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 2025-04-26.

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%	113119
	Night	13% 10% 4 8% 5% 5% 5% 4 5% 6% 35%	115547
Transparency	Day	47% 9% 14% 19% 10% 1	116603
	Night	48% 8% 13% 20% 10% 1	120228
Seeing	Day	22% 13% 31% 23% 9% 2	16002
	Night	21% 9% 27% 32% 10% 1	24467

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	12% 21% 5% 8% 4 4 4 3 5% 5% 28%	3461
	11th to 20th	Day	6% 12% 5% 12% 6% 6% 7% 4 5% 8% 28%	3570
	21st to end	Day	4 5% 3 10% 6% 6% 6% 4 6% 6% 41%	3960
February	1st to 10th	Day	6% 10% 4 9% 9% 6% 7% 4 7% 6% 32%	1621
	11th to 20th	Day	8% 12% 4 13% 8% 5% 5% 3 5% 6% 31%	1880
	21st to end	Day	10% 11% 3 8% 6% 5% 6% 5% 6% 6% 35%	1650
March	1st to 10th	Day	16% 14% 4 10% 6% 4 5% 3 5% 5% 29%	4476
	11th to 20th	Day	9% 13% 4 8% 5% 5% 5% 4 7% 7% 32%	4830
	21st to end	Day	12% 15% 4 9% 5% 5% 5% 3 4 5% 33%	5712
April	1st to 10th	Day	18% 14% 3 8% 5% 4 4 4 5% 5% 30%	2400
	11th to 20th	Day	15% 9% 2 8% 5% 4 5% 3 5% 5% 38%	2515
	21st to end	Day	20% 11% 2 6% 5% 4 5% 3 5% 5% 33%	2535
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 5% 6% 35%	4168
	11th to 20th	Day	17% 8% 2 7% 5% 6% 5% 4 5% 7% 34%	4398
	21st to end	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 8% 33%	5109
June	1st to 10th	Day	19% 7% 3 6% 6% 6% 6% 5% 6% 7% 30%	2850
	11th to 20th	Day	17% 6% 2 6% 5% 6% 5% 4 6% 7% 36%	2850
	21st to end	Day	22% 8% 3 6% 5% 5% 5% 4 5% 8% 28%	2850
July	1st to 10th	Day	14% 6% 2 7% 6% 6% 7% 5% 8% 9% 32%	4740
	11th to 20th	Day	11% 5% 2 7% 6% 6% 6% 6% 8% 9% 33%	4440
	21st to end	Day	11% 5% 2 6% 5% 6% 6% 5% 7% 9% 39%	4814
August	1st to 10th	Day	11% 5% 2 7% 5% 7% 7% 6% 8% 10% 31%	4369
	11th to 20th	Day	13% 6% 3 7% 6% 6% 7% 6% 7% 8% 30%	4045
	21st to end	Day	16% 5% 2 5% 4 5% 5% 4 6% 7% 41%	4441
September	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 5% 8% 42%	2275
	11th to 20th	Day	13% 5% 2 5% 4 4 4 3 6% 6% 48%	2090
	21st to end	Day	19% 4 2 6% 4 4 4 4 5% 8% 41%	2090
October	1st to 10th	Day	17% 7% 2 7% 5% 5% 5% 3 5% 6% 39%	2081
	11th to 20th	Day	20% 8% 2 7% 5% 5% 6% 4 7% 6% 31%	1740
	21st to end	Day	20% 10% 3 7% 4 4 4 3 5% 7% 32%	1881
November	1st to 10th	Day	12% 10% 4 9% 5% 5% 4 3 6% 5% 36%	1761
	11th to 20th	Day	15% 11% 4 8% 5% 5% 4 4 5% 7% 32%	1725
	21st to end	Day	16% 15% 4 9% 5% 5% 4 4 5% 6% 28%	1418
December	1st to 10th	Day	11% 14% 5% 9% 4 5% 4 3 4 6% 34%	2280
	11th to 20th	Day	15% 9% 4 9% 6% 5% 5% 4 4 5% 33%	2618
	21st to end	Day	14% 11% 3 9% 5% 5% 5% 4 6% 6% 31%	3476
January	1st to 10th	Night	11% 18% 6% 10% 6% 5% 5% 3 4 5% 27%	6486
	11th to 20th	Night	6% 13% 7% 15% 8% 6% 5% 4 5% 6% 26%	6315
	21st to end	Night	4 8% 4 13% 6% 6% 5% 4 6% 8% 38%	6996
February	1st to 10th	Night	6% 10% 3 10% 6% 5% 5% 4 5% 7% 38%	2777
	11th to 20th	Night	7% 13% 5% 11% 7% 5% 5% 3 6% 5% 33%	2405
	21st to end	Night	10% 12% 4 8% 5% 5% 4 4 6% 7% 36%	1980
March	1st to 10th	Night	16% 14% 4 10% 5% 4 5% 4 5% 5% 27%	5364
	11th to 20th	Night	10% 12% 4 10% 6% 6% 6% 4 6% 8% 28%	5128
	21st to end	Night	14% 16% 4 9% 4 3 4 3 4 4 34%	5203
April	1st to 10th	Night	17% 15% 4 7% 4 4 4 2 5% 4 34%	2000
	11th to 20th	Night	16% 8% 3 8% 5% 4 6% 3 5% 8% 35%	1832
	21st to end	Night	21% 11% 3 6% 4 3 4 3 5% 5% 36%	1762
May	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 5% 6% 38%	2802
	11th to 20th	Night	18% 9% 2 7% 5% 5% 5% 4 5% 7% 34%	2495
	21st to end	Night	17% 6% 2 5% 4 4 5% 5% 6% 9% 39%	2472
June	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 4 5% 7% 34%	1330
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 3 6% 8% 42%	1330
	21st to end	Night	19% 8% 3 8% 4 5% 5% 4 6% 5% 33%	1330
July	1st to 10th	Night	14% 6% 2 6% 4 5% 5% 4 7% 8% 40%	2226
	11th to 20th	Night	14% 5% 2 5% 4 4 5% 4 6% 8% 41%	2520
	21st to end	Night	11% 4 2 3 4 4 4 4 6% 9% 49%	2739
August	1st to 10th	Night	15% 6% 2 6% 5% 5% 6% 4 6% 7% 40%	2528
	11th to 20th	Night	13% 5% 2 8% 6% 7% 6% 5% 7% 7% 34%	2782
	21st to end	Night	15% 6% 3 7% 3 4 4 3 4 7% 44%	3212
September	1st to 10th	Night	14% 5% 1 5% 5% 4 4 3 5% 6% 48%	1905
	11th to 20th	Night	15% 6% 2 6% 3 5% 3 4 5% 6% 45%	2090
	21st to end	Night	16% 6% 2 7% 4 4 4 3 4 6% 44%	2090
October	1st to 10th	Night	15% 7% 3 6% 4 4 4 3 5% 6% 44%	2099
	11th to 20th	Night	21% 9% 2 6% 3 4 5% 3 4 6% 36%	2437
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 7% 37%	2717
November	1st to 10th	Night	13% 9% 2 7% 5% 4 4 5% 5% 7% 39%	2537
	11th to 20th	Night	16% 9% 3 7% 4 4 4 4 5% 6% 36%	2675
	21st to end	Night	15% 13% 4 7% 6% 4 4 3 5% 6% 32%	2980
December	1st to 10th	Night	10% 12% 5% 12% 6% 5% 4 4 4 6% 33%	4910
	11th to 20th	Night	17% 10% 3 7% 5% 5% 5% 4 6% 7% 32%	5591
	21st to end	Night	14% 14% 5% 9% 5% 5% 5% 4 5% 6% 30%	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	57% 6% 11% 19% 7%	3462
	11th to 20th	Day	53% 7% 12% 20% 7% 1	3600
	21st to end	Day	39% 9% 10% 19% 23% 1	3960
February	1st to 10th	Day	49% 8% 11% 16% 12% 3	1621
	11th to 20th	Day	54% 7% 12% 15% 11% 2	1888
	21st to end	Day	47% 9% 9% 19% 13% 3	1650
March	1st to 10th	Day	56% 8% 9% 10% 11% 5%	4479
	11th to 20th	Day	47% 10% 18% 15% 8% 1	4830
	21st to end	Day	53% 5% 8% 17% 13% 5%	5712
April	1st to 10th	Day	54% 7% 10% 14% 11% 3	2400
	11th to 20th	Day	46% 6% 12% 21% 12% 3	2515
	21st to end	Day	52% 7% 10% 16% 12% 2	2535
May	1st to 10th	Day	46% 9% 15% 16% 13% 1	4182
	11th to 20th	Day	46% 9% 17% 20% 6% 1	4402
	21st to end	Day	45% 10% 13% 19% 12%	5109
June	1st to 10th	Day	49% 12% 13% 19% 7% 1	2850
	11th to 20th	Day	44% 10% 14% 20% 10% 1	2850
	21st to end	Day	51% 11% 13% 16% 8%	2850
July	1st to 10th	Day	42% 11% 18% 21% 8%	4665
	11th to 20th	Day	41% 12% 19% 24% 5%	4365
	21st to end	Day	38% 12% 18% 22% 10%	4661
August	1st to 10th	Day	42% 14% 19% 18% 8%	4355
	11th to 20th	Day	45% 13% 20% 17% 4	4084
	21st to end	Day	40% 9% 19% 19% 12% 1	4441
September	1st to 10th	Day	39% 10% 16% 23% 12%	2275
	11th to 20th	Day	36% 8% 18% 27% 10%	2090
	21st to end	Day	41% 10% 14% 23% 11%	2090
October	1st to 10th	Day	50% 9% 13% 22% 6%	3458
	11th to 20th	Day	43% 8% 18% 26% 6%	2865
	21st to end	Day	42% 7% 22% 17% 12% 1	3081
November	1st to 10th	Day	48% 9% 12% 20% 9% 1	1761
	11th to 20th	Day	52% 8% 10% 21% 8% 1	1725
	21st to end	Day	57% 7% 14% 15% 6% 1	1418
December	1st to 10th	Day	52% 6% 12% 18% 13%	2280
	11th to 20th	Day	49% 8% 13% 16% 13% 1	2618
	21st to end	Day	50% 10% 13% 17% 9% 2	3476
January	1st to 10th	Night	59% 6% 12% 15% 7% 1	6486
	11th to 20th	Night	57% 8% 12% 15% 6% 1	6360
	21st to end	Night	42% 7% 11% 21% 18% 1	6996
February	1st to 10th	Night	45% 8% 11% 20% 14% 2	2777
	11th to 20th	Night	53% 6% 11% 15% 14% 1	2408
	21st to end	Night	47% 7% 10% 20% 13% 2	1980
March	1st to 10th	Night	56% 7% 10% 12% 13% 2	5374
	11th to 20th	Night	52% 9% 16% 14% 7% 3	5129
	21st to end	Night	53% 5% 8% 20% 13% 2	5203
April	1st to 10th	Night	54% 5% 8% 17% 14% 3	2000
	11th to 20th	Night	47% 8% 10% 21% 13% 1	1832
	21st to end	Night	52% 5% 9% 19% 12% 2	1762
May	1st to 10th	Night	45% 8% 14% 20% 12% 1	2808
	11th to 20th	Night	45% 10% 15% 19% 10% 1	2496
	21st to end	Night	40% 9% 16% 21% 13% 1	2473
June	1st to 10th	Night	48% 8% 14% 21% 7% 1	1330
	11th to 20th	Night	39% 8% 14% 23% 13% 3	1330
	21st to end	Night	51% 10% 12% 17% 10%	1330
July	1st to 10th	Night	39% 8% 23% 26% 4	2191
	11th to 20th	Night	39% 9% 21% 26% 5%	2472
	21st to end	Night	31% 9% 25% 23% 11%	2651
August	1st to 10th	Night	42% 9% 17% 24% 7%	2518
	11th to 20th	Night	45% 13% 20% 13% 9%	2809
	21st to end	Night	40% 8% 13% 27% 12%	3212
September	1st to 10th	Night	36% 9% 16% 25% 13% 1	1905
	11th to 20th	Night	39% 8% 16% 27% 9% 1	2090
	21st to end	Night	41% 8% 13% 25% 12% 1	2090
October	1st to 10th	Night	44% 7% 13% 28% 8% 1	3502
	11th to 20th	Night	43% 6% 13% 31% 6%	4051
	21st to end	Night	40% 8% 20% 19% 13% 1	4467
November	1st to 10th	Night	44% 9% 14% 23% 9% 1	2537
	11th to 20th	Night	48% 8% 12% 20% 10% 1	2675
	21st to end	Night	53% 9% 11% 18% 8% 1	2980
December	1st to 10th	Night	53% 6% 11% 22% 7%	4910
	11th to 20th	Night	49% 7% 11% 22% 11% 1	5592
	21st to end	Night	53% 8% 13% 16% 9% 1	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	22% 8% 35% 28% 7% 1	200
	11th to 20th	Day	25% 14% 32% 21% 8% 1	200
	21st to end	Day	15% 16% 40% 20% 7% 3	220
February	1st to 10th	Day	18% 16% 43% 16% 6%	221
	11th to 20th	Day	20% 16% 38% 20% 5%	461
	21st to end	Day	18% 17% 37% 23% 5% 1	414
March	1st to 10th	Day	24% 18% 30% 21% 6%	500
	11th to 20th	Day	20% 18% 33% 20% 9% 1	500
	21st to end	Day	23% 21% 29% 19% 8% 1	550
April	1st to 10th	Day	30% 18% 27% 18% 6% 1	500
	11th to 20th	Day	25% 16% 31% 20% 7% 1	492
	21st to end	Day	29% 15% 35% 15% 4 2	480
May	1st to 10th	Day	28% 11% 29% 22% 9% 1	463
	11th to 20th	Day	23% 18% 28% 21% 9% 2	487
	21st to end	Day	22% 17% 32% 21% 6% 1	684
June	1st to 10th	Day	26% 11% 26% 25% 10% 1	680
	11th to 20th	Day	23% 12% 27% 24% 13% 1	680
	21st to end	Day	24% 10% 28% 25% 10% 2	680
July	1st to 10th	Day	18% 9% 30% 30% 11% 2	663
	11th to 20th	Day	16% 9% 35% 27% 10% 2	484
	21st to end	Day	16% 8% 34% 30% 11% 2	532
August	1st to 10th	Day	15% 10% 33% 25% 13% 3	486
	11th to 20th	Day	15% 8% 29% 31% 13% 4	479
	21st to end	Day	17% 13% 31% 19% 16% 3	539
September	1st to 10th	Day	18% 13% 29% 26% 10% 4	490
	11th to 20th	Day	18% 9% 31% 25% 11% 5%	490
	21st to end	Day	22% 11% 27% 22% 13% 5%	490
October	1st to 10th	Day	19% 15% 27% 26% 9% 3	485
	11th to 20th	Day	27% 13% 27% 19% 12% 2	390
	21st to end	Day	31% 12% 27% 23% 7%	429
November	1st to 10th	Day	26% 12% 32% 23% 6% 2	395
	11th to 20th	Day	26% 11% 27% 27% 9% 1	400
	21st to end	Day	30% 18% 29% 17% 5%	218
December	1st to 10th	Day	23% 12% 32% 20% 11% 3	200
	11th to 20th	Day	27% 17% 32% 18% 6% 2	200
	21st to end	Day	25% 16% 27% 24% 6% 2	220
January	1st to 10th	Night	21% 10% 30% 33% 6% 1	830
	11th to 20th	Night	18% 9% 33% 32% 8%	800
	21st to end	Night	12% 8% 28% 42% 9% 1	880
February	1st to 10th	Night	13% 8% 38% 31% 9% 1	800
	11th to 20th	Night	16% 12% 36% 27% 8% 1	783
	21st to end	Night	18% 9% 31% 33% 8%	660
March	1st to 10th	Night	20% 13% 30% 31% 6% 1	800
	11th to 20th	Night	19% 13% 31% 29% 8% 1	800
	21st to end	Night	22% 14% 28% 28% 8% 1	880
April	1st to 10th	Night	28% 10% 31% 25% 6% 1	800
	11th to 20th	Night	24% 10% 28% 29% 9%	648
	21st to end	Night	30% 10% 26% 26% 8% 1	588
May	1st to 10th	Night	26% 8% 28% 30% 7%	551
	11th to 20th	Night	26% 7% 21% 32% 13%	374
	21st to end	Night	27% 8% 27% 29% 8% 1	411
June	1st to 10th	Night	23% 5% 22% 36% 13% 1	380
	11th to 20th	Night	17% 3 23% 38% 17% 2	380
	21st to end	Night	22% 5% 21% 38% 13% 1	380
July	1st to 10th	Night	20% 5% 22% 42% 11% 1	370
	11th to 20th	Night	14% 3 19% 45% 17% 1	368
	21st to end	Night	15% 4 19% 47% 15% 1	416
August	1st to 10th	Night	14% 4 14% 47% 20% 2	387
	11th to 20th	Night	17% 4 18% 40% 21% 1	562
	21st to end	Night	16% 5% 23% 36% 18% 1	676
September	1st to 10th	Night	16% 6% 24% 39% 14% 1	760
	11th to 20th	Night	21% 4 22% 36% 15% 3	760
	21st to end	Night	19% 6% 20% 38% 16% 1	760
October	1st to 10th	Night	20% 10% 24% 34% 11% 2	760
	11th to 20th	Night	26% 8% 23% 31% 12% 1	760
	21st to end	Night	26% 9% 26% 30% 9%	836
November	1st to 10th	Night	22% 12% 27% 29% 9% 1	780
	11th to 20th	Night	23% 12% 27% 27% 8% 1	837
	21st to end	Night	25% 13% 24% 29% 7% 1	900
December	1st to 10th	Night	23% 11% 29% 28% 8% 1	900
	11th to 20th	Night	22% 13% 29% 28% 8%	900
	21st to end	Night	24% 11% 27% 26% 10% 1	990

Page generated at 2025-04-23 09:25:02. Version ac07

Copyright © 2025 Attilla Danko.