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 2024-05-13.

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	15% 9% 3 7% 5% 5% 5% 4 6% 7% 34%	83056
	Night	16% 11% 4 8% 5% 4 4 3 5% 6% 34%	86451
Transparency	Day	48% 10% 14% 18% 9% 1	82754
	Night	51% 8% 14% 17% 9% 1	86288
Seeing	Day	21% 16% 31% 22% 8% 2	14802
	Night	21% 10% 30% 29% 9% 1	23248

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% 14% 4 8% 5% 5% 4 3 6% 8% 30%	2397
	11th to 20th	Day	11% 10% 4 10% 7% 5% 6% 4 6% 7% 31%	2520
	21st to end	Day	7% 8% 4 10% 5% 5% 6% 4 7% 9% 37%	2772
February	1st to 10th	Day	6% 12% 4 10% 8% 5% 5% 4 6% 7% 31%	1546
	11th to 20th	Day	9% 13% 4 12% 6% 5% 4 4 5% 5% 31%	1775
	21st to end	Day	11% 10% 3 9% 7% 6% 5% 4 6% 5% 34%	1570
March	1st to 10th	Day	16% 11% 4 8% 5% 4 5% 4 5% 5% 33%	3116
	11th to 20th	Day	13% 16% 4 9% 6% 4 5% 4 5% 6% 28%	3377
	21st to end	Day	12% 15% 5% 8% 5% 4 4 3 4 5% 36%	3970
April	1st to 10th	Day	20% 12% 4 8% 6% 4 4 4 4 6% 30%	2280
	11th to 20th	Day	16% 8% 2 7% 5% 5% 4 3 4 6% 38%	2394
	21st to end	Day	21% 10% 3 6% 5% 4 4 4 5% 6% 33%	2470
May	1st to 10th	Day	20% 8% 3 7% 4 5% 4 4 5% 6% 35%	2498
	11th to 20th	Day	19% 7% 2 7% 5% 5% 5% 4 5% 7% 36%	2509
	21st to end	Day	17% 6% 2 6% 5% 4 5% 4 6% 7% 37%	2885
June	1st to 10th	Day	19% 7% 2 7% 5% 5% 6% 4 6% 7% 30%	2700
	11th to 20th	Day	18% 6% 2 6% 4 5% 5% 5% 6% 7% 36%	2700
	21st to end	Day	22% 6% 3 7% 5% 5% 5% 5% 6% 8% 29%	2700
July	1st to 10th	Day	14% 6% 2 6% 6% 5% 5% 4 7% 10% 34%	2700
	11th to 20th	Day	11% 6% 2 8% 6% 6% 6% 5% 6% 9% 34%	2528
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 4 8% 9% 36%	2744
August	1st to 10th	Day	14% 5% 2 7% 5% 6% 7% 5% 8% 10% 30%	2471
	11th to 20th	Day	12% 5% 3 6% 5% 5% 5% 5% 7% 9% 38%	2304
	21st to end	Day	13% 5% 2 7% 5% 4 4 4 6% 9% 40%	2534
September	1st to 10th	Day	14% 6% 3 6% 4 4 5% 4 6% 8% 42%	2152
	11th to 20th	Day	15% 5% 2 5% 3 4 4 3 5% 6% 48%	1980
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 8% 43%	1980
October	1st to 10th	Day	18% 6% 2 7% 5% 4 3 3 5% 7% 40%	1965
	11th to 20th	Day	21% 8% 3 8% 6% 4 5% 4 6% 7% 30%	1656
	21st to end	Day	23% 11% 3 7% 4 4 4 3 5% 6% 30%	1782
November	1st to 10th	Day	15% 10% 4 9% 4 4 4 4 5% 5% 36%	1671
	11th to 20th	Day	16% 11% 4 8% 4 6% 4 3 6% 6% 32%	1627
	21st to end	Day	16% 17% 4 7% 6% 4 4 3 5% 6% 27%	1354
December	1st to 10th	Day	16% 16% 4 9% 6% 4 5% 3 5% 5% 27%	1330
	11th to 20th	Day	18% 12% 4 9% 6% 3 6% 2 5% 4 31%	1668
	21st to end	Day	19% 13% 4 7% 5% 4 3 3 4 5% 35%	2431
January	1st to 10th	Night	11% 15% 5% 10% 6% 5% 5% 4 6% 6% 27%	4549
	11th to 20th	Night	9% 13% 5% 10% 7% 6% 5% 3 5% 6% 30%	4440
	21st to end	Night	7% 12% 6% 11% 6% 6% 6% 3 6% 7% 30%	4884
February	1st to 10th	Night	7% 11% 4 9% 6% 5% 5% 4 5% 7% 36%	2632
	11th to 20th	Night	9% 13% 5% 12% 6% 5% 4 3 5% 5% 32%	2287
	21st to end	Night	13% 12% 3 8% 5% 4 4 3 6% 7% 35%	1884
March	1st to 10th	Night	18% 13% 4 9% 5% 4 4 3 5% 6% 29%	3724
	11th to 20th	Night	12% 16% 5% 11% 6% 6% 6% 4 5% 5% 24%	3581
	21st to end	Night	17% 14% 4 7% 4 4 4 3 5% 5% 34%	3658
April	1st to 10th	Night	21% 14% 4 7% 4 3 4 3 4 5% 33%	1900
	11th to 20th	Night	20% 8% 3 7% 4 5% 5% 4 5% 6% 34%	1735
	21st to end	Night	23% 10% 2 6% 4 4 4 4 5% 4 35%	1710
May	1st to 10th	Night	23% 9% 3 6% 4 4 3 4 3 5% 35%	1698
	11th to 20th	Night	23% 8% 2 4 4 4 4 4 6% 5% 37%	1439
	21st to end	Night	22% 8% 2 5% 3 2 5% 4 4 6% 38%	1390
June	1st to 10th	Night	19% 10% 3 7% 5% 4 4 4 6% 6% 33%	1260
	11th to 20th	Night	15% 6% 3 5% 5% 5% 4 4 6% 8% 41%	1260
	21st to end	Night	20% 8% 2 7% 5% 5% 4 3 6% 6% 33%	1260
July	1st to 10th	Night	17% 7% 3 5% 4 5% 4 4 6% 7% 39%	1260
	11th to 20th	Night	12% 7% 2 6% 5% 5% 5% 4 7% 9% 37%	1429
	21st to end	Night	13% 7% 2 5% 5% 6% 6% 4 6% 9% 38%	1574
August	1st to 10th	Night	13% 4 2 6% 4 4 5% 4 6% 9% 42%	1448
	11th to 20th	Night	15% 5% 2 6% 7% 5% 5% 4 6% 7% 37%	1596
	21st to end	Night	14% 6% 2 6% 3 3 5% 3 4 8% 46%	1820
September	1st to 10th	Night	15% 5% 2 6% 4 4 4 4 6% 7% 45%	1808
	11th to 20th	Night	18% 6% 2 6% 3 4 5% 3 5% 5% 44%	1980
	21st to end	Night	18% 6% 2 5% 4 4 3 3 4 5% 46%	1980
October	1st to 10th	Night	15% 8% 3 7% 4 3 4 3 4 5% 44%	1994
	11th to 20th	Night	23% 8% 3 5% 5% 3 4 3 4 6% 35%	2301
	21st to end	Night	22% 9% 2 5% 5% 4 4 3 5% 6% 35%	2574
November	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 5% 7% 38%	2407
	11th to 20th	Night	18% 9% 3 7% 4 4 4 4 5% 6% 35%	2550
	21st to end	Night	16% 13% 4 8% 5% 5% 4 3 5% 6% 32%	2822
December	1st to 10th	Night	17% 15% 5% 9% 6% 5% 3 3 5% 4 29%	2850
	11th to 20th	Night	18% 12% 4 7% 5% 4 4 3 6% 6% 30%	3531
	21st to end	Night	18% 16% 5% 8% 4 3 3 2 5% 4 31%	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	52% 8% 18% 16% 5%	2398
	11th to 20th	Day	51% 8% 11% 21% 8% 1	2520
	21st to end	Day	43% 12% 14% 17% 14%	2772
February	1st to 10th	Day	51% 9% 13% 16% 10% 2	1546
	11th to 20th	Day	54% 8% 12% 15% 10% 2	1783
	21st to end	Day	48% 9% 10% 19% 11% 3	1570
March	1st to 10th	Day	51% 10% 14% 12% 9% 3	3119
	11th to 20th	Day	54% 10% 15% 13% 7% 1	3377
	21st to end	Day	50% 7% 9% 17% 12% 5%	3970
April	1st to 10th	Day	54% 8% 12% 13% 10% 3	2280
	11th to 20th	Day	45% 8% 12% 21% 11% 3	2394
	21st to end	Day	50% 7% 12% 16% 13% 2	2470
May	1st to 10th	Day	48% 9% 12% 16% 13% 2	2498
	11th to 20th	Day	46% 10% 11% 21% 11% 1	2513
	21st to end	Day	45% 11% 11% 22% 10% 1	2885
June	1st to 10th	Day	49% 11% 12% 19% 8% 1	2700
	11th to 20th	Day	45% 11% 13% 20% 10% 2	2700
	21st to end	Day	50% 11% 14% 16% 8% 1	2700
July	1st to 10th	Day	41% 13% 18% 21% 7%	2625
	11th to 20th	Day	42% 13% 16% 21% 7%	2452
	21st to end	Day	42% 14% 17% 20% 7%	2591
August	1st to 10th	Day	43% 17% 18% 18% 4	2457
	11th to 20th	Day	37% 15% 19% 21% 8%	2304
	21st to end	Day	37% 12% 19% 23% 9%	2534
September	1st to 10th	Day	38% 10% 17% 22% 12%	2152
	11th to 20th	Day	36% 10% 16% 27% 11%	1980
	21st to end	Day	40% 11% 15% 24% 11%	1980
October	1st to 10th	Day	43% 8% 15% 23% 10%	1965
	11th to 20th	Day	53% 9% 17% 17% 4	1656
	21st to end	Day	55% 9% 13% 16% 6% 1	1782
November	1st to 10th	Day	49% 10% 13% 19% 8% 1	1671
	11th to 20th	Day	53% 8% 14% 18% 7% 1	1627
	21st to end	Day	58% 7% 16% 13% 5%	1354
December	1st to 10th	Day	62% 7% 12% 12% 7%	1330
	11th to 20th	Day	58% 6% 10% 16% 9% 1	1668
	21st to end	Day	54% 5% 15% 17% 6% 3	2431
January	1st to 10th	Night	57% 9% 15% 11% 8% 1	4549
	11th to 20th	Night	54% 8% 15% 15% 7% 1	4440
	21st to end	Night	49% 9% 13% 17% 11% 1	4884
February	1st to 10th	Night	49% 8% 13% 18% 11% 1	2632
	11th to 20th	Night	55% 6% 13% 14% 12% 1	2290
	21st to end	Night	49% 7% 13% 18% 12% 2	1884
March	1st to 10th	Night	55% 9% 11% 17% 7% 1	3734
	11th to 20th	Night	59% 11% 10% 13% 6% 1	3582
	21st to end	Night	53% 7% 10% 19% 10% 2	3658
April	1st to 10th	Night	55% 4 10% 15% 12% 3	1900
	11th to 20th	Night	49% 8% 11% 19% 12% 1	1735
	21st to end	Night	53% 6% 9% 19% 12% 2	1710
May	1st to 10th	Night	52% 7% 11% 17% 12% 2	1698
	11th to 20th	Night	47% 8% 12% 19% 13% 1	1440
	21st to end	Night	47% 7% 13% 18% 14% 1	1391
June	1st to 10th	Night	51% 7% 14% 19% 7% 1	1260
	11th to 20th	Night	42% 8% 13% 22% 13% 3	1260
	21st to end	Night	51% 10% 13% 16% 10%	1260
July	1st to 10th	Night	43% 10% 18% 23% 6%	1225
	11th to 20th	Night	42% 11% 15% 24% 8%	1382
	21st to end	Night	43% 10% 19% 21% 7%	1486
August	1st to 10th	Night	38% 11% 22% 25% 4	1438
	11th to 20th	Night	42% 15% 17% 18% 8%	1596
	21st to end	Night	38% 9% 16% 28% 8%	1820
September	1st to 10th	Night	39% 8% 17% 24% 12% 1	1808
	11th to 20th	Night	41% 9% 17% 23% 9% 1	1980
	21st to end	Night	41% 8% 13% 25% 12% 1	1980
October	1st to 10th	Night	43% 7% 13% 25% 11% 1	1994
	11th to 20th	Night	52% 8% 15% 20% 5%	2301
	21st to end	Night	51% 8% 16% 17% 8% 1	2574
November	1st to 10th	Night	48% 8% 17% 19% 8% 1	2407
	11th to 20th	Night	48% 9% 18% 16% 9% 1	2550
	21st to end	Night	56% 7% 15% 13% 8% 1	2822
December	1st to 10th	Night	61% 6% 14% 12% 6% 1	2850
	11th to 20th	Night	55% 9% 14% 13% 9% 1	3532
	21st to end	Night	57% 8% 13% 13% 8% 2	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	18% 17% 38% 22% 4 1	180
	11th to 20th	Day	23% 21% 36% 14% 7%	180
	21st to end	Day	14% 23% 37% 18% 6% 2	198
February	1st to 10th	Day	19% 23% 36% 17% 4	206
	11th to 20th	Day	17% 20% 39% 19% 4	420
	21st to end	Day	18% 21% 36% 19% 6% 1	382
March	1st to 10th	Day	23% 22% 29% 20% 5%	460
	11th to 20th	Day	21% 19% 32% 20% 8% 1	460
	21st to end	Day	22% 21% 29% 19% 7% 1	506
April	1st to 10th	Day	29% 20% 27% 18% 6% 1	460
	11th to 20th	Day	23% 20% 29% 19% 6% 2	452
	21st to end	Day	28% 18% 33% 16% 3 2	460
May	1st to 10th	Day	26% 14% 31% 20% 8% 1	463
	11th to 20th	Day	20% 20% 30% 21% 8% 2	456
	21st to end	Day	22% 18% 32% 20% 7% 1	645
June	1st to 10th	Day	23% 14% 31% 23% 7% 1	630
	11th to 20th	Day	22% 13% 27% 23% 13% 1	630
	21st to end	Day	23% 11% 29% 26% 10% 1	630
July	1st to 10th	Day	17% 10% 32% 29% 10% 1	620
	11th to 20th	Day	15% 12% 31% 29% 11% 2	451
	21st to end	Day	15% 10% 35% 28% 10% 1	488
August	1st to 10th	Day	14% 15% 29% 26% 13% 3	446
	11th to 20th	Day	15% 8% 31% 30% 14% 3	439
	21st to end	Day	16% 16% 31% 18% 15% 4	495
September	1st to 10th	Day	17% 14% 30% 25% 10% 4	450
	11th to 20th	Day	20% 12% 31% 24% 9% 4	450
	21st to end	Day	20% 15% 25% 25% 11% 4	450
October	1st to 10th	Day	20% 16% 31% 22% 10% 2	442
	11th to 20th	Day	26% 14% 28% 21% 10% 2	360
	21st to end	Day	33% 12% 30% 20% 5% 1	396
November	1st to 10th	Day	27% 16% 31% 22% 3 1	365
	11th to 20th	Day	24% 16% 30% 21% 9% 1	370
	21st to end	Day	27% 25% 26% 19% 2	204
December	1st to 10th	Day	24% 14% 30% 18% 12% 2	180
	11th to 20th	Day	30% 18% 31% 15% 4 2	180
	21st to end	Day	28% 16% 29% 22% 4 1	198
January	1st to 10th	Night	22% 11% 37% 25% 5% 1	792
	11th to 20th	Night	19% 10% 38% 27% 5%	760
	21st to end	Night	13% 10% 33% 35% 8% 1	836
February	1st to 10th	Night	15% 10% 39% 28% 7% 1	760
	11th to 20th	Night	17% 14% 38% 24% 7% 1	743
	21st to end	Night	19% 11% 35% 28% 5% 1	628
March	1st to 10th	Night	19% 14% 34% 26% 6% 1	760
	11th to 20th	Night	20% 13% 35% 24% 7% 1	760
	21st to end	Night	22% 13% 33% 26% 6%	836
April	1st to 10th	Night	28% 11% 31% 24% 7%	760
	11th to 20th	Night	23% 13% 29% 26% 8%	611
	21st to end	Night	29% 11% 27% 26% 7%	570
May	1st to 10th	Night	26% 8% 31% 29% 5%	558
	11th to 20th	Night	23% 10% 24% 31% 11%	360
	21st to end	Night	27% 9% 29% 28% 7%	389
June	1st to 10th	Night	23% 5% 25% 38% 9% 1	360
	11th to 20th	Night	18% 5% 24% 36% 16% 1	360
	21st to end	Night	22% 5% 28% 33% 11% 1	360
July	1st to 10th	Night	19% 6% 32% 33% 11% 1	350
	11th to 20th	Night	13% 4 24% 43% 15% 1	348
	21st to end	Night	15% 4 23% 46% 11% 2	394
August	1st to 10th	Night	12% 5% 17% 47% 18% 1	374
	11th to 20th	Night	17% 4 22% 39% 17% 1	532
	21st to end	Night	16% 6% 25% 36% 15% 2	632
September	1st to 10th	Night	16% 6% 27% 36% 14% 1	720
	11th to 20th	Night	21% 8% 23% 32% 13% 2	720
	21st to end	Night	19% 7% 26% 34% 13% 1	720
October	1st to 10th	Night	20% 9% 29% 30% 9% 2	720
	11th to 20th	Night	26% 8% 27% 27% 12% 1	720
	21st to end	Night	27% 9% 31% 25% 8%	792
November	1st to 10th	Night	23% 14% 29% 26% 7%	740
	11th to 20th	Night	23% 14% 31% 23% 8% 1	798
	21st to end	Night	25% 14% 29% 25% 6% 1	850
December	1st to 10th	Night	25% 12% 32% 24% 7% 1	850
	11th to 20th	Night	22% 16% 32% 23% 6%	850
	21st to end	Night	26% 13% 32% 21% 8% 1	935

Page generated at 2024-05-10 12:39:01. Version ac07

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