Observatoire Isis
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Isis 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	17% 11% 4 9% 5% 5% 4 3 5% 7% 31%	113666
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	114920
Transparency	Day	51% 6% 12% 20% 10% 1	117179
	Night	53% 6% 11% 19% 10% 1	119571
Seeing	Day	25% 16% 31% 19% 8% 2	14963
	Night	24% 13% 28% 25% 9% 1	24202

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	13% 18% 7% 16% 8% 5% 4 2 3 4 19%	3597
	11th to 20th	Day	12% 16% 6% 16% 8% 6% 4 3 4 5% 20%	3570
	21st to end	Day	10% 16% 7% 14% 6% 6% 4 3 4 5% 21%	4098
February	1st to 10th	Day	13% 16% 6% 13% 8% 5% 4 3 4 6% 23%	1800
	11th to 20th	Day	13% 16% 6% 12% 6% 5% 4 3 4 5% 27%	1757
	21st to end	Day	19% 12% 4 11% 7% 5% 4 4 5% 5% 24%	1650
March	1st to 10th	Day	20% 15% 4 11% 6% 5% 4 3 5% 5% 24%	4476
	11th to 20th	Day	13% 13% 5% 9% 6% 5% 5% 4 6% 7% 29%	4970
	21st to end	Day	18% 11% 3 8% 6% 4 3 3 4 5% 35%	5780
April	1st to 10th	Day	22% 12% 3 8% 5% 4 3 3 4 6% 31%	2400
	11th to 20th	Day	23% 10% 3 7% 4 3 3 3 4 5% 35%	2374
	21st to end	Day	22% 11% 3 9% 5% 5% 4 3 4 5% 30%	2487
May	1st to 10th	Day	21% 9% 3 8% 5% 4 5% 4 6% 6% 30%	4426
	11th to 20th	Day	18% 8% 3 8% 5% 4 4 3 5% 7% 33%	4398
	21st to end	Day	17% 8% 2 6% 5% 5% 5% 3 5% 7% 36%	4837
June	1st to 10th	Day	23% 8% 3 8% 5% 4 5% 4 5% 7% 29%	2660
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 4 6% 9% 37%	2660
	21st to end	Day	20% 7% 2 6% 4 5% 6% 5% 7% 7% 31%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 10% 34%	4440
	11th to 20th	Day	15% 5% 2 7% 4 5% 5% 5% 6% 8% 39%	4440
	21st to end	Day	12% 6% 2 5% 4 4 4 4 6% 8% 46%	4814
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 6% 4 6% 8% 35%	4374
	11th to 20th	Day	17% 5% 2 7% 4 4 4 4 7% 9% 37%	3994
	21st to end	Day	14% 5% 2 6% 5% 4 5% 4 7% 9% 40%	4180
September	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 5% 8% 43%	2280
	11th to 20th	Day	14% 5% 3 5% 4 4 5% 3 5% 7% 47%	2279
	21st to end	Day	20% 6% 2 5% 4 4 4 3 5% 8% 39%	2029
October	1st to 10th	Day	22% 8% 2 7% 4 4 4 3 5% 6% 34%	1900
	11th to 20th	Day	24% 10% 3 7% 5% 5% 5% 3 5% 6% 28%	1900
	21st to end	Day	26% 10% 4 7% 5% 5% 4 3 4 5% 27%	1929
November	1st to 10th	Day	15% 10% 3 8% 4 5% 5% 4 7% 6% 34%	1761
	11th to 20th	Day	19% 13% 4 9% 5% 5% 5% 4 5% 6% 25%	1600
	21st to end	Day	23% 14% 4 10% 6% 5% 4 3 5% 5% 20%	1600
December	1st to 10th	Day	20% 21% 6% 11% 5% 3 5% 4 5% 6% 14%	2600
	11th to 20th	Day	18% 16% 6% 11% 5% 5% 4 3 3 4 25%	2986
	21st to end	Day	22% 19% 5% 10% 5% 4 4 3 3 5% 21%	3960
January	1st to 10th	Night	13% 23% 8% 13% 5% 4 4 3 4 5% 19%	6349
	11th to 20th	Night	11% 16% 7% 14% 7% 6% 5% 3 4 5% 22%	6315
	21st to end	Night	12% 17% 7% 14% 7% 7% 5% 3 4 5% 21%	6840
February	1st to 10th	Night	13% 15% 5% 12% 7% 5% 5% 4 5% 5% 24%	2600
	11th to 20th	Night	13% 14% 6% 11% 7% 5% 5% 3 4 5% 28%	2528
	21st to end	Night	16% 14% 4 8% 6% 5% 5% 4 4 6% 28%	1980
March	1st to 10th	Night	22% 15% 5% 10% 4 4 4 3 3 3 28%	5364
	11th to 20th	Night	12% 14% 4 9% 5% 5% 5% 4 6% 7% 29%	4984
	21st to end	Night	18% 11% 4 9% 5% 4 3 3 4 6% 33%	5151
April	1st to 10th	Night	21% 11% 3 8% 5% 4 3 3 4 6% 31%	2000
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 5% 5% 35%	1975
	21st to end	Night	22% 8% 3 8% 4 4 3 3 5% 6% 34%	1825
May	1st to 10th	Night	20% 9% 4 8% 4 5% 4 4 5% 6% 30%	2514
	11th to 20th	Night	21% 7% 2 7% 4 4 4 3 5% 6% 36%	2495
	21st to end	Night	16% 6% 3 6% 4 5% 5% 4 6% 6% 38%	2738
June	1st to 10th	Night	18% 6% 2 7% 5% 4 5% 5% 7% 7% 35%	1520
	11th to 20th	Night	15% 5% 2 7% 4 4 5% 4 6% 6% 42%	1520
	21st to end	Night	19% 7% 2 5% 6% 5% 5% 4 5% 8% 35%	1520
July	1st to 10th	Night	13% 7% 3 7% 5% 6% 6% 4 6% 7% 36%	2520
	11th to 20th	Night	15% 5% 2 6% 5% 3 4 4 6% 8% 41%	2520
	21st to end	Night	11% 6% 2 5% 4 4 5% 4 7% 8% 45%	2739
August	1st to 10th	Night	13% 7% 3 7% 5% 5% 4 4 6% 8% 39%	2551
	11th to 20th	Night	16% 8% 2 5% 3 4 4 3 6% 8% 42%	2803
	21st to end	Night	15% 5% 2 6% 5% 4 4 4 5% 7% 41%	3476
September	1st to 10th	Night	12% 5% 2 6% 4 3 4 4 6% 6% 47%	1900
	11th to 20th	Night	15% 6% 1 6% 3 3 4 4 5% 5% 48%	1900
	21st to end	Night	18% 6% 2 6% 5% 3 3 3 4 6% 44%	2147
October	1st to 10th	Night	20% 7% 2 7% 3 4 3 4 5% 5% 40%	2280
	11th to 20th	Night	22% 8% 2 7% 6% 5% 4 4 5% 6% 32%	2280
	21st to end	Night	23% 10% 3 8% 5% 5% 4 3 5% 6% 28%	2669
November	1st to 10th	Night	19% 11% 3 6% 5% 5% 4 4 5% 6% 33%	2537
	11th to 20th	Night	19% 11% 4 10% 5% 5% 5% 3 4 5% 30%	2796
	21st to end	Night	21% 13% 4 10% 5% 4 5% 4 4 6% 23%	2800
December	1st to 10th	Night	18% 18% 5% 11% 6% 5% 5% 4 6% 6% 16%	4580
	11th to 20th	Night	24% 13% 5% 11% 5% 4 3 2 3 4 26%	5208
	21st to end	Night	24% 19% 4 10% 4 3 3 3 4 4 22%	6996

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	69% 6% 8% 13% 4 1	3598
	11th to 20th	Day	63% 5% 11% 13% 8%	3600
	21st to end	Day	63% 6% 10% 11% 8% 1	4098
February	1st to 10th	Day	60% 5% 10% 18% 6% 1	1800
	11th to 20th	Day	59% 4 12% 15% 9% 1	1766
	21st to end	Day	60% 5% 10% 13% 10% 2	1650
March	1st to 10th	Day	61% 5% 8% 12% 11% 3	4479
	11th to 20th	Day	52% 6% 17% 15% 10% 1	4970
	21st to end	Day	52% 4 5% 19% 17% 2	5780
April	1st to 10th	Day	54% 4 8% 18% 13% 3	2400
	11th to 20th	Day	51% 4 6% 21% 15% 2	2374
	21st to end	Day	57% 4 8% 15% 13% 4	2487
May	1st to 10th	Day	51% 7% 8% 22% 10% 2	4440
	11th to 20th	Day	48% 8% 14% 19% 9% 2	4402
	21st to end	Day	45% 7% 11% 21% 14% 2	4837
June	1st to 10th	Day	53% 6% 11% 18% 11% 1	2660
	11th to 20th	Day	42% 8% 12% 23% 12% 2	2660
	21st to end	Day	49% 9% 13% 20% 10% 1	2660
July	1st to 10th	Day	43% 7% 14% 24% 11%	4370
	11th to 20th	Day	40% 7% 15% 30% 8%	4365
	21st to end	Day	35% 8% 15% 31% 12%	4661
August	1st to 10th	Day	45% 7% 16% 23% 9%	4360
	11th to 20th	Day	41% 8% 15% 28% 8%	4033
	21st to end	Day	39% 6% 17% 25% 12% 1	4180
September	1st to 10th	Day	39% 7% 14% 28% 12%	2280
	11th to 20th	Day	36% 6% 14% 31% 12% 1	2279
	21st to end	Day	42% 7% 14% 26% 10%	2029
October	1st to 10th	Day	47% 7% 12% 28% 7%	3160
	11th to 20th	Day	46% 7% 15% 24% 7%	3140
	21st to end	Day	49% 5% 19% 20% 7%	3154
November	1st to 10th	Day	48% 7% 14% 23% 7% 1	1761
	11th to 20th	Day	56% 6% 13% 16% 9% 1	1600
	21st to end	Day	64% 7% 12% 10% 6% 1	1600
December	1st to 10th	Day	69% 6% 13% 10% 2	2600
	11th to 20th	Day	63% 4 10% 16% 7%	2986
	21st to end	Day	66% 5% 8% 15% 5%	3960
January	1st to 10th	Night	69% 5% 11% 12% 4	6349
	11th to 20th	Night	64% 7% 11% 13% 5%	6360
	21st to end	Night	65% 5% 8% 10% 11% 1	6840
February	1st to 10th	Night	61% 5% 10% 16% 7% 1	2600
	11th to 20th	Night	59% 4 10% 19% 8% 1	2530
	21st to end	Night	56% 6% 9% 19% 10% 1	1980
March	1st to 10th	Night	61% 4 6% 14% 12% 3	5374
	11th to 20th	Night	52% 7% 15% 16% 8% 3	4985
	21st to end	Night	52% 4 7% 21% 13% 2	5151
April	1st to 10th	Night	53% 5% 9% 17% 14% 2	2000
	11th to 20th	Night	50% 6% 7% 20% 16% 1	1975
	21st to end	Night	51% 5% 9% 17% 16% 3	1825
May	1st to 10th	Night	52% 9% 8% 19% 9% 3	2520
	11th to 20th	Night	47% 9% 10% 20% 12% 2	2496
	21st to end	Night	43% 7% 12% 19% 17% 2	2739
June	1st to 10th	Night	46% 7% 12% 24% 10% 1	1520
	11th to 20th	Night	40% 8% 11% 24% 16% 2	1520
	21st to end	Night	46% 9% 12% 19% 14% 1	1520
July	1st to 10th	Night	45% 9% 14% 19% 13% 1	2480
	11th to 20th	Night	40% 7% 13% 32% 9%	2472
	21st to end	Night	37% 8% 15% 26% 14%	2651
August	1st to 10th	Night	41% 9% 15% 25% 10%	2541
	11th to 20th	Night	40% 7% 14% 31% 8%	2830
	21st to end	Night	41% 8% 15% 23% 13% 2	3476
September	1st to 10th	Night	36% 8% 11% 30% 15% 1	1900
	11th to 20th	Night	35% 8% 12% 34% 10% 1	1900
	21st to end	Night	42% 5% 12% 30% 11%	2147
October	1st to 10th	Night	39% 8% 13% 28% 11%	3800
	11th to 20th	Night	43% 7% 14% 27% 8%	3779
	21st to end	Night	46% 6% 18% 20% 9%	4394
November	1st to 10th	Night	51% 6% 15% 20% 9%	2537
	11th to 20th	Night	55% 6% 12% 17% 10% 1	2796
	21st to end	Night	61% 6% 12% 15% 5%	2800
December	1st to 10th	Night	66% 6% 13% 11% 3	4580
	11th to 20th	Night	62% 4 7% 15% 11% 1	5208
	21st to end	Night	67% 4 7% 12% 9%	6996

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	28% 17% 36% 16% 4	200
	11th to 20th	Day	28% 18% 39% 12% 3 2	200
	21st to end	Day	22% 20% 36% 16% 4	250
February	1st to 10th	Day	35% 24% 31% 8% 2	300
	11th to 20th	Day	31% 28% 35% 6% 1	304
	21st to end	Day	33% 22% 30% 13% 2	414
March	1st to 10th	Day	29% 19% 33% 13% 5%	500
	11th to 20th	Day	28% 21% 33% 14% 3 1	500
	21st to end	Day	33% 18% 30% 13% 5% 1	550
April	1st to 10th	Day	33% 23% 26% 14% 4 1	500
	11th to 20th	Day	36% 19% 23% 13% 7% 1	492
	21st to end	Day	33% 21% 29% 12% 5% 1	480
May	1st to 10th	Day	26% 16% 32% 16% 6% 3	463
	11th to 20th	Day	19% 18% 34% 19% 8% 2	487
	21st to end	Day	23% 15% 33% 20% 8% 2	529
June	1st to 10th	Day	26% 11% 35% 20% 7% 1	490
	11th to 20th	Day	19% 15% 31% 22% 11% 3	490
	21st to end	Day	26% 11% 31% 20% 9% 3	490
July	1st to 10th	Day	14% 8% 32% 32% 11% 3	485
	11th to 20th	Day	14% 10% 35% 27% 11% 3	484
	21st to end	Day	14% 7% 35% 29% 12% 3	532
August	1st to 10th	Day	16% 9% 28% 30% 14% 3	486
	11th to 20th	Day	19% 4 27% 30% 13% 8%	479
	21st to end	Day	17% 11% 29% 25% 12% 6%	539
September	1st to 10th	Day	18% 11% 31% 22% 12% 7%	490
	11th to 20th	Day	14% 10% 26% 21% 19% 10%	490
	21st to end	Day	22% 12% 27% 21% 14% 4	490
October	1st to 10th	Day	25% 16% 23% 22% 10% 3	490
	11th to 20th	Day	27% 18% 32% 16% 7% 1	490
	21st to end	Day	33% 16% 29% 19% 4	454
November	1st to 10th	Day	28% 17% 30% 18% 6% 1	395
	11th to 20th	Day	27% 27% 30% 14% 3	200
	21st to end	Day	31% 23% 30% 12% 3 2	200
December	1st to 10th	Day	34% 14% 38% 12% 3	200
	11th to 20th	Day	28% 23% 37% 10% 3	200
	21st to end	Day	34% 19% 32% 12% 2 1	220
January	1st to 10th	Night	29% 16% 33% 18% 4	800
	11th to 20th	Night	30% 17% 26% 21% 6% 1	800
	21st to end	Night	20% 15% 36% 25% 5%	880
February	1st to 10th	Night	22% 21% 31% 24% 3	800
	11th to 20th	Night	23% 21% 35% 17% 4	783
	21st to end	Night	25% 18% 29% 23% 4	660
March	1st to 10th	Night	27% 16% 31% 22% 4	800
	11th to 20th	Night	25% 19% 31% 20% 5%	800
	21st to end	Night	27% 17% 28% 21% 7%	880
April	1st to 10th	Night	30% 16% 27% 24% 4	800
	11th to 20th	Night	25% 19% 28% 20% 7% 1	791
	21st to end	Night	27% 15% 27% 23% 8%	651
May	1st to 10th	Night	25% 10% 27% 29% 8% 2	380
	11th to 20th	Night	24% 9% 28% 30% 10%	374
	21st to end	Night	18% 8% 36% 27% 10% 1	411
June	1st to 10th	Night	19% 9% 27% 34% 10%	380
	11th to 20th	Night	16% 9% 26% 34% 14% 1	380
	21st to end	Night	21% 7% 23% 35% 14%	380
July	1st to 10th	Night	13% 3 28% 38% 19%	370
	11th to 20th	Night	13% 4 22% 43% 17% 1	368
	21st to end	Night	10% 5% 24% 42% 18% 2	416
August	1st to 10th	Night	13% 5% 19% 41% 22% 1	397
	11th to 20th	Night	14% 5% 18% 36% 24% 3	569
	21st to end	Night	15% 6% 25% 34% 18% 2	836
September	1st to 10th	Night	15% 7% 29% 29% 19% 1	760
	11th to 20th	Night	17% 7% 20% 30% 22% 4	760
	21st to end	Night	22% 8% 25% 26% 18% 2	760
October	1st to 10th	Night	23% 11% 26% 23% 15% 1	760
	11th to 20th	Night	27% 12% 27% 25% 8% 1	760
	21st to end	Night	29% 14% 28% 24% 5%	836
November	1st to 10th	Night	28% 15% 23% 25% 8% 1	780
	11th to 20th	Night	26% 17% 31% 16% 10%	800
	21st to end	Night	30% 19% 23% 22% 5%	800
December	1st to 10th	Night	30% 16% 29% 21% 4	800
	11th to 20th	Night	29% 18% 27% 20% 5% 1	800
	21st to end	Night	34% 15% 30% 16% 4	880

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