Ibis Observatoire
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ibis Observatoire clear sky chart from 2005-11-06 to 2026-02-01.

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% 11% 4 9% 6% 5% 5% 4 5% 6% 28%	127256
	Night	14% 11% 4 10% 6% 5% 5% 4 5% 6% 29%	124817
Transparency	Day	55% 7% 11% 18% 9% 1	130720
	Night	54% 7% 12% 18% 9% 1	129440
Seeing	Day	25% 20% 31% 16% 6% 1	17009
	Night	22% 18% 31% 21% 7% 1	25479

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% 17% 7% 15% 8% 5% 5% 4 4 6% 20%	3615
	11th to 20th	Day	7% 14% 7% 20% 9% 6% 5% 4 4 4 20%	3650
	21st to end	Day	6% 8% 5% 18% 7% 6% 7% 4 5% 5% 28%	4048
February	1st to 10th	Day	9% 14% 5% 14% 9% 9% 6% 5% 5% 5% 19%	1661
	11th to 20th	Day	11% 16% 6% 14% 8% 5% 6% 3 5% 4 21%	1884
	21st to end	Day	10% 14% 4 14% 7% 6% 5% 3 4 6% 27%	1650
March	1st to 10th	Day	16% 19% 6% 11% 5% 5% 4 3 3 5% 23%	4476
	11th to 20th	Day	12% 17% 4 10% 5% 5% 5% 4 6% 6% 26%	4857
	21st to end	Day	15% 16% 6% 10% 5% 4 4 3 4 4 28%	5665
April	1st to 10th	Day	20% 16% 5% 9% 5% 4 4 3 4 5% 25%	2400
	11th to 20th	Day	16% 13% 3 9% 5% 4 4 3 5% 6% 33%	2517
	21st to end	Day	21% 14% 3 8% 5% 4 3 3 4 6% 29%	2600
May	1st to 10th	Day	22% 13% 4 9% 5% 5% 5% 4 5% 6% 24%	5886
	11th to 20th	Day	23% 10% 3 8% 6% 5% 5% 4 5% 6% 26%	6289
	21st to end	Day	17% 10% 3 8% 5% 6% 5% 5% 6% 7% 28%	7260
June	1st to 10th	Day	21% 10% 3 7% 5% 5% 5% 4 5% 8% 26%	3000
	11th to 20th	Day	20% 8% 3 5% 5% 5% 5% 4 6% 8% 31%	3000
	21st to end	Day	23% 9% 4 9% 6% 6% 4 3 5% 6% 25%	3000
July	1st to 10th	Day	17% 8% 3 8% 6% 6% 7% 5% 8% 8% 25%	6689
	11th to 20th	Day	14% 5% 2 7% 5% 6% 6% 5% 7% 9% 33%	6360
	21st to end	Day	12% 6% 2 6% 5% 4 5% 5% 7% 9% 40%	6926
August	1st to 10th	Day	15% 6% 3 8% 5% 5% 6% 4 8% 10% 30%	4439
	11th to 20th	Day	16% 8% 3 8% 7% 6% 7% 6% 6% 7% 26%	4175
	21st to end	Day	18% 7% 3 6% 5% 5% 5% 4 6% 7% 35%	4581
September	1st to 10th	Day	15% 7% 3 8% 5% 5% 4 3 6% 8% 37%	2377
	11th to 20th	Day	16% 7% 3 5% 3 4 4 3 5% 7% 44%	2200
	21st to end	Day	16% 9% 2 7% 5% 4 4 4 5% 7% 38%	2200
October	1st to 10th	Day	17% 10% 3 7% 4 5% 5% 3 5% 6% 34%	2185
	11th to 20th	Day	19% 11% 4 9% 5% 5% 4 4 5% 6% 27%	1861
	21st to end	Day	21% 16% 4 10% 5% 5% 4 3 4 5% 23%	1980
November	1st to 10th	Day	16% 15% 4 9% 6% 5% 4 3 4 5% 27%	1851
	11th to 20th	Day	16% 14% 5% 10% 7% 7% 5% 3 5% 6% 24%	1836
	21st to end	Day	16% 19% 5% 10% 6% 5% 5% 4 5% 5% 21%	1547
December	1st to 10th	Day	12% 17% 6% 13% 7% 5% 5% 3 6% 5% 21%	2350
	11th to 20th	Day	14% 15% 5% 11% 8% 4 4 3 5% 5% 25%	2688
	21st to end	Day	16% 15% 6% 11% 7% 5% 4 3 4 5% 24%	3553
January	1st to 10th	Night	9% 15% 8% 14% 7% 7% 6% 4 5% 6% 19%	6538
	11th to 20th	Night	6% 14% 7% 19% 10% 8% 6% 4 5% 5% 17%	6455
	21st to end	Night	5% 8% 6% 16% 6% 6% 6% 5% 7% 7% 29%	7150
February	1st to 10th	Night	7% 14% 5% 12% 9% 6% 5% 5% 6% 5% 25%	2738
	11th to 20th	Night	9% 13% 6% 14% 7% 6% 5% 3 7% 5% 24%	2402
	21st to end	Night	11% 13% 4 10% 6% 6% 5% 4 4 6% 31%	1980
March	1st to 10th	Night	16% 16% 4 10% 5% 5% 5% 3 4 6% 25%	5364
	11th to 20th	Night	11% 12% 4 10% 7% 7% 7% 5% 7% 6% 24%	5101
	21st to end	Night	16% 14% 4 10% 6% 5% 4 3 4 5% 28%	5252
April	1st to 10th	Night	16% 15% 5% 7% 5% 5% 4 4 5% 5% 29%	2000
	11th to 20th	Night	15% 13% 3 7% 4 4 4 4 7% 8% 31%	1832
	21st to end	Night	19% 13% 3 9% 4 4 4 3 4 5% 31%	1800
May	1st to 10th	Night	21% 12% 3 7% 4 4 5% 4 5% 6% 29%	4054
	11th to 20th	Night	22% 10% 3 6% 5% 4 5% 3 5% 5% 32%	3634
	21st to end	Night	17% 8% 2 6% 4 4 4 5% 5% 8% 37%	3627
June	1st to 10th	Night	21% 11% 2 6% 6% 4 6% 4 7% 7% 28%	1400
	11th to 20th	Night	14% 8% 3 7% 4 4 6% 4 5% 8% 38%	1400
	21st to end	Night	19% 10% 3 7% 5% 5% 6% 4 6% 7% 30%	1400
July	1st to 10th	Night	13% 7% 2 7% 5% 5% 6% 5% 7% 10% 32%	3281
	11th to 20th	Night	17% 6% 2 6% 4 5% 4 4 6% 8% 37%	3600
	21st to end	Night	9% 5% 2 6% 5% 6% 5% 5% 6% 8% 43%	3926
August	1st to 10th	Night	17% 7% 3 7% 5% 4 5% 4 6% 8% 35%	2677
	11th to 20th	Night	14% 6% 3 9% 6% 5% 6% 4 6% 8% 33%	2872
	21st to end	Night	15% 6% 2 6% 5% 5% 5% 3 5% 7% 40%	3313
September	1st to 10th	Night	14% 6% 2 6% 4 5% 4 4 5% 7% 43%	2022
	11th to 20th	Night	15% 6% 2 5% 4 4 4 4 5% 6% 44%	2200
	21st to end	Night	16% 7% 3 6% 4 5% 5% 3 5% 6% 41%	2200
October	1st to 10th	Night	15% 8% 3 6% 4 4 4 3 5% 7% 41%	2215
	11th to 20th	Night	19% 10% 3 6% 5% 5% 5% 3 5% 6% 32%	2535
	21st to end	Night	18% 12% 4 8% 5% 5% 4 4 5% 6% 28%	2860
November	1st to 10th	Night	16% 11% 4 9% 4 6% 5% 4 6% 7% 27%	2667
	11th to 20th	Night	14% 11% 4 10% 6% 5% 5% 4 6% 7% 29%	2783
	21st to end	Night	14% 15% 6% 11% 6% 5% 5% 4 5% 6% 23%	3071
December	1st to 10th	Night	11% 11% 5% 13% 8% 8% 6% 5% 7% 6% 20%	5060
	11th to 20th	Night	15% 15% 6% 11% 7% 6% 5% 3 4 5% 22%	5741
	21st to end	Night	15% 16% 5% 11% 6% 4 4 3 4 4 27%	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	68% 5% 8% 13% 6%	3616
	11th to 20th	Day	69% 5% 7% 12% 7% 1	3680
	21st to end	Day	58% 5% 6% 12% 18% 1	4048
February	1st to 10th	Day	65% 7% 8% 10% 9% 2	1661
	11th to 20th	Day	65% 6% 9% 11% 8% 1	1892
	21st to end	Day	59% 5% 7% 18% 8% 3	1650
March	1st to 10th	Day	66% 4 6% 10% 11% 4	4479
	11th to 20th	Day	58% 7% 14% 14% 8% 1	4857
	21st to end	Day	60% 3 6% 16% 12% 3	5665
April	1st to 10th	Day	62% 5% 9% 13% 10% 2	2400
	11th to 20th	Day	52% 6% 9% 19% 12% 2	2517
	21st to end	Day	57% 5% 8% 16% 11% 3	2600
May	1st to 10th	Day	61% 6% 7% 15% 10% 1	5897
	11th to 20th	Day	57% 8% 12% 15% 6% 2	6293
	21st to end	Day	53% 8% 10% 18% 9% 2	7260
June	1st to 10th	Day	55% 7% 10% 19% 7% 1	3000
	11th to 20th	Day	49% 8% 13% 20% 9% 1	3000
	21st to end	Day	59% 8% 10% 16% 7%	3000
July	1st to 10th	Day	53% 8% 13% 18% 7%	6616
	11th to 20th	Day	43% 10% 16% 27% 4	6285
	21st to end	Day	39% 8% 15% 30% 8%	6728
August	1st to 10th	Day	47% 10% 18% 19% 7%	4425
	11th to 20th	Day	53% 11% 16% 16% 4	4214
	21st to end	Day	45% 9% 15% 19% 10% 1	4581
September	1st to 10th	Day	44% 8% 14% 22% 10% 1	2377
	11th to 20th	Day	40% 6% 13% 28% 12%	2200
	21st to end	Day	45% 7% 11% 24% 11%	2200
October	1st to 10th	Day	52% 6% 11% 24% 7%	3562
	11th to 20th	Day	49% 8% 15% 23% 5%	3012
	21st to end	Day	50% 6% 16% 19% 10%	3180
November	1st to 10th	Day	57% 7% 13% 16% 7%	1851
	11th to 20th	Day	61% 8% 9% 15% 7% 1	1836
	21st to end	Day	65% 9% 8% 13% 5%	1547
December	1st to 10th	Day	63% 7% 10% 11% 10%	2350
	11th to 20th	Day	60% 6% 10% 14% 11%	2688
	21st to end	Day	62% 5% 8% 10% 11% 2	3553
January	1st to 10th	Night	66% 7% 9% 13% 5%	6538
	11th to 20th	Night	68% 6% 9% 12% 4 1	6500
	21st to end	Night	53% 6% 8% 15% 17% 1	7150
February	1st to 10th	Night	59% 8% 9% 13% 10% 1	2738
	11th to 20th	Night	61% 7% 9% 13% 9% 1	2404
	21st to end	Night	55% 6% 8% 17% 12% 2	1980
March	1st to 10th	Night	61% 5% 7% 13% 13% 1	5374
	11th to 20th	Night	57% 8% 15% 12% 7% 1	5102
	21st to end	Night	59% 4 5% 18% 13% 1	5252
April	1st to 10th	Night	57% 6% 8% 14% 12% 3	2000
	11th to 20th	Night	50% 8% 10% 19% 12% 1	1832
	21st to end	Night	55% 6% 8% 18% 11% 2	1800
May	1st to 10th	Night	54% 6% 11% 17% 11% 1	4060
	11th to 20th	Night	52% 7% 13% 20% 8%	3635
	21st to end	Night	45% 8% 15% 18% 12% 2	3628
June	1st to 10th	Night	53% 8% 12% 20% 7%	1400
	11th to 20th	Night	44% 8% 13% 22% 11% 2	1400
	21st to end	Night	52% 9% 14% 17% 9%	1400
July	1st to 10th	Night	44% 9% 21% 21% 5%	3244
	11th to 20th	Night	43% 8% 21% 24% 4	3552
	21st to end	Night	37% 9% 21% 25% 8%	3809
August	1st to 10th	Night	46% 9% 17% 25% 4	2667
	11th to 20th	Night	47% 10% 19% 17% 6%	2899
	21st to end	Night	42% 9% 14% 22% 12% 1	3313
September	1st to 10th	Night	39% 10% 14% 27% 10% 1	2022
	11th to 20th	Night	39% 8% 17% 27% 9%	2200
	21st to end	Night	43% 9% 11% 24% 13% 1	2200
October	1st to 10th	Night	43% 6% 12% 29% 9% 1	3618
	11th to 20th	Night	47% 7% 12% 28% 6%	4123
	21st to end	Night	46% 7% 17% 18% 11%	4610
November	1st to 10th	Night	54% 10% 12% 18% 6%	2667
	11th to 20th	Night	55% 8% 11% 16% 9% 1	2783
	21st to end	Night	61% 9% 12% 13% 5%	3071
December	1st to 10th	Night	62% 7% 13% 15% 3	5060
	11th to 20th	Night	63% 6% 6% 17% 7%	5742
	21st to end	Night	61% 5% 8% 13% 12% 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	25% 32% 33% 9% 1	220
	11th to 20th	Day	29% 27% 33% 8% 3	220
	21st to end	Day	22% 24% 37% 12% 4	242
February	1st to 10th	Day	28% 29% 30% 11% 3	261
	11th to 20th	Day	26% 27% 37% 9% 2	463
	21st to end	Day	24% 28% 32% 13% 3	414
March	1st to 10th	Day	29% 23% 31% 13% 3	500
	11th to 20th	Day	27% 26% 30% 11% 6%	500
	21st to end	Day	29% 24% 27% 14% 5% 1	550
April	1st to 10th	Day	35% 24% 23% 12% 5% 1	500
	11th to 20th	Day	29% 24% 28% 14% 3 1	492
	21st to end	Day	34% 20% 30% 11% 3 2	500
May	1st to 10th	Day	30% 16% 37% 14% 3 1	503
	11th to 20th	Day	27% 17% 31% 18% 6% 1	527
	21st to end	Day	24% 19% 34% 17% 5%	702
June	1st to 10th	Day	24% 17% 35% 17% 7% 1	730
	11th to 20th	Day	22% 18% 28% 23% 8% 1	730
	21st to end	Day	27% 14% 30% 21% 6% 1	730
July	1st to 10th	Day	20% 16% 36% 20% 8%	669
	11th to 20th	Day	19% 15% 41% 18% 6% 1	524
	21st to end	Day	19% 15% 37% 22% 7% 1	576
August	1st to 10th	Day	18% 16% 32% 23% 8% 3	526
	11th to 20th	Day	17% 12% 34% 24% 11% 2	519
	21st to end	Day	19% 17% 30% 21% 9% 4	583
September	1st to 10th	Day	17% 19% 29% 20% 11% 3	530
	11th to 20th	Day	21% 13% 30% 21% 10% 5%	530
	21st to end	Day	20% 17% 32% 17% 10% 5%	530
October	1st to 10th	Day	23% 19% 25% 20% 11% 2	522
	11th to 20th	Day	27% 20% 28% 15% 8% 1	420
	21st to end	Day	34% 23% 27% 11% 4	462
November	1st to 10th	Day	33% 19% 32% 13% 3	425
	11th to 20th	Day	26% 23% 29% 15% 7% 1	430
	21st to end	Day	32% 27% 29% 9% 2	297
December	1st to 10th	Day	30% 17% 34% 15% 2 1	220
	11th to 20th	Day	35% 25% 31% 8% 1	220
	21st to end	Day	32% 24% 26% 14% 4	242
January	1st to 10th	Night	23% 25% 36% 13% 2	851
	11th to 20th	Night	23% 23% 34% 17% 3	840
	21st to end	Night	15% 23% 38% 20% 4	924
February	1st to 10th	Night	16% 30% 36% 14% 4	801
	11th to 20th	Night	23% 25% 36% 13% 3	783
	21st to end	Night	22% 20% 37% 17% 3	660
March	1st to 10th	Night	23% 22% 35% 17% 3	800
	11th to 20th	Night	21% 23% 34% 16% 5% 1	800
	21st to end	Night	22% 20% 31% 21% 6%	880
April	1st to 10th	Night	29% 21% 30% 15% 5%	800
	11th to 20th	Night	25% 18% 31% 20% 6%	648
	21st to end	Night	33% 13% 28% 20% 6% 1	600
May	1st to 10th	Night	24% 14% 32% 23% 7%	600
	11th to 20th	Night	23% 11% 29% 28% 8% 1	427
	21st to end	Night	23% 13% 32% 25% 6% 1	433
June	1st to 10th	Night	24% 9% 34% 26% 8%	400
	11th to 20th	Night	17% 13% 27% 34% 10% 1	400
	21st to end	Night	23% 9% 32% 29% 8% 1	400
July	1st to 10th	Night	16% 11% 33% 29% 10%	390
	11th to 20th	Night	18% 7% 33% 31% 10% 1	388
	21st to end	Night	15% 5% 32% 37% 10%	438
August	1st to 10th	Night	15% 6% 32% 32% 15% 1	450
	11th to 20th	Night	17% 7% 29% 34% 13% 1	592
	21st to end	Night	15% 10% 29% 31% 13% 2	711
September	1st to 10th	Night	17% 11% 31% 29% 11% 1	800
	11th to 20th	Night	19% 9% 28% 27% 15% 2	800
	21st to end	Night	20% 12% 30% 24% 14% 1	800
October	1st to 10th	Night	20% 16% 30% 23% 10% 1	800
	11th to 20th	Night	27% 15% 30% 20% 8% 1	800
	21st to end	Night	28% 20% 31% 16% 5%	880
November	1st to 10th	Night	28% 21% 29% 17% 4	820
	11th to 20th	Night	24% 22% 27% 19% 6% 1	868
	21st to end	Night	26% 24% 31% 14% 5% 1	950
December	1st to 10th	Night	25% 22% 33% 15% 5% 1	950
	11th to 20th	Night	26% 27% 26% 17% 5%	950
	21st to end	Night	27% 25% 28% 16% 4	1045

Page generated at 2026-01-29 01:21:43. Version ac07

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