Download by Arnett Stanley Dennis, B.Sc. Being a thesis

INTERACTIONS BEl'WEEN CONTINUOUS AND SHOWERY PRECIPITATION
by
Arnett Stanley Dennis, B.Sc.
Being a thesis submitted to the Dean of the Faculty of Graduate
Studies and Research, McGill University, in partial fulfilment
of the requirements for the degree of Master of Science.
April, 1953
Physics Department
MOntreal, Quebec
AC Kl.~OHLEDGMENTS
This thesis was prepared under the direction of Professor J.S.
Marshall, whose sound advice and continued interest are deeply appreciated.
Professor K.L.S. Gunn read several chapters and offered valuable suggestions.
Advice, regarding the printing of photographs, was received from Mr. M.P.
Langleben.
Meteorological records, which were used in many phases of the work,
were made available by the Meteorological Division of Canada.
MT. H.M.
Hutchon, Officer-in-Charge of the Main Meteorological Office, Montreal
Airport, was especially helpful in providing necessary data.
The film
records used were prepared by various members of the Stormy 'o/eather
Research Group, NcGill University, using equipment provided by the Defence
Research Board of Canada.
To aIl of the above, and to Professor
author is most grateful.
~hrshaIl
in particular, the
ABSTRACT
Examination of radar records shows that snow trails frequently
occur around the tops of showers.
can be initiated by snow trails.
Evidence has been sought that showers
Cumulus clouds with tops above the
freezing level often fail to precipitate.
Theoretical studies show
that snow trails could act as seeding agents for cumulus clouds, entrainment providing the mechanism whereby snow enters a cloud.
Certain observed radar patterns suggest strongly that showers were
produced in this manner.
As a check, polar diagrams can be used to
organize observations made on different bearings.
Those constructed
show showers occurring in regions of snow trails more frequently than
elsewhere.
This is further evidence that snow can act as a seeding
agent in cumulus clouds.
TABLE OF CONTENTS
Chapter
Page
Title
1
INTRODUCTION
1
2
SHOWERS
4
:3
CONTINUOUS PRECIPITATION
10
4
POSSIBLE !l10DES OF INTERACTION
13
5
VERTICAL SECTION
18
6
PLAN PATTERNS
25
7
SUMMARY AND CONCLUSIONS
30
REFERENCES
31
APPENDIX
PATTElù~S
DUE TO SEEDING
Chapter l
INTRODUCTION
During World War II it was found that radar echoes were returned
from precipitation particles.
The first theoretical treatment of this
new subject, which proved remarkably complete and accurate, had been
published by Ryde in 1941.
Fol1owing the re1ease of radar equipment
at the end of the war, research in radar meteoro1ogy was undertaken
in several countries.
It was brought out early that radar echoes from showers and from
continuous precipitation are different.
On scopes showing a plan view,
areas of continuous precipitation appear as patches of quite uniform
echo.
Showers, on the other hand, show as distinct small cells which
are often arranged in larger-scale patterns.
(Fig. 4.3).
On radar
pictures in vertical section, showers appear as intense, near1y vertical
echoes of the order of 3 - 10 miles width.
Echoes from continuous
precipitation are generally more diffuse and extensive.
often show shallow slopes as a result of wind shear.
The patterns
(Fig. 1.1).
With continuous rain another distinctive characteristic appears.
This
is a region of intense echo, at or just below the freezing level, known
as the bright band.
An excellent account of the radar patterns associated
with various meteoro1ogical situations is given by Ligda in the "Compendium of Meteorology" (1951).
-2-
/
./
20
40
RANGE
(MILES )
VERTICAL SECTION BEARING 215 0 , 1·{)NTREAL AIRPORT, 2134 GeT
3 AUGUST 1951
A trail of continuous snow appears as a streak sloping from 20,000'
at extreme left to approximately 12,000' at 35 miles.
Shower echoes
appear at 25 to 35 and at 40 to 50 miles.
Using film records of radar weather observations, members of the
Storrny Weather Research Group at HcGill University have studied both
showers (Gunn, 1947; Rigby (Smith), 1948) and continuous precipitation
(Marshall, et al., 1952).
In some cases the two types of echo appeared
in close conjunction, as in Fig. 1.1, leading to suggestions of possible
significant interactions. (Marshall, 1953).
11arshall's first suggestion
is that the lightning discharges, which are sometimes observed by radar
around the tops of thunderstorrns, occur between the thunderclouds and
surrounding trails of falling snow.
The second suggestion is that snow
can act as a seeding agent to initiate showers in cumulus clouds.
The investigation of these and other possible modes of interaction
between showery and continuous precipitation is the object of this
thesis .
In Chapters 2 and 3 the atmospheric processes leading to
-3-
different precipitation forms are reviewed in the light of radar
observations.
The possible interactions are dealt with in the later
Chapters.
The 3 cm. radar set in use at the McGil1 Radar Weather Observatory,
Montreal Airport for the past 4 years is not suitable for the detection
of lightning echoes.
As a result, no new data have been obtained, and
Marshall's study remains the most complete to date.
Patterns resulting from seeding of cumulus clouds by snow are
derived for simple cases in Chapters 5 and 6.
Radar pictures of similar
form are presented as evidence that the process does occur.
The importance of continuous precipitation in modifying the air
around developing cumulus clouds is also noted.
The conclusion is reached
that, in general, the effect of passing trails of continuous precipitation
is to promote shower activity.
Plan diagrams, prepared from available
records and showing cases in which shower activity i5 concentrated in
regions of snow trails, are included.
This concentration could be
explained on the basis of either seeding of the cumulus clouds or
moi stening of the envi ronme nt al air.
It is generally assumed by forecasters in temperate zones that
showers will not occur until cumulus tops reach a layer with a temperature of -IOC or less.
If seeding by snow were to prove important, the
figure -10 would have to be modified toward higher temperatures in cases
where higher clouds aie present to produce snow.
The moistening effect
acts by increasing the height cumulus clouds can reach in a given situation;
but the modification of forecasting rules to account for this effect would
be more complex.
-4Chapter 2
SHO\fERS
Sho\ver acti vi ty is associated vli th currents of rising air.
The
word unstable is used to describe air in which vertical currents tend
to develop; and stable to describe air in which vertical motions tend
to be damped out.
In general, unstable air is characterized by cU1lluli-
form clouds, a possibility of showers or thunderstorms, and bU1llpy flying
condi tions.
Stable air is characterized by smooth flying conditions and,
frequently, layers of smolce or haze.
Any clouds present tend to be of
the stratiform class.
The stability of the atmosphere is a function of the vertical temperature distribution.
The IIlapse rate" is defined as
bT
Y - - bz·
Since temperature usually decreases with height, the lapse rate is generally
positi ve.
A small body of air (referred to as a parcel), when lifted a short
distance, will continue to rise if it is warmer than its new surround1ngs.
To study this factor it 1s necessary to lcnow the temperature changes that
occur when a parcel of air moves up or down to a region of different
pressure.
For dry air the temperature change rate proves to be very
nearly a constant.
given by
r
~
It is known as the dry adiabatic lapse rate and is
30 C/lOOO ft.
(Hewson and Longley, 1944)
The adiabatic lapse rate for moist air i5 a function of the water
vapor content.
However, for unsaturated air it is, for practical purposes,
the same as the dry adiabatic lapse rate.
-5\-le
have, for unsaturated air,
y
~
r
y= r
y>r
air stable
air neutral
air unstable
In the stable case a parcel of air displaced ei ther upwards or dO\ffiWards
tends to return to its original position.
In unstable air a displaced
parcel is accelerated in such a 'ltray as to displace i t further.
In the
neutral case a displaced parcel is not accelerated in any direction.
If the ascending air is cooled to i ts dew-point, condensation begins.
The de'ltr-point i tself decreases slightly wi th height (He\.Json and Longley,
1944).
Once condensation begins, cooling of the rising air is sloued by
the release of latent heat of condensation.
As the saturation vapor
pressure is a function of pressure and temperature, the amount of heat
released varies 'ltrith the circumstances.
be assigned to
ri,
Therefore no constant value can
the saturated adiabatic lapse rate.
The stability of
air with respect to rising (or sinking) currents of saturated air is
determined as follO'ltTs.
r'
stable
y = r'
neutral
y
y
If
r
~
Y"l
r' ,
<::
> ri
unstable
the air i s stable 'ltd th respect to currents of dry
air, but unstable with respect to saturated air.
This situation is
called conditional instability.
A straightforward application of the above criteria is known as
the parcel method of forecasting. (Austin, 1951).
convective currents
Carl
The height to 'ltrhich
rise through an undisturbed environment is
determined from thermodynamic diagrams.
If the rising currents pass
-6-
'00
"":'
IIQ
t
'--'
Iq
ca:.
':)
\1)
800
II)
lU
~
Il..
o
-20
T
E
M
P
(c)
FIG. 2.1
SIMPLIFIElJ THERMJDYNAMIC
DIAGRAl\f SHOWING SAMPLE
VERTICAL SOUNDING .
A-B UNSTABLE
B-C STABLE
C-D CONDITIONALLY UNSTABLE
their condensation level, cumulus clouds are to be expected.
The height
at which the rising currents become colder than their environment is
taken as the top of the cumuliform clouds.
If cumulus clouds develop to a thickness of several thousand feet,
showers become possible.
Following the acceptance of the Bergeron theory,
which attributes precipitation to the presence of ice in clouds, it was
generally assumed that showers never occurred until the cloud top passed
-7-
the freezing level.
Cases to the contrary have been reported in recent
years. (Hoaghton, 1951).
Turbulence has been suggested as the factor
responsible for rain formation in these cases. (Marshall, 1952).
On
the other hand, many cumulus clouds with tops above the freezing level
fail to pro duce raine
This point will be dealt with further in a later
chapter.
Thunderstorms are likely to occur when large amounts of condensed
water are present and the temperature at the cloud top is below -20 C.
(Byers, 1951).
Studies made during the Thunderstorm Project show that
each thunderstorm consists of a number of cells.
Each cell undergoes a
life cycle which can be divided into three stages.
The first, the cumulus
stage, is characterized by an updraft throughout the celle
The second,
or mature stage, begins when the first rain falls out of the cloud.
In
this stage downdrafts develop, at least in the lower half of the celle
the dissipating stage weak downdrafts are present throughout the celle
.....
t
30,000
1
~
t
t
J
-+
---
t
· . . r- -
JO,OOO
~
t ~
~
J.
~
\".
JI
E
,
__O_C_
~
../
\ ': \
--:
/'/lAJ,J"
\,"
,t
.'
,
5
. . i -- ,-'
*
ri
1
,
....
1.
l,
,
1
'"
MIL ES
FIG. 2.2
CROSS-SECTION THROUGH
THUNDERSTORM CELL IN MATURE STAGE
In
-8-
One defect of the parc el method of forecasting is its assumption
of an undisturbed environment.
The "slice" method eliminates this error
b,y assuming that envi ronme nt al air sinks adiabatically in compensation
for the rising current.
(Petterssen, 1940).
The slice method still does not allow for exchange of air between
the rising current and i ts environment.
entrainment.
(Austin, 1951).
This mixing is referred to as
"Observed inflow rates in American thunder-
storms show that the cumulus cloud which develops into a thunderstorm
entrains environmental air at a rate ••••• which doubles i ts mass as i t
rises through a pressure decrease of 500 mb."
(Byers, 1951).
This mixing
tends to reduce the difference between in-cloud and environmental temperatures.
Furthermore, if the entrained air is dry, some cloud particles
evaporate into it, causing further cooling of the cloud.
It has been noted
that thunderstorm tops sometimes ascend in a series of steps, each step
appearing as a turret, or proturberance, on the cloud.
Apparently each
turret helps the following ones b,y increasing the moi sture in the air which
must be entrained b,y the succeeding turret. (Byers, 1951).
Where \.n.nd speed increases wi th height, entrainment is greater on the
upwind side.
On the dowm,1ind side it is possible for liquid cloud to be
detrained fro!ll the field of rising motion.
(Halkus, 1949).
The concept
of entrainment provides an explanation for the commonly observed fact that
shower development is less probable when the air aloft is dry, or when
marked wind shear is present.
If cloud air, cooled at a rate greater than the saturated adiabatic
lapse rate during its ascent, is dragged downward by falling rain, it will
warm at only the saturated adiabatic lapse rate.
This is because saturation
is very nearly maintained b,y evaporation from the raine
Thus the descending
air will reach a position at which it is colder than the surrounding cloud-
filled air.
From there it will continue downward with accelerated
motion to reach the ground as an outflow of cold air.
This is the
explanation for the temperature fall and gusty winds observed even
in local thunderstorms.
(Byers, 1951).
-10-
Coopter 3
CONTINUOUS PRECIPITATION
An air mass is an extensive body of air ..Ti th approximately uniform
properties in any gi ven horizontal plane.
Here "extensi ve" means that
the body of air covers at least several hundred thousand square miles of
the earth's surface.
AlI the processes mentioned in the previous chapter
occur wi thin any gi ven air mass; and the circulations descri bed are on a
relatively small scale.
For example, a typical thunderstorm covers only
about 10 sq. miles.
In addition to convective overturning, air masses are subject to
rising or sinking motions as a . . lhole.
These vertical motions are less
rapid than those associated with convection.
Ascending motion is generally found in regions of low pressure as
a result of convergence.
Wide-spread ascent is also found on the wind-
ward slopes of mountain ranges.
When two air masses having different temperatures, and hence different
densities, occupy adjacent areas, the cold air mass forms a wedge undercutting the
WarIn
air.
The surface of separation between the two air masses
is called a frontal surface.
Its slope is a function of the temperature
difference between the air masses and of the wind shear through the surface.
Slopes of frontal surfaces usually lie in the range .004 - .02.
The line of intersection between a frontal surface and the earth's
surface is known as a front.
If the cold air is receding, its trailing
edge is called a warm front; if cold air is advancing, its leading edge
is called a cold front.
Various other terms are used to describe discon-
tinuities in the slopes of frontal surfaces, intersections between frontal
surfaces, etc.
-11-
The temperature transition between two air masses is not actual1y
concentrated at a surface, but rather over a band of perhaps 50 or 100
miles width.
This region of large temperature gradient is referred to
as the frontal zone.
For practical purposes, the frontal zone is regarded
a.s part of the cold air.
The frontal surface is then defined as the surface
separating the frontal zone and the warm air.
In any frontal situation,
convergence, if present, results in lifting of the warm air mass involved.
(Ref. above paragraphs: Byers, 1944; Hewson and Long1ey, 1944).
-IOC
-
-
-
-
z
oc
~
WARM AIR
'--'1
...,:
::r.
Zoo
100
M/Lf;S
FIG. 3.1
CROSS-SECTION THROUGH A FROlIT AL SYSTEH
The weather conditions associated \dth any of the lifting processes
described depend mainly upon the moisture content and degree of stability
of the 1ifted air.
If it is stable (and sufficiently mOist), precipitation
occurs as light rain or snow.
Recent radar studies indicate that in such a case the precipitation
originates as snow in compact generating ce11s at altitudes usually between
12,000 and 30,000 ft. (Marshall et al., 1952).
these cel1s move
\.JÏ th
In some cases at least,
the wind at their own level.
A falling snowflake
[.ssumes, at any time, a horizontal ve10city equa1 to the wind ve10city of
the air around i t.
It fo11ows that, if the wind varies wi th height, snow-
flakes are disp1aced horizontal1y with respect to their generating ce11s.
-12-
Theoretical studies have shown that a continuously generating element
produces a pattern, or trail, which maintains its shape and moves with the
velocity of the generating element.
This holds whether or not the cell
velocity is equal to the wind at the cell height.
If the cell does move
with the wind, the trail slope at any level is inversely proportional to
the difference in wind between that level and the generating level.
Further-
more, if the reference axes are considered as moving with the generating
cell, the trajectory of an indi vi dual snowflake becomes identical wi th
this trail.
These patterns have been observed by means of radar.
The fact that
sorne trails extend down tbrough as much as 20,000 ft. shows that an
individual cell can exist for over an hour.
.After a fall of 2 or 3 miles,
the trails usually become nearly horizontal (typical slope, 0.05).
The
trails have a vertical extent of the order of 3,000 ft. due to variations
in fall velocity for snowflakes of different sizes.
The figures quoted
give Il miles as the length of the surface precipitation band due to one
generating element.
Plots made during this project show that snow trails
commonly have widths of the order of 10 miles.
Thus a single cell can
produce precipitation over an area of more than 100 sq. miles at one time.
Precipitation from neighbouring cells frequently merges to cause the large
areas of continuous rain or snow often found on weather maps.
We shall
use the word "continuous" to describe precipitation originating in such
generating cells, even though it is sometimes discontinuous in space at
the surface and always is so aloft.
vIhen low-level temperatures are above freezing, the snow mel ts just
below the freezing level.
This causes an intensification of the radar
echo known as the bright band.
1952).
(Austin and Bemis, 1950; Gunn and East,
The bright band is useful in determining the origin of precipitation,
-12a-
20, 000 1/
10,000
1. . ' : : ' ; i ! i i I U ! ! ! ! ! ! J
RANGE (~lILES)
FIG. J.2
SNO 1 TRAILS IN VERTICAL SECTION
Bearing 2600 from I-bntreal Airport, 2052GCT, 2 November, 1951
as shower echoes do not show it, except for a weak effect in their later
stages (Harshall, 1952).
This residual effect may be due to snow falling
from the upper part of the cloud after the updrafts die out.
~Ti th
the
exception of this situation, the bright band is a definite indication of
snow above the freezing level.
m.;:
20,0001 ~__~~=-
10,000 1
______~~~__~~~~~!!~
40
20
RANGE
(MILES )
FIG. J.J
BRIGHT BAND AND cmrrINUOUS RAIll
Bearing 1800 from Montreal Airport, 22J8GCT, 8 June 1952
-1)-
Chapter 4
POSSIBLE HODES OF INTERACTION
\-Ihen an air mass being lifted by any means 1s unstable as 'WeIl as
moi st, the pattern of gradual ascent is complicated by the presence of
convective currents.
Such developments occur quite frequently, especially
as sorne air masses are rendered more unstable by lifting.
A common result
of such instability is the presence of cumulonimbus clouds embedded in
the cloud deck above a 'Warm frontal surface.
The cumulonimbus clouds,
moving with a velocity roughly equal to that of the 'WarID air wind, cause
local intensifications to move across an otherwise rather uniform precipitation area.
10,000'
l' ,
'.,1
Il
1.
•
l
'
Il
l
, ,1 .' .
, ",1' •
. 'l'·
1
,',
","
'
1
FIG. 4.1
CUMULONIMBUS EMB:IDDED IN LAYER CLOUD DECK
It is also possible to have a cloud system giving continuous precipitation with broken cloud belo'W causing local intensifications.
The inten-
sification here could be due to further growth of precipitation particles
in the 10'Wer cloud, or to independent sho'Wer development in the 10'Wer levels.
These patterns move with the wind at the level of the 10'W clouds.
If the
10'W clouds have much vertical development their velocity is given by the
wind roughly mid'Way bet'Ween the bases and tops.
Various patterns due to
combinations of continuous precipitation and sho'Wers have been discussed
by previous writers. (Ligda, 1952).
-14-
10 , 000'-
,
'
:~:~
1 ~
1
t_ (
• Il. ,
"
\ '.
lf ( l'
1 ! .:
,
:
FIG. 4.2
STRATOCUMULUS BELOW LAYER CLOUD DECK
Although convection can occur in either the cold or warm air in a
frontal situation, it should be noted that convective currents cannot
penetrate a well-defined frontal zone.
In the frontal zone the lapse
rate is smaller than in either air mass, and can even be negative through
a layer one thousand feet or more thick.
Therefore the frontal zone is
a region of marked stability.
We have already noted that the entrainment of dry air reduces the
development of cumulus clouds.
In cases where isolated snow generating
cells at high levels pass over a relatively dry air mass, localized bands
of increased humidity will result.
Cumulus clouds forming in these areas
will tend to reach a larger size than those surrounded by drier environmèntal air.
The probability of showers should increase accordingly in
these areas.
Above the freezing level evaporation from snow can only bring the
relative humidity (with respect to water saturation) up to that required
for ice saturation.
The relative humidity required for ice saturation is
a function of temperature, reaching a minimum of 85% at -12 C for spherical
ice crystals.
For the common types of snow crystals, the saturation point
is presumably higher than for ice spheres.
If air initially has a relative
hummdi ty near 100%, snow can dry i tout slightly through sublimation on
the flakes.
This drying affect, however, is limited to air with a relative
-15-
humidity between 85 and 100%.
Since cumuli often push up through 1ayers
with relative humidities of 40 or 50%, the moistening effect can be much
more pronounced.
With snow fal1ing into 1ayers containing cumulus c1ouds, variations
in the normal atmospheric e1ectric field are possible.
Researches in
atmospheric e1ectricity have shown that a great variety of situations
are found in the atmosphere at different times. (Gunn, 1951).
Fal1ing
snow might be expected to possess a positive potential with respect to
its environment, as the upper atmosphere is positive with respect to the
earth. (Gish, 1951).
Sorne investigators have measured positive charge
on snow; but others have found snow that possessed free negative charge.
(Gunn, 1951).
In showers and thunderstorms the potentials vary with
time and position, particular1y in the lower parts.
However, a general
tendency towards concentration of positive charge on the tops of thunderstorms has been noted.
(Gish, 1951).
In a recent paper Marshall has cal1ed attention to the possibility
of lightning discbarges between snow trai1s and the tops of showers.
Lightning echoes observed on a 10 cm. radar set at Ottawa on 5 July 1948
were attributed to this process.
Unfortunate1y, the low sensitivity of
the 10 cm. set did not al10w the snow trai1s to be recorded.
The shape
of the trai1s was determined approximate1y, using the position of the
associated bright band and the upper winds. (}furshal1, 1953).
Ligda (1950) has also reported lightning echoes on 10 cm. sets, but
only rare1y on 3 cm. sets.
No 1ightning echoes have been observed on
the 3 cm. set operated by the McGi11 Radar Weather Observatory since
1949.
However, the 10 cm. sets general1y do not detect the snow trai1s
in these cases.
The possibi1ity of obtaining echoes from showers, snow
-16-
FIG. 4.3
A plan view of precipitation at Ottawa 5 July 1948. North is
at top; range markers at 20 and 40 miles. Frontal thunderstorms
and continuous rain extend from 2200 clockwise to 020 0 • Air-mass
thunderstorms show from 0800 to 1200 • The snow melting to give
bright band NW originated E, the trails extending past the airmass and frontal thunderstorms. Range-height indicator showed
lightning echoes in sno\.f-filled regions around the thunderstorms.
and lightning on one picture therefore appears small.
In spite of the fact that rain sometimes falls from clouds with
above-freezing temperatures throughout, a more important consideration
in temperate zones is the large number of cumuli with tops above the
freezing 1eve1 wInch never produce precipitation.
Observations on 399
c10uds in New Mexico during 1950 revealed no precipitation for c10uds
whose tops had temperatures between 0 and -12 C.
Even with cloud-top
temperatures as low as -25 C, a significant proportion of the observed
. clouds failed to return a radar echo.
1951).
(Braham, Reynolds and Harrell,
In such cases the cloud tops consist of super-cooled water droplets.
The actual percentage of such cases appears to be a function of the dropsize distribution, types of freezing nuclei available, and the degree of
turbulence present.
-17-
A noteworthy result of the
abov.~
investigation was the fact that,
for various temperature ranges below -12 C, the percentage of clouds
showing radar echoes was greater on generally eloudy days.
The authors
have suggested that cirrus produced by one storm might seed neighbouring
clouds, which had not reached the height at which ordinary glaciation
would oeeur.
Similar effects could result from cirrus left over from
storms on the previous day.
(Braham, et al., 1951).
The suggestion that seeding by continuous snow can initiate showers
in cumulus clouds has been put fo~ward by Marshall (1953).
It could
only be effective where a cloud top had pushed at least a short distance
above the freezing level.
Since many cumuli undergo nucleation in any
event when the temperature at their tops falls to -12 or -15 C, the
seeding process could be important over a narrow range only.
The possibility of such a process and the patterns which would result
from it will now be considered in sorne detail.
-18-
Chapter 5
VERTICAL SECTION PATTERNS DUE TO SEEDING
The patterns resulting from the seeding of cumuli by snow trails
could be very complexe
However, consideration of a few hypothetical cases
shows the pattern type to be expected.
Consider a case in which wind direction is invariant wi th height,
but wind speed increases linearly with height.
A radar set pointed in
the direction of the wind "sees" a snow trail as a parabolic figure, such
as curve a in Fig. 5.1.
Assume the freezing level to be 10,000 ft., a non-
precipitating cumulus cloud based at 5000 ft. and topped at 15,000 ft. and
moving with the 10,000 ft. wind, a snow-generating cell at 24,000 ft. and a
wind shear of 2.5 mi hr-l per 1000 ft.
To allow simplification of the
diagram (Fig. 5.1), the reference axes are considered as moving with the
10,000 ft. wind.
The cumulus thus remains fixed on the diagram.
Snow-
flake trajectories calculated are the actual trajectories relative to the
cloud.
The cumulus cloud is subject to shearing effects and currents
inside it, but we regard it as a fixed objecte
The shape of the snow
trail is a function of rate of fall for the flakes and the wind shear
only, and so is independent of the choice of reference axes.
A calculation
of trajectories using a fall velocity of 4 ft sec- l reveals the pattern
at successive times.
As the cumulus cloud does not return an echo until it precipitates,
the first observed effect will be its shadow appearing as a gap in the
snow trail.
In cases where the radar set is not pointed parallel to the
wind shear (which is in the same direction as the wind for this example),
the gap will not appear.
Echo will be received from snow on either side
of the cumulus being investigated, the "shadow" of the cumulus extending
from i t at an angle to the radar beam.
-19-
o(V'\
~
//---/'
//71~
Il
~
~
/~
11
/
~
~
1
/
1
/
1
1
1
· 0
(\/
_J
-
-20-
The region around the cloud top is one of divergence with rising
currents inside the cloud.
It is therefore unlikely that any snow could
enter the cloud froID above.
Fig. 5.1 (curve b) shows that snow comes in
contact with the upwind side of the cloud, the point of contact descending
as time passes.
Since this brings snow into regions of maximum entrainment,
snow is likely to be carried into the cloud.
The snowflakes and the frag-
ments broken off them by turbulence may grow by condensation, coalescence,
or by some combination of the two processes.
In any event, the development
of a shower appears likely.
Once a shower is in progress, attenuation by rain becomes important,
if the wave-length used is as short as 3 cm.
The part of the trail beyond
the shower, as seen froID the radar set, could easily be lost from view.
Film records of observations made at the McGill RadarWeather Observatory, Montreal Airport, from 1949-1952 have been examined for evidence of
interaction between continuous and showery precipitation.
R.H.I. pictures
(radar pictures in vertical section) resembling stage b or stage c of Fig.
5.1 have been found for l day in 1951 and Il days in 1952.
The me te oro-
logical data for each of these cases have been collected and used to check
movements of precipitation areas, slopes of snow trails, etc.
Estimates of the temperatures at the tops of the cumulus clouds
involved have been made by two methods.
In the first, the degree of
instability present was estimated with the aid of available tephigrams.
Cloud top temperatures were then taken directly from the tephigrams.
In
sorne cases, an unstable layer near the surface was capped by a weak or
shallow stable layer just above the freezing level, with another unstable
layer aloft.
In such a situation, sorne cumulus will stop at the first
stable layer, but others may rise through it and the second unstable layer.
-21-
Seeding would be of more importance for the first class of clouds, and
the temperatures given are for such clouds.
The second method consisted of determining from the radar records
the heights of young showers apparently caused by seeding.
It was realized
that this falls a little short of the cloud tops, but as the methods contain
considerable error no correction was made for this particular factor.
The
temperatures at the measured heights were then taken from the upper-air data.
The values obtained by the two methods agree reasonably well in each
case.
Of greater significance is the fact that all the values determined
fall in the range -3 C to -13 C.
These are reasonable limits for the
range over which seeding could assume importance.
TABLE 5.1
Date
Temp.
First 11ethod
1951 Aug. 3
1952 May 29
June 6
June 10
July 19
July 23
July 26
July 27
July" 28
Aug. 4
Aug. 21
Aug. 29
Cumulus
Tops (C)
Second Method
-8
-5
-4
-13
-5
-Il
-5
-8
-5
-5
-3
-6
:i*
-6
-6 to -12
-5
-7
-10:t
-3*
:t
-4
-5
-3
ü
-8
-8
doubtful
only pictures of later stages available.
VERTICAL SECTION B~~ING 1700 , 1756 GCT 29 May 1952
-22-
As an example, the situation of 29 May 1952 will now be considered.
A vertical section on a bearing of 1700 at 1756 GCT shows a shower at
]0-34 miles and a snow trail out to 28 nilles.
A plan view, obtained by
combining observations on different bearings, and a summary of the weather
data are given on page iii of the Appendix.
The
sno~
was being formed
just north and northeast of the radar set at an altitude of about 22,000
ft.
The trail shown, if projected, would cut the shower around 12,000 ft.
The hourly reports at Montreal Airport showed the base of the low
c10uds to be around 4000 ft.
As the shower shown reached 18,000 ft., it
is reasonable to assume it was moving with the 11,000 ft. wind.
shear from Il,000 1 to 18,000' was from approximate1y 1700 •
The wind
Therefore the
"shadow" of the shower extended from it towards the north, producing the
gap in the snow trai1 from 28 to ]0.5 miles.
The absence of echo beyond
the shower is attributed to attenuation.
The only sequence showing the deve10pment of a shower, from its ear1y
stages to maturity, on a snow trail was obtained on 26 July 1952 (Fig. 5.3).
A smal1 shower echo, embedded in a snow trai1, first appeared at 16-21,000
ft.
The top showed only minor fluctuations, but the shower deve10ped
rapidly dovmwards.
Rain at the ground, as indicated by radar, began 8.5
minutes after the first echo was detected.
Sorne pictures were taken on
reduced gain but no bright band was detected at any stage.
Successive positions of the center of the shower were plotted on a
polar diagram.
Its ve10city was found to be 54 mi. hr.-1 from 2500 •
the change in wind "Ji th height was smal1, no steering 1eve1 can be
assigned.
(See page viii of Appendix).
The component of the thermal wind (change in wind with height)
perpendicular to the radar bearing of 2400 was very sma~.
This means
that the sides of the shower were near1y vertical, as shearing effects
As
-_...-_------. -.-_
-
..........
.......
- 23-
5.3
FIG.
VERTICAL SECTION THROUGH DEVELOPING SHOWER
26 July 1952
BEARING 2400
1911 GCT
snow
21,000'
.(--- 16,000'
(
10
20
30
BEARING 2380
1915 GCT
21,000'
10,000'
10
20
30
BEARING 2310
.1919 GCT
~
10
l,
20
RANGE (MILES)
30
-24-
were smal1.
The radar was pointed very near1y into the wind, and the
bearing changed slight1y to fo11ow the shower as it progressed.
It is
therefore conc1uded that changes in the echo represent changes in the
shower and not a movement of different parts of the shower across the
radar beam.
A sample R.H.I. picture for each of the 12 days listed is given
in the Appendix.
For days such as 27 July 1952, where only pictures of later stages
were available, care was taken ta avoid mistaking a cumulonimbus anvil
for a snow trail.
Byers (1951) states that anvils and other lateral
extensions of cumulonimbi usually do not return radar echoes.
However,
he does not rule out the possibility.
In the usua1 case of wind increasing with height, the only direction
in which anvils extend an appreciable distance from cumulonimbus clouds
is downwind.
In most cases it was possible ta find pictures showing an
extensive trail on bath sides of the shower.
In other examples, plots
on polar diagrams were used ta confirm the existence of snow trails
from generating elements at higher levels.
-25Chapter
(l
PL./IN PATTERNS
If a seeded cloud develops into a shower which passes through a
normal cycle and dissipates, a plan view of the patterns produced by
snow seeding can be calculated.
Various conceivable complications, such
as the seeding of a second cloud by the mature shower, will not be discussed
here.
B
D
A
FIG. 6.1
;
MOVEl.fIDlt OF SHOWER. LlNE DUE TO SEWING
1
Consider an air Ii mass containing numerous cumulus clouds wi th tops
1
1
above the freezing level, the clouds moving wi th the wind veloci ty CV)
at a height roughly ~dway between their bases and tops.
1
If a snow trail
1
moves across the
vdll move with
j
re~on
,
;
and seeds a particular cloud, the shower produced
velo~ityv.
If the seeding occurs at point A (Fig. 6.1)
!
and the shower lasts for a time t, it will be dissipating as it passes B.
\
In a usual situation t would be of the order of 1 hour.
By time t, the point of contact between the trail and cumulus clouds
will have moved
vn. th
the veloci ty of the snovl-generating element to C.
We shall consider the case where this veloci ty is equal to W, the wind at
the height of the snow generating element.
New showers will be forming
-26-
at C and the line BC will be marked by a line of showers in different
stages.
At 2t, the shower line will extend from D to E.
although indi vi dual showers move wi th veloei ty
has a veloei ty equal to
v,
It is seen that,
the system as a whole
w.
This mechanism may be responsible for sorne observed cases in whieh
air-mass thunderstorms oceur in linesrather than as seattered storms,
and for whieh no explanation has been offered.
Out of 31 days studied in
Ohio during the Thunderstorm Projeet, with no fronts or squall lines
present, the thunderstorms lined up on 7 days. (Byers , 1951).
The length (t) of the shower line is a function of t, v and w.
g
Let
be the angle between w and v, and w and v be the magnitudes of w and v
respectively.
Then
g
will usually be an acute angle, and v be less than w.
t :: t [w2 + v2 - 2vw cos
Q1
1/2
For the order of magnitudes involved, consider v
2300 , w :: 60 mi hr- l from 270 0 , and t :: 50 minutes.
t ::
t [3600 + 625 -
= 25 mi
hr- l from
Then
3000 x .77] 1/2 mi. :: 37 miles
If vdnd direction is invariant with height, cos g :: l and the equation
simplifies to
t :: t (w - v)
The angle cr as shovm in Fig. 6.1 1s given by
cr :: cos-1 [
2 2 - v cos g
(w + v - 2vw cos
g)
1/2]
As t is not known exaetly for a partieular situation, the function cr
would provide a bet ter check than t as to whether or not a shovler line was
the result of snow seeding.
observati ons.
cr would also he easier to determine from
-27-
Since snow trails are of the order of 5 - 10 miles in width, a band
of showers should be expected, rather than a sharp line.
Variations in
the heights of the cumulus clouds and different rates of development for
the showers would make the boundaries of the showery region diffuse.
The photographie records made by the McGill Radar Weather Observatory
during 1951 and 1952 all show vertical sections.
Even when a vertical
section shows showers developing along a snow trail, the possibility exists
that showers are occurring randornly, and that the density of showers is as
great in other sectors not affected by snow.
To check this, radar data,
obtained on different bearings of the set, were plotted on polar diagrams.
It was found that satisfactory plots could be made if successive
observations differed in aximuth by less than 15 0 •
The rotation of the
beam renders uncertain the location of the boundaries of precipitation
regions by an amount
rœ[.
Here r is the distance of the observed object
from the radar, w is the angular velocity of the rotation of the beam, and
T is the exposure time being used in photographing the scope.
This error
often completely outwèighs the error in position introduced by the beam
width.
However, only a certain amount of detail can be plotted, no matter
how close together the successive bearings are.
If an azimuthal sweep of
3600 takes more than 3 to 4 minutes, an accurate picture of the situation
is hard to obtain.
Changes in the situation introduce a discontinuity in
the diagram at the bearing on which the sweep begins and ends.
The construction of such a diagram requires 30 minutes to l hour,
depending upon the accuracy and amount of detail required.
The result is
more useful than an ordinary photograph of a plan-position indicator radar
scope.
Echoes are obtained on the sample bearings from all elevations up
to 25 0 •
A blind spot extends from the origin to about 10 miles, its area
being 1/25 of the region within range of the radar.
- 28FIG.
6. 2
,s
1
T
/
1
1
1
1
1
1
/
1
1
/7
T
1
.~'-'If..
1
T
JO
"
()
B.B.
\
\
i
20 mi.
2700 r---~~~~~~r--H~~+------r-------------~------------------
90°
1
J
IS
T
/3
\
\
---,
\
"-
'\
'\
13
T
\
,
\
1000
2125-2130 GCT, 3 AUGUST 1951
~
10,000 FT. CONTOURS OF SHOWERS
./8.8.') OUTLINE OF AREAS OF CONTINUOUS PRECIPITATION
~ ~ - - OUTLINE OF SNOW TRAILS - HT. TOP ABOVE "T" AND HT. BASE BELOW
-'
(r.
"
CONVECTIVE CELL IN EARLY STAGE
-29-
Such diagrams have been prepared from 811 suitable sequences recorded
on the 12 days listed in Table 5.1. (See Fig. 6.2).
completed for each day except 19 July 1952.
included in the Appendix.
At least l has been
Each of the other 11 days is
It should be noted that no attempt to correct
for attenuation has been made in plotting the observations.
In most of the cases studied, shower activity is definitely eoneentrated in the regions affeeted by snow trails.
This could be due either '
to seeding by snow or to moistening of the air around the cumuliform clouds.
On the other hand, such juxtaposition of showers and eontinuous precipitation, in a partieular case, might be due to orographie factors, or to
the synoptic situation.
-)0-
Chapter 7
SUMMARY AND CONCLUSIONS
Snow trails occur frequently above the freezing level and MOye with
velocities generally different from those of cumulus clouds.
Examination
of radar records shows that snow trails and showers occur together quite
frequently.
Now, cumulus clouds with tops weIl above the freezing level
often fail to undergo glaciation.
Evidence has been sought that entrain-
ment of snow from the trails into cumulus clouds induced shower formation.
Certain sequences, such as 26 July 1952, do suggest strongly that showers
were caused in thi s way.
Plan diagrams can be used to organize observations in vertical section,
giving a general view of a situation.
Those constructed show concentrations
of showers in regions affected b,y snow trails.
This provides further
evidence that showers can be initiated by snow entrained into cumulus clouds.
An empirical rule, commonly used by meteorologists in the temperate
zones, states that shower activity commences when cumulus tops extend 4,000
ft. above the freezing level.
-10 C.
This corresponds to a temperature of about
Tabulation of temperatures at the tops of showers associated with
snow trails shows sorne as warm as -) C.
It appears that, with snow present,
showers become possible more quickly than when cumulus clouds develop in
an otherwise clear atmosphere.
This suggests that some consideration of
high clouds anticipated should be made in deciding whether or not cumulus
clouds, expected to extend just above the freezing level will produce
showers.
-31REFERENCES
1. AUSTIN, J.M. (1951): "Cumulus Convection and Entrainment", p. 694,
Compendium of Meteoro1ogy, American Meteoro1ogical Society.
2. AUSTIN, P.M. and BEMIS, A.C. (1950): "A Quantitative Study of the
Bright Band in Precipitation Echoes", Journal of Meteoro1ogy, 1: 135.
3. BRAHAM, R.R., REYNOLDS, S.E. and HARRELL, J.H. (1951): "Possibilities
for Cloud Seeding as Determined by a Study of Cloud Height versus Precipitation", Journal of Meteoro1ogy, ~ : 416.
4.
BYERS, H.R. (1944): "General Meteoro1ogy", McGraw-Hi11 Book Company, Inc.
5.
BYERS, H.R. (1951): "Thunderstorms", Compendium of Meteoro1ogy, p. 681.
6. GISH, O.H. (1951): "Uni versal Aspects of Atmospheric Electrici ty",
Compendium of V~teorology, p. 101.
7. GUNN, K.L.S. (1947): "Radar Echoes from Rain Showers", M.Sc. thesis,
MbGi11 University.
8.
GUNN, K.L.S. and EAST, T.W.R. (1952): "Microwave Properties of Precip-
i tation Particles", p. F-1, Proceedings of the Third Radar Weather Conference,
McGi11 University.
9. GUNN, R. (1951): "Precipitation Electricity", Compendium of Meteorology,
p. 128.
10. HEL-lSON, E.W. ,and LûNGLEY, R.W. (1944): "Meteorology Theoretical and
Applied", John Wi1ey and Sons, Inc.
Il. HOUGHTON, H.G. (1951): "On the Physics of C10uds and Precipitation",
Compendium of Meteoro1ogy, p. 165.
12. LIGDA, MeG.H. (1950): "Lightning Detection by Radar", Bulletin American
Meteoro1ogical Society, 11 : ,279.
13. LIGDA, M.G.H. (1951): "Radar Storm Observation", Compendium of Meteorology, p. 1265.
14. LIGDA, M.G.H. (1952): "The Horizontal Hotion of Small Precipitation
Areas as Observed by Radar", p. D-41, Proceedings of the Third Radar
Weather Conference, }~Gi11 University.
15. MALIruS, J .S. (1949): "Effects of Wind Shear on Some Aspects of Convection", Transactions of the American Geophysical Union, .JQ : 19.
16. MARSHALL, J.S. (1952): "Brussels Meeting of the Joint Commission on
Radar Meteorology", p. 205, Proceedings of Conference (1951) on Water
Resources, Illinois State Water Survey.
17. MARSHALL, J.S. (1953): "Frontal Precipitation and Lightning Observed
by Radar", Canadian Journal of Physics, 1l : 194.
-32-
18. MARSHALL, J.S., LANGLEBEN, M.P., and RIGBY (SMITH), E.C. (1952):
"Precipitation Trajectories and Patterns", Stormy Weather Research Group,
McGil1 University, Scientific Report MW-8 under U.S.A.F. Contract No.
AF-19(122)-217.
19. PEl'Tms8 EN , S. (1940): "Weather Analysis and Forecasting", McGrawHill Book Company, Inc.
20. RIGBY (SMITH), E.C. (1948): liA Study of Vertical Motions in Radar
Patterns of Rain", M.Sc. thesis, McGill University.
21. RYDE, J .W. (1941): IIEcho Intensi ties and Attenuation Due to Clouds,
Rain, Hail, Sand and Dust Storms ll , General Electric Company (British),
Report No. 7831, 1941.
APPENDIX
In this appendix, a sample vertical section and plan diagram, and
a summary of the meteorological data are given for each of the days
studied, which showed evidence of interaction.
On the plan patterns the fo1lowing conventions were used:
10,000 1 ft. contours of showers
areas of continuous precipitation
1.<.
outline of trails aloft, height
T
JO
of top above liT" and ht. base
below
echo of shower which has not yet
reached the ground, ht. top above
dash and ht. base below.
The code used for recording wind and temperature data can best be
exp1aiped by an examp1e. 10/250/20/3 is read as fo11ows:
The wind at
10,000 1 above sea-1eve1 was from 2500 true at 20 knots, and the temperature was +.3
c.
( i i)
3 August 19 31
2134 GCT
Rt. in
thsds. ft
30
20
10
~
20
Ra nge (mile s)
2 1)+1 ~- 215 1 GGT
270
\
,
180
WINDS J..ND TEMPERATURES :
SFC /180/10 / 19
20/240 /60/-1 5
10/ 2 50 /42/0
301245/90/- 37
SYNOPSIS:
A col d front preceded by moist Ma ritime Polar air
and followed by a strong outbreak of Continental.Pola r air
passed Mont rea l about 2200 GGT. Tr opical air ass ociated
with a frontal system along the Atlantic coast wa s present
at hif,h levels.
tiii)
29 May 1952
1756 GCT
Rt.
10,000-
20
Range (miles)
1753-1755 GCT
12
T
17
1
1
WINDS AND
"
TEr,PERA TURES :
SFC/180/3!19
20/210/50/-18
SYNOPSIS:
2.1
T
10/230/36/- 3
30/220/100/-40 (app.)
A weak occlusion east of Montreal \'las moving east'ward.
Montreal "'Tas in a Vleak circula tion of moist Polar air, wi th
a strong southwesterly flow of Tropioal air aloft.
(i v)
6 June 1952
BEA RING 315 0
2146 GOT
Ht.
40,000,
20,000'
20
Range (miles)
2141-2143 GOT
40
o
t,
T
...
'NINDS AND
--
'5
,.
,
f
TE~lPERATURES:
SFO /2 30}10 /20
20/300/40 /-16
10/290/28/0
30/300/73/-:-40
SYNOPSIS:
A deve10ping loV! pressure area moved eastward acrOdS
southern ~uebec during the day. The associated frontal system
was qui te comp1ex. rrhe thunderstorms in the Montreal area
were apparent1y caused by the passage of a trough of uns table
tropical air aloft.
( v)
10 June 1952
1959 GCT
BEARmG 355
0
Ut.
1~0
,000 ,.-
20,000' -
20
Range (miles)
40
2239-2244 GCT
~'INDS
AND TEMPERATURES:
SFC /250 /B /19
20/240 /3B /-23
10/255/28/-2
30/250/55/-44
SYNOPSIS:
A deep low pressure area was centreè east of James Bay.
Montreal was covered by a westsouthwesterly flow of unstable
Continental Polar air with Maritime Polar air aloft.
(vi)
19 July 1952
0330 GCT 20 July
Rt.
40 ,000' 20,000' -
(miles)
No sequence of pictures suitable for the construction ot
a plan diagram was taken.
VITNDS AND
TEr.~ERA.TURES:
SFC /310/8/23
20/290/601-11
10/)10/.30/5
30/290/70/- 36
SYNOPSIS:
A cold front extending in an east-west line across
northern New England was causing a few showers in southern
Quebec. Continental Polar air was present north ot the front
with Tropical air above the frontal surface.
( vii)
23 July 1952
2316 GOT
Ht.
20,000' _
10,000 '
Range (miles)
2251-2255 GOT
,7
T
'7 1
.-T"1-'--_
,
\1
8. B.
/
.
WINDS AND TBMPERATURES:
SFO/200/2/28
20/245/55/-10
SYNOPSIS:
10/250/38/7
30/245/65/- 32
The precipitation in the Montreal area occurred in
a stream of Tropical air just ahead of a cold front moving
eastward from Ontario. The front was followed by a moderately
strong flow of Continental Polar air.
(viii)
26 July 1952
BEARING 235 0
1915 GCT
Ht.
20,000'10 ,000'-
(miles)
1944-1946 GCT
WlliDS AND TEMPERATURES:
SFC /220/7/27
20/260/40/-8
10/250/35/10
30/240/751- 33
SYNOPSIS:
A high pressure area centred south of Nantucket vms
causing a southwesterly circulation over the region. A shallow
layer of warm modified Polar air was present from the surface
to approximately 5,000', with very warm Tropical air aloft. A
slow1y moving cold front extended in a northeast-southwest
line near Ottawa.
llX'
27 Ju1y 1952
BEARING 040 0
1952 GOT
Ht.
20,000' _
10,000' 20
Range (miles)
40
1916-1924 GOT
360°
\
\
,
,. 1
t 1
270t----eaI-mt..----+----+-------I09Q0
WINDS AND TEMPERATURES:
SFO /200/6/26
20/270/25/-12
10/280/20 I!+
30/270/45/- 37
SYNOPSIS:
A cold front, which was near Ottawa on July 26, lay
Just south of Montreal by 1500 GOT on the 27. It continued
to move very slowly southeastward. There VIas a shallow layer
of modified Polar a ir over Montreal with Tropical air above
the frontal surface.
( x)
28 July 1952
BEARING 340 0
0015 GCT 29 July
lIt.
20,000' 10.000' 10
Range (miles)
0040-0046 GCT, 29 July
.....
...-.,
"-
8. 8.
\.
VoTINDS AND TEMPERATURES:
SFC/220/6/22
20/260/55/-13
10/260/37/!+
30/260/90/- 35
SYNOPSIS:
A quasistationary front lay along the New England
coast with Maritime Tropical air to the south and greatly
modified Polar air to the north. A deep low pressure area
moving eastward aeross northern Quebee ~~s followed by a
strong outbreak of fresh Continental Polar air. The passagb
of the cold front was aceompanied by rain and thunderstorms
in the Montreal area.
( ~i)
4 August 1952
BEARING 300 0
1841 GCT
Rt.
20,000 t
-
10,000 t
-
20
Range (miles)
40
1623-1629 GCT
360°
WINDS AND Tll1.1PERATURES:
SFC /060/10/19
20/235/40 /-11
SYNOPSIS:
10/210/25/5
30/260/40 /- 35
A low pressure area was centred over Lake Huron w1th
a warm front running eastward from the centre to Portland,
Maine. Southern Q,uebec was in an easterly flow of Continental
Polar air with a southwesterly flow of Maritime Polar and
Tropical air above the frontal surface.
(xii)
21 August 1952
1756 GCT
3&:..RING
280 0
Ht.
20,000''''
10,000'20
Hange (miles)
40
1312-1324 GCT
o
/,-
--'
/-.-")
/'
~-r--","-
(
/'
270 0
,"
)
,1....
b,...../~.)=---"!,.,-f;;;,r'--.nr--+----->,c--r=-:-+-----t 0900
/
WINDS AND TEMPERATURES:
SFC/120/5/18
20/230 /32/-16
10/220/25/2
30/210/501- 38
SYNOPSIS:
A deep low pressure are a was moving across northern
Quebec towards Labrador. An occlusion extended southward trom
the low to a point about 350 miles north of Montreal;from
there a co~d front ran southwestward to near Toronto. A trough
of warm air aloft preceded the cold front by about 100 miles.
Unstable 'l'ropical air in this trough gave rise to extensive
shower and thunderstorm development in southern Quebec.
\x~li)
29 August 1952
JjEARING 110 0
2306 GCT
Ht.
20,000'
10,000'
.Range (miles)
2022-2029 (JCT
o
1'lINDS AND TEMPERf-\.TURES:
SFC /300/8/25
20/260/301-13
10/280/25/5
30/260/45/- 33
S".{NOPSIS:
A cold front on a line through Anticosti, Ottawa
and Chicago a t 1530 CrCT passed Montreal about 2200 UCT. r:t.the
air to the south was warm modified Polar air, while the air
behind the front was fresh Continental Polar air. 'l'ropical
air was present above 12,000'.