Observatoire Iris
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Iris clear sky chart from 2005-11-06 to 2026-08-30.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	144199
	Night	13% 10% 4 9% 5% 5% 5% 4 5% 7% 35%	138111
Transparency	Day	46% 9% 14% 20% 10% 1	147637
	Night	47% 7% 13% 20% 11% 1	142763
Seeing	Day	22% 14% 30% 23% 9% 2	17854
	Night	21% 9% 27% 32% 10% 1	26031

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	10% 18% 5% 8% 4 3 4 3 6% 7% 32%	4488
	11th to 20th	Day	6% 13% 5% 12% 6% 6% 6% 3 5% 8% 29%	4650
	21st to end	Day	3 5% 4 11% 5% 6% 6% 5% 7% 8% 40%	5124
February	1st to 10th	Day	5% 10% 4 9% 9% 6% 7% 4 7% 6% 33%	1702
	11th to 20th	Day	7% 12% 4 13% 7% 5% 5% 3 5% 6% 32%	1977
	21st to end	Day	9% 11% 3 9% 6% 5% 6% 5% 6% 6% 35%	1730
March	1st to 10th	Day	15% 13% 4 10% 6% 4 5% 3 5% 5% 29%	4576
	11th to 20th	Day	9% 13% 4 8% 5% 5% 5% 4 7% 7% 32%	4936
	21st to end	Day	12% 15% 5% 9% 5% 5% 4 3 4 5% 33%	5838
April	1st to 10th	Day	18% 14% 3 8% 5% 4 4 4 5% 6% 29%	2520
	11th to 20th	Day	15% 9% 2 8% 5% 4 5% 3 5% 5% 38%	2642
	21st to end	Day	19% 11% 2 6% 5% 4 5% 3 5% 5% 34%	2730
May	1st to 10th	Day	20% 10% 3 8% 5% 5% 5% 4 5% 6% 30%	7755
	11th to 20th	Day	19% 9% 3 8% 5% 5% 5% 4 5% 6% 32%	8221
	21st to end	Day	16% 9% 3 9% 5% 5% 5% 4 5% 8% 30%	9556
June	1st to 10th	Day	19% 7% 3 6% 6% 5% 6% 5% 7% 7% 31%	3150
	11th to 20th	Day	17% 6% 3 6% 5% 6% 5% 5% 6% 7% 34%	3150
	21st to end	Day	22% 8% 3 6% 5% 6% 5% 4 6% 8% 28%	3150
July	1st to 10th	Day	12% 6% 2 7% 5% 6% 6% 4 8% 9% 35%	8846
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 6% 8% 9% 34%	8280
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 5% 7% 9% 39%	8993
August	1st to 10th	Day	11% 5% 2 7% 5% 6% 7% 5% 8% 10% 33%	4645
	11th to 20th	Day	13% 6% 3 7% 6% 6% 7% 5% 7% 8% 31%	4305
	21st to end	Day	16% 5% 2 6% 4 5% 5% 4 6% 7% 41%	4697
September	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 5% 8% 43%	2395
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2200
	21st to end	Day	18% 4 2 5% 4 4 4 4 6% 7% 41%	2200
October	1st to 10th	Day	16% 6% 2 7% 5% 5% 5% 3 5% 6% 41%	2190
	11th to 20th	Day	19% 7% 2 7% 5% 5% 6% 4 6% 6% 32%	1831
	21st to end	Day	20% 10% 3 7% 4 4 4 3 5% 7% 32%	1980
November	1st to 10th	Day	12% 10% 4 9% 5% 6% 5% 3 6% 5% 36%	1851
	11th to 20th	Day	14% 11% 4 8% 5% 5% 4 4 5% 7% 32%	1811
	21st to end	Day	16% 15% 4 9% 5% 5% 4 4 5% 6% 28%	1489
December	1st to 10th	Day	11% 14% 5% 9% 5% 5% 4 3 4 6% 34%	2350
	11th to 20th	Day	14% 9% 4 9% 6% 5% 5% 4 4 6% 33%	2688
	21st to end	Day	14% 11% 3 9% 5% 5% 5% 4 6% 6% 31%	3553
January	1st to 10th	Night	9% 16% 5% 10% 6% 5% 5% 3 5% 6% 30%	8411
	11th to 20th	Night	6% 14% 7% 15% 7% 6% 5% 4 5% 6% 25%	8235
	21st to end	Night	3 7% 5% 15% 6% 6% 5% 4 5% 7% 37%	9057
February	1st to 10th	Night	6% 10% 3 10% 6% 5% 5% 4 5% 7% 39%	2916
	11th to 20th	Night	7% 12% 5% 11% 7% 5% 5% 3 6% 5% 34%	2528
	21st to end	Night	10% 12% 4 7% 5% 5% 4 4 6% 7% 37%	2076
March	1st to 10th	Night	16% 13% 4 10% 5% 5% 5% 4 5% 5% 28%	5484
	11th to 20th	Night	10% 12% 4 10% 6% 6% 6% 4 6% 7% 28%	5242
	21st to end	Night	14% 16% 5% 9% 4 3 4 3 4 5% 34%	5319
April	1st to 10th	Night	17% 15% 4 7% 4 4 4 2 5% 4 33%	2100
	11th to 20th	Night	17% 8% 3 8% 5% 4 5% 3 5% 8% 34%	1925
	21st to end	Night	20% 11% 3 6% 4 3 4 4 4 5% 36%	1890
May	1st to 10th	Night	19% 9% 3 8% 5% 4 4 3 5% 6% 33%	5200
	11th to 20th	Night	21% 10% 3 7% 5% 4 4 3 4 6% 32%	4639
	21st to end	Night	17% 9% 3 6% 4 3 4 4 5% 7% 38%	4627
June	1st to 10th	Night	18% 8% 3 7% 5% 4 5% 4 5% 7% 34%	1470
	11th to 20th	Night	15% 6% 2 6% 4 5% 4 4 6% 8% 40%	1470
	21st to end	Night	18% 8% 3 7% 4 5% 5% 4 6% 6% 32%	1470
July	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 3 6% 9% 43%	4126
	11th to 20th	Night	13% 5% 2 5% 4 4 5% 4 7% 9% 43%	4680
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 8% 48%	5084
August	1st to 10th	Night	14% 6% 2 6% 4 4 5% 4 6% 7% 41%	2690
	11th to 20th	Night	12% 5% 2 8% 6% 7% 6% 5% 7% 7% 36%	2962
	21st to end	Night	15% 7% 3 7% 3 4 4 3 4 7% 44%	3397
September	1st to 10th	Night	13% 5% 1 5% 5% 4 4 3 5% 6% 49%	2005
	11th to 20th	Night	15% 6% 2 6% 3 5% 3 4 5% 6% 46%	2200
	21st to end	Night	16% 6% 2 7% 4 4 4 3 4 6% 45%	2200
October	1st to 10th	Night	14% 7% 3 5% 4 4 4 3 5% 6% 45%	2210
	11th to 20th	Night	20% 8% 2 6% 3 4 5% 3 4 7% 37%	2565
	21st to end	Night	19% 8% 3 7% 4 4 4 3 5% 7% 36%	2860
November	1st to 10th	Night	14% 9% 2 7% 5% 4 4 4 5% 7% 39%	2667
	11th to 20th	Night	16% 10% 3 7% 4 4 4 4 5% 6% 36%	2809
	21st to end	Night	15% 13% 4 8% 6% 4 4 4 5% 6% 32%	3129
December	1st to 10th	Night	9% 11% 5% 12% 6% 5% 4 4 4 6% 33%	5060
	11th to 20th	Night	17% 10% 3 7% 5% 5% 5% 4 6% 7% 32%	5741
	21st to end	Night	14% 14% 5% 9% 5% 5% 5% 4 5% 6% 30%	7667

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	51% 6% 13% 20% 10%	4489
	11th to 20th	Day	51% 7% 12% 20% 9% 1	4680
	21st to end	Day	36% 9% 13% 20% 21% 1	5124
February	1st to 10th	Day	48% 8% 11% 18% 12% 3	1702
	11th to 20th	Day	53% 7% 11% 16% 12% 2	1985
	21st to end	Day	47% 9% 9% 21% 12% 3	1730
March	1st to 10th	Day	55% 8% 10% 10% 11% 5%	4579
	11th to 20th	Day	48% 10% 18% 15% 8% 1	4936
	21st to end	Day	53% 5% 8% 16% 13% 5%	5838
April	1st to 10th	Day	54% 8% 10% 14% 11% 3	2520
	11th to 20th	Day	47% 6% 12% 21% 11% 2	2642
	21st to end	Day	51% 7% 10% 17% 12% 2	2730
May	1st to 10th	Day	50% 8% 13% 17% 11% 1	7767
	11th to 20th	Day	49% 8% 12% 21% 7% 2	8225
	21st to end	Day	49% 9% 14% 16% 13%	9557
June	1st to 10th	Day	48% 11% 12% 20% 7% 1	3150
	11th to 20th	Day	45% 10% 14% 20% 10% 1	3150
	21st to end	Day	51% 11% 13% 16% 8% 1	3150
July	1st to 10th	Day	40% 9% 17% 23% 12%	8771
	11th to 20th	Day	39% 11% 17% 26% 8%	8205
	21st to end	Day	37% 11% 19% 25% 8%	8795
August	1st to 10th	Day	41% 13% 19% 18% 9%	4631
	11th to 20th	Day	44% 13% 20% 18% 5%	4344
	21st to end	Day	40% 9% 19% 19% 11% 1	4697
September	1st to 10th	Day	38% 10% 17% 23% 12%	2395
	11th to 20th	Day	35% 8% 18% 28% 11% 1	2200
	21st to end	Day	40% 10% 14% 24% 12%	2200
October	1st to 10th	Day	48% 9% 13% 23% 7%	3567
	11th to 20th	Day	42% 8% 18% 26% 6%	2956
	21st to end	Day	42% 7% 22% 18% 12% 1	3180
November	1st to 10th	Day	48% 9% 13% 20% 9% 1	1851
	11th to 20th	Day	52% 8% 10% 21% 8% 1	1811
	21st to end	Day	56% 7% 14% 15% 7% 1	1489
December	1st to 10th	Day	52% 6% 12% 17% 13%	2350
	11th to 20th	Day	49% 8% 13% 16% 12% 1	2688
	21st to end	Day	50% 10% 13% 17% 9% 2	3553
January	1st to 10th	Night	55% 7% 12% 16% 9% 1	8411
	11th to 20th	Night	57% 7% 12% 16% 8% 1	8280
	21st to end	Night	44% 6% 11% 20% 18% 1	9057
February	1st to 10th	Night	44% 7% 11% 22% 14% 2	2916
	11th to 20th	Night	52% 6% 11% 16% 14% 1	2531
	21st to end	Night	46% 7% 10% 21% 14% 2	2076
March	1st to 10th	Night	56% 7% 10% 12% 13% 2	5494
	11th to 20th	Night	52% 9% 15% 14% 7% 3	5243
	21st to end	Night	53% 5% 8% 20% 13% 2	5319
April	1st to 10th	Night	54% 5% 8% 17% 14% 3	2100
	11th to 20th	Night	47% 8% 10% 21% 12% 2	1925
	21st to end	Night	51% 5% 10% 20% 12% 2	1890
May	1st to 10th	Night	49% 7% 13% 18% 13% 1	5206
	11th to 20th	Night	49% 8% 12% 20% 11%	4640
	21st to end	Night	43% 7% 13% 20% 15% 3	4628
June	1st to 10th	Night	47% 8% 13% 22% 8% 1	1470
	11th to 20th	Night	40% 8% 14% 23% 12% 3	1470
	21st to end	Night	50% 11% 12% 17% 9%	1470
July	1st to 10th	Night	36% 7% 21% 26% 10%	4091
	11th to 20th	Night	35% 8% 21% 29% 7%	4632
	21st to end	Night	34% 9% 22% 27% 9%	4967
August	1st to 10th	Night	41% 9% 18% 24% 8%	2680
	11th to 20th	Night	43% 13% 20% 15% 9%	2989
	21st to end	Night	40% 8% 14% 27% 11%	3397
September	1st to 10th	Night	35% 9% 16% 27% 13% 1	2005
	11th to 20th	Night	38% 8% 16% 28% 9% 1	2200
	21st to end	Night	40% 8% 13% 26% 13% 1	2200
October	1st to 10th	Night	43% 7% 14% 28% 8% 1	3613
	11th to 20th	Night	43% 6% 13% 31% 7%	4179
	21st to end	Night	40% 8% 20% 19% 12% 1	4610
November	1st to 10th	Night	45% 8% 14% 23% 9% 1	2667
	11th to 20th	Night	48% 8% 12% 21% 10% 1	2809
	21st to end	Night	53% 9% 11% 18% 8% 1	3129
December	1st to 10th	Night	52% 6% 11% 22% 7% 1	5060
	11th to 20th	Night	49% 7% 11% 21% 11% 1	5742
	21st to end	Night	53% 8% 12% 16% 9% 1	7667

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% 7% 37% 28% 8% 1	220
	11th to 20th	Day	25% 15% 32% 20% 7% 1	220
	21st to end	Day	13% 17% 40% 20% 7% 3	242
February	1st to 10th	Day	16% 15% 42% 19% 7% 1	242
	11th to 20th	Day	18% 16% 37% 22% 6%	498
	21st to end	Day	17% 17% 37% 23% 4 1	446
March	1st to 10th	Day	24% 21% 29% 20% 6%	540
	11th to 20th	Day	21% 18% 32% 20% 8% 1	540
	21st to end	Day	22% 22% 30% 18% 7% 1	594
April	1st to 10th	Day	30% 19% 27% 17% 6% 1	540
	11th to 20th	Day	26% 16% 30% 20% 7% 1	532
	21st to end	Day	28% 14% 34% 17% 5% 2	540
May	1st to 10th	Day	30% 12% 28% 22% 8% 1	543
	11th to 20th	Day	24% 19% 27% 20% 8% 2	567
	21st to end	Day	22% 17% 31% 22% 6% 1	788
June	1st to 10th	Day	25% 12% 26% 27% 10% 1	780
	11th to 20th	Day	23% 13% 27% 24% 12% 1	780
	21st to end	Day	23% 11% 27% 26% 10% 2	780
July	1st to 10th	Day	17% 11% 30% 29% 11% 2	762
	11th to 20th	Day	16% 12% 36% 25% 10% 2	564
	21st to end	Day	16% 9% 34% 28% 10% 2	620
August	1st to 10th	Day	14% 10% 33% 25% 14% 4	566
	11th to 20th	Day	14% 8% 29% 31% 14% 3	559
	21st to end	Day	18% 15% 29% 19% 16% 4	619
September	1st to 10th	Day	18% 13% 30% 25% 10% 5%	530
	11th to 20th	Day	18% 10% 31% 25% 12% 6%	530
	21st to end	Day	21% 12% 26% 22% 13% 5%	530
October	1st to 10th	Day	19% 15% 28% 25% 10% 3	524
	11th to 20th	Day	25% 13% 28% 20% 11% 2	420
	21st to end	Day	31% 13% 27% 22% 7%	462
November	1st to 10th	Day	26% 13% 32% 22% 5% 2	425
	11th to 20th	Day	25% 11% 27% 27% 9% 1	430
	21st to end	Day	30% 18% 31% 17% 5%	239
December	1st to 10th	Day	21% 15% 33% 19% 10% 3	220
	11th to 20th	Day	27% 17% 32% 16% 6% 1	220
	21st to end	Day	24% 16% 29% 23% 6% 2	242
January	1st to 10th	Night	20% 9% 30% 34% 5% 1	873
	11th to 20th	Night	18% 10% 33% 32% 7%	840
	21st to end	Night	12% 9% 28% 41% 10% 1	924
February	1st to 10th	Night	13% 7% 37% 31% 11% 1	840
	11th to 20th	Night	15% 12% 36% 28% 8% 1	823
	21st to end	Night	17% 9% 32% 33% 7%	692
March	1st to 10th	Night	20% 14% 30% 30% 6% 1	840
	11th to 20th	Night	19% 13% 31% 29% 7% 1	840
	21st to end	Night	21% 14% 28% 28% 7% 1	924
April	1st to 10th	Night	28% 10% 30% 25% 6%	840
	11th to 20th	Night	25% 10% 28% 28% 9%	681
	21st to end	Night	29% 9% 27% 27% 8%	630
May	1st to 10th	Night	26% 9% 27% 31% 6%	609
	11th to 20th	Night	25% 7% 23% 31% 13%	414
	21st to end	Night	26% 8% 27% 30% 8% 1	455
June	1st to 10th	Night	22% 5% 21% 38% 13%	420
	11th to 20th	Night	18% 5% 22% 38% 15% 2	420
	21st to end	Night	22% 5% 21% 38% 13% 1	420
July	1st to 10th	Night	20% 5% 21% 44% 10% 1	410
	11th to 20th	Night	14% 4 21% 44% 16%	408
	21st to end	Night	15% 4 19% 47% 14% 1	460
August	1st to 10th	Night	14% 4 13% 46% 20% 2	429
	11th to 20th	Night	16% 4 18% 41% 21% 1	622
	21st to end	Night	17% 6% 22% 37% 17% 1	741
September	1st to 10th	Night	16% 6% 23% 39% 15% 1	800
	11th to 20th	Night	20% 4 22% 35% 17% 3	800
	21st to end	Night	19% 6% 21% 38% 16% 1	800
October	1st to 10th	Night	20% 10% 24% 35% 11% 2	800
	11th to 20th	Night	25% 8% 22% 32% 13% 1	800
	21st to end	Night	26% 10% 25% 30% 9%	880
November	1st to 10th	Night	23% 13% 27% 29% 8% 1	820
	11th to 20th	Night	23% 12% 27% 28% 9% 1	881
	21st to end	Night	25% 14% 24% 29% 7% 1	950
December	1st to 10th	Night	22% 11% 30% 28% 8% 1	950
	11th to 20th	Night	22% 14% 29% 27% 8%	950
	21st to end	Night	24% 11% 27% 26% 10% 1	1045

Page generated at 2026-08-27 04:07:12. Version ac07

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