The Clear Nights Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Clear Nights 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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	120035
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 45%	118756
Transparency	Day	38% 7% 11% 19% 19% 6%	123852
	Night	36% 6% 10% 19% 21% 8%	123063
Seeing	Day	15% 10% 22% 24% 19% 9%	15804
	Night	15% 9% 22% 29% 21% 4	25843

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	16% 7% 2 4 3 3 3 3 4 4 50%	4056
	11th to 20th	Day	10% 7% 3 4 3 2 3 3 5% 7% 54%	4060
	21st to end	Day	21% 8% 3 7% 4 3 3 3 4 6% 40%	4649
February	1st to 10th	Day	16% 7% 2 7% 5% 4 5% 4 5% 7% 40%	2000
	11th to 20th	Day	20% 8% 1 6% 4 3 4 3 4 5% 42%	1948
	21st to end	Day	17% 7% 2 7% 5% 4 4 3 5% 4 42%	1650
March	1st to 10th	Day	20% 8% 3 7% 4 4 4 3 5% 5% 37%	4679
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 7% 52%	4980
	21st to end	Day	15% 6% 3 7% 4 3 4 3 4 5% 45%	5465
April	1st to 10th	Day	11% 6% 2 5% 5% 4 3 4 5% 6% 48%	2210
	11th to 20th	Day	11% 5% 1 6% 4 3 4 3 6% 7% 50%	2374
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 5% 7% 51%	2567
May	1st to 10th	Day	12% 6% 2 7% 5% 5% 4 4 7% 9% 39%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 5% 7% 9% 30%	5861
	21st to end	Day	10% 5% 2 7% 4 4 5% 4 7% 9% 43%	6433
June	1st to 10th	Day	10% 6% 2 8% 6% 6% 7% 6% 8% 9% 33%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 7% 5% 8% 11% 42%	2600
	21st to end	Day	6% 5% 2 6% 5% 6% 8% 6% 7% 10% 39%	2600
July	1st to 10th	Day	12% 6% 3 9% 7% 7% 7% 7% 9% 11% 23%	4185
	11th to 20th	Day	6% 5% 3 7% 7% 6% 7% 6% 9% 12% 31%	4120
	21st to end	Day	9% 8% 3 10% 7% 8% 8% 6% 10% 11% 20%	4466
August	1st to 10th	Day	10% 5% 2 7% 5% 6% 7% 6% 9% 12% 31%	4104
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 4 7% 8% 47%	3799
	21st to end	Day	8% 4 2 5% 4 4 5% 4 6% 8% 49%	4148
September	1st to 10th	Day	12% 6% 3 6% 6% 5% 6% 4 7% 8% 37%	2090
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 5% 7% 7% 39%	2090
	21st to end	Day	14% 6% 2 5% 4 3 4 3 5% 5% 48%	2090
October	1st to 10th	Day	12% 5% 1 4 3 3 3 3 4 7% 55%	2090
	11th to 20th	Day	12% 4 1 3 3 2 3 2 5% 6% 57%	1974
	21st to end	Day	11% 4 1 5% 3 4 3 3 4 6% 55%	2090
November	1st to 10th	Day	14% 5% 2 4 4 3 4 3 4 5% 53%	1826
	11th to 20th	Day	14% 5% 2 3 4 3 4 3 4 6% 53%	1800
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 7% 47%	1800
December	1st to 10th	Day	15% 6% 3 5% 3 3 3 3 5% 6% 48%	2930
	11th to 20th	Day	17% 8% 3 5% 4 3 4 2 4 5% 45%	3373
	21st to end	Day	22% 7% 2 5% 3 3 3 3 4 6% 42%	4466
January	1st to 10th	Night	14% 4 2 5% 4 4 3 3 5% 6% 51%	5889
	11th to 20th	Night	12% 6% 2 6% 4 3 4 4 4 6% 48%	5900
	21st to end	Night	17% 9% 2 5% 4 3 4 3 5% 6% 42%	6291
February	1st to 10th	Night	14% 7% 2 6% 6% 4 3 3 5% 6% 44%	2400
	11th to 20th	Night	20% 7% 3 6% 4 3 3 3 4 6% 43%	2338
	21st to end	Night	16% 6% 2 6% 3 5% 4 3 4 6% 45%	1980
March	1st to 10th	Night	20% 8% 3 6% 4 3 4 3 5% 6% 37%	5172
	11th to 20th	Night	10% 4 2 4 3 3 4 4 6% 7% 53%	4980
	21st to end	Night	15% 5% 2 5% 4 4 4 3 5% 6% 47%	5467
April	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 7% 49%	2200
	11th to 20th	Night	12% 5% 2 4 4 3 3 4 5% 7% 51%	1981
	21st to end	Night	7% 5% 2 5% 3 4 3 3 6% 8% 53%	1830
May	1st to 10th	Night	9% 6% 2 6% 5% 5% 6% 4 7% 11% 37%	4033
	11th to 20th	Night	18% 5% 2 6% 5% 4 5% 4 6% 9% 34%	4032
	21st to end	Night	10% 4 2 5% 4 4 4 4 6% 11% 45%	4421
June	1st to 10th	Night	6% 6% 3 7% 6% 5% 5% 5% 8% 10% 39%	1800
	11th to 20th	Night	5% 5% 2 6% 5% 5% 5% 4 8% 12% 45%	1800
	21st to end	Night	5% 5% 2 6% 5% 5% 5% 6% 9% 11% 40%	1800
July	1st to 10th	Night	11% 7% 3 9% 6% 6% 5% 5% 9% 10% 28%	2885
	11th to 20th	Night	6% 5% 2 7% 6% 6% 7% 7% 11% 14% 30%	2840
	21st to end	Night	9% 8% 3 9% 6% 7% 7% 6% 9% 11% 23%	3087
August	1st to 10th	Night	8% 6% 2 7% 6% 7% 7% 6% 9% 12% 30%	2825
	11th to 20th	Night	7% 4 2 5% 5% 6% 4 4 8% 10% 47%	3026
	21st to end	Night	7% 5% 2 4 4 4 4 4 7% 8% 52%	3476
September	1st to 10th	Night	9% 6% 3 6% 5% 5% 6% 5% 7% 10% 39%	2090
	11th to 20th	Night	13% 5% 3 7% 4 4 5% 4 5% 6% 44%	2090
	21st to end	Night	12% 5% 2 5% 5% 4 4 4 6% 7% 46%	2090
October	1st to 10th	Night	12% 4 2 5% 3 4 3 3 5% 6% 53%	2090
	11th to 20th	Night	11% 4 1 4 3 3 3 3 4 5% 59%	2204
	21st to end	Night	11% 4 1 4 4 3 3 3 4 6% 57%	2508
November	1st to 10th	Night	13% 5% 2 4 3 3 3 3 5% 5% 54%	2469
	11th to 20th	Night	13% 6% 2 4 4 3 3 2 4 5% 54%	2600
	21st to end	Night	14% 5% 2 5% 4 3 4 2 5% 6% 50%	2600
December	1st to 10th	Night	13% 6% 2 5% 3 2 3 3 4 5% 53%	4250
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 7% 47%	4822
	21st to end	Night	20% 6% 3 6% 4 4 4 3 4 6% 41%	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	37% 3 6% 19% 27% 8%	4057
	11th to 20th	Day	32% 3 5% 21% 30% 10%	4060
	21st to end	Day	46% 2 5% 16% 21% 9%	4649
February	1st to 10th	Day	44% 5% 8% 17% 19% 6%	2000
	11th to 20th	Day	44% 4 6% 18% 21% 6%	1958
	21st to end	Day	44% 5% 7% 14% 21% 9%	1650
March	1st to 10th	Day	48% 5% 7% 11% 22% 7%	4679
	11th to 20th	Day	31% 5% 7% 14% 22% 21%	4980
	21st to end	Day	41% 4 6% 20% 19% 9%	5465
April	1st to 10th	Day	36% 5% 9% 22% 20% 8%	2210
	11th to 20th	Day	33% 4 8% 21% 24% 10%	2374
	21st to end	Day	32% 7% 10% 24% 24% 4	2567
May	1st to 10th	Day	40% 8% 14% 21% 16% 2	5897
	11th to 20th	Day	47% 9% 15% 19% 10% 1	5865
	21st to end	Day	36% 8% 12% 22% 20% 2	6447
June	1st to 10th	Day	42% 11% 14% 20% 11% 2	2600
	11th to 20th	Day	32% 10% 18% 23% 15% 1	2600
	21st to end	Day	35% 12% 17% 20% 12% 3	2600
July	1st to 10th	Day	48% 14% 18% 14% 4 1	4120
	11th to 20th	Day	39% 15% 19% 22% 5% 1	4051
	21st to end	Day	52% 17% 14% 13% 5%	4324
August	1st to 10th	Day	38% 12% 17% 28% 5%	4091
	11th to 20th	Day	32% 9% 14% 30% 14% 2	3837
	21st to end	Day	31% 8% 15% 26% 17% 3	4148
September	1st to 10th	Day	42% 9% 14% 15% 15% 4	2090
	11th to 20th	Day	40% 9% 11% 16% 20% 3	2090
	21st to end	Day	37% 5% 10% 21% 20% 6%	2090
October	1st to 10th	Day	26% 4 7% 14% 36% 13%	3480
	11th to 20th	Day	17% 3 5% 17% 30% 29%	3253
	21st to end	Day	24% 3 6% 18% 36% 13%	3428
November	1st to 10th	Day	35% 4 7% 14% 28% 12%	1826
	11th to 20th	Day	33% 3 7% 18% 27% 13%	1800
	21st to end	Day	36% 5% 8% 17% 26% 8%	1800
December	1st to 10th	Day	38% 5% 6% 18% 26% 8%	2930
	11th to 20th	Day	42% 4 9% 18% 21% 7%	3370
	21st to end	Day	42% 4 8% 21% 22% 3	4466
January	1st to 10th	Night	35% 3 5% 21% 28% 8%	5889
	11th to 20th	Night	36% 4 6% 19% 26% 9%	5900
	21st to end	Night	44% 3 6% 14% 27% 6%	6291
February	1st to 10th	Night	41% 4 7% 19% 20% 10%	2400
	11th to 20th	Night	44% 4 5% 17% 21% 9%	2339
	21st to end	Night	40% 5% 8% 16% 19% 13%	1980
March	1st to 10th	Night	47% 4 8% 14% 16% 10%	5172
	11th to 20th	Night	29% 5% 9% 12% 26% 19%	4980
	21st to end	Night	38% 4 6% 19% 21% 11%	5467
April	1st to 10th	Night	33% 7% 9% 21% 20% 10%	2200
	11th to 20th	Night	31% 5% 7% 22% 25% 11%	1981
	21st to end	Night	29% 5% 13% 23% 23% 6%	1830
May	1st to 10th	Night	36% 10% 17% 22% 13% 3	4060
	11th to 20th	Night	46% 8% 15% 21% 10% 1	4033
	21st to end	Night	31% 8% 15% 24% 20% 3	4429
June	1st to 10th	Night	37% 10% 17% 20% 15% 1	1800
	11th to 20th	Night	30% 10% 18% 28% 12% 2	1800
	21st to end	Night	32% 10% 22% 19% 14% 3	1800
July	1st to 10th	Night	47% 13% 20% 14% 4 2	2840
	11th to 20th	Night	36% 16% 24% 16% 6% 1	2786
	21st to end	Night	50% 16% 17% 12% 4 1	2988
August	1st to 10th	Night	41% 12% 17% 25% 5%	2814
	11th to 20th	Night	30% 9% 17% 27% 15% 2	3054
	21st to end	Night	27% 9% 15% 26% 21% 2	3476
September	1st to 10th	Night	37% 10% 16% 16% 16% 4	2090
	11th to 20th	Night	40% 9% 10% 19% 18% 4	2090
	21st to end	Night	36% 8% 11% 17% 20% 8%	2090
October	1st to 10th	Night	29% 5% 8% 13% 30% 14%	3480
	11th to 20th	Night	17% 2 4 15% 31% 31%	3651
	21st to end	Night	22% 3 5% 20% 31% 19%	4120
November	1st to 10th	Night	33% 4 8% 15% 27% 13%	2469
	11th to 20th	Night	34% 3 5% 17% 27% 14%	2600
	21st to end	Night	36% 5% 6% 18% 26% 9%	2600
December	1st to 10th	Night	34% 4 6% 19% 29% 9%	4250
	11th to 20th	Night	37% 4 11% 17% 24% 7%	4824
	21st to end	Night	45% 3 6% 23% 20% 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	23% 25% 38% 13% 1	300
	11th to 20th	Day	23% 24% 30% 17% 6% 1	300
	21st to end	Day	23% 25% 36% 11% 4 1	330
February	1st to 10th	Day	20% 21% 38% 16% 4	300
	11th to 20th	Day	38% 15% 31% 14% 1	300
	21st to end	Day	24% 20% 38% 16% 1	249
March	1st to 10th	Day	23% 19% 30% 22% 5% 1	382
	11th to 20th	Day	22% 15% 33% 22% 7%	500
	21st to end	Day	18% 19% 34% 22% 7% 1	550
April	1st to 10th	Day	17% 18% 30% 24% 10% 1	500
	11th to 20th	Day	13% 17% 28% 26% 14% 2	492
	21st to end	Day	13% 12% 24% 29% 19% 3	500
May	1st to 10th	Day	11% 12% 29% 28% 14% 6%	503
	11th to 20th	Day	14% 8% 26% 23% 20% 9%	527
	21st to end	Day	14% 5% 19% 27% 23% 11%	573
June	1st to 10th	Day	11% 3 19% 26% 29% 12%	530
	11th to 20th	Day	6% 4 16% 28% 25% 21%	530
	21st to end	Day	8% 2 14% 30% 26% 19%	530
July	1st to 10th	Day	8% 2 11% 22% 34% 23%	505
	11th to 20th	Day	8% 1 8% 21% 35% 27%	484
	21st to end	Day	5% 3 9% 22% 39% 22%	532
August	1st to 10th	Day	10% 3 10% 28% 27% 21%	486
	11th to 20th	Day	9% 2 9% 23% 36% 21%	479
	21st to end	Day	15% 2 7% 27% 32% 17%	539
September	1st to 10th	Day	12% 3 12% 25% 31% 17%	490
	11th to 20th	Day	18% 2 12% 27% 29% 11%	490
	21st to end	Day	13% 3 18% 29% 27% 10%	490
October	1st to 10th	Day	16% 6% 20% 28% 22% 8%	490
	11th to 20th	Day	13% 9% 23% 28% 20% 7%	490
	21st to end	Day	14% 10% 23% 29% 19% 5%	539
November	1st to 10th	Day	23% 7% 26% 26% 15% 4	364
	11th to 20th	Day	17% 17% 38% 20% 6% 2	300
	21st to end	Day	15% 17% 33% 23% 11% 2	300
December	1st to 10th	Day	25% 18% 30% 19% 7%	300
	11th to 20th	Day	26% 18% 28% 21% 7%	300
	21st to end	Day	28% 17% 30% 18% 5% 2	330
January	1st to 10th	Night	23% 19% 30% 22% 6% 1	800
	11th to 20th	Night	20% 16% 29% 27% 8%	800
	21st to end	Night	22% 15% 35% 24% 4	880
February	1st to 10th	Night	18% 14% 29% 33% 7%	800
	11th to 20th	Night	24% 16% 30% 27% 2	783
	21st to end	Night	19% 16% 35% 27% 3	660
March	1st to 10th	Night	17% 14% 32% 31% 6%	800
	11th to 20th	Night	19% 13% 32% 27% 10%	800
	21st to end	Night	17% 13% 30% 29% 10%	880
April	1st to 10th	Night	14% 14% 31% 29% 12%	800
	11th to 20th	Night	14% 12% 25% 30% 18% 1	599
	21st to end	Night	8% 10% 24% 36% 22% 1	600
May	1st to 10th	Night	8% 8% 26% 38% 20% 2	600
	11th to 20th	Night	12% 4 22% 33% 25% 5%	591
	21st to end	Night	13% 2 13% 33% 33% 6%	650
June	1st to 10th	Night	8% 2 10% 38% 37% 5%	600
	11th to 20th	Night	5% 2 15% 29% 37% 13%	600
	21st to end	Night	7% 2 8% 34% 41% 9%	600
July	1st to 10th	Night	9% 2 8% 24% 43% 14%	570
	11th to 20th	Night	7% 1 4 24% 49% 15%	552
	21st to end	Night	6% 1 7% 27% 47% 12%	624
August	1st to 10th	Night	8% 1 6% 27% 45% 13%	567
	11th to 20th	Night	10% 2 5% 27% 47% 9%	564
	21st to end	Night	15% 2 9% 25% 38% 11%	627
September	1st to 10th	Night	9% 2 12% 26% 40% 11%	760
	11th to 20th	Night	14% 2 13% 28% 39% 4	760
	21st to end	Night	14% 2 14% 31% 35% 3	760
October	1st to 10th	Night	15% 3 17% 33% 26% 7%	760
	11th to 20th	Night	13% 5% 22% 33% 24% 3	760
	21st to end	Night	13% 8% 21% 34% 22% 2	836
November	1st to 10th	Night	15% 8% 22% 33% 22% 1	780
	11th to 20th	Night	15% 13% 26% 30% 14% 2	800
	21st to end	Night	15% 14% 25% 32% 13% 1	800
December	1st to 10th	Night	19% 13% 29% 28% 10% 1	800
	11th to 20th	Night	21% 11% 29% 30% 10% 1	800
	21st to end	Night	26% 14% 25% 28% 7%	880

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