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 2024-11-05.

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% 11% 7% 5% 5% 4 5% 5% 22%	101535
	Night	13% 16% 6% 13% 7% 6% 5% 4 5% 5% 20%	97518
Transparency	Day	60% 8% 10% 14% 7% 1	104979
	Night	62% 8% 10% 13% 6% 1	102164
Seeing	Day	27% 23% 30% 14% 5% 1	15343
	Night	24% 24% 30% 16% 5% 1	23681

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	6% 16% 7% 17% 12% 8% 6% 4 6% 5% 11%	2469
	11th to 20th	Day	5% 11% 8% 18% 13% 13% 5% 4 6% 5% 13%	2520
	21st to end	Day	5% 9% 6% 22% 8% 7% 5% 4 6% 6% 21%	2772
February	1st to 10th	Day	5% 13% 6% 15% 11% 10% 10% 5% 5% 5% 15%	1520
	11th to 20th	Day	6% 15% 7% 18% 10% 6% 7% 5% 6% 5% 15%	1787
	21st to end	Day	6% 14% 8% 16% 9% 7% 7% 5% 6% 5% 18%	1570
March	1st to 10th	Day	9% 16% 6% 14% 8% 5% 5% 4 5% 5% 24%	3116
	11th to 20th	Day	7% 24% 9% 17% 8% 5% 5% 2 4 4 16%	3316
	21st to end	Day	7% 17% 6% 15% 7% 5% 5% 3 4 4 27%	4060
April	1st to 10th	Day	15% 22% 6% 12% 7% 5% 3 3 3 4 19%	2280
	11th to 20th	Day	14% 19% 7% 11% 4 4 3 3 4 5% 27%	2361
	21st to end	Day	16% 17% 6% 10% 6% 4 3 4 4 4 25%	2470
May	1st to 10th	Day	18% 17% 5% 10% 5% 5% 5% 4 5% 5% 22%	4260
	11th to 20th	Day	16% 12% 4 9% 5% 5% 6% 4 6% 7% 26%	4398
	21st to end	Day	19% 12% 4 9% 6% 5% 5% 4 6% 6% 25%	5028
June	1st to 10th	Day	23% 13% 3 8% 6% 4 5% 4 6% 6% 21%	2850
	11th to 20th	Day	22% 11% 3 7% 5% 4 5% 4 6% 7% 25%	2850
	21st to end	Day	27% 10% 4 10% 5% 4 4 4 5% 5% 22%	2850
July	1st to 10th	Day	22% 10% 4 9% 6% 6% 5% 4 6% 7% 23%	4668
	11th to 20th	Day	20% 10% 4 9% 6% 6% 6% 5% 6% 6% 22%	4440
	21st to end	Day	17% 9% 3 8% 5% 6% 6% 4 6% 7% 29%	4814
August	1st to 10th	Day	18% 10% 3 10% 5% 7% 7% 5% 7% 7% 21%	4423
	11th to 20th	Day	24% 12% 4 9% 5% 7% 6% 5% 6% 6% 17%	4056
	21st to end	Day	22% 11% 4 9% 5% 4 4 3 5% 6% 26%	4377
September	1st to 10th	Day	20% 13% 4 8% 4 3 4 4 5% 6% 29%	2280
	11th to 20th	Day	19% 13% 3 6% 3 3 5% 3 6% 6% 33%	2114
	21st to end	Day	23% 16% 3 6% 4 4 4 4 4 6% 27%	2090
October	1st to 10th	Day	20% 18% 5% 7% 5% 4 4 3 4 5% 24%	2077
	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% 6% 4 3 2 2 3 17%	1708
	11th to 20th	Day	13% 21% 8% 18% 8% 5% 4 2 3 3 14%	1645
	21st to end	Day	10% 23% 8% 17% 9% 6% 4 4 4 4 11%	1330
December	1st to 10th	Day	11% 21% 9% 15% 11% 6% 5% 3 3 4 11%	1330
	11th to 20th	Day	12% 16% 7% 14% 13% 7% 6% 3 4 3 16%	1668
	21st to end	Day	13% 17% 7% 15% 6% 6% 6% 4 4 6% 15%	2431
January	1st to 10th	Night	7% 16% 8% 19% 8% 9% 10% 6% 6% 4 7%	4461
	11th to 20th	Night	5% 11% 5% 20% 12% 15% 10% 4 5% 4 10%	4440
	21st to end	Night	4 10% 6% 19% 9% 9% 8% 6% 7% 6% 16%	4884
February	1st to 10th	Night	5% 12% 5% 15% 10% 10% 10% 6% 6% 6% 14%	2660
	11th to 20th	Night	6% 16% 7% 16% 10% 7% 8% 6% 6% 4 13%	2275
	21st to end	Night	5% 15% 7% 17% 9% 5% 9% 5% 6% 5% 18%	1884
March	1st to 10th	Night	10% 21% 5% 13% 6% 5% 5% 6% 6% 5% 19%	3724
	11th to 20th	Night	9% 18% 9% 19% 9% 6% 5% 3 4 4 12%	3642
	21st to end	Night	10% 17% 6% 15% 8% 5% 4 3 4 4 24%	3572
April	1st to 10th	Night	14% 20% 7% 13% 7% 5% 4 3 3 3 21%	1900
	11th to 20th	Night	15% 18% 6% 12% 5% 4 4 4 4 4 25%	1768
	21st to end	Night	14% 17% 7% 9% 7% 4 5% 3 3 5% 26%	1710
May	1st to 10th	Night	18% 13% 4 9% 6% 4 5% 3 4 6% 29%	2710
	11th to 20th	Night	14% 14% 4 8% 4 4 4 4 5% 7% 33%	2495
	21st to end	Night	20% 12% 4 6% 4 4 5% 4 6% 7% 27%	2539
June	1st to 10th	Night	20% 14% 4 7% 6% 5% 5% 5% 5% 6% 25%	1330
	11th to 20th	Night	17% 11% 3 8% 5% 5% 4 4 5% 7% 33%	1330
	21st to end	Night	21% 11% 4 9% 5% 5% 4 5% 5% 7% 25%	1330
July	1st to 10th	Night	19% 12% 3 9% 5% 5% 5% 5% 5% 7% 25%	2294
	11th to 20th	Night	20% 13% 4 8% 4 5% 5% 4 6% 7% 23%	2520
	21st to end	Night	17% 9% 2 6% 4 4 5% 4 6% 7% 37%	2739
August	1st to 10th	Night	17% 11% 3 8% 5% 4 5% 4 7% 7% 29%	2506
	11th to 20th	Night	21% 16% 4 8% 5% 5% 4 3 5% 6% 23%	2740
	21st to end	Night	19% 12% 4 8% 4 4 5% 3 5% 7% 29%	3275
September	1st to 10th	Night	21% 14% 4 6% 4 2 4 3 5% 5% 32%	1900
	11th to 20th	Night	20% 16% 4 7% 4 3 4 3 4 5% 31%	2065
	21st to end	Night	19% 18% 4 6% 4 3 3 2 3 4 32%	2090
October	1st to 10th	Night	16% 19% 5% 8% 5% 4 4 3 4 5% 27%	2099
	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% 16% 6% 3 3 3 3 5% 16%	2470
	11th to 20th	Night	13% 19% 7% 18% 9% 5% 4 3 4 4 14%	2529
	21st to end	Night	10% 19% 8% 21% 11% 8% 5% 3 3 3 10%	2850
December	1st to 10th	Night	10% 19% 9% 19% 11% 7% 4 3 3 4 10%	2850
	11th to 20th	Night	13% 17% 7% 17% 11% 8% 6% 4 5% 3 11%	3531
	21st to end	Night	12% 19% 8% 16% 8% 7% 5% 3 5% 3 14%	5236

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	68% 8% 8% 11% 5%	2470
	11th to 20th	Day	69% 8% 5% 9% 8% 1	2520
	21st to end	Day	58% 7% 6% 11% 13% 5%	2772
February	1st to 10th	Day	65% 7% 7% 10% 8% 2	1520
	11th to 20th	Day	68% 6% 7% 7% 10% 2	1796
	21st to end	Day	62% 6% 6% 15% 7% 3	1570
March	1st to 10th	Day	60% 6% 9% 11% 10% 4	3119
	11th to 20th	Day	69% 7% 8% 8% 7% 1	3316
	21st to end	Day	59% 4 7% 14% 11% 5%	4060
April	1st to 10th	Day	68% 6% 8% 10% 7% 3	2280
	11th to 20th	Day	59% 5% 8% 16% 9% 2	2361
	21st to end	Day	61% 6% 9% 11% 12% 2	2470
May	1st to 10th	Day	58% 9% 11% 12% 9% 1	4274
	11th to 20th	Day	53% 10% 14% 18% 5%	4402
	21st to end	Day	56% 8% 9% 16% 10%	5028
June	1st to 10th	Day	61% 8% 10% 16% 5% 1	2850
	11th to 20th	Day	56% 9% 13% 15% 7% 1	2850
	21st to end	Day	63% 7% 10% 12% 8%	2850
July	1st to 10th	Day	57% 8% 11% 20% 4	4595
	11th to 20th	Day	56% 10% 17% 14% 3	4365
	21st to end	Day	51% 10% 12% 20% 7%	4661
August	1st to 10th	Day	57% 10% 15% 15% 4	4409
	11th to 20th	Day	62% 10% 15% 11% 2	4095
	21st to end	Day	56% 8% 14% 12% 9% 2	4377
September	1st to 10th	Day	54% 8% 12% 19% 8%	2280
	11th to 20th	Day	49% 7% 14% 22% 7% 1	2114
	21st to end	Day	57% 7% 10% 16% 9%	2090
October	1st to 10th	Day	64% 7% 7% 18% 4	3454
	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	1708
	11th to 20th	Day	73% 6% 6% 10% 4 1	1645
	21st to end	Day	75% 7% 7% 9% 2	1330
December	1st to 10th	Day	75% 6% 6% 8% 5%	1330
	11th to 20th	Day	70% 5% 5% 9% 10%	1668
	21st to end	Day	64% 8% 8% 9% 9% 1	2431
January	1st to 10th	Night	72% 8% 8% 8% 3 1	4461
	11th to 20th	Night	72% 7% 5% 9% 5% 2	4440
	21st to end	Night	60% 10% 7% 9% 11% 4	4884
February	1st to 10th	Night	61% 9% 7% 12% 9% 2	2660
	11th to 20th	Night	67% 8% 8% 8% 7% 2	2277
	21st to end	Night	63% 6% 7% 12% 9% 2	1884
March	1st to 10th	Night	63% 6% 9% 11% 8% 4	3734
	11th to 20th	Night	70% 8% 8% 8% 5% 1	3643
	21st to end	Night	61% 6% 8% 15% 7% 2	3572
April	1st to 10th	Night	67% 5% 7% 11% 8% 2	1900
	11th to 20th	Night	58% 8% 8% 15% 9% 1	1768
	21st to end	Night	60% 6% 7% 16% 9% 1	1710
May	1st to 10th	Night	52% 9% 11% 17% 9% 1	2716
	11th to 20th	Night	47% 10% 16% 18% 8%	2496
	21st to end	Night	51% 9% 13% 16% 10% 1	2540
June	1st to 10th	Night	57% 8% 11% 17% 5% 1	1330
	11th to 20th	Night	50% 7% 14% 17% 9% 3	1330
	21st to end	Night	57% 9% 12% 15% 7%	1330
July	1st to 10th	Night	55% 10% 13% 19% 3	2257
	11th to 20th	Night	55% 11% 18% 13% 3	2472
	21st to end	Night	43% 10% 20% 18% 10%	2651
August	1st to 10th	Night	50% 9% 19% 16% 5%	2496
	11th to 20th	Night	59% 11% 15% 12% 3	2767
	21st to end	Night	51% 9% 14% 20% 6%	3275
September	1st to 10th	Night	52% 8% 10% 21% 8%	1900
	11th to 20th	Night	56% 6% 13% 19% 5%	2065
	21st to end	Night	55% 6% 11% 20% 7% 1	2090
October	1st to 10th	Night	59% 6% 9% 20% 5% 1	3502
	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	2470
	11th to 20th	Night	71% 6% 8% 8% 5%	2529
	21st to end	Night	76% 7% 8% 6% 3	2850
December	1st to 10th	Night	77% 6% 6% 7% 4 1	2850
	11th to 20th	Night	73% 6% 5% 7% 8% 1	3532
	21st to end	Night	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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	19% 29% 42% 8% 2	180
	11th to 20th	Day	21% 27% 36% 12% 4	180
	21st to end	Day	14% 26% 36% 18% 4 3	198
February	1st to 10th	Day	23% 39% 28% 7% 1 1	180
	11th to 20th	Day	21% 33% 32% 10% 3	415
	21st to end	Day	27% 31% 29% 10% 3 1	382
March	1st to 10th	Day	22% 28% 35% 12% 3	460
	11th to 20th	Day	25% 26% 30% 12% 6% 1	460
	21st to end	Day	28% 25% 30% 11% 6%	506
April	1st to 10th	Day	30% 27% 27% 12% 3 1	460
	11th to 20th	Day	31% 24% 30% 9% 4 2	452
	21st to end	Day	29% 28% 30% 9% 3 1	460
May	1st to 10th	Day	30% 22% 30% 10% 7% 1	463
	11th to 20th	Day	23% 19% 33% 18% 5% 1	487
	21st to end	Day	28% 25% 27% 14% 5% 1	642
June	1st to 10th	Day	29% 19% 28% 18% 5%	680
	11th to 20th	Day	28% 21% 25% 17% 7% 1	680
	21st to end	Day	29% 18% 27% 19% 6%	680
July	1st to 10th	Day	21% 18% 36% 20% 5%	619
	11th to 20th	Day	23% 19% 37% 15% 6%	484
	21st to end	Day	22% 18% 35% 19% 5%	532
August	1st to 10th	Day	24% 19% 32% 17% 7% 2	486
	11th to 20th	Day	21% 16% 31% 21% 8% 2	479
	21st to end	Day	27% 19% 26% 15% 11% 3	539
September	1st to 10th	Day	28% 17% 30% 16% 6% 2	490
	11th to 20th	Day	24% 16% 31% 19% 8% 3	490
	21st to end	Day	27% 22% 26% 13% 7% 4	490
October	1st to 10th	Day	26% 24% 28% 12% 7% 2	490
	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	34% 28% 28% 8% 2	378
	11th to 20th	Day	26% 26% 32% 10% 5% 2	337
	21st to end	Day	28% 32% 29% 9% 2 1	180
December	1st to 10th	Day	29% 21% 30% 14% 6%	180
	11th to 20th	Day	28% 33% 29% 5% 3 2	180
	21st to end	Day	32% 25% 29% 10% 5% 1	198
January	1st to 10th	Night	20% 27% 38% 13% 2	769
	11th to 20th	Night	21% 31% 30% 14% 3 1	760
	21st to end	Night	14% 25% 38% 17% 4 1	836
February	1st to 10th	Night	16% 33% 34% 13% 3 1	760
	11th to 20th	Night	20% 29% 35% 11% 4	743
	21st to end	Night	18% 30% 36% 12% 3	628
March	1st to 10th	Night	20% 28% 35% 13% 3	760
	11th to 20th	Night	23% 29% 31% 11% 5% 1	760
	21st to end	Night	22% 26% 33% 16% 4	836
April	1st to 10th	Night	27% 26% 30% 13% 4	760
	11th to 20th	Night	28% 23% 29% 17% 4	644
	21st to end	Night	27% 22% 31% 15% 5%	570
May	1st to 10th	Night	27% 17% 29% 21% 5%	498
	11th to 20th	Night	21% 15% 33% 25% 6%	374
	21st to end	Night	30% 19% 28% 18% 5%	411
June	1st to 10th	Night	23% 14% 30% 24% 8%	380
	11th to 20th	Night	21% 18% 28% 26% 7% 1	380
	21st to end	Night	23% 16% 29% 25% 6% 1	380
July	1st to 10th	Night	23% 15% 32% 23% 6%	370
	11th to 20th	Night	21% 15% 31% 25% 7% 1	368
	21st to end	Night	22% 11% 33% 25% 10%	416
August	1st to 10th	Night	16% 13% 33% 28% 9% 1	378
	11th to 20th	Night	24% 11% 29% 22% 13% 1	532
	21st to end	Night	24% 16% 28% 19% 11% 1	713
September	1st to 10th	Night	28% 17% 28% 20% 7% 1	760
	11th to 20th	Night	27% 17% 27% 17% 10% 2	760
	21st to end	Night	26% 19% 26% 18% 9% 1	760
October	1st to 10th	Night	26% 21% 27% 18% 6% 1	760
	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	31% 25% 30% 9% 3 1	760
	11th to 20th	Night	27% 29% 28% 10% 6% 1	774
	21st to end	Night	24% 31% 30% 11% 3 1	850
December	1st to 10th	Night	26% 28% 26% 14% 5%	850
	11th to 20th	Night	25% 30% 25% 15% 4	850
	21st to end	Night	27% 28% 28% 13% 4 1	935

Page generated at 2024-11-03 02:11:08. Version ac07

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