Observatoire Ébouliborg
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Ébouliborg clear sky chart from 2005-11-06 to 2025-09-19.

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% 9% 3 7% 5% 5% 5% 4 6% 7% 34%	126298
	Night	15% 11% 4 8% 5% 5% 4 3 5% 6% 34%	122851
Transparency	Day	47% 9% 15% 18% 9% 1	129746
	Night	50% 8% 14% 18% 9% 1	127491
Seeing	Day	21% 15% 30% 23% 9% 2	16668
	Night	21% 10% 30% 29% 9% 1	24844

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	15% 20% 5% 8% 4 4 3 3 5% 6% 26%	3438
	11th to 20th	Day	10% 14% 5% 11% 8% 7% 6% 4 5% 6% 25%	3570
	21st to end	Day	5% 7% 4 11% 6% 5% 6% 4 6% 7% 39%	3960
February	1st to 10th	Day	6% 12% 4 10% 8% 6% 6% 5% 6% 7% 31%	1628
	11th to 20th	Day	9% 14% 4 12% 6% 5% 4 4 5% 5% 32%	1871
	21st to end	Day	11% 10% 3 9% 7% 5% 5% 4 6% 5% 35%	1650
March	1st to 10th	Day	19% 13% 4 9% 5% 4 4 4 4 5% 29%	4476
	11th to 20th	Day	12% 13% 3 7% 5% 5% 5% 5% 6% 7% 30%	4835
	21st to end	Day	15% 15% 4 8% 5% 4 4 3 4 5% 33%	5690
April	1st to 10th	Day	19% 12% 3 8% 6% 4 4 3 4 6% 31%	2400
	11th to 20th	Day	16% 8% 3 7% 5% 5% 4 3 5% 6% 38%	2522
	21st to end	Day	22% 10% 3 6% 5% 4 4 4 5% 6% 33%	2600
May	1st to 10th	Day	21% 9% 3 7% 4 4 4 4 5% 6% 32%	5968
	11th to 20th	Day	23% 7% 2 7% 5% 4 5% 3 5% 6% 32%	6318
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 4 6% 8% 32%	7361
June	1st to 10th	Day	20% 7% 2 6% 5% 5% 6% 4 6% 7% 31%	3000
	11th to 20th	Day	17% 6% 2 6% 4 5% 5% 5% 6% 6% 37%	3000
	21st to end	Day	22% 6% 3 7% 5% 4 5% 4 6% 8% 29%	3000
July	1st to 10th	Day	17% 6% 2 8% 6% 6% 6% 4 7% 9% 31%	6820
	11th to 20th	Day	13% 6% 3 7% 5% 5% 6% 5% 7% 8% 34%	6368
	21st to end	Day	11% 5% 2 5% 4 5% 5% 4 6% 8% 43%	6926
August	1st to 10th	Day	12% 5% 2 7% 5% 6% 7% 5% 8% 9% 34%	4499
	11th to 20th	Day	14% 6% 3 7% 6% 6% 6% 5% 8% 8% 32%	4175
	21st to end	Day	18% 4 2 6% 4 4 4 3 5% 7% 42%	4590
September	1st to 10th	Day	13% 6% 3 5% 4 4 5% 4 6% 8% 43%	2390
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 6% 50%	2178
	21st to end	Day	18% 5% 2 6% 4 3 4 3 5% 8% 42%	2090
October	1st to 10th	Day	18% 6% 2 7% 5% 4 3 3 5% 7% 39%	2074
	11th to 20th	Day	21% 7% 2 7% 6% 4 5% 4 6% 7% 31%	1748
	21st to end	Day	23% 10% 3 6% 4 4 4 3 5% 6% 32%	1881
November	1st to 10th	Day	15% 10% 4 9% 4 4 4 4 5% 5% 36%	1761
	11th to 20th	Day	16% 11% 4 8% 4 5% 4 3 5% 6% 32%	1712
	21st to end	Day	18% 18% 4 7% 5% 4 4 3 5% 6% 26%	1425
December	1st to 10th	Day	14% 14% 5% 8% 5% 4 5% 3 5% 6% 32%	2280
	11th to 20th	Day	17% 11% 4 8% 7% 6% 5% 3 4 4 32%	2618
	21st to end	Day	16% 10% 4 9% 6% 7% 6% 3 4 5% 30%	3476
January	1st to 10th	Night	14% 19% 6% 10% 5% 4 4 3 5% 5% 25%	6511
	11th to 20th	Night	8% 16% 7% 14% 7% 5% 4 3 4 6% 25%	6315
	21st to end	Night	5% 9% 5% 11% 6% 6% 6% 3 6% 7% 37%	6996
February	1st to 10th	Night	7% 11% 4 10% 6% 5% 5% 4 5% 7% 36%	2770
	11th to 20th	Night	10% 13% 5% 12% 6% 5% 4 3 5% 6% 31%	2410
	21st to end	Night	13% 12% 4 8% 5% 4 4 3 6% 7% 35%	1980
March	1st to 10th	Night	18% 13% 4 10% 5% 4 4 4 5% 6% 27%	5364
	11th to 20th	Night	12% 13% 4 10% 6% 6% 6% 4 6% 7% 27%	5123
	21st to end	Night	18% 16% 4 7% 4 4 3 3 4 5% 33%	5238
April	1st to 10th	Night	20% 14% 4 7% 4 4 4 3 4 5% 33%	2000
	11th to 20th	Night	19% 8% 3 7% 4 5% 5% 4 5% 6% 33%	1827
	21st to end	Night	24% 10% 2 6% 4 3 4 4 4 4 35%	1800
May	1st to 10th	Night	20% 8% 3 7% 4 4 4 4 5% 6% 35%	4015
	11th to 20th	Night	25% 8% 2 6% 5% 4 4 3 6% 6% 31%	3575
	21st to end	Night	18% 7% 3 6% 4 4 5% 4 5% 8% 36%	3532
June	1st to 10th	Night	19% 9% 3 7% 5% 4 4 4 6% 6% 34%	1400
	11th to 20th	Night	15% 6% 2 5% 5% 5% 4 4 6% 8% 41%	1400
	21st to end	Night	21% 8% 3 7% 5% 5% 4 3 6% 7% 32%	1400
July	1st to 10th	Night	14% 6% 3 6% 5% 5% 5% 4 6% 9% 38%	3160
	11th to 20th	Night	17% 6% 2 6% 4 4 5% 4 6% 7% 40%	3589
	21st to end	Night	11% 5% 2 4 4 4 4 3 5% 8% 49%	3926
August	1st to 10th	Night	15% 6% 2 6% 4 4 5% 4 6% 8% 39%	2617
	11th to 20th	Night	14% 5% 2 8% 7% 6% 6% 5% 6% 7% 35%	2872
	21st to end	Night	17% 7% 3 7% 3 3 4 3 4 7% 43%	3305
September	1st to 10th	Night	14% 5% 2 5% 4 4 4 3 6% 7% 46%	2010
	11th to 20th	Night	16% 5% 2 5% 3 4 5% 3 5% 6% 45%	2179
	21st to end	Night	18% 6% 2 5% 4 4 3 3 4 5% 45%	2090
October	1st to 10th	Night	16% 8% 3 7% 4 3 4 3 5% 5% 43%	2105
	11th to 20th	Night	23% 8% 3 5% 5% 3 4 3 4 6% 35%	2429
	21st to end	Night	22% 9% 2 5% 5% 4 4 3 5% 6% 36%	2717
November	1st to 10th	Night	15% 9% 3 7% 4 4 4 3 5% 7% 38%	2537
	11th to 20th	Night	18% 9% 3 7% 4 4 4 4 5% 6% 35%	2685
	21st to end	Night	17% 13% 4 8% 5% 5% 4 3 5% 6% 31%	2971
December	1st to 10th	Night	12% 13% 6% 10% 6% 5% 4 3 5% 5% 31%	4910
	11th to 20th	Night	20% 12% 3 9% 5% 4 4 3 5% 6% 30%	5591
	21st to end	Night	16% 14% 5% 9% 6% 7% 5% 3 5% 4 28%	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	59% 7% 13% 15% 5%	3439
	11th to 20th	Day	56% 9% 11% 17% 6% 1	3600
	21st to end	Day	41% 11% 12% 17% 19%	3960
February	1st to 10th	Day	50% 9% 12% 15% 11% 3	1628
	11th to 20th	Day	54% 8% 12% 15% 10% 1	1879
	21st to end	Day	48% 9% 10% 20% 11% 3	1650
March	1st to 10th	Day	56% 8% 11% 9% 12% 4	4479
	11th to 20th	Day	50% 11% 17% 14% 7% 1	4835
	21st to end	Day	53% 6% 8% 16% 12% 5%	5690
April	1st to 10th	Day	53% 7% 11% 14% 11% 3	2400
	11th to 20th	Day	45% 8% 11% 21% 12% 3	2522
	21st to end	Day	51% 7% 12% 16% 13% 2	2600
May	1st to 10th	Day	51% 9% 12% 15% 12% 1	5979
	11th to 20th	Day	50% 9% 14% 18% 6% 3	6322
	21st to end	Day	46% 10% 15% 17% 11%	7361
June	1st to 10th	Day	49% 11% 11% 20% 8% 1	3000
	11th to 20th	Day	44% 10% 14% 20% 10% 1	3000
	21st to end	Day	50% 11% 14% 16% 9% 1	3000
July	1st to 10th	Day	47% 9% 17% 21% 7%	6745
	11th to 20th	Day	41% 12% 20% 21% 5%	6292
	21st to end	Day	35% 10% 18% 28% 9%	6728
August	1st to 10th	Day	40% 13% 20% 19% 8%	4485
	11th to 20th	Day	43% 13% 20% 18% 5%	4214
	21st to end	Day	39% 10% 20% 18% 12%	4590
September	1st to 10th	Day	37% 10% 18% 24% 12%	2390
	11th to 20th	Day	34% 10% 16% 29% 10%	2178
	21st to end	Day	40% 11% 15% 24% 11%	2090
October	1st to 10th	Day	48% 11% 14% 21% 6%	3451
	11th to 20th	Day	45% 7% 19% 25% 4	2886
	21st to end	Day	44% 8% 21% 17% 10%	3081
November	1st to 10th	Day	49% 10% 13% 19% 8% 1	1761
	11th to 20th	Day	53% 8% 13% 18% 7% 1	1712
	21st to end	Day	60% 7% 16% 12% 5%	1425
December	1st to 10th	Day	55% 8% 14% 12% 11%	2280
	11th to 20th	Day	52% 7% 13% 15% 11% 1	2618
	21st to end	Day	57% 9% 12% 13% 7% 2	3476
January	1st to 10th	Night	61% 8% 13% 12% 6% 1	6511
	11th to 20th	Night	61% 8% 13% 12% 5% 1	6360
	21st to end	Night	43% 9% 12% 20% 16% 1	6996
February	1st to 10th	Night	49% 8% 12% 18% 12% 2	2770
	11th to 20th	Night	55% 6% 13% 14% 12% 1	2413
	21st to end	Night	49% 7% 12% 18% 12% 2	1980
March	1st to 10th	Night	57% 7% 9% 13% 12% 2	5374
	11th to 20th	Night	54% 10% 13% 14% 6% 2	5124
	21st to end	Night	54% 5% 8% 20% 11% 1	5238
April	1st to 10th	Night	55% 4 10% 15% 13% 3	2000
	11th to 20th	Night	49% 8% 11% 19% 12% 1	1827
	21st to end	Night	53% 6% 9% 19% 12% 2	1800
May	1st to 10th	Night	49% 8% 13% 16% 13% 1	4021
	11th to 20th	Night	50% 9% 13% 19% 8% 1	3576
	21st to end	Night	43% 8% 17% 18% 12% 2	3533
June	1st to 10th	Night	50% 7% 13% 21% 8% 1	1400
	11th to 20th	Night	41% 8% 14% 21% 13% 3	1400
	21st to end	Night	51% 10% 13% 16% 10%	1400
July	1st to 10th	Night	41% 9% 22% 23% 5%	3125
	11th to 20th	Night	42% 9% 21% 24% 4	3542
	21st to end	Night	33% 8% 22% 27% 9%	3809
August	1st to 10th	Night	42% 9% 19% 23% 7%	2607
	11th to 20th	Night	46% 13% 18% 13% 9%	2899
	21st to end	Night	42% 8% 15% 24% 11%	3305
September	1st to 10th	Night	38% 8% 17% 25% 12% 1	2010
	11th to 20th	Night	39% 9% 17% 26% 9% 1	2179
	21st to end	Night	42% 8% 13% 25% 12% 1	2090
October	1st to 10th	Night	45% 7% 13% 27% 7%	3508
	11th to 20th	Night	46% 6% 14% 28% 5%	4030
	21st to end	Night	43% 8% 20% 17% 11% 1	4467
November	1st to 10th	Night	47% 7% 17% 20% 8% 1	2537
	11th to 20th	Night	49% 9% 17% 17% 9% 1	2685
	21st to end	Night	58% 7% 15% 12% 7% 1	2971
December	1st to 10th	Night	58% 7% 13% 16% 6%	4910
	11th to 20th	Night	54% 8% 11% 19% 7% 1	5592
	21st to end	Night	59% 9% 11% 12% 7% 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% 16% 37% 20% 5% 2	200
	11th to 20th	Day	22% 19% 35% 16% 8% 1	200
	21st to end	Day	13% 22% 38% 21% 5% 2	220
February	1st to 10th	Day	17% 21% 36% 20% 5%	228
	11th to 20th	Day	18% 19% 39% 20% 4	456
	21st to end	Day	16% 20% 35% 21% 7%	414
March	1st to 10th	Day	24% 22% 27% 22% 5%	500
	11th to 20th	Day	20% 17% 31% 23% 8% 1	500
	21st to end	Day	23% 20% 28% 21% 7% 1	550
April	1st to 10th	Day	27% 20% 27% 20% 5% 1	500
	11th to 20th	Day	23% 20% 29% 21% 7% 1	492
	21st to end	Day	28% 17% 32% 17% 3 2	500
May	1st to 10th	Day	27% 13% 30% 21% 9% 1	503
	11th to 20th	Day	20% 19% 29% 21% 9% 2	527
	21st to end	Day	22% 18% 30% 21% 8% 2	751
June	1st to 10th	Day	24% 13% 29% 24% 9% 1	730
	11th to 20th	Day	21% 12% 27% 26% 13% 1	730
	21st to end	Day	23% 10% 28% 28% 10% 1	730
July	1st to 10th	Day	17% 10% 30% 30% 12% 1	720
	11th to 20th	Day	15% 12% 29% 31% 12% 2	531
	21st to end	Day	13% 10% 33% 31% 12% 1	576
August	1st to 10th	Day	13% 13% 26% 29% 14% 4	526
	11th to 20th	Day	14% 7% 28% 31% 17% 3	519
	21st to end	Day	16% 14% 30% 18% 17% 4	583
September	1st to 10th	Day	16% 14% 28% 26% 12% 4	530
	11th to 20th	Day	18% 10% 28% 25% 13% 6%	522
	21st to end	Day	20% 14% 24% 24% 13% 5%	490
October	1st to 10th	Day	19% 16% 30% 22% 10% 3	481
	11th to 20th	Day	25% 14% 29% 22% 9% 2	390
	21st to end	Day	31% 11% 30% 21% 6%	429
November	1st to 10th	Day	26% 17% 30% 23% 3 2	395
	11th to 20th	Day	24% 16% 29% 22% 9% 1	400
	21st to end	Day	29% 25% 25% 18% 3	225
December	1st to 10th	Day	24% 14% 31% 18% 11% 3	200
	11th to 20th	Day	28% 19% 31% 16% 6% 2	200
	21st to end	Day	26% 15% 30% 23% 5% 1	220
January	1st to 10th	Night	22% 11% 36% 25% 5% 1	835
	11th to 20th	Night	19% 10% 38% 27% 7% 1	800
	21st to end	Night	13% 10% 34% 35% 8% 1	880
February	1st to 10th	Night	15% 10% 39% 30% 7% 1	800
	11th to 20th	Night	17% 14% 38% 25% 7% 1	783
	21st to end	Night	19% 11% 34% 29% 5% 1	660
March	1st to 10th	Night	20% 14% 34% 27% 6% 1	800
	11th to 20th	Night	19% 13% 34% 26% 7% 1	800
	21st to end	Night	23% 12% 32% 26% 6%	880
April	1st to 10th	Night	28% 11% 31% 24% 6%	800
	11th to 20th	Night	23% 13% 28% 27% 8%	643
	21st to end	Night	30% 11% 26% 27% 7%	600
May	1st to 10th	Night	26% 8% 31% 30% 5%	587
	11th to 20th	Night	23% 10% 23% 32% 11%	394
	21st to end	Night	26% 9% 27% 30% 8% 1	433
June	1st to 10th	Night	22% 6% 23% 39% 10% 1	400
	11th to 20th	Night	17% 5% 23% 37% 17% 1	400
	21st to end	Night	22% 6% 27% 33% 12% 1	400
July	1st to 10th	Night	18% 6% 29% 35% 12% 1	390
	11th to 20th	Night	14% 4 23% 44% 14% 1	388
	21st to end	Night	15% 3 21% 47% 12% 2	438
August	1st to 10th	Night	12% 4 16% 47% 19% 2	416
	11th to 20th	Night	16% 3 21% 38% 21% 1	592
	21st to end	Night	16% 6% 24% 36% 16% 2	703
September	1st to 10th	Night	15% 6% 26% 37% 15% 1	800
	11th to 20th	Night	20% 7% 21% 31% 17% 4	793
	21st to end	Night	19% 6% 25% 34% 14% 1	760
October	1st to 10th	Night	21% 9% 29% 30% 9% 3	760
	11th to 20th	Night	26% 8% 27% 27% 12% 1	760
	21st to end	Night	26% 10% 31% 26% 8%	836
November	1st to 10th	Night	22% 15% 29% 27% 7%	780
	11th to 20th	Night	23% 14% 30% 24% 8% 1	843
	21st to end	Night	26% 15% 29% 24% 6% 1	900
December	1st to 10th	Night	24% 11% 32% 24% 7% 1	900
	11th to 20th	Night	23% 16% 32% 24% 6%	900
	21st to end	Night	25% 13% 31% 22% 8% 1	990

Page generated at 2025-09-16 01:49:16. Version ac07

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