINNING G
LIDING Third Ed
ition
Derek Pig
gott
GLID
ING Sevent
h Edition
Derek
Piggott
GLIDING S F
ETY
Derek
Piggott
METEOROLOGY ND
FLIGHT Third
Edition
A Pilot sGuide to Weath
er
Tom Bradbury
UNDERS
T NDING G
LIDING Third Edition
Derek
Piggott
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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nderstanding
F
ly
i
n
g
W
e
a
t
h
e
r
e
cond
e
diti
on
erek
Piggott
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Publishe
d
by
A
C
Black (Publishers) L
td
35 BedfordRow,
London W C1 R 4JH
First
edition 1988
Reprinted 1991
Second edition 1996
Reprinted 1999
1988, 996DerekPiggott
ISBN 0
7136
4346
3
All rights
reserved. N o part of
this
publication may
be
reprodu
ced
in
any form or by any
means
grap
hic,
electronic or mechanical,
in cluding photocopying,
recording, tapingor info
rmation storage and retrieval
systems
without
th e
prior
permission
in
writing
of
the
publishers.
Derek Piggott has asserted
his
rights under the Copyright,
Design and Pat
ents
Act 1988, to
be identified as
the
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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CONTENTS
Author s
note
7
Introduction 9
The airmass 11
2 The pressure pattern 14
High
and
low
pressure
areas 15
Buys-Ballot s
Law
16
Veering
and backing
16
Interpreting the isobars 17
Wind strength
18
Wind gradient 22
3
Depressions
and anticyclones
23
Depressions
23
Occlusions
31
Anticyclones 3 4
Timing
3 5
Shipping forecasts 3 7
4 The
atmosphere 41
Water vapour
41
Stability and
instability
of the atmosphere
43
5
Clouds
4 9
The
formation of
clouds 4 9
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6 Local topography
59
Sea
breeze
effects
59
High ground
eff
ects
61
7 Soaring
conditions
6
3
Seasonal effects 63
Thermals 64
Visibility
68
Wave
conditions
7
Ridge
lift
an
d hill soaring 75
8 dvice on using so
aring condition
s
76
n powered
aircraft
76
n
gliders and low
speed aircraft 8
dditional hazard
s
for
gli er pilots
converting
to
powered aircraft 85
ltimeter
settings
86
Revision
questions 9
nswers
92
Index 96
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Author
s N
ote
My
aim
in
writing
this
book
has
been
to provide
a
simple
in-
trodu
ction
t
o Met
eorology
for peopl
e learn
ing to fl
y glide
rs or
other kinds o
f airc
raft. Hopef
ully,the
bookshoul
dh
elp
the
re
ader
to
un
derstand
the
current
wea
ther, to
interpre
t
and ma
ke
bet
ter
use o
f
for
ecasts,and t
o recogn
ise the
better days for
flying
and
the
e
ver presen
t haza
rds associat
ed w
ith fly
ing
in
to bad
weathe
r.
Altho
ugh the book
is
larg
ely gl
ider and
soari
ng
o
rientate
d, am
sure
that
all
pilots would
be
safer
and
have
more
fun
from
their
flyin
g if th
ey understo
od mo
re of the
s
ubtleties
ofso
aring condi-
tions On
many
daysany
knowl
edgeable
pilot can u
se
thermal
s
and hill or
wav
e l
ift
to
improv
e the climb
ing
ability
and even
to
increase
t
he
average
cruis
ing sp
eed
and economy
of a li
ght
air-
cra
ft
With
this
in
mind
,
have
i
ncluded
some notes
on how to
mak
e
use
of
thermal
activity
when
flying
a
light
aircraft
and
also some
advic
e for the
les
s experien
ced
gl
ider p
ilot who
m
ay
b
e
h
aving
diffic
ulties
in staying
up.
For a
long
time ha
ve
known
h
ow
difficult
it is fo
r gliderpilots
reading
for t
heir Bro
nze
C
It
usually
involv
es searchin
g throug
h
number
of
books, n
ever kn
owing
what is essent
ial for the
test.
No one met
eorology
book
seems to cover
wh
at
the
pilot
s
r
eally
need
to
know
in
an
easily
understandable
manner. hope
that
this
boo
k will
solveth
eir pro
blems
a
nd
ha
ve
i
ncluded
question
s
and
answ
ers coveri
ng th
at syll
abus.The
se should
also be
useful
to
the
rea
der w
ho
is
star
ting
to study
for a Pr
ivate Pilot
Licence
I
would
l
ike
to
thank
John
Finlater
and To
m Bradbu
ry,
both
v
ery
experi
enced
forecas
ters, for
the
ir enco
uragem
ent and for
their hel
p
i
n checkin
g
m y te
xt
and
sugge
sting
what should
be
in cluded
D
erek Pig
gott
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IN T
RO
U T
ION
Meteo
rology
is a
comple
x
subject
and th
erefore
to simp
lify
it
m eans
discussing
only
the
basic
factors.
Each
is
affected
and
modi-
f
ied bythe
others
so that
all of them
need
to b
e
taken
in
toaccou
nt
wh
en co
nsidering
what
is happen
ing to
the
weat
her.
In
t
rying to
und
erstand the
weathe
r from
a pilot s poi
ntof
vie
w
the
main factors
invol
ved ar
e:
w
here the airm
ass
has come
from an
d the
type of
terrain
over
w
hich
it
h
as tra
velled;
t
he pressure
di
stributio
n and the
signif
icance o
f
the
pres
sure
p
attern;
the
characte
ristics
of depressi
ons,
fron
tsand anti
cyclones
;
4
th
e stabili
ty of
the airma
ss;
5 th
e local
topo
graphy;
6 the
tim
e of year and
th
e
time of da
y.
All
of these
factors and
many
others
have
to
be
considered
by
a
forec
aster if
a
re
asonable
p
rediction
i
s
to b
e mad
e. How
ever,
p
ilots do
not norm
ally ha
ve detail
ed inf
ormation
a
nd mu
st r
ly
mainly
o
n the pr
ofession
al foreca
ster to
interp
ret the
report
s
and
meas
urement
s
availa
ble
to him.
Altho
ugh this
in
formati
on
i
s up-
d
ated
every fe
w h
ours, many of
the
we
ather ch
anges are
rapid,
making
th
e fo
recasts u
ncertain
. Eve
n
a
pat
ch of hig
h cloud
or an
unexpected
shower
can
change the soaring conditions and wreck
an
o
therwise
acc
urate forecas
t.
This makes i
t particu
larly
impor-
tant fo
r
th
e soaring
p
ilot to
ma
ke
a good
assess
ment of the
l
ikely
co
nditions
by look
ing
at
the
forecast
s availa
ble i
n c
onjuncti
on
wi
th
his ownob
servatio
ns.
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T H
E
A IR
M A
S S
An ai
rmass
is
a b
ody
of air in
which
h
orizon
tal chan
ges of tem
perature andhumidity are
slight.
It
may
extend for
many hun
d
reds
of
k
ilometr
es.
Its c
haracte
ristics
are
ac
quired
by
m
oving
ov
er
a d
istant
source
reg
ion f
or lo
ng
peri
od
befo
rebe
ing mo
ved
o
n to affe
ct ot
her areas.
Differe
nt
air
masses
are
separ
ated by
fro
nts whe
re h
orizon
tal cha
nges o
f te
mpera
ture
and hu
midity
m
ay be
sharp
.
A
front is slo
ping su
rface
sep
arating
twoairm
asses
havi
ng
different
temperature
and humidity. Surprisingly
even
slightly
d
ifferen
t
a
irmass
es do
not
easily
mix
tog
ether. Th
e fr
ontal
zone
whe
re the
two airm
asses
meet
is some
times
only
few
mile
s
across
altho
ugh it m
ay
on
occas
ion
stre
tch for
50 miles o
r
m
ore.
The sour
ce of the
airm
assdet
ermine
s
its char
acteris
tics and
in
partic
ular its
temp
erature
. H
oweve
r the path
over
whic
h the
airm
assha
s travell
edd
etermin
es ho
w
m
uch
thea
irmass
is m
odi
fied
before
it
reaches
us.
Unfortunately TVandnewspaper weath
er
map
s
sel
dom
give any
detai
l
on
the
air
masses
.
Po
lar
airmas
ses come
from
the n
orth
and sta
rt cold o
r cool.
Cold
air
c
annot
hold mu
ch
moi
sture
and
therefo
re
pol
ar a
ir-
m
assesare
likely
t
o
be
relativ
ely dry
.
Arc
tic mar
itime Am
).
This
is
pola
r air w
hich has
on
ly h
ad
short sea
tra
ck. T
hewar
ming ove
r the sea mak
es i
t
very un
stable
pr
oducin
g frequ
ent show
ers in th
e n
ortho
f
E
ngland
.
Apo
lar airmas
swhich
ha
s travel
led
over larg
e areas
of
se will
be
called
Po
lar
m
aritime
airm
ass
Pm). By
o
riginat
ing
i
n the
nort
h Polar
mar
itimeair
reach
es us
by mo
ving so
uthwa
rds over
a pro
gressiv
ely
w
armer sea
thus
warm
ing and
m
oisten
ing th
e
ather
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2 IN
TRO U
TION
Fig
Theo
rigin of
the airm
asses.
m
Arctic Marit
ime; P
ra Pola
r
Maritim
e; P Po
lar
Contine
ntal;
T
c Tro
pical
Conti
nental;
rPm
Return
ing
P
olar
Mari
time.
over co
lder sea
which
stab
ilises t
he low
er layers
giv
ing s
tratus
c
louds.
W
hen
this air reac
hes
the
UK
itmay th
en
be
hea
ted
as
i
t
mo
ves ov
erland
. The
lo
w level
stabilit
y isde
stroyed
causi
ng
deep
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I N TRODUCT I ON
3
more
moisture,
it can
be
dry
or
have a
high
humidity, depending
on the route it
has
taken to
reach us.
A
tropical
airmass
which has moved over large areas of sea will
have
picked
up
a
large
amount of moisture. The
relatively
low
sea
temperatures
will
have cooled
the lower
layers
producing stable,
cloudy conditions. Near windward
coasts
low
cloud is
liable to
persist all day.
This kind
of
airmass is
then
known
as Tropical
maritime Tm) and where it travels over land in summer,
the
cloud will break up leaving
rather stable
muggy weather and poor
visibility.
Where tropical air moves in from the
continent
in summer it
will
be
hot,
hazy
and
dry
and
known
as
a
Tropical continental
Tc)
airmass.
On a much smaller scale the airmass is being continuously
modified during the day and night.
Even
a slight change in the
wind direction, so that the air has travelled over more land, will
often result
in
a dramatic change in soaring conditions. The air
will be drier, giving a high
cloud
base and
better
thermals. For
example,
in
England
a
small change
in
the
wind direction of
northerly winds can
result
in marked improvement and better
soaring.
Instead
of
blowing in
almost
directly
from the sea, the
rather
moist cool
air
will have
dried
out by
travelling
over the
length of
the
country before it reaches central
England.
This will
reduce the amount of
cloud
cover and
raise
the
cloud
base.
With such
a small
island
the proximity of the
coastline is always
a limitation for
long distance
glider flights. The coast
line is only
20 miles or so
from
the
Lasham
Gliding Centre
in
Hampshire
(midway between London and Southampton) and with south
or
southwest winds,
cloud
base will remain low all day and thermals
will usually be weak unless good cloud streets
are formed.
20
miles
further
inland at Booker and Dunstable the cloud base will
be
sever l
thousand feet higher, giving far
better
conditions.
Sim-
ilar
conditions occur near
most
coastlines all over the world, and
have
a significant
effect
on
soaring conditions
whenever
the winds
are
coming inland off the
sea, particularly
in summer.
In
winter
the
sea and
land
temperatures
are much closer
to
each other and
sea breeze effects are insignificant.
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THE
PR SSUR
P TTERN
Si
nce
th
e a
ir
it
sel
f has
wei
ght
the
ai
r
n
ea
r
t
he su
rfa
ce
is
com
pressed
by
theweight of
all
theair above
it.
Thegreaterthe
height
t
he le
ss
we
igh
t
of air
ab
ove
t
h t
l
eve
l an
dthe
ref
ore
t
he
low
er
th
e
p
re
ssu
re. B
el
ow
ab
out
5
00
0
f
eet
the
p
res
sur
e is
r
edu
ced
by a
bo
ut
1
mi
llib
ar
fo
r ev
ery
30
f
eet
of
e
xtr
a
h
eig
ht.
T
his
is
a
n
ave
rag
e
fi
gur
an
dis
im
por
tan
t
to re
me
mb
er
for
c
alcu
lat
ing
he
igh
ts
w
hen
u
sin
g
a
n alti
me
ter.
S
ee
fig.
2
.)
He
f
eet
3
5 0
00-
30
000
-
2
0 0
00-
10
,000
-
5
000
-
ight
:::
Pressure
m
etre
s
mill
iba
rs
1
0,0
00
; ; x
250
5,50
0
500
-
3 0
0 0
j ; ; i ; ;
; ; j
7
0 0
-
1
,50
0
- |
| -850
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T H
E PR E S
S U R E
PA T
T E R N
5
Lifti
ng
th
e
air cau
ses co
olingbeca
use t
he at
mosph
eric pres
sure
de
creases
with
heig
ht, and
any
reduc
tion in p
ressure
o
f
gas
resul
ts ina
n expan
sion
so
tha
t
the
same am
ount of
he
at is co
n
taine
d
in
la
rger
volum
e.
T
hisprinc
iple
is
dem
onstrat
edin many
types
of refrigerator
where
gas
is
allowed to expand, lowering
the tempe
rature;
conv
ersely,
with
bicyc
le pum
p,
co
mpress
ing
t
he
air into
smalle
r vol
ume res
ults
in
a
ris
e in
tempe
rature.
Variat
ions of
p
ressur
e
in
theu
pper
atmos
phere cause
gradu
al
asce
nt
or
subsid
ence in
the
low
er
levels
resu
lting
in
chan
ges
in
the
press
ure at
the
surface
.
I
f air
is
heat
ed,
it expa
ndswhen
free todo
so
.This resu
lts in
reduc
tion of
its densit
y and
pre
ssure. U
neven
heati
ng o
f
the
earth
s
s
urface
will there
fore g
ive r
ise
to
pressu
re
varia
tions from
place
to p
lace. W
inds
are
th
e resu
lt
of
th
e
movem
ents o
f
the
a
ir as
it
tr
ies to ev
en
out th
ese
differen
ces in pre
ssure.
H
igh
n
d
ow
pr
essure
re
s
The
pres
sure
dist
ributio
n
d
etermi
nes
the
d
irectio
n a
ndstren
gth of
the
winds.
Without
the
earth s rotation,
air
would
always
tend to
flo
w
dire
ctly
from
any h
igher pre
ssure
region
to ev
en o
ut any
pressu
re d
ifferen
ces. (Th
is do
es happe
n near
the
E
quato
r). How
e
ver, the
E
arth ro
tates and
this
causes
air mov
ing from
high
to
low
er pressu
re
i
nthe
North
ern hemi
sphere
to
be
deflec
ted
t
o
th
e
right
until
bala
nce is
achie
ved
bet
ween
the p
ressure
gradie
nt
force
and the d
eflectin
g for
ce due to
th
ero
tation
of the
earth
. This
is
known
as
the Coriolis
effect,
and
it causes
the
air to
flow in
clockw
ise direc
tion
aroun
d ana
nticycl
one o
r
hig
hpress
ure are
a,
and
in an anticl
ockwis
e
directi
on aro
und dep
ressio
n
or lo
w
pressu
re area.
See
fig
3.
In
t
he
sou
thern
h
emisp
here th
e
rever
se
is true
an
d
th
e a
ir m
oves clo
ckwise
ro
und the
dep
ression
s. Since
the
wi
nds
are the res
ult
of these
pre
ssure pa
tterns,
to unde
rstand
th
em and th
e mov
ements
of the
we
ather syste
ms
we nee
d
t
o
know
more
about
press
ure
syste
ms.
The
readin
gs of
the
loca
l
pressu
re ar
e recor
ded by
stati
ons all
over the
wor
ld bu
t
it
i
s
only
ove
r thel
and ma
sses that
the
re
a
re
en
ough
statio
ns to
form
fair
ly accu
rate p
icture
of the
ever-
cha
nging p
ressure
patt
erns. In re
cent y
ears
th
e
cha
nges
in
pressu
re
e been
p
redicte
d large
ly by
com
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6 T
HE PR ESSU
R E
PATTERN
Fig 3
Th
e
earth s rotation
causes the air toc
irculate
anti-clockwise
rou
nd
a ow pres
sure and clockwi
se rou
nd a highpressure with
the
surface
winds at about
30 degrees to the iso
bars.
it is well worth
while learni
ng to interpret thes
e
charts.
I
t may
b
e surpri
sing to learn that for
all practical purposes
the
winds at 2 3
fe
et abo
ve
the
surfac
e
b
low along these lines of
equal pressure.
Therea
son
fo
r this
is
not simple
to explain
and
the
ave
rage read
er
need
only
remember that
this
is
a
fact.
Near
t
he surface, however, air
is
retarded b
y
s
urface fri
ction
caused
by
trees,
vegetation,structures and topography
so
that
the
wind
s
peed
and therefore
the deflecting
force caused
by th
e
earth s
rotat
ion
are reduced. Th
is
makes
the s
urface
wi
nd flow
at an
angle
a
cross
th
e isobars towards
the low
pressure
,
ofte
n
about
3
degrees
overland and 1 degrees over
th
e
sea
wherethe f
riction
is
less.
Whenthe
isobars are close tog
ether the pressure
gradient and
the
wind
are strong. Whenthe
isobars
are
widely
spaced
and
the
pressure
g
radient is
weak, the
winds
are
light.
Buys-Ballot s
w
Seefig.
4. This
useful
law
states that:
i
n th
e
northern h
emisphere,
if
you stand with
you
r
back
to
the
wind,
the low pressure will
always
be
to your
left
and
high
pressure to your
right. It
is
also
us
eful t
o note
that if
you stan
d with your
back
to the surface
wind,
the upper wind
at about 2000 feet
will be blo
wing about
15 20
degre
es further round
in a clock
wise
direction.
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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TH
E P
RESSUR
E
P
ATTERN
7
Fig Buy
s-Ballot
s Law
. Standi
ng back
t
o
t
he wi
nd,
lo
w press
ure
is
alwa
ys to
the left
in th
e northe
rn hem
isphere
).
backin
g anti
-clockw
ise)
veering
clockwise)
Fig
5 V
eering and bac
king.
nterp
reting
theisob
ars
Fig.
6
see
next
page)
shows
a
weather
map
with
only the
isobars
dra
wn
in
. Eac
h
line co
nnects p
laces
with e
qual
mean sea
level
pr
essurea
nd in
t
his case
is
m
arkedw
ith the
pressu
re
in
m
illibars
.
The
ma
in
fe
ature
is th
e
d
epress
ion
centre
d
ov
erNort
hern
Ir
eland
w
ith a w
eak
r
idge
of
high pres
sure o
ver E
urope.
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8
THE
PRESS
URE
P
ATTER
N
F
ig
Interp
reting the isob
ars.
Isob
ars
link
place
s w
ith
e
qual
pressu
re
and a
re
us
ually spa
ced
at
4
mil
libar
interv
als. W
hich directio
n
are the
surfa
ce
an
d
up
per wi
nds at
Bri
stol?
mor
e sout
herly becau
se it
woul
d
b
e flowi
ng
a
litt
le m
ore towa
rds
th
e
centre
of
the low
.
T
ry
the Buy
s-Ballo
ts law
for Bri
stol. I
m
agin
e
your b
ackto
th
ewind,Yes,
the
l
owpress
ure
is toyou
r left.
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
18/31
THE PRESS
URE
PATTE
R N 9
Fig
High
and
lo
w
press
ure areas anticy
clones
and dep
ressions)
.
at a
ce
rtain
height by
mea
suring this press
ure g
radient.
special
ly
calibrate
d
scale is
used to measu
re
th
e dist
ance
betwe
en
th
eselines
an
d
g
ives h
im an e
stimate of the
wind
s
peeds
at
bo
ut 200
0feet.
This
is know
n as the geo
strophic
wind.)
At this
heigh
t
the
actual
wind
blow
s
al
ong the isob
ars
b
ut
v
aries from the
geo
strophic
wind
in
some circum
stances
an
d this
is
known as the
gr
adient
w
ind. N
ote th t
thi
s is not
hing to do w
ith vert
ical wind
gradien
ts
w
hich are
caused
by t
he friction
of the
g
round onthe w
ind and
cause us
so much
trouble
during
take-of
f and
landing
in
strong
winds.
In fa
ct,
the
grad
ient wi
nd is
the
wind
bove
the
influe
nce
of the s
urface fric
tion.
At low
levels
the
airf
low
is distu
rbed by
t
rees, build
ings and
hills,
so
th t
it
isne
ver steady
. In
strong
w
inds, gusts of
at lea
st the stren
gthof the
up
per wind shou
ld
be
expectedbecausesome
of the momentumof
th t
upper
wind
is
bro
ught down
bythe
effects
of
conv
ectionand
turbulen
ce.
Fig. 7
sh
ows a
low
a
nd
high
p
ressure
region. In the hi
gh
press
ure area
a
n
an
ticyclon
e)
the i
sobars
are
furt
her p r
t
indica
ting
mu
ch lighter
winds.
Wind
s alw
ays blow
clockwi
se
ro
unda
high.
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
19/31
2 THE
PRES SU R
E PATTERN
a.
m.
stable
air
12
a.m
.
mix ing
7 p.m.
stable
a
ir
a
.m .
12 a.m.
7
p.m.
su
rface
wind
5
Fig 8 Ch
anges in the
surface a
nd upper win
ds during
the d
ay. The
surface winds increa
se a
nd veer as the
thermal activity
c
auses
m
ixing
and decrease
and
back in the e
venings as
the a
ir becomes
stable.
swin
g
a
littl
e
close
r in
d
irection
to
the wi
nd
at
200
0 feet or so.
See
fig.
8.)
A
knowledge
able
D
uty
I
nstructor
at
a g
liding site will
all
ow for thi
s
in
evitable ve
eringand
strength
ening
o
f the
surface
w
ind
whe
nhe s
ites thepositi
on
o
fthe winches
in the e
arly
mo
rning.
For example,
if
the surface wind
is
a
light
westerly
at
8
a.m.
it
w
ill
most likely increase
by 5
knots or so a
nd
sw
ing
to a more
northweste
rly direct
ion later
in the morning.
Ho
wever,
t
his
e
ffect
ca
n be
complete
ly swam
ped by any rapid
chan
ge
in the
weather,
such as the app
roach o
f
a
de
pression.
The
surface wind
m
ay al
so increa
se and
ch
ange
in dir
ection
if
i
t
is
reinforc
ed by
the
sea
br
eeze. Th
e
timing of thi
s
inc
rease wi
ll
depend
on the
distance
the air has
to
come
from
the
coast. For
e
xample, a
bout 25 m
iles fr
om the
c
oastline
an onshor
e wind can
be
expected
to
in
crease
someti
me aft
er mid-day,
bringing in
cooler,m
oister airan
d
prob
ably spoiling
an
y the
rmal ac
tivity. S
ea
breezes
may
also advan
ce inlan
d against
a gene
rally lig
ht bu
t
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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THE PRESS
URE P
TTERN
evening
k
atabatic winds
Fig Anabat
ic an
d katabatic
winds.
very close
to the hill
slo
pe.
Anaba
tic winds
r
e
most
evid
ent on
sun-facing and le
e slopes
where the surfac
e gets the grea
test heat
-
ing
Conversely
in
the
evening
and
particularly
on
a
clear
night
the
eart
h
radia
tes
heat ou
t
into
space
coolin
g the groun
d
an
d the a
ir
c
lose
to it n hilly
country
this
cooler
air f
lo ws dow
n the
s
ides
of
any hill
s or mount
ains into the vall
eys.
This is
known asa k t -
b
tic w
ind.
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THE PRESSURE PATTE
R N
a
short distance up or downwind of these
areas the wi
nd can be
verystrong
indeed. Even if it is a light wind
on the g
round
prec
au
tions
alwa
ys
need
to
be taken to tie down gliders properly,
as the
position of
the wave
m
ay change without warning, resultin
g in a
sudden unexpectedly
strong
wind.
Lee
waves
are
explained
in
a
la
ter chapter.
W ind gra
dient
See
fig
10.) The
layers of air
c
lose to
the
ground are slowed down
by the friction betw
een
the
air
and
the ground. Obviously, the
stronger thewind, the grea
ter the change
in
the wind
speed which
is
possible due
to
this
effect.
The friction
will
also
be
greater
if
the
ground is rough or
covered
by
obstructions. If the
air is unstable
and
the area
isin the
lee
of obstructions such as trees o
r buildings,
some of
th
e
faster
moving air
fro
m
above
may be
brought
down
to ground level
caus
ing
m
omentary stronger winds
or gusts.
These r
apid changes in
wind
speed
combin
ed
with
the wind gra
dient
often
make
take-offs and landings critical
in
wind
y weather.
Flying
downinto
a
rapidly
decreasing wind
speed
during
the
final
s
tages
of
an approach causes a sudden loss of
airspeed
and
a
rapid
loss of height which
can be
dange
rous in a gli
der
or light
aircraft.
Th
is effect
is caused
by
the win gradient Turbulence near
the
ground al
ways
in
dicates rapid changes in
wind
speed and direc
tion and ex
tra flying speed is essential to ensure
that there is
adequate
speed
and
control
at
all
times.
F
ig
10
The wind gradien
t. Surface fr
iction
reduces
the wind
s
peed
n
ear the ground.
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22/31
DEPRES
S IONS
AND
AN
T ICYCLONES
In thepast few
years,
with
the developme
nts in
radar
and the
use
of
powerful computers and weather
satellites,
the
mechanisms
by
which
the
depre
ssions develop
have
been
studied
in much more
deta
il than was
ever possible
in the past.
It
is
now kn
own that movements occurring
in the upper
atmo
sphere
are the cause
of the high
and
low
pres
sure
systems
forming
near the
surface
whic
h dominate
our
weather.
W
ith
the
air
circulating
round
t
he low
pressure area,
air
mus
t
be
rising
and
being
removed
at
high
level
or
the low
would
fill
in
very quickly.
Similarly,
fora
high
pressure
system to
persist
there
has
t
o be a
des
cent of theair to m
aintain the higher pressure. It
is
these general
m
ovements
up
and do
wn, started
at
high level
w
hich
dictate
where
systems will
form and
which cause the
characteristic weather associated with
them.
b ).
Fig 11 Symbolsused on weather m
aps, a) A warm front,
b )
A cold
front, c) An occlusion.
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4
D
EPRE
SSIO
NS
ND
NT
ICYC
LONE
S
pola
r
reg
ions
meet
s
the
warm
er tr
opica
l air
along
a
fro
nt an
d
dep
ress
ions
form
alo
ng this
b
ound
ary.
It
is
kno
wn as a
wa
rm f
rontif th
e
wa
rmer
air
mas
s
is
mov
ing
fo
rwar
d
ov
er the
su
rface
and
a c
old
fron
t
if
i
t
is
th
e
coole
r
a
ir
mass
moving
forward
over
the
surface.
In
some
circumstances the
m
ovem
ent of
a f
ront
may
be
reve
rsed so
th t a w
arm
fro
nt be-
come
s
a
co
ld on
e.
Th
e maj
ority o
fo
ur
weat
her
sys
tems
fo
rm
out int
he A
tlant
ic
alon
g the
fro
ntal
zon
e
b
etwe
en t
he
Arc
tic
a
ir
m
ass and
thewa
rmer
Trop
ical mar
itime
a
ir
tothe
sout
h.
W
hen
two air
mas
ses
of
diffe
rent den
sity l
iesi
de
by s
ide
they
indu
ce
a str
ong
cu
rren
t
of air
t
o flo
w par
allel
and
onthe
c
old air
s
ide
of the
fron
t at
very
high
a
ltitud
es.
T
his
i
s
kn
own
as a
jet
s
tream
a
ndis
seve
ralm
iles
dee
p
a
nd
trave
ls at
s
peed
s
o
f
100
t
o 2
mi
les an
hour
.
See
fig.
12
a).)
D
istu
rban
ces in
this
up
per
flow
c
ause t
he
jet
strea
m
to ch
ange
di
recti
on a
nd
swin
g
fr
om
side
to s
ide
and
th
is re
sults
in area
s
w
here
thef
lowis
sp
eede
d up
or
slow
edd
own
. S
ee fig
s 12 b
) n
d
c
).) Wh
ere th
ef
low
s
peeds
u
p
b
ecaus
eof
di
verg
ence th
e pre
ssure
drops
sligh
tly indu
cing
an
upw
ard
flow
of
air.
This
con
tinu
ing up
war
d
m
ove
men
t resu
lts
in
a
cha
nge
o
f
pr
ess-
u
re
at
th
e
su
rface
w
hich
form
s the
d
epres
sion.
D
urin
g
the
de-
vel
opme
nt
of a
d
epres
sion
air
is
flow
ing
in
n
ear th
e su
rface
thou
gh not
fa
steno
ugh
to
stop
th
e
depr
essio
n de
epen
ing.
See fig .
a
)
j t
str m
co
ld a
ir
fron
t
w
armair
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24/31
a
DEPR
ESSIONS
ND N
TICYCLONE
S 5
upperconver
gence
j
st
surface pres
sure
rises
b )
upp
er diverge
nce
ac
celerating^
fast
surface
pressure
Fig
3 a)
U
pper converg
encecauses an
increase
in
pressure and
a
descending
fl
ow forming
an
area of high
pressu
re
anticyc
lone),
b )
Upper divergence
causes
a drop
in p
ressure and an upw
ard
f
low,
reduc
ing the surf
ace
pressure and
forming
an
area of
low
pressure
dep
ression).
Si
milarly, anticyclo
nes
start with
convergent
flow
slowing
down
the air
flow
a
t heig
ht and causing an
increase in pres
sure
a
nd
a
downwa
rd
mo
vement
of air. This res
ults in an
area of
hi
gher
pressure
andan
outward
flow at low
levels. See
fig. 13 a).
)
h
e life o
adepressio
n
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26
DEP
RESSION S ND
NTICYCLO
NES
See
figs 14
c) nd
d).)
B
ecause
of the eff
ects of the r
otation of
the
earth this
infl
ow
is div
erted i
nto a circu
lation
in
an
anti-clock
wise directio
n.
Wherea
s at
f
irst the
fro
ntal
surfa
cewas
more or l
ess stationary
th
e circu
lation starts a
wave
in
the
front. The
colddense a
ir moves
a
nd starts
to
u
ndercut th
e warmer
tropical
air formi
ng a
cold
front. At the
same
time the
warmer a
ir
is
moved
up ov
er
the
colder
air m
ass forming
a warm
front d)
Se
e figs 14 e) nd f).
) As
the air iscontinuo
usly
ex
tracted
at the
topof the
syste
m
so
the
pressure at
the sur
face continue
s to
d
rop
increa
sing
the ci
rculation and the
wind s
peeds. The nticloc
kwise
cir
culation
is gradually spread
up to highe
r levels
a
nd the upw
ard
movem
entof the w
armer m
oist air
causes
adiabati
c
cooling
a
nd
the
formatio
n
of
clouds rele
asingmor
e
h
eatenergy as
the mois
ture conde
nses in
to wate
r droplets.
The
cold front
moves mo
re quickly
than the w
arm one
and
catches i
t up
fo
rming an occlusi
on
with the w
arm
se
ctor at
th
e
surface
being
reduc
ed in
size.
S
ee figs
1
4 g) nd
h). The w
arm air con
tinues to
be
carrie
d
round
in
a
spiral
by
the
winds
but
the
upper
extractor
effect
has
cea
sed so
tha
t the
circulat
ion
runs
dow
n with the
surface l
ow
pr
essure
area gra
dually filling. Howev
er the
chang
es in
th
eupper
flow caused
by the
last stage
s of the life o
f this
d
epression
often
c
reate a
furtheraccelerati
on of the
flow
in
the
jet
strea
m. T
his sets
of
f the
p
rocess
again form
ing a
new depres
sion.
Som
etimes a whole
familyof dep
ressions f
orms givi
ng
a
per
iod
of almost
continuous
unsettled
or
bad weather.
On other
occa
sions the uppe
r flow w
ill
be
slowed d
own c using
an
increase
in
press
ure
a
nd a g
radual descending
flow
which
starts t
he
form -
tion
of an anticyclon
e a
nd more se
ttled w
eather.
The meand
ering of
th
e je
t
stream
to the
north and south
as
it
flo
ws
ac
ross
the Atlantic
largely
de
termines
the pattern
of our
we
ather and
thepositi
on
an
d inten
sity of the
areas of hig
h and
low
pressure.
a
rm
a
nd co
ld
fro
nts
Both warm and
cold fronts
m
ay vary
in
the w
eather cond
itions
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DEPRESS I ONS AND ANT I CYCLONES 27
fT
a)
(b)
c
(d )
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28
DEPRESSIO
N S ND NTICY
CLONES
Ns
warm sec
to r
Cs
As
20
0
front
rain or sn
ow
Fig
15
A
section
through
one type
of warm front.
4
500
thun
derstorms
and man
y of
the cha
racteristics
of a
cold
front.
Because of
the many
variations
in fronts an
d depres
sions, I have
not a
ttempted
to explain
all the
types of f
ront.
It
is importa
nt to rea
lisethat
it
is
very
rare
inde
ed to
have
bo
th
an active war
m
f
ront
and an
act
ive co
ld
fro
nt
on the same depress
-
ion.
This
means
that
if
there
is
a
large
amount
ofrainfall
onthe
warm front
there will be lit
tle
activity on t
he cold one.
The norma
l warm
front
has a sha
llow s
lope with
the
a
ir
risin
g
very grad
ually over a
dist
ance
of man
y hu
ndreds of miles.
This
lifti
ng p
roduces the
gradually
thickeni
ng layer of cloud wh
ich
results
in
steady ra
in
ne
ar the fron
t. See f
ig. 15.}
Usually
the
h
ighcloud cuts
off
the sun s heating
, but
if
the
high
cloud
arrives
after
the
thermal
activity
hasbecome
well
establish-
ed it is
not
uncommo
m for s
oaring condit
ions to pers
ist for
several
hours
after the sun
ha
s become
partially o
bscured.
Th
e warm
fr
ont
drizzle and
rain sometime
s
s
tart
sporad
ically,
but
once the
cloud
has
thi
ckened an
d continuo
us ra
in
begins
it is
usually the e
nd of
f
lying
for many hours
, ifnot
for
th
ewh
ole day.
Fig. 16
shows
a
ty
pical depression
with war
m and cold
fronts
and
their probable rain
areas.
In
reality
only
one of the
fronts
is
likel
y
t
o
be act
ive. The
first
indication
of
the appr
oaching
bad
w
eather will
be the h
igh cirru
s cloud, possibly
creat
ing a halo
around the sun
or mo
on. The
cloud will
grad
ually thicken to
altostr
atus
and
n
imbostratu
s,
with rain
beginn
ing
5-
10 ho
urs
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
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DEP
RES
SION
S
ND
N
TICY
CLO
NES
9
Fig
16
W
arm
a
nd
cold
fro
nts
on a
dep
ressi
on,
show
ing
are
as
of
pr
obab
le
rain a
nd th
e sur
face
win
ds.
in
th
e i
soba
rs
at the
fron
t ind
icat
es the
ch
ang
e in
win
d dire
ctio
n.
he w
arm
s
ector
be
twe
en th
e w
arm
and
c
old fro
nts
s typ
ified
by
relat
ively
wa
rm h
umi
d c
ond
ition
s
and
lo
w clou
d.
It
m
ay
pro
duce
driz
zle,
e
spec
ially
in
win
ter,
but ther
e
are ofte
n g
ood
b
reak
s in
the c
loud
al
low
ing
trai
ning
fligh
ts to
c
onti
nue
al
thou
gh
the
re wl
l
be
f
ewif
an
y the
rmal
s.
The
wa
rm
secto
r ma
y als
o h
ave
s
uitab
le cond
ition
s
foru
sabl
e
wav
es to f
orm
in
the
lee
o
f
hills
a
nd
m
oun
tain
s. Le
e wa
ves ar
ee
xpla
ined
late
r.
No
tice th
at
the
col
d
fron
t s a
re
lativ
ely
stee
p
w
edg
e
of
a
ir
sho
velli
ng u
p
th
e w
arm
erair
r
apid
ly.
See
fig 17
.)
The
pa
ssag
e of
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
29/31
3
EPR
ESSIO
NS
ND
NTIC
YCLO
NES
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
30/31
lying
Weather
lying Weather
Derek Piggott 's introduction
for gli er and
aeroplane
pilots. In
simple
and
accessible
terms
it
how
atmospheric phenomena can be used
to
flying performance
and
to
make
flying both
and
enjoyable.
into account advances in satellite and computer
the book describes pressure patterns,
cloud
and
how
soaring conditions can be used
to
a s
well
as
detailing
the effects of
depressions,
and local topography.
uncomplicated text
and
diagrams cover the syllabus
the British Gliding Association Bronze 'C Certificate
so
will
be
particularly relevant to those studying for
important
examination.
Piggott M E
was Chief
Flying Instructor a t Lasham
1953
to
1989. He
is very highly respected throughout
gliding world
and
is the author of several
standard
books.
7/25/2019 Understanding Flying Weather - Sample - Derek Piggot
31/31
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