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 2025-10-04.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	16% 14% 5% 12% 7% 6% 5% 4 5% 5% 22%	126427
	Night	12% 15% 6% 14% 7% 6% 6% 4 5% 5% 20%	123012
Transparency	Day	61% 7% 10% 14% 7% 1	129853
	Night	63% 7% 9% 12% 7% 1	127674
Seeing	Day	27% 23% 30% 14% 5% 1	16532
	Night	24% 24% 30% 16% 5% 1	24823

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	7% 17% 8% 19% 11% 8% 8% 4 5% 4 9%	3536
	11th to 20th	Day	4 10% 7% 19% 15% 14% 5% 4 5% 5% 12%	3570
	21st to end	Day	3 7% 4 19% 11% 8% 5% 6% 10% 6% 21%	3960
February	1st to 10th	Day	5% 12% 6% 15% 12% 10% 10% 5% 5% 6% 14%	1600
	11th to 20th	Day	6% 16% 8% 18% 10% 6% 7% 5% 6% 5% 15%	1884
	21st to end	Day	5% 13% 8% 16% 8% 7% 7% 5% 6% 5% 19%	1650
March	1st to 10th	Day	9% 19% 7% 17% 8% 5% 4 3 4 4 21%	4476
	11th to 20th	Day	7% 21% 7% 14% 7% 5% 5% 3 5% 5% 21%	4746
	21st to end	Day	8% 19% 6% 13% 7% 5% 5% 3 4 4 26%	5822
April	1st to 10th	Day	15% 21% 6% 13% 7% 5% 3 2 3 4 20%	2400
	11th to 20th	Day	14% 18% 7% 11% 4 4 3 3 4 5% 27%	2487
	21st to end	Day	16% 18% 6% 10% 6% 4 3 4 4 4 25%	2600
May	1st to 10th	Day	22% 17% 4 9% 5% 4 4 3 4 4 22%	6110
	11th to 20th	Day	20% 12% 3 9% 5% 5% 5% 4 5% 6% 25%	6318
	21st to end	Day	20% 13% 4 9% 6% 5% 5% 3 5% 6% 24%	7224
June	1st to 10th	Day	23% 13% 3 8% 6% 4 5% 4 6% 6% 21%	3000
	11th to 20th	Day	23% 12% 3 7% 5% 4 5% 4 6% 6% 25%	3000
	21st to end	Day	27% 10% 4 10% 5% 4 4 4 5% 5% 22%	3000
July	1st to 10th	Day	23% 10% 4 9% 6% 5% 5% 4 6% 6% 20%	6686
	11th to 20th	Day	18% 10% 3 9% 6% 6% 6% 5% 7% 7% 22%	6360
	21st to end	Day	15% 9% 3 7% 5% 5% 7% 5% 7% 7% 31%	6926
August	1st to 10th	Day	18% 10% 3 10% 5% 6% 7% 5% 7% 7% 22%	4563
	11th to 20th	Day	23% 11% 4 9% 5% 7% 5% 5% 6% 6% 18%	4186
	21st to end	Day	22% 11% 4 9% 5% 5% 4 3 5% 6% 26%	4515
September	1st to 10th	Day	19% 12% 4 8% 4 4 4 4 5% 6% 29%	2400
	11th to 20th	Day	18% 13% 2 6% 3 3 5% 4 6% 6% 34%	2225
	21st to end	Day	22% 16% 3 6% 4 4 4 4 4 6% 28%	2200
October	1st to 10th	Day	20% 18% 5% 7% 5% 4 4 3 4 5% 25%	2110
	11th to 20th	Day	18% 27% 7% 11% 5% 3 3 2 3 5% 15%	1726
	21st to end	Day	19% 24% 8% 11% 4 3 3 2 3 5% 18%	1881
November	1st to 10th	Day	15% 22% 9% 16% 5% 4 3 2 2 3 17%	1761
	11th to 20th	Day	13% 21% 8% 17% 8% 4 4 2 3 3 15%	1731
	21st to end	Day	10% 24% 9% 17% 9% 6% 4 3 4 4 11%	1400
December	1st to 10th	Day	8% 18% 8% 19% 9% 10% 6% 4 4 3 10%	2280
	11th to 20th	Day	10% 15% 7% 16% 12% 9% 5% 3 5% 4 14%	2618
	21st to end	Day	11% 16% 9% 15% 6% 6% 6% 4 4 6% 19%	3476
January	1st to 10th	Night	6% 18% 8% 22% 11% 8% 10% 5% 5% 3 5%	6395
	11th to 20th	Night	4 10% 7% 23% 14% 15% 9% 4 5% 3 7%	6315
	21st to end	Night	3 7% 5% 17% 9% 11% 9% 9% 8% 6% 16%	6996
February	1st to 10th	Night	5% 12% 5% 15% 11% 10% 9% 6% 6% 6% 15%	2800
	11th to 20th	Night	6% 16% 7% 16% 9% 7% 9% 6% 6% 4 13%	2397
	21st to end	Night	5% 14% 7% 17% 9% 6% 9% 5% 6% 5% 18%	1980
March	1st to 10th	Night	8% 20% 7% 16% 7% 6% 5% 6% 6% 4 14%	5364
	11th to 20th	Night	7% 15% 7% 17% 10% 7% 6% 4 6% 6% 15%	5212
	21st to end	Night	10% 16% 6% 15% 7% 5% 5% 3 4 5% 25%	5110
April	1st to 10th	Night	14% 21% 7% 13% 7% 5% 4 3 3 4 21%	2000
	11th to 20th	Night	14% 18% 6% 11% 5% 4 4 4 5% 4 25%	1862
	21st to end	Night	14% 17% 7% 9% 7% 4 5% 3 4 5% 25%	1800
May	1st to 10th	Night	20% 15% 4 8% 5% 3 4 3 4 5% 29%	3874
	11th to 20th	Night	18% 16% 4 8% 4 3 3 3 5% 7% 29%	3575
	21st to end	Night	19% 15% 4 6% 4 4 4 3 5% 7% 29%	3649
June	1st to 10th	Night	20% 14% 4 7% 6% 4 5% 4 5% 6% 26%	1400
	11th to 20th	Night	17% 11% 3 7% 5% 5% 4 4 5% 7% 32%	1400
	21st to end	Night	21% 11% 4 9% 5% 5% 4 4 5% 7% 26%	1400
July	1st to 10th	Night	18% 13% 3 8% 6% 6% 5% 5% 6% 7% 23%	3283
	11th to 20th	Night	20% 13% 4 7% 4 5% 5% 4 6% 7% 27%	3600
	21st to end	Night	17% 10% 2 6% 4 5% 4 4 6% 6% 34%	3926
August	1st to 10th	Night	17% 11% 3 8% 5% 4 5% 4 7% 7% 30%	2586
	11th to 20th	Night	21% 16% 4 8% 5% 5% 4 3 5% 6% 24%	2829
	21st to end	Night	19% 12% 4 8% 4 4 5% 3 5% 7% 29%	3379
September	1st to 10th	Night	20% 13% 4 6% 4 2 4 3 5% 5% 33%	2000
	11th to 20th	Night	19% 16% 4 7% 4 4 4 3 4 5% 31%	2174
	21st to end	Night	19% 17% 4 6% 4 3 3 2 3 4 33%	2200
October	1st to 10th	Night	16% 19% 5% 8% 4 4 4 3 5% 5% 27%	2133
	11th to 20th	Night	20% 27% 7% 8% 4 3 3 2 3 4 18%	2453
	21st to end	Night	20% 25% 7% 11% 5% 4 3 2 3 3 16%	2717
November	1st to 10th	Night	13% 23% 8% 17% 6% 3 3 3 3 5% 16%	2537
	11th to 20th	Night	12% 19% 8% 18% 9% 5% 3 3 4 4 15%	2663
	21st to end	Night	10% 20% 9% 21% 10% 7% 5% 3 3 3 9%	3000
December	1st to 10th	Night	7% 14% 8% 25% 15% 10% 7% 3 3 3 7%	4910
	11th to 20th	Night	13% 15% 8% 16% 10% 9% 6% 3 4 3 14%	5591
	21st to end	Night	10% 17% 9% 16% 7% 6% 4 3 4 4 19%	7502

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	73% 7% 6% 9% 5%	3537
	11th to 20th	Day	70% 7% 4 8% 8% 3	3600
	21st to end	Day	55% 6% 5% 13% 18% 3	3960
February	1st to 10th	Day	65% 7% 7% 10% 8% 3	1600
	11th to 20th	Day	68% 6% 8% 7% 9% 2	1893
	21st to end	Day	62% 6% 6% 15% 7% 3	1650
March	1st to 10th	Day	65% 5% 7% 8% 9% 6%	4479
	11th to 20th	Day	62% 7% 11% 9% 9% 1	4746
	21st to end	Day	60% 4 6% 12% 14% 4	5822
April	1st to 10th	Day	68% 6% 7% 10% 7% 3	2400
	11th to 20th	Day	59% 5% 8% 16% 9% 2	2487
	21st to end	Day	61% 6% 9% 11% 11% 2	2600
May	1st to 10th	Day	61% 7% 8% 12% 10% 2	6121
	11th to 20th	Day	58% 8% 11% 16% 5% 2	6322
	21st to end	Day	58% 8% 9% 15% 10%	7224
June	1st to 10th	Day	60% 8% 10% 16% 5% 1	3000
	11th to 20th	Day	57% 8% 12% 15% 7% 1	3000
	21st to end	Day	62% 7% 10% 12% 8%	3000
July	1st to 10th	Day	59% 8% 11% 18% 4	6613
	11th to 20th	Day	54% 10% 18% 15% 3	6285
	21st to end	Day	47% 9% 13% 23% 7%	6728
August	1st to 10th	Day	55% 10% 16% 15% 4	4549
	11th to 20th	Day	61% 10% 15% 12% 3	4225
	21st to end	Day	56% 8% 14% 13% 8% 1	4515
September	1st to 10th	Day	53% 8% 13% 19% 7%	2400
	11th to 20th	Day	48% 7% 14% 23% 7% 1	2225
	21st to end	Day	57% 7% 10% 16% 9%	2200
October	1st to 10th	Day	63% 7% 7% 18% 4	3487
	11th to 20th	Day	66% 6% 8% 15% 4	2838
	21st to end	Day	63% 5% 14% 13% 4	3081
November	1st to 10th	Day	71% 6% 7% 12% 3	1761
	11th to 20th	Day	73% 6% 5% 11% 5% 1	1731
	21st to end	Day	76% 7% 7% 9% 2	1400
December	1st to 10th	Day	74% 6% 7% 10% 4	2280
	11th to 20th	Day	70% 4 5% 12% 9%	2618
	21st to end	Day	62% 7% 7% 8% 14% 3	3476
January	1st to 10th	Night	79% 6% 6% 6% 2	6395
	11th to 20th	Night	76% 7% 5% 7% 4 1	6360
	21st to end	Night	57% 8% 7% 12% 14% 3	6996
February	1st to 10th	Night	61% 9% 7% 12% 9% 2	2800
	11th to 20th	Night	67% 8% 8% 8% 7% 2	2399
	21st to end	Night	63% 6% 7% 12% 10% 2	1980
March	1st to 10th	Night	68% 5% 7% 9% 9% 3	5374
	11th to 20th	Night	65% 8% 11% 9% 6% 1	5213
	21st to end	Night	60% 5% 7% 15% 11% 2	5110
April	1st to 10th	Night	67% 5% 7% 11% 9% 2	2000
	11th to 20th	Night	58% 8% 7% 16% 9% 2	1862
	21st to end	Night	61% 6% 8% 16% 9% 1	1800
May	1st to 10th	Night	55% 7% 9% 15% 12% 2	3880
	11th to 20th	Night	53% 8% 14% 18% 6%	3576
	21st to end	Night	52% 7% 13% 17% 11% 1	3650
June	1st to 10th	Night	57% 8% 11% 18% 5% 1	1400
	11th to 20th	Night	52% 7% 13% 16% 9% 3	1400
	21st to end	Night	56% 9% 12% 16% 7%	1400
July	1st to 10th	Night	56% 10% 14% 17% 3	3246
	11th to 20th	Night	53% 10% 21% 14% 2	3552
	21st to end	Night	45% 10% 19% 18% 8%	3809
August	1st to 10th	Night	49% 9% 19% 17% 6%	2576
	11th to 20th	Night	58% 10% 15% 13% 3	2856
	21st to end	Night	52% 9% 14% 20% 6%	3379
September	1st to 10th	Night	51% 8% 11% 22% 8%	2000
	11th to 20th	Night	55% 6% 12% 21% 5%	2174
	21st to end	Night	54% 6% 10% 21% 8% 1	2200
October	1st to 10th	Night	59% 6% 9% 20% 5% 1	3536
	11th to 20th	Night	65% 5% 9% 14% 7%	4080
	21st to end	Night	67% 6% 10% 12% 4 1	4467
November	1st to 10th	Night	71% 6% 8% 11% 4	2537
	11th to 20th	Night	71% 6% 8% 8% 6%	2663
	21st to end	Night	77% 7% 7% 6% 3	3000
December	1st to 10th	Night	81% 6% 4 6% 2	4910
	11th to 20th	Night	73% 4 4 8% 10% 1	5592
	21st to end	Night	66% 5% 7% 8% 10% 4	7502

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 28% 43% 9% 2	200
	11th to 20th	Day	20% 26% 36% 13% 5% 1	200
	21st to end	Day	15% 28% 35% 17% 4 2	220
February	1st to 10th	Day	24% 38% 31% 7% 2 1	200
	11th to 20th	Day	22% 33% 31% 10% 3	452
	21st to end	Day	26% 31% 30% 9% 3	414
March	1st to 10th	Day	23% 27% 35% 12% 3	500
	11th to 20th	Day	27% 27% 29% 11% 5% 1	500
	21st to end	Day	29% 24% 30% 11% 6%	550
April	1st to 10th	Day	32% 26% 26% 12% 3 1	500
	11th to 20th	Day	31% 24% 30% 9% 4 2	492
	21st to end	Day	30% 27% 30% 9% 3 1	500
May	1st to 10th	Day	31% 21% 30% 10% 7% 1	503
	11th to 20th	Day	25% 19% 32% 17% 5% 1	527
	21st to end	Day	28% 24% 27% 14% 6% 1	692
June	1st to 10th	Day	30% 20% 27% 18% 5%	730
	11th to 20th	Day	29% 21% 26% 17% 7% 1	730
	21st to end	Day	30% 18% 27% 19% 5%	730
July	1st to 10th	Day	22% 19% 34% 19% 5% 1	666
	11th to 20th	Day	23% 20% 37% 15% 6%	524
	21st to end	Day	22% 20% 35% 18% 5%	576
August	1st to 10th	Day	22% 18% 33% 17% 7% 2	526
	11th to 20th	Day	21% 16% 31% 22% 8% 2	519
	21st to end	Day	26% 19% 26% 15% 10% 3	583
September	1st to 10th	Day	27% 17% 31% 16% 6% 3	530
	11th to 20th	Day	23% 17% 30% 19% 8% 3	530
	21st to end	Day	28% 22% 25% 13% 7% 4	530
October	1st to 10th	Day	26% 24% 28% 12% 8% 2	502
	11th to 20th	Day	35% 27% 24% 9% 4 1	397
	21st to end	Day	34% 28% 26% 11% 2	429
November	1st to 10th	Day	36% 28% 27% 8% 2	395
	11th to 20th	Day	26% 26% 32% 10% 4 2	365
	21st to end	Day	29% 32% 30% 8% 2 1	200
December	1st to 10th	Day	27% 20% 34% 14% 6%	200
	11th to 20th	Day	28% 33% 31% 5% 4 2	200
	21st to end	Day	30% 24% 30% 10% 5% 2	220
January	1st to 10th	Night	20% 28% 37% 12% 2	810
	11th to 20th	Night	20% 30% 30% 15% 5% 1	800
	21st to end	Night	15% 25% 38% 17% 4 1	880
February	1st to 10th	Night	16% 33% 34% 13% 3 1	800
	11th to 20th	Night	21% 29% 36% 11% 4	783
	21st to end	Night	17% 30% 37% 12% 3	660
March	1st to 10th	Night	20% 28% 35% 13% 3	800
	11th to 20th	Night	23% 29% 31% 11% 5% 1	800
	21st to end	Night	21% 26% 32% 17% 4	880
April	1st to 10th	Night	28% 26% 29% 13% 4	800
	11th to 20th	Night	28% 22% 28% 18% 4	678
	21st to end	Night	28% 22% 31% 15% 4	600
May	1st to 10th	Night	27% 18% 28% 22% 5%	524
	11th to 20th	Night	21% 15% 34% 25% 6%	394
	21st to end	Night	30% 19% 27% 18% 6%	433
June	1st to 10th	Night	23% 14% 30% 25% 8%	400
	11th to 20th	Night	22% 17% 27% 27% 7% 1	400
	21st to end	Night	24% 17% 29% 25% 6% 1	400
July	1st to 10th	Night	24% 15% 32% 23% 6%	390
	11th to 20th	Night	21% 15% 31% 25% 7% 1	388
	21st to end	Night	22% 12% 33% 24% 9%	438
August	1st to 10th	Night	16% 12% 33% 28% 10% 1	398
	11th to 20th	Night	24% 12% 28% 24% 12% 1	561
	21st to end	Night	24% 16% 27% 20% 11% 1	751
September	1st to 10th	Night	27% 17% 27% 20% 8% 1	800
	11th to 20th	Night	26% 17% 27% 17% 11% 2	800
	21st to end	Night	26% 19% 26% 19% 9% 1	800
October	1st to 10th	Night	26% 21% 28% 18% 6% 1	773
	11th to 20th	Night	33% 25% 25% 10% 6% 1	760
	21st to end	Night	36% 25% 24% 12% 4	836
November	1st to 10th	Night	32% 25% 30% 9% 3 1	780
	11th to 20th	Night	26% 29% 27% 10% 6% 1	816
	21st to end	Night	24% 31% 30% 11% 3 1	900
December	1st to 10th	Night	26% 28% 26% 15% 5%	900
	11th to 20th	Night	26% 30% 25% 15% 4	900
	21st to end	Night	27% 27% 28% 13% 4 1	990

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