Ex Nihilo Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ex Nihilo Observatory clear sky chart from 2005-11-06 to 2025-07-06.

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	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	120011
	Night	14% 6% 2 5% 4 4 4 4 5% 7% 44%	118775
Transparency	Day	39% 7% 10% 19% 19% 6%	123816
	Night	37% 6% 10% 19% 21% 7%	123107
Seeing	Day	18% 10% 22% 23% 18% 8%	15828
	Night	17% 9% 22% 29% 20% 3	25878

By Month

Percentage Cloud Cover SE (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	17% 8% 3 5% 3 3 3 2 4 5% 48%	4056
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 5% 56%	4060
	21st to end	Day	22% 9% 3 6% 3 3 2 2 3 5% 42%	4530
February	1st to 10th	Day	16% 7% 3 6% 4 5% 4 4 5% 6% 41%	2000
	11th to 20th	Day	23% 7% 2 4 3 3 4 3 4 5% 43%	1948
	21st to end	Day	19% 7% 2 6% 3 4 4 3 4 7% 41%	1650
March	1st to 10th	Day	21% 8% 3 8% 4 4 3 3 4 5% 37%	4709
	11th to 20th	Day	9% 5% 2 5% 4 4 3 3 4 7% 53%	4980
	21st to end	Day	17% 6% 2 6% 5% 4 3 3 3 4 45%	5465
April	1st to 10th	Day	13% 4 2 6% 5% 4 4 4 6% 7% 45%	2200
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 48%	2364
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 49%	2600
May	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 4 6% 9% 37%	5862
	11th to 20th	Day	17% 8% 3 7% 4 5% 6% 4 7% 8% 30%	5861
	21st to end	Day	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	6433
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 7% 4 8% 10% 34%	2600
	11th to 20th	Day	6% 4 2 5% 5% 6% 7% 6% 9% 11% 41%	2600
	21st to end	Day	6% 4 2 7% 5% 6% 7% 5% 8% 11% 40%	2600
July	1st to 10th	Day	8% 6% 2 9% 7% 7% 8% 7% 9% 11% 28%	4185
	11th to 20th	Day	8% 5% 2 8% 7% 7% 7% 6% 9% 10% 31%	4120
	21st to end	Day	10% 8% 4 10% 8% 8% 8% 7% 8% 9% 20%	4466
August	1st to 10th	Day	10% 6% 2 6% 5% 6% 6% 5% 9% 11% 32%	4104
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 11% 40%	3899
	21st to end	Day	8% 5% 2 6% 4 4 4 4 7% 8% 48%	4122
September	1st to 10th	Day	11% 6% 3 7% 6% 5% 5% 5% 7% 7% 39%	2090
	11th to 20th	Day	17% 6% 3 5% 4 4 4 4 7% 7% 39%	2090
	21st to end	Day	15% 5% 2 6% 3 4 4 3 4 6% 47%	2090
October	1st to 10th	Day	14% 5% 2 4 3 2 3 2 5% 5% 56%	2090
	11th to 20th	Day	13% 4 1 4 3 3 3 2 4 5% 58%	1945
	21st to end	Day	13% 4 2 5% 3 3 4 2 4 6% 56%	2090
November	1st to 10th	Day	14% 4 2 5% 3 2 3 3 3 5% 55%	1833
	11th to 20th	Day	14% 5% 1 4 2 3 3 3 5% 6% 53%	1800
	21st to end	Day	14% 5% 2 5% 3 3 4 4 4 7% 50%	1800
December	1st to 10th	Day	17% 7% 2 5% 3 4 3 3 5% 6% 47%	2930
	11th to 20th	Day	17% 6% 2 7% 4 4 4 3 5% 6% 42%	3373
	21st to end	Day	23% 6% 2 4 3 3 3 3 4 7% 42%	4466
January	1st to 10th	Night	16% 5% 2 4 3 3 3 2 4 5% 52%	5889
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 5% 50%	5900
	21st to end	Night	19% 7% 2 5% 3 3 4 2 4 5% 45%	6421
February	1st to 10th	Night	16% 6% 2 6% 4 4 3 4 5% 5% 47%	2400
	11th to 20th	Night	22% 7% 2 5% 4 3 3 3 4 4 44%	2338
	21st to end	Night	19% 5% 2 5% 4 4 4 3 4 6% 44%	1980
March	1st to 10th	Night	22% 7% 2 6% 5% 4 3 4 5% 5% 37%	5143
	11th to 20th	Night	9% 4 1 4 3 3 4 4 6% 8% 53%	4980
	21st to end	Night	16% 6% 2 6% 4 3 4 4 5% 6% 45%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 49%	2200
	11th to 20th	Night	13% 4 2 5% 3 3 3 3 5% 7% 51%	1999
	21st to end	Night	10% 5% 2 5% 4 4 4 3 6% 7% 50%	1800
May	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 4 7% 10% 37%	4033
	11th to 20th	Night	18% 7% 2 7% 4 5% 5% 4 6% 8% 33%	4032
	21st to end	Night	13% 5% 2 5% 4 4 4 4 6% 10% 43%	4421
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 7% 6% 9% 11% 37%	1800
	11th to 20th	Night	5% 5% 2 5% 6% 5% 5% 5% 9% 12% 42%	1800
	21st to end	Night	7% 4 2 5% 5% 5% 6% 6% 8% 11% 41%	1800
July	1st to 10th	Night	10% 7% 3 9% 7% 5% 6% 5% 7% 11% 30%	2885
	11th to 20th	Night	6% 5% 2 8% 7% 6% 7% 6% 10% 14% 30%	2840
	21st to end	Night	11% 9% 3 9% 6% 8% 6% 6% 9% 11% 21%	3087
August	1st to 10th	Night	10% 6% 2 7% 5% 5% 6% 5% 8% 13% 32%	2825
	11th to 20th	Night	10% 5% 2 7% 5% 5% 5% 4 7% 9% 41%	2912
	21st to end	Night	9% 4 1 5% 4 4 5% 4 6% 8% 49%	3500
September	1st to 10th	Night	10% 5% 2 6% 5% 5% 6% 4 6% 10% 41%	2090
	11th to 20th	Night	15% 7% 2 5% 4 4 4 4 6% 7% 42%	2090
	21st to end	Night	14% 4 2 5% 4 4 4 4 6% 6% 46%	2090
October	1st to 10th	Night	13% 4 1 5% 3 3 3 2 5% 5% 54%	2090
	11th to 20th	Night	11% 5% 2 4 3 4 3 2 3 4 58%	2233
	21st to end	Night	11% 4 2 4 2 3 3 3 3 6% 59%	2508
November	1st to 10th	Night	14% 6% 2 4 3 3 3 2 4 5% 54%	2460
	11th to 20th	Night	16% 4 1 4 3 3 2 3 4 6% 53%	2600
	21st to end	Night	15% 5% 1 4 3 4 4 3 6% 6% 49%	2600
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 4 52%	4250
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 5% 7% 45%	4822
	21st to end	Night	21% 6% 2 5% 3 4 3 3 5% 5% 42%	6490

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	39% 4 5% 19% 24% 10%	4057
	11th to 20th	Day	30% 3 6% 20% 29% 11%	4060
	21st to end	Day	47% 2 4 17% 21% 10%	4530
February	1st to 10th	Day	44% 4 8% 20% 19% 6%	2000
	11th to 20th	Day	44% 4 6% 18% 22% 6%	1958
	21st to end	Day	44% 4 8% 15% 21% 9%	1650
March	1st to 10th	Day	50% 4 6% 12% 19% 9%	4709
	11th to 20th	Day	32% 4 7% 14% 24% 20%	4980
	21st to end	Day	42% 4 6% 21% 21% 6%	5465
April	1st to 10th	Day	38% 5% 9% 21% 22% 6%	2200
	11th to 20th	Day	36% 5% 8% 19% 23% 9%	2364
	21st to end	Day	31% 7% 10% 27% 22% 3	2600
May	1st to 10th	Day	41% 8% 14% 21% 14% 2	5897
	11th to 20th	Day	49% 9% 13% 19% 9% 1	5865
	21st to end	Day	40% 7% 11% 21% 18% 2	6447
June	1st to 10th	Day	41% 11% 15% 21% 10% 2	2600
	11th to 20th	Day	32% 11% 19% 24% 14% 1	2600
	21st to end	Day	35% 11% 17% 21% 13% 3	2600
July	1st to 10th	Day	43% 14% 17% 19% 5% 1	4120
	11th to 20th	Day	41% 13% 22% 18% 4 2	4051
	21st to end	Day	54% 14% 15% 13% 5%	4324
August	1st to 10th	Day	39% 11% 15% 28% 6%	4091
	11th to 20th	Day	34% 10% 18% 21% 14% 2	3938
	21st to end	Day	32% 9% 14% 22% 19% 4	4122
September	1st to 10th	Day	40% 10% 14% 16% 17% 4	2090
	11th to 20th	Day	42% 8% 10% 17% 21% 2	2090
	21st to end	Day	38% 4 11% 20% 20% 7%	2090
October	1st to 10th	Day	26% 4 7% 14% 37% 12%	3480
	11th to 20th	Day	21% 2 5% 15% 28% 29%	3211
	21st to end	Day	25% 4 5% 20% 32% 14%	3428
November	1st to 10th	Day	34% 3 8% 14% 29% 12%	1833
	11th to 20th	Day	32% 3 8% 19% 28% 10%	1800
	21st to end	Day	35% 5% 7% 19% 27% 7%	1800
December	1st to 10th	Day	39% 4 5% 17% 25% 9%	2930
	11th to 20th	Day	43% 5% 7% 18% 19% 8%	3370
	21st to end	Day	41% 5% 8% 24% 19% 2	4466
January	1st to 10th	Night	34% 4 6% 20% 29% 8%	5889
	11th to 20th	Night	36% 4 6% 19% 27% 9%	5900
	21st to end	Night	42% 3 5% 15% 29% 7%	6421
February	1st to 10th	Night	40% 4 9% 19% 21% 8%	2400
	11th to 20th	Night	45% 3 6% 17% 21% 8%	2339
	21st to end	Night	43% 3 7% 17% 19% 11%	1980
March	1st to 10th	Night	47% 5% 8% 13% 20% 7%	5143
	11th to 20th	Night	27% 5% 9% 13% 28% 18%	4980
	21st to end	Night	39% 4 6% 21% 21% 10%	5467
April	1st to 10th	Night	34% 5% 10% 20% 23% 8%	2200
	11th to 20th	Night	32% 5% 7% 21% 24% 10%	1999
	21st to end	Night	32% 7% 10% 25% 22% 5%	1800
May	1st to 10th	Night	39% 9% 16% 20% 13% 2	4060
	11th to 20th	Night	48% 7% 15% 18% 11% 1	4033
	21st to end	Night	35% 8% 14% 23% 18% 2	4429
June	1st to 10th	Night	36% 12% 17% 20% 13% 2	1800
	11th to 20th	Night	31% 11% 18% 25% 13% 2	1800
	21st to end	Night	32% 12% 19% 20% 13% 4	1800
July	1st to 10th	Night	45% 12% 20% 17% 5% 1	2840
	11th to 20th	Night	36% 17% 23% 17% 5% 1	2786
	21st to end	Night	53% 13% 17% 12% 4 1	2988
August	1st to 10th	Night	40% 11% 19% 24% 5%	2814
	11th to 20th	Night	36% 11% 20% 19% 14% 1	2939
	21st to end	Night	32% 9% 13% 22% 22% 2	3500
September	1st to 10th	Night	37% 10% 15% 17% 17% 4	2090
	11th to 20th	Night	40% 8% 12% 18% 19% 3	2090
	21st to end	Night	36% 7% 12% 17% 20% 7%	2090
October	1st to 10th	Night	27% 5% 8% 15% 36% 9%	3480
	11th to 20th	Night	19% 3 5% 17% 30% 28%	3706
	21st to end	Night	23% 3 5% 21% 29% 18%	4120
November	1st to 10th	Night	33% 4 7% 18% 25% 13%	2460
	11th to 20th	Night	33% 3 7% 17% 28% 13%	2600
	21st to end	Night	35% 5% 7% 19% 25% 9%	2600
December	1st to 10th	Night	37% 4 5% 17% 31% 7%	4250
	11th to 20th	Night	39% 5% 10% 16% 22% 7%	4824
	21st to end	Night	43% 4 7% 26% 18% 2	6490

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	29% 21% 35% 14% 1	300
	11th to 20th	Day	29% 20% 30% 15% 5% 1	300
	21st to end	Day	30% 18% 39% 11% 2	330
February	1st to 10th	Day	26% 15% 36% 20% 3	300
	11th to 20th	Day	41% 21% 26% 10% 2	300
	21st to end	Day	33% 20% 31% 13% 3	249
March	1st to 10th	Day	27% 16% 28% 19% 9% 1	399
	11th to 20th	Day	25% 16% 34% 18% 6% 1	500
	21st to end	Day	24% 15% 33% 19% 8% 1	550
April	1st to 10th	Day	23% 13% 32% 20% 11% 1	500
	11th to 20th	Day	16% 14% 31% 23% 14% 1	492
	21st to end	Day	17% 9% 28% 26% 16% 4	500
May	1st to 10th	Day	15% 15% 29% 22% 13% 6%	503
	11th to 20th	Day	16% 7% 22% 30% 15% 10%	527
	21st to end	Day	17% 7% 18% 26% 21% 11%	573
June	1st to 10th	Day	13% 4 17% 30% 27% 9%	530
	11th to 20th	Day	8% 4 17% 27% 27% 18%	530
	21st to end	Day	9% 5% 15% 32% 25% 15%	530
July	1st to 10th	Day	12% 4 7% 26% 29% 23%	505
	11th to 20th	Day	10% 3 9% 24% 32% 22%	484
	21st to end	Day	7% 2 8% 26% 37% 20%	532
August	1st to 10th	Day	12% 5% 14% 23% 27% 18%	486
	11th to 20th	Day	14% 3 8% 25% 34% 16%	479
	21st to end	Day	16% 3 7% 25% 30% 19%	539
September	1st to 10th	Day	11% 4 13% 29% 29% 14%	490
	11th to 20th	Day	20% 3 11% 25% 28% 13%	490
	21st to end	Day	17% 5% 15% 25% 28% 10%	490
October	1st to 10th	Day	19% 4 19% 29% 19% 9%	490
	11th to 20th	Day	13% 9% 26% 23% 24% 5%	490
	21st to end	Day	17% 9% 23% 25% 22% 4	539
November	1st to 10th	Day	22% 6% 26% 29% 15% 2	371
	11th to 20th	Day	19% 17% 34% 21% 8% 2	300
	21st to end	Day	18% 14% 33% 25% 9% 2	300
December	1st to 10th	Day	31% 18% 28% 19% 4	300
	11th to 20th	Day	26% 13% 34% 20% 6% 2	300
	21st to end	Day	34% 15% 29% 13% 6% 3	330
January	1st to 10th	Night	26% 18% 28% 21% 7%	800
	11th to 20th	Night	22% 16% 31% 25% 6%	800
	21st to end	Night	22% 17% 34% 24% 3	880
February	1st to 10th	Night	21% 13% 31% 31% 5%	800
	11th to 20th	Night	26% 16% 32% 23% 3	783
	21st to end	Night	24% 16% 33% 26% 3	660
March	1st to 10th	Night	21% 15% 29% 30% 5%	800
	11th to 20th	Night	19% 12% 34% 26% 9% 1	800
	21st to end	Night	19% 14% 31% 28% 8%	880
April	1st to 10th	Night	16% 14% 31% 28% 11%	800
	11th to 20th	Night	15% 11% 29% 30% 15%	623
	21st to end	Night	12% 7% 27% 34% 20% 1	600
May	1st to 10th	Night	11% 9% 26% 33% 20% 1	600
	11th to 20th	Night	13% 6% 21% 32% 23% 4	591
	21st to end	Night	14% 3 13% 33% 32% 5%	650
June	1st to 10th	Night	10% 2 16% 35% 32% 6%	600
	11th to 20th	Night	6% 2 17% 30% 35% 11%	600
	21st to end	Night	8% 1 11% 34% 37% 9%	600
July	1st to 10th	Night	10% 3 9% 26% 38% 13%	570
	11th to 20th	Night	9% 2 5% 24% 48% 13%	552
	21st to end	Night	7% 4 6% 31% 44% 8%	624
August	1st to 10th	Night	13% 3 6% 28% 40% 10%	567
	11th to 20th	Night	14% 2 7% 26% 40% 10%	564
	21st to end	Night	15% 3 7% 26% 37% 11%	638
September	1st to 10th	Night	10% 3 10% 26% 39% 11%	760
	11th to 20th	Night	19% 2 11% 27% 36% 5%	760
	21st to end	Night	16% 3 14% 32% 32% 3	760
October	1st to 10th	Night	14% 4 18% 31% 26% 6%	760
	11th to 20th	Night	14% 7% 23% 32% 22% 3	760
	21st to end	Night	14% 8% 21% 33% 22% 2	836
November	1st to 10th	Night	18% 7% 21% 32% 21% 1	780
	11th to 20th	Night	18% 11% 26% 31% 14%	800
	21st to end	Night	19% 11% 25% 30% 15% 1	800
December	1st to 10th	Night	21% 14% 28% 29% 8%	800
	11th to 20th	Night	21% 13% 26% 29% 10% 1	800
	21st to end	Night	28% 11% 26% 29% 6%	880

Page generated at 2025-07-03 01:30:15. Version ac07

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