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 2024-01-14.

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 8% 7% 4 5% 5% 5% 6% 28%	74562
	Night	15% 11% 4 8% 7% 4 5% 5% 5% 6% 30%	75204
Transparency	Day	56% 8% 11% 17% 8% 1	74263
	Night	54% 8% 11% 17% 9% 1	75037
Seeing	Day	25% 20% 31% 16% 6% 1	14367
	Night	23% 18% 31% 20% 7% 1	22847

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% 15% 7% 13% 10% 4 6% 5% 4 6% 18%	1493
	11th to 20th	Day	12% 17% 7% 11% 9% 4 5% 4 4 5% 24%	1464
	21st to end	Day	8% 11% 7% 9% 9% 6% 6% 6% 5% 5% 28%	1584
February	1st to 10th	Day	10% 14% 6% 12% 10% 7% 7% 5% 5% 5% 18%	1494
	11th to 20th	Day	11% 16% 6% 12% 10% 3 6% 4 4 4 21%	1691
	21st to end	Day	11% 13% 5% 12% 9% 4 6% 4 4 6% 27%	1480
March	1st to 10th	Day	12% 15% 6% 10% 7% 5% 5% 5% 4 6% 25%	1797
	11th to 20th	Day	15% 14% 5% 9% 6% 4 5% 5% 5% 4 28%	1927
	21st to end	Day	14% 13% 5% 10% 6% 4 5% 5% 4 5% 28%	2253
April	1st to 10th	Day	21% 15% 5% 8% 6% 3 4 3 4 6% 24%	2160
	11th to 20th	Day	17% 12% 3 8% 5% 3 4 4 4 5% 35%	2262
	21st to end	Day	21% 13% 4 7% 6% 3 3 4 4 6% 28%	2340
May	1st to 10th	Day	20% 11% 4 7% 4 3 5% 4 5% 6% 31%	2340
	11th to 20th	Day	20% 10% 3 6% 6% 3 4 4 4 6% 33%	2463
	21st to end	Day	18% 7% 4 7% 6% 4 5% 5% 6% 8% 30%	2840
June	1st to 10th	Day	22% 9% 3 6% 6% 3 6% 5% 5% 8% 25%	2700
	11th to 20th	Day	21% 8% 4 5% 5% 3 6% 5% 5% 8% 31%	2700
	21st to end	Day	23% 9% 4 8% 7% 4 5% 4 5% 6% 24%	2700
July	1st to 10th	Day	16% 8% 4 6% 6% 5% 5% 6% 7% 8% 29%	2653
	11th to 20th	Day	16% 7% 3 7% 7% 5% 6% 6% 7% 8% 28%	2520
	21st to end	Day	18% 8% 4 6% 6% 4 6% 6% 6% 8% 29%	2744
August	1st to 10th	Day	18% 7% 4 7% 6% 4 6% 6% 7% 9% 25%	2441
	11th to 20th	Day	15% 8% 4 7% 7% 4 6% 6% 5% 8% 31%	2304
	21st to end	Day	14% 7% 4 6% 6% 4 5% 6% 7% 8% 34%	2525
September	1st to 10th	Day	16% 7% 3 7% 6% 4 4 4 5% 8% 35%	2140
	11th to 20th	Day	18% 7% 3 5% 4 3 3 4 5% 7% 42%	1980
	21st to end	Day	17% 9% 2 6% 5% 3 4 5% 5% 7% 37%	1980
October	1st to 10th	Day	18% 9% 3 7% 6% 4 5% 5% 5% 6% 33%	1967
	11th to 20th	Day	21% 11% 4 8% 7% 4 5% 5% 5% 6% 25%	1675
	21st to end	Day	22% 16% 4 9% 7% 4 4 4 4 5% 22%	1782
November	1st to 10th	Day	18% 13% 5% 8% 6% 3 4 5% 4 5% 28%	1671
	11th to 20th	Day	17% 12% 5% 9% 8% 5% 5% 5% 5% 6% 23%	1662
	21st to end	Day	16% 17% 5% 8% 7% 4 5% 5% 5% 4 23%	1401
December	1st to 10th	Day	17% 17% 9% 12% 8% 3 5% 3 4 3 19%	1330
	11th to 20th	Day	17% 15% 7% 10% 7% 3 5% 4 5% 5% 24%	1668
	21st to end	Day	19% 15% 6% 9% 7% 4 5% 4 4 4 22%	2431
January	1st to 10th	Night	12% 15% 8% 14% 9% 4 6% 5% 5% 4 18%	2670
	11th to 20th	Night	11% 13% 6% 11% 9% 5% 6% 5% 5% 6% 24%	2574
	21st to end	Night	7% 9% 7% 10% 8% 4 6% 7% 7% 8% 27%	2772
February	1st to 10th	Night	8% 14% 6% 11% 10% 5% 6% 6% 6% 6% 24%	2464
	11th to 20th	Night	10% 12% 6% 13% 8% 4 5% 5% 6% 6% 23%	2155
	21st to end	Night	12% 12% 5% 9% 7% 4 6% 5% 4 6% 30%	1776
March	1st to 10th	Night	13% 12% 6% 9% 7% 3 5% 4 5% 7% 29%	2150
	11th to 20th	Night	13% 11% 5% 8% 6% 4 6% 6% 5% 6% 31%	2031
	21st to end	Night	13% 12% 5% 8% 7% 5% 5% 5% 5% 5% 30%	2102
April	1st to 10th	Night	17% 14% 6% 7% 6% 3 5% 5% 5% 5% 28%	1800
	11th to 20th	Night	15% 11% 3 6% 6% 2 4 5% 7% 8% 33%	1647
	21st to end	Night	20% 13% 3 8% 5% 3 4 4 4 5% 30%	1620
May	1st to 10th	Night	20% 10% 4 5% 5% 3 5% 5% 4 6% 32%	1620
	11th to 20th	Night	20% 8% 3 6% 6% 3 4 4 4 5% 37%	1446
	21st to end	Night	19% 7% 3 5% 6% 4 5% 5% 5% 6% 36%	1433
June	1st to 10th	Night	21% 10% 3 6% 7% 2 6% 5% 7% 7% 27%	1260
	11th to 20th	Night	15% 8% 3 6% 5% 2 5% 6% 4 7% 38%	1260
	21st to end	Night	19% 9% 3 6% 5% 3 6% 5% 5% 7% 32%	1260
July	1st to 10th	Night	14% 8% 3 6% 6% 3 5% 5% 6% 9% 36%	1303
	11th to 20th	Night	14% 6% 2 6% 5% 4 6% 6% 6% 9% 36%	1440
	21st to end	Night	13% 6% 3 6% 6% 4 6% 6% 6% 8% 37%	1574
August	1st to 10th	Night	14% 5% 3 5% 5% 3 4 5% 7% 9% 40%	1478
	11th to 20th	Night	15% 7% 3 7% 5% 3 5% 7% 6% 8% 35%	1596
	21st to end	Night	12% 7% 3 5% 5% 4 5% 5% 5% 8% 41%	1827
September	1st to 10th	Night	15% 6% 2 5% 5% 4 5% 5% 5% 8% 40%	1819
	11th to 20th	Night	16% 6% 2 5% 5% 3 4 5% 4 7% 42%	1980
	21st to end	Night	16% 7% 3 5% 5% 3 5% 4 5% 6% 41%	1980
October	1st to 10th	Night	16% 8% 3 5% 5% 3 4 5% 5% 7% 39%	1993
	11th to 20th	Night	20% 10% 3 6% 6% 4 5% 5% 5% 6% 30%	2281
	21st to end	Night	19% 12% 4 7% 7% 4 4 5% 5% 6% 27%	2574
November	1st to 10th	Night	16% 10% 4 8% 5% 5% 6% 5% 6% 7% 28%	2407
	11th to 20th	Night	14% 10% 4 8% 7% 4 5% 5% 6% 7% 30%	2517
	21st to end	Night	15% 13% 6% 9% 7% 4 5% 5% 5% 7% 24%	2778
December	1st to 10th	Night	17% 13% 6% 11% 8% 4 6% 5% 5% 5% 19%	2850
	11th to 20th	Night	17% 13% 6% 10% 8% 4 5% 5% 5% 5% 22%	3531
	21st to end	Night	18% 18% 5% 9% 6% 3 4 4 4 4 25%	5236

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% 6% 8% 11% 6%	1494
	11th to 20th	Day	65% 6% 6% 14% 8% 2	1464
	21st to end	Day	56% 6% 8% 16% 13% 1	1584
February	1st to 10th	Day	67% 7% 7% 10% 8% 1	1494
	11th to 20th	Day	65% 7% 8% 11% 8% 1	1699
	21st to end	Day	59% 5% 7% 18% 8% 3	1480
March	1st to 10th	Day	61% 5% 8% 14% 11% 2	1800
	11th to 20th	Day	59% 6% 8% 15% 11% 1	1927
	21st to end	Day	60% 4 9% 15% 9% 3	2253
April	1st to 10th	Day	63% 5% 9% 12% 9% 2	2160
	11th to 20th	Day	52% 6% 8% 20% 12% 2	2262
	21st to end	Day	58% 5% 8% 16% 11% 3	2340
May	1st to 10th	Day	54% 7% 9% 16% 13% 1	2340
	11th to 20th	Day	52% 7% 11% 20% 10%	2467
	21st to end	Day	51% 10% 12% 19% 9%	2840
June	1st to 10th	Day	56% 8% 11% 18% 7% 1	2700
	11th to 20th	Day	50% 8% 12% 20% 9% 1	2700
	21st to end	Day	60% 8% 9% 15% 7%	2700
July	1st to 10th	Day	51% 10% 15% 18% 6%	2580
	11th to 20th	Day	49% 12% 14% 19% 6%	2445
	21st to end	Day	53% 10% 12% 18% 5% 1	2591
August	1st to 10th	Day	53% 11% 16% 17% 3	2427
	11th to 20th	Day	50% 10% 14% 19% 7%	2304
	21st to end	Day	45% 11% 14% 22% 7%	2525
September	1st to 10th	Day	46% 9% 14% 21% 10% 1	2140
	11th to 20th	Day	43% 6% 13% 26% 12%	1980
	21st to end	Day	46% 8% 12% 24% 11%	1980
October	1st to 10th	Day	51% 7% 11% 23% 8%	1967
	11th to 20th	Day	58% 9% 14% 15% 3	1675
	21st to end	Day	64% 7% 8% 14% 6% 1	1782
November	1st to 10th	Day	57% 7% 13% 16% 7%	1671
	11th to 20th	Day	62% 8% 9% 13% 7% 1	1662
	21st to end	Day	64% 9% 9% 14% 5%	1401
December	1st to 10th	Day	70% 6% 8% 11% 5%	1330
	11th to 20th	Day	63% 6% 8% 14% 9% 1	1668
	21st to end	Day	66% 6% 9% 11% 6% 2	2431
January	1st to 10th	Night	69% 8% 7% 12% 5% 1	2670
	11th to 20th	Night	63% 7% 10% 12% 8% 1	2574
	21st to end	Night	52% 9% 9% 17% 11% 2	2772
February	1st to 10th	Night	60% 9% 9% 13% 9% 1	2464
	11th to 20th	Night	62% 8% 9% 13% 8% 1	2157
	21st to end	Night	56% 6% 8% 16% 13% 1	1776
March	1st to 10th	Night	56% 6% 9% 18% 10% 2	2160
	11th to 20th	Night	52% 9% 11% 16% 10% 2	2032
	21st to end	Night	55% 6% 9% 17% 10% 3	2102
April	1st to 10th	Night	58% 7% 8% 14% 11% 3	1800
	11th to 20th	Night	48% 8% 11% 20% 13% 1	1647
	21st to end	Night	56% 6% 8% 17% 11% 2	1620
May	1st to 10th	Night	53% 7% 10% 16% 13% 1	1620
	11th to 20th	Night	49% 7% 11% 19% 13% 1	1447
	21st to end	Night	50% 8% 11% 17% 14%	1434
June	1st to 10th	Night	54% 9% 13% 18% 6%	1260
	11th to 20th	Night	45% 7% 13% 22% 11% 2	1260
	21st to end	Night	52% 9% 14% 16% 9%	1260
July	1st to 10th	Night	43% 12% 17% 22% 6%	1266
	11th to 20th	Night	42% 11% 18% 22% 6%	1392
	21st to end	Night	44% 11% 17% 20% 7%	1486
August	1st to 10th	Night	38% 12% 20% 28% 2	1468
	11th to 20th	Night	45% 11% 18% 21% 6%	1596
	21st to end	Night	38% 10% 15% 28% 8%	1827
September	1st to 10th	Night	40% 11% 15% 24% 10% 1	1819
	11th to 20th	Night	41% 9% 17% 24% 9% 1	1980
	21st to end	Night	43% 9% 11% 24% 13% 1	1980
October	1st to 10th	Night	44% 7% 12% 24% 11% 1	1993
	11th to 20th	Night	54% 10% 12% 19% 5%	2281
	21st to end	Night	57% 9% 11% 14% 8% 1	2574
November	1st to 10th	Night	54% 10% 12% 18% 6%	2407
	11th to 20th	Night	54% 9% 11% 16% 9% 1	2517
	21st to end	Night	60% 9% 12% 14% 5%	2778
December	1st to 10th	Night	67% 8% 10% 12% 4	2850
	11th to 20th	Night	64% 7% 7% 14% 8%	3532
	21st to end	Night	63% 6% 8% 14% 9% 1	5236

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% 8% 1 1	180
	11th to 20th	Day	29% 28% 32% 7% 4 1	166
	21st to end	Day	20% 26% 37% 14% 3 1	176
February	1st to 10th	Day	29% 30% 29% 10% 2	214
	11th to 20th	Day	26% 27% 37% 7% 2	390
	21st to end	Day	24% 29% 29% 14% 4	346
March	1st to 10th	Day	27% 24% 33% 13% 3	420
	11th to 20th	Day	27% 26% 29% 10% 7%	420
	21st to end	Day	27% 24% 26% 16% 6% 1	462
April	1st to 10th	Day	33% 25% 22% 13% 5% 1	420
	11th to 20th	Day	28% 25% 28% 15% 4 1	412
	21st to end	Day	34% 19% 31% 11% 4 2	420
May	1st to 10th	Day	29% 16% 38% 14% 2 1	423
	11th to 20th	Day	26% 18% 33% 18% 4 1	447
	21st to end	Day	23% 19% 34% 18% 5% 1	600
June	1st to 10th	Day	25% 17% 35% 16% 7% 1	630
	11th to 20th	Day	21% 19% 27% 23% 9%	630
	21st to end	Day	27% 13% 30% 22% 7% 1	630
July	1st to 10th	Day	20% 16% 35% 21% 9% 1	575
	11th to 20th	Day	18% 14% 42% 18% 7%	444
	21st to end	Day	20% 14% 37% 22% 6% 1	488
August	1st to 10th	Day	20% 16% 31% 23% 8% 2	446
	11th to 20th	Day	18% 12% 36% 24% 10% 1	439
	21st to end	Day	20% 17% 30% 20% 9% 4	495
September	1st to 10th	Day	17% 20% 29% 20% 11% 3	450
	11th to 20th	Day	24% 14% 32% 18% 9% 3	450
	21st to end	Day	20% 18% 33% 15% 9% 5%	450
October	1st to 10th	Day	23% 19% 25% 20% 11% 2	444
	11th to 20th	Day	29% 19% 28% 15% 7% 1	360
	21st to end	Day	35% 23% 26% 12% 4	396
November	1st to 10th	Day	33% 17% 33% 13% 3	365
	11th to 20th	Day	25% 24% 28% 16% 7% 1	370
	21st to end	Day	29% 27% 30% 10% 3	251
December	1st to 10th	Day	32% 17% 32% 16% 3 1	180
	11th to 20th	Day	36% 24% 31% 8% 1 1	180
	21st to end	Day	36% 25% 25% 13% 1	198
January	1st to 10th	Night	23% 26% 36% 12% 2	769
	11th to 20th	Night	23% 23% 35% 16% 2	736
	21st to end	Night	15% 23% 38% 20% 4	792
February	1st to 10th	Night	17% 30% 36% 13% 4	720
	11th to 20th	Night	23% 26% 35% 14% 3	703
	21st to end	Night	23% 21% 36% 17% 4	592
March	1st to 10th	Night	22% 22% 36% 17% 3	720
	11th to 20th	Night	22% 23% 33% 15% 6% 1	720
	21st to end	Night	21% 20% 31% 21% 6% 1	792
April	1st to 10th	Night	28% 20% 31% 15% 5%	720
	11th to 20th	Night	25% 18% 31% 20% 6%	583
	21st to end	Night	32% 13% 27% 21% 7% 1	540
May	1st to 10th	Night	24% 13% 32% 23% 8%	540
	11th to 20th	Night	23% 11% 30% 27% 7% 1	385
	21st to end	Night	23% 13% 32% 25% 7% 1	389
June	1st to 10th	Night	24% 10% 34% 24% 8%	360
	11th to 20th	Night	18% 11% 28% 33% 10% 1	360
	21st to end	Night	22% 9% 31% 29% 8% 1	360
July	1st to 10th	Night	16% 10% 35% 29% 10%	350
	11th to 20th	Night	16% 7% 35% 30% 11% 1	348
	21st to end	Night	16% 6% 30% 37% 11%	394
August	1st to 10th	Night	15% 6% 33% 31% 15% 1	404
	11th to 20th	Night	17% 7% 30% 32% 13% 1	532
	21st to end	Night	15% 10% 30% 31% 13% 2	639
September	1st to 10th	Night	17% 11% 32% 29% 10% 1	720
	11th to 20th	Night	21% 10% 29% 25% 13% 2	720
	21st to end	Night	21% 13% 30% 24% 13% 1	720
October	1st to 10th	Night	20% 17% 29% 24% 9% 1	720
	11th to 20th	Night	28% 15% 29% 19% 8% 1	720
	21st to end	Night	29% 20% 30% 15% 5%	792
November	1st to 10th	Night	27% 21% 29% 18% 4	740
	11th to 20th	Night	25% 24% 25% 19% 6% 1	782
	21st to end	Night	25% 24% 32% 14% 4 1	850
December	1st to 10th	Night	27% 23% 31% 14% 5% 1	850
	11th to 20th	Night	25% 26% 27% 17% 5%	850
	21st to end	Night	28% 26% 27% 16% 3	935

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