The Mistral is normally experienced in Winter and Spring with the worst conditions
experienced in the seasonal
transition. It funnels between the Central Massif and the Alps which is why
it appears south of Lyon. It is a strong
cold north westerly. It is at it's strongest, where it has to funnel between
the Alps and the Cevennes. It is what is known
as a katabatic wind.
It is well researched and very predictable. It occurs when upper level northerly
winds are in phase with lower level
northerly wind flow.
To quote one of the experts :
"A favored sequence for mistral development is passage of a cold front
into the Gulf of Genoa in advance of a short
wave trough at upper levels. The upper- level trough produces southwesterly
flow aloft and positive vorticity advection
over the position of the cold front in the Gulf. This pattern leads to cyclogenesis
in the Gulf of Genoa. As the upper
level trough advances so that northerly winds aloft are in phase with the
northerly surface flow over Southern France,
strong mistral winds occur "
To put it in more layman's terms :
"A complex set of conditions gives rise to a Mistral, but it's essentially
a kind of föhn wind. When a high-pressure
system grips the plateau of the Massif Central, and a low-pressure area squats
over the Mediterranean, the cold mountain
air can begin to flow downhill, accelerating dramatically as it roars through
the gap of the Rhône valley, before it
turns and breaks eastward up the coast."
The presence of a cold vortex ( cyclone) in the gulf of Genoa ( to the east
of the Gulf of Lyon) in winter and early
spring is a certain sign of an impending mistral
As to duration, the local French say it always lasts an odd number of days.
The more scientific data suggest the peak
winds (> 30 knots) normally last a max of 65 hours.
Strength? They can average 100kph with windgusts of 150 kph. Highest speeds
measured in Gulf of Lyon.
Mistral come from what the Roman's called it "Magistralis"...Masterful.
Rhone flow rates
The flow rates on the Rhone are given not as a current
but as a volume/minute. The measure is cubic meters per second.
They are issued each day on the French Minitel system by the Compagnie Nationale
du Rhone. Minitel is a bit like Ceefax info that comes down your telephone
line to a small terminal.
The Minitel address for this information is 3617 inforhone.
There is a second general flood warning number at 3617 infocrues. This can
be accessed from a PC in France
via a landline. There is no way of doing this outside France as it is a premium
service that you are charged for on usage on your French phone bill. If you
are in France Minitel services can be accessed at most post offices. This
service gives actual debit numbers for all over the Saone/Rhone basin as well
historic data
The Compagnie Nationale
du Rhone have a web site. The
information on this site now replicates that obtainable on the old Minitel
service. To view their web site go to our links page.
In addition, for 0.12 Euros/min you can dial 08 20 10 10 20 you can receive
the latest information over a voicemail system. It is somewhat verbose but
you have a choice of English at the beginning.
The “debits” have to be interpreted carefully, as the current at the point
of measurement is calculated in the knowledge of the cross section of the
river at that point. The trouble is that as the river rises
the cross section increases too, so the conversion from flow (debit) to current
goes pear shaped when the river gets very high. Normally by that stage you
are not allowed to travel anyway!
There are four points that are used for measurement of these "debits"
that are then put on panels on the lockside at the measuring points. These
four points are at Ternay near Pierre Benite lock just below the confluence
of the Rhone and Saone at Lyon,
then Valence, Viviers and Beaucaire
.
| |
Ternay (Lyon)
|
Valence
|
Viviers
|
Beaucaire
|
| Approximate current at the PHEN level in kms/hr |
19
|
12
|
??
|
14
|
| Average annual flow cu m /sec |
1100
|
1400
|
1500
|
1700
|
| Plus Haute Eau Navigable (PHEN) cu m/sec |
2700
|
3400
|
3450
|
4100
|
| Minimum (10 days/yr) cu m/sec |
400
|
600
|
600
|
700
|
For an approximate idea of the current in kms/hr multiply debit by .0072
for Ternay/Pierre Benite and .0036 for Beaucaire and Bourg les
Valence
The debit measurement is also used to decide when to close the river to navigation.
Each section has a maximum debit figure allowable before they close the locks.
This corresponds to when the river has reached its PHEN level "Plus Haute
Eau Navigable" (highest navigable level). As you approach each lock you
will see markers on the side telling you where this level is.
The flow rates are the real measure of whether the river is navigable. The
following flow rates represent those that you can expect to experience if
the river is at its PHEN level.
|
Between
|
and
|
PHEN flow rate cu m/sec
|
|
PK1
|
PK86
|
2700
|
|
PK86
|
PK101.5
|
2750
|
|
PK101.5
|
PK124.3
|
3400
|
|
PK124.3
|
PK164.3
|
3450
|
|
PK164.3
|
PK215
|
3580
|
|
PK215
|
PK239
|
3650
|
|
PK239
|
PK248
|
3800
|
|
PK248
|
PK323
|
4100
|
Confluences
Understanding the geography helps. You can be fighting against the current
for hours and suddenly pick up speed. This can be because you have just passed
a confluence.The debit figures for the tributaries are available from the
3617 Inforhone site. Armed with this you can get some idea of what difference
you will feel in current as you pass the confluences. If the tributary flows
in from the East then flow rates are governed by rain in the Alps and the
Drome. If the tributary flows in from the West then flow rates are governed
by rain in the Ardeche.
| Tributary |
Confluence at PK |
East or West bank |
| Saone |
1 |
West |
| Isere |
101 |
East |
| Eyrieux |
126.5 |
West |
| Drome |
132 |
East |
| Roubion |
158 |
East |
| Ardeche |
200.5 |
West |
| Atgues |
218 |
West |
| Durance |
247.5 |
East |
| Gard |
269 |
West |
Historic flow rate data......Rhone Ternay Period 1920-1996
| Month |
Minimum |
Maximum |
Average |
| January |
259 |
5060 |
1298 |
| February |
297 |
4960 |
1350 |
| March |
254 |
3710 |
1240 |
| April |
290 |
3890 |
1160 |
| May |
274 |
4700 |
1005 |
| June |
219 |
3670 |
988 |
| July |
254 |
2870 |
863 |
| August |
231 |
2990 |
747 |
| September |
250 |
3720 |
737 |
| October |
193 |
4341 |
794 |
| November |
210 |
4730 |
1072 |
| December |
190 |
4490 |
1178 |
Historic flow rate data.* Period 1920-1990 ** Period 1997
| |
Isere ** |
Saone * |
Ardeche * |
Rhone * |
Rhone * |
Rhone * |
Rhone * |
Rhone * |
| Month |
Beau.Mont |
Couzon |
Sauze |
Perrache |
Ternay |
Valence |
Viviers |
Beaucaire |
| January |
337 |
718 |
100 |
579 |
1298 |
159 |
1687 |
1950 |
| February |
287 |
718 |
97 |
630 |
1350 |
1670 |
1772 |
2040
|
| March |
??? |
609 |
87 |
630 |
1240 |
1603 |
1717 |
2015 |
| April |
275 |
518 |
75 |
640 |
1160 |
1599 |
1709 |
1952 |
| May |
360 |
374 |
59 |
627 |
1005 |
1554 |
1649 |
1902 |
| June |
370 |
285 |
37 |
704 |
988 |
1573 |
1646 |
1822 |
| July |
439 |
192 |
14 |
672 |
863 |
1314 |
1349 |
1390 |
| August |
272 |
162 |
14 |
583 |
747 |
1057 |
1083 |
1103 |
| September |
229 |
206 |
36 |
529 |
737 |
1030 |
1079 |
1158 |
| October |
219 |
306 |
101 |
485 |
794 |
1101 |
1189 |
1432 |
| November |
233 |
503 |
103 |
568 |
1072 |
1411 |
1526 |
1868 |
| December |
283 |
609 |
82 |
567 |
1178 |
1490 |
1572 |
1878 |