Portes de l'enfer
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Portes de l'enfer clear sky chart from 2005-11-06 to 2024-01-12.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
16%
22707
8%
74921
4%
22656
22%
22656
8%
59828

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
8%
769
7%
2668
2%
766
14%
766
4%
2285
January11th to 20thNight
12%
725
10%
2535
3%
725
20%
725
5%
2037
January21st to endNight
14%
792
10%
2772
3%
792
24%
792
8%
2136
February1st to 10thNight
9%
720
8%
2520
3%
720
17%
720
6%
2002
February11th to 20thNight
9%
703
7%
2154
1%
700
16%
700
6%
1784
February21st to endNight
11%
592
12%
1776
4%
592
21%
592
9%
1421
March1st to 10thNight
12%
720
10%
2160
4%
720
20%
720
9%
1738
March11th to 20thNight
14%
720
9%
2073
4%
720
19%
720
8%
1687
March21st to endNight
14%
792
12%
2044
3%
792
20%
792
11%
1636
April1st to 10thNight
13%
720
9%
1800
2%
720
17%
720
9%
1509
April11th to 20thNight
17%
610
12%
1674
5%
610
22%
610
12%
1301
April21st to endNight
17%
540
12%
1620
4%
540
24%
540
11%
1260
May1st to 10thNight
20%
472
11%
1552
6%
472
27%
472
11%
1160
May11th to 20thNight
23%
354
13%
1416
9%
354
36%
354
11%
1009
May21st to endNight
19%
389
9%
1443
4%
388
29%
388
11%
1059
June1st to 10thNight
25%
360
5%
1260
3%
360
28%
360
11%
955
June11th to 20thNight
29%
360
8%
1260
5%
360
35%
360
14%
864
June21st to endNight
26%
360
6%
1260
4%
360
29%
360
11%
952
July1st to 10thNight
25%
350
6%
1268
3%
350
31%
350
13%
936
July11th to 20thNight
29%
348
6%
1392
8%
348
32%
348
11%
1020
July21st to endNight
28%
394
7%
1486
10%
372
31%
372
11%
1074
August1st to 10thNight
32%
358
9%
1422
7%
356
39%
356
13%
1025
August11th to 20thNight
29%
503
9%
1567
5%
503
33%
503
13%
1196
August21st to endNight
27%
675
11%
1863
6%
675
29%
675
17%
1410
September1st to 10thNight
25%
720
11%
1800
4%
720
29%
720
19%
1370
September11th to 20thNight
22%
720
9%
1956
3%
720
26%
720
15%
1516
September21st to endNight
22%
720
14%
1980
7%
720
29%
720
16%
1485
October1st to 10thNight
20%
720
14%
1988
7%
720
26%
720
14%
1539
October11th to 20thNight
14%
720
7%
2323
3%
720
18%
720
7%
1959
October21st to endNight
9%
792
6%
2574
2%
792
14%
792
5%
2181
November1st to 10thNight
8%
740
8%
2407
2%
740
15%
740
6%
2013
November11th to 20thNight
12%
774
8%
2529
5%
774
16%
774
6%
2139
November21st to endNight
10%
850
5%
2850
3%
850
12%
850
4%
2505
December1st to 10thNight
11%
850
5%
2850
3%
850
13%
850
4%
2507
December11th to 20thNight
11%
850
7%
3497
3%
848
16%
848
3%
2963
December21st to endNight
8%
925
5%
5182
3%
907
13%
907
2%
4195

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
17%14%6%10%8%45%5%5%6%21%
74527
Night
14%15%7%11%9%46%5%5%5%19%
75177
TransparencyDay
62%7%9%13%7%1
74229
Night
63%7%9%12%7%1
75010
SeeingDay
27%23%30%14%5%1
14195
Night
24%24%30%16%5%1
22717

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
8%17%9%13%12%6%7%6%6%5%11%
1493
11th to 20thDay
7%13%9%17%10%5%5%5%7%8%16%
1448
21st to endDay
49%7%13%10%7%8%7%8%8%19%
1584
February1st to 10thDay
5%11%8%14%12%7%12%7%5%6%13%
1440
11th to 20thDay
6%15%8%15%12%5%7%6%6%5%15%
1690
21st to endDay
6%13%8%15%10%49%6%6%5%18%
1480
March1st to 10thDay
8%14%6%14%12%6%6%5%5%5%20%
1797
11th to 20thDay
10%16%8%12%9%47%5%4422%
1886
21st to endDay
10%17%7%11%8%45%5%4424%
2312
April1st to 10thDay
16%19%8%11%9%4433418%
2160
11th to 20thDay
14%18%7%9%5%33445%28%
2235
21st to endDay
17%17%7%8%7%345%4425%
2340
May1st to 10thDay
19%16%5%7%5%35%45%5%27%
2424
11th to 20thDay
16%13%5%7%6%35%5%5%5%29%
2478
21st to endDay
22%11%5%7%6%35%45%5%26%
2832
June1st to 10thDay
24%12%37%7%35%5%6%6%21%
2700
11th to 20thDay
23%10%46%5%35%5%6%7%26%
2700
21st to endDay
27%9%5%8%6%35%446%23%
2700
July1st to 10thDay
25%8%47%6%5%45%6%8%22%
2650
11th to 20thDay
22%10%48%7%46%6%6%6%21%
2520
21st to endDay
21%11%48%7%5%5%5%6%7%22%
2744
August1st to 10thDay
22%12%49%7%5%7%6%6%6%17%
2506
11th to 20thDay
24%11%47%6%35%6%6%7%20%
2315
21st to endDay
22%11%47%6%36%5%5%7%23%
2489
September1st to 10thDay
21%12%47%5%24446%29%
2160
11th to 20thDay
20%13%26%4345%5%6%31%
2003
21st to endDay
24%16%45%5%345%36%27%
1980
October1st to 10thDay
21%18%6%6%5%34435%25%
1968
11th to 20thDay
19%26%8%10%6%34335%14%
1636
21st to endDay
19%24%9%9%6%3333417%
1782
November1st to 10thDay
15%20%11%14%7%3432417%
1671
11th to 20thDay
14%19%9%16%10%3433314%
1645
21st to endDay
10%21%9%15%11%5%5%44412%
1330
December1st to 10thDay
12%19%11%13%13%5%6%43411%
1330
11th to 20thDay
12%14%8%13%14%5%6%35%316%
1668
21st to endDay
13%16%8%14%8%47%5%46%15%
2431
January1st to 10thNight
8%15%10%18%12%5%8%7%7%5%6%
2668
11th to 20thNight
7%12%8%16%12%6%8%7%7%6%13%
2535
21st to endNight
5%10%6%12%11%7%10%9%9%7%14%
2772
February1st to 10thNight
6%10%6%14%12%7%11%8%6%6%14%
2520
11th to 20thNight
7%16%8%14%11%5%8%7%6%413%
2152
21st to endNight
5%14%8%15%10%49%6%6%5%17%
1776
March1st to 10thNight
9%13%7%14%10%5%8%7%6%6%17%
2150
11th to 20thNight
10%15%8%11%10%6%7%6%4419%
2072
21st to endNight
9%15%7%13%10%5%5%44423%
2044
April1st to 10thNight
15%18%9%12%10%4443419%
1800
11th to 20thNight
15%16%6%10%6%345%5%426%
1674
21st to endNight
14%16%7%9%7%35%436%26%
1620
May1st to 10thNight
20%14%6%8%6%34335%29%
1552
11th to 20thNight
16%14%5%7%6%445%46%29%
1415
21st to endNight
23%11%5%6%5%34445%29%
1442
June1st to 10thNight
21%12%5%7%6%45%5%5%6%24%
1260
11th to 20thNight
18%9%46%6%35%5%5%6%34%
1260
21st to endNight
20%10%48%6%45%6%5%7%26%
1260
July1st to 10thNight
21%9%36%6%446%6%8%26%
1305
11th to 20thNight
20%9%47%5%45%7%6%8%26%
1440
21st to endNight
20%10%36%7%35%5%6%6%29%
1574
August1st to 10thNight
17%11%46%6%25%5%7%7%30%
1432
11th to 20thNight
21%11%47%5%35%5%6%7%26%
1567
21st to endNight
21%13%36%5%3446%8%28%
1863
September1st to 10thNight
21%12%46%5%2445%5%32%
1800
11th to 20thNight
21%16%46%5%33435%29%
1956
21st to endNight
20%17%46%5%3334433%
1980
October1st to 10thNight
16%18%5%7%5%25%445%28%
1988
11th to 20thNight
21%25%8%8%5%3333417%
2323
21st to endNight
21%24%9%9%6%3333316%
2574
November1st to 10thNight
14%21%10%15%7%34335%16%
2407
11th to 20thNight
13%17%9%16%11%4434414%
2529
21st to endNight
10%18%8%18%14%6%6%43310%
2850
December1st to 10thNight
11%18%10%17%13%6%5%44410%
2850
11th to 20thNight
13%15%8%15%12%6%7%5%5%311%
3531
21st to endNight
12%19%9%14%10%5%6%45%314%
5236

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
67%9%7%10%6%
1494
11th to 20thDay
64%7%6%12%9%2
1448
21st to endDay
56%7%7%12%15%3
1584
February1st to 10thDay
66%8%7%9%8%2
1440
11th to 20thDay
67%7%8%7%9%2
1699
21st to endDay
63%6%6%15%7%3
1480
March1st to 10thDay
63%7%6%11%10%3
1800
11th to 20thDay
62%7%7%10%11%2
1886
21st to endDay
61%6%8%13%9%3
2312
April1st to 10thDay
69%6%8%9%6%3
2160
11th to 20thDay
58%5%8%16%10%2
2235
21st to endDay
61%6%8%10%12%2
2340
May1st to 10thDay
58%7%8%13%12%2
2424
11th to 20thDay
56%6%10%19%9%
2482
21st to endDay
58%7%8%18%8%
2832
June1st to 10thDay
61%8%10%15%5%1
2700
11th to 20thDay
57%8%12%15%7%1
2700
21st to endDay
63%7%10%12%8%
2700
July1st to 10thDay
57%9%14%16%4
2577
11th to 20thDay
59%10%13%13%5%
2445
21st to endDay
60%10%11%16%4
2591
August1st to 10thDay
65%10%11%11%2
2492
11th to 20thDay
60%10%15%12%4
2315
21st to endDay
58%9%13%15%4
2489
September1st to 10thDay
54%8%12%18%8%
2160
11th to 20thDay
51%7%14%19%7%1
2003
21st to endDay
58%7%10%16%9%
1980
October1st to 10thDay
61%6%9%16%7%1
1968
11th to 20thDay
72%8%8%10%2
1636
21st to endDay
72%48%12%41
1782
November1st to 10thDay
71%6%7%12%3
1671
11th to 20thDay
73%6%6%10%41
1645
21st to endDay
75%7%7%9%2
1330
December1st to 10thDay
75%6%6%8%5%
1330
11th to 20thDay
70%5%5%9%10%
1668
21st to endDay
64%8%8%9%9%1
2431
January1st to 10thNight
73%7%6%8%5%1
2668
11th to 20thNight
66%7%7%10%7%3
2535
21st to endNight
58%10%8%11%11%1
2772
February1st to 10thNight
62%9%7%12%8%2
2520
11th to 20thNight
66%9%8%8%8%2
2154
21st to endNight
64%6%6%12%10%2
1776
March1st to 10thNight
64%6%7%13%7%2
2160
11th to 20thNight
64%8%8%11%8%2
2073
21st to endNight
61%6%10%12%7%3
2044
April1st to 10thNight
69%5%7%10%7%2
1800
11th to 20thNight
57%8%8%16%10%1
1674
21st to endNight
60%6%7%16%9%1
1620
May1st to 10thNight
57%7%10%13%10%2
1552
11th to 20thNight
54%8%10%17%10%1
1416
21st to endNight
56%7%10%16%10%1
1443
June1st to 10thNight
58%8%12%16%5%1
1260
11th to 20thNight
50%7%13%18%9%3
1260
21st to endNight
56%9%12%16%7%
1260
July1st to 10thNight
52%11%15%17%4
1268
11th to 20thNight
52%13%14%16%6%
1392
21st to endNight
53%10%13%18%5%
1486
August1st to 10thNight
50%13%17%17%3
1422
11th to 20thNight
53%12%16%14%5%
1567
21st to endNight
52%10%14%21%3
1863
September1st to 10thNight
52%8%10%21%9%
1800
11th to 20thNight
58%6%13%18%5%1
1956
21st to endNight
54%6%11%20%8%1
1980
October1st to 10thNight
58%6%11%16%8%1
1988
11th to 20thNight
70%7%8%10%4
2323
21st to endNight
73%5%7%9%5%1
2574
November1st to 10thNight
71%6%8%11%4
2407
11th to 20thNight
71%6%8%8%5%
2529
21st to endNight
76%7%8%6%3
2850
December1st to 10thNight
77%6%6%7%41
2850
11th to 20thNight
73%6%5%7%8%1
3532
21st to endNight
70%6%8%8%6%1
5236

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
19%29%42%8%2
180
11th to 20thDay
21%25%36%14%4
162
21st to endDay
14%26%35%19%42
176
February1st to 10thDay
24%40%29%5%11
160
11th to 20thDay
22%33%32%9%31
378
21st to endDay
28%29%28%11%31
346
March1st to 10thDay
22%27%36%12%3
420
11th to 20thDay
26%25%31%11%6%1
420
21st to endDay
27%24%30%12%6%
462
April1st to 10thDay
31%27%26%12%31
420
11th to 20thDay
31%23%30%9%42
412
21st to endDay
29%28%29%10%41
420
May1st to 10thDay
31%22%30%11%6%
423
11th to 20thDay
24%20%34%17%41
447
21st to endDay
27%25%27%14%5%1
592
June1st to 10thDay
30%20%28%17%5%
630
11th to 20thDay
29%20%24%18%7%1
630
21st to endDay
29%17%27%20%6%
630
July1st to 10thDay
20%19%35%20%5%1
572
11th to 20thDay
23%18%38%15%6%
444
21st to endDay
23%17%36%19%5%
488
August1st to 10thDay
24%17%33%17%7%2
446
11th to 20thDay
21%16%32%21%8%2
439
21st to endDay
26%20%26%14%11%3
495
September1st to 10thDay
28%17%29%17%6%3
450
11th to 20thDay
26%18%30%17%7%2
450
21st to endDay
28%23%25%13%7%4
450
October1st to 10thDay
27%25%27%12%7%2
450
11th to 20thDay
35%26%25%9%41
367
21st to endDay
33%28%26%11%2
396
November1st to 10thDay
34%28%28%7%2
365
11th to 20thDay
26%26%32%10%5%2
337
21st to endDay
28%32%29%9%21
180
December1st to 10thDay
29%21%30%14%6%
180
11th to 20thDay
28%33%29%5%32
180
21st to endDay
32%25%29%10%5%1
198
January1st to 10thNight
19%28%38%13%2
769
11th to 20thNight
21%31%30%15%41
725
21st to endNight
14%25%39%17%41
792
February1st to 10thNight
17%34%34%13%31
720
11th to 20thNight
21%29%34%12%4
703
21st to endNight
18%30%36%13%31
592
March1st to 10thNight
19%28%36%13%3
720
11th to 20thNight
24%29%30%11%5%1
720
21st to endNight
21%25%33%16%41
792
April1st to 10thNight
27%27%30%12%4
720
11th to 20thNight
27%23%29%17%4
610
21st to endNight
27%22%31%15%5%
540
May1st to 10thNight
27%18%29%21%6%
472
11th to 20thNight
21%15%33%24%6%
354
21st to endNight
30%19%27%18%5%
389
June1st to 10thNight
24%13%32%23%8%
360
11th to 20thNight
21%18%27%27%7%1
360
21st to endNight
22%16%30%26%6%1
360
July1st to 10thNight
23%16%32%22%7%
350
11th to 20thNight
21%15%29%26%7%1
348
21st to endNight
22%10%33%25%10%
394
August1st to 10thNight
16%13%32%29%9%1
358
11th to 20thNight
25%11%29%22%12%1
503
21st to endNight
24%16%29%19%11%1
675
September1st to 10thNight
27%17%27%21%8%1
720
11th to 20thNight
28%18%28%16%9%2
720
21st to endNight
27%19%26%18%8%1
720
October1st to 10thNight
26%20%29%18%6%1
720
11th to 20thNight
33%25%24%10%7%1
720
21st to endNight
36%25%24%11%4
792
November1st to 10thNight
31%26%30%9%31
740
11th to 20thNight
27%29%28%10%6%1
774
21st to endNight
24%31%30%11%31
850
December1st to 10thNight
26%28%26%14%5%
850
11th to 20thNight
25%30%25%15%4
850
21st to endNight
27%28%28%13%41
935
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