The Stocker AstroScience Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Stocker AstroScience Center clear sky chart from 2005-11-06 to 2025-07-21.

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	7% 5% 2 7% 6% 6% 7% 6% 8% 11% 35%	119728
	Night	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	119395
Transparency	Day	38% 9% 18% 21% 11% 3	123436
	Night	35% 8% 19% 23% 12% 3	123824
Seeing	Day	8% 6% 24% 30% 19% 13%	16989
	Night	8% 4 18% 32% 27% 11%	27114

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	6% 5% 2 5% 4 5% 5% 4 7% 11% 46%	4056
	11th to 20th	Day	15% 7% 3 8% 6% 6% 5% 4 7% 9% 28%	4060
	21st to end	Day	8% 8% 3 8% 5% 5% 5% 4 6% 7% 39%	4581
February	1st to 10th	Day	8% 6% 3 6% 6% 6% 5% 5% 6% 10% 41%	2000
	11th to 20th	Day	7% 4 2 5% 4 5% 4 4 7% 9% 50%	2073
	21st to end	Day	6% 4 2 6% 6% 5% 6% 5% 7% 10% 44%	1815
March	1st to 10th	Day	4 4 2 6% 6% 6% 6% 6% 8% 10% 44%	4935
	11th to 20th	Day	3 3 1 3 3 3 4 4 6% 10% 59%	4980
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 8% 10% 43%	5465
April	1st to 10th	Day	4 4 2 5% 5% 4 6% 5% 10% 12% 45%	2200
	11th to 20th	Day	6% 4 2 7% 5% 6% 7% 5% 8% 10% 41%	2259
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 9% 11% 44%	2400
May	1st to 10th	Day	2 2 1 3 3 4 4 5% 8% 12% 56%	5405
	11th to 20th	Day	7% 4 2 5% 5% 5% 6% 5% 6% 10% 46%	5422
	21st to end	Day	4 4 2 6% 6% 7% 7% 7% 10% 13% 35%	6433
June	1st to 10th	Day	13% 10% 3 11% 8% 9% 7% 6% 9% 9% 15%	2600
	11th to 20th	Day	9% 8% 4 10% 8% 9% 9% 7% 9% 10% 18%	2600
	21st to end	Day	6% 8% 3 10% 8% 9% 9% 8% 10% 11% 18%	2600
July	1st to 10th	Day	5% 7% 3 8% 7% 7% 7% 6% 9% 14% 27%	4250
	11th to 20th	Day	4 5% 2 9% 7% 7% 8% 7% 10% 14% 25%	4250
	21st to end	Day	3 4 2 6% 5% 7% 8% 7% 11% 15% 31%	4466
August	1st to 10th	Day	4 5% 3 11% 8% 10% 10% 8% 11% 13% 19%	4013
	11th to 20th	Day	6% 5% 2 8% 6% 7% 8% 7% 11% 16% 23%	3730
	21st to end	Day	5% 4 2 8% 8% 8% 10% 8% 11% 16% 19%	3842
September	1st to 10th	Day	7% 5% 3 10% 8% 10% 9% 8% 10% 11% 18%	2090
	11th to 20th	Day	7% 8% 3 11% 9% 9% 10% 6% 8% 9% 20%	2090
	21st to end	Day	12% 9% 4 11% 7% 8% 7% 6% 8% 9% 20%	2090
October	1st to 10th	Day	9% 7% 3 11% 8% 7% 7% 6% 10% 11% 20%	2005
	11th to 20th	Day	10% 6% 2 8% 7% 7% 8% 6% 9% 11% 26%	1900
	21st to end	Day	11% 7% 2 8% 6% 6% 5% 5% 7% 9% 34%	2090
November	1st to 10th	Day	8% 5% 3 7% 6% 6% 7% 5% 9% 9% 37%	1957
	11th to 20th	Day	8% 7% 3 9% 7% 7% 7% 6% 7% 8% 33%	2000
	21st to end	Day	7% 6% 2 7% 6% 6% 7% 5% 6% 10% 39%	2000
December	1st to 10th	Day	5% 4 2 5% 4 3 4 4 7% 8% 53%	3232
	11th to 20th	Day	9% 6% 2 7% 6% 5% 6% 5% 8% 11% 37%	3373
	21st to end	Day	11% 7% 3 9% 6% 6% 6% 6% 8% 9% 28%	4466
January	1st to 10th	Night	6% 4 2 6% 5% 5% 6% 5% 7% 9% 44%	5889
	11th to 20th	Night	14% 9% 3 9% 5% 5% 5% 4 5% 7% 34%	5900
	21st to end	Night	10% 8% 3 7% 5% 4 4 4 6% 8% 41%	6370
February	1st to 10th	Night	7% 5% 3 6% 4 4 4 4 6% 9% 49%	2400
	11th to 20th	Night	6% 4 1 5% 3 4 5% 3 5% 7% 57%	2211
	21st to end	Night	6% 2 1 4 3 3 4 3 6% 9% 57%	1815
March	1st to 10th	Night	3 4 2 6% 4 5% 5% 4 6% 8% 52%	4918
	11th to 20th	Night	4 3 1 4 3 3 3 4 5% 8% 62%	4980
	21st to end	Night	5% 4 1 5% 5% 5% 6% 4 6% 8% 51%	5467
April	1st to 10th	Night	4 3 1 3 3 3 4 3 7% 10% 58%	2200
	11th to 20th	Night	5% 3 1 5% 4 4 5% 4 6% 8% 55%	2087
	21st to end	Night	6% 4 1 4 4 4 4 4 7% 10% 52%	2000
May	1st to 10th	Night	2 2 1 2 3 3 4 3 6% 10% 64%	4490
	11th to 20th	Night	5% 4 2 5% 4 4 4 3 6% 9% 55%	4490
	21st to end	Night	4 4 1 5% 4 5% 5% 6% 9% 12% 44%	4429
June	1st to 10th	Night	11% 8% 2 9% 5% 6% 8% 6% 8% 11% 25%	1800
	11th to 20th	Night	7% 5% 2 6% 5% 7% 6% 6% 9% 13% 34%	1800
	21st to end	Night	4 5% 2 6% 7% 6% 7% 8% 12% 13% 30%	1800
July	1st to 10th	Night	3 5% 2 7% 5% 5% 6% 6% 9% 14% 37%	2930
	11th to 20th	Night	3 3 2 6% 5% 5% 6% 6% 10% 15% 37%	2930
	21st to end	Night	3 4 2 5% 5% 5% 6% 6% 9% 15% 41%	3087
August	1st to 10th	Night	3 4 3 7% 6% 7% 8% 7% 9% 13% 34%	2884
	11th to 20th	Night	3 3 2 6% 5% 6% 7% 6% 10% 16% 37%	3097
	21st to end	Night	4 4 3 7% 6% 6% 8% 8% 12% 17% 27%	3793
September	1st to 10th	Night	5% 4 2 7% 7% 6% 8% 7% 10% 12% 33%	2090
	11th to 20th	Night	7% 8% 3 8% 7% 6% 7% 5% 9% 11% 28%	2090
	21st to end	Night	7% 6% 3 9% 6% 8% 7% 6% 8% 11% 27%	2090
October	1st to 10th	Night	9% 6% 2 8% 7% 7% 8% 6% 9% 10% 28%	2174
	11th to 20th	Night	8% 6% 2 7% 6% 5% 6% 5% 7% 12% 36%	2280
	21st to end	Night	10% 6% 3 7% 5% 6% 5% 4 6% 8% 41%	2508
November	1st to 10th	Night	7% 5% 2 5% 5% 4 4 4 7% 9% 47%	2341
	11th to 20th	Night	8% 7% 3 7% 5% 6% 6% 4 7% 9% 39%	2400
	21st to end	Night	7% 4 3 6% 5% 5% 5% 4 5% 9% 47%	2400
December	1st to 10th	Night	5% 5% 2 5% 3 3 4 3 6% 10% 54%	3943
	11th to 20th	Night	11% 8% 2 7% 5% 4 4 4 6% 9% 40%	4822
	21st to end	Night	9% 8% 3 10% 6% 6% 6% 5% 7% 8% 31%	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	31% 5% 10% 31% 17% 7%	4057
	11th to 20th	Day	51% 7% 13% 19% 8% 2	4060
	21st to end	Day	41% 5% 8% 26% 15% 4	4581
February	1st to 10th	Day	37% 6% 10% 26% 17% 4	2000
	11th to 20th	Day	28% 6% 11% 27% 22% 6%	2083
	21st to end	Day	31% 6% 10% 22% 24% 7%	1815
March	1st to 10th	Day	31% 9% 13% 21% 22% 4	4935
	11th to 20th	Day	18% 6% 15% 30% 19% 11%	4980
	21st to end	Day	32% 7% 12% 28% 17% 5%	5465
April	1st to 10th	Day	27% 8% 15% 31% 16% 3	2200
	11th to 20th	Day	32% 9% 14% 21% 18% 5%	2259
	21st to end	Day	29% 8% 12% 25% 22% 4	2400
May	1st to 10th	Day	17% 7% 18% 36% 19% 3	5437
	11th to 20th	Day	30% 9% 18% 34% 8% 1	5426
	21st to end	Day	33% 10% 20% 30% 7%	6447
June	1st to 10th	Day	60% 11% 16% 11% 2	2600
	11th to 20th	Day	53% 13% 19% 13% 2	2600
	21st to end	Day	52% 13% 23% 10% 1	2600
July	1st to 10th	Day	40% 12% 32% 16%	4185
	11th to 20th	Day	42% 12% 36% 10%	4181
	21st to end	Day	33% 12% 39% 16% 1	4324
August	1st to 10th	Day	49% 15% 25% 10% 1	4000
	11th to 20th	Day	39% 13% 33% 14%	3766
	21st to end	Day	45% 13% 34% 7% 1	3842
September	1st to 10th	Day	51% 16% 21% 11% 1	2090
	11th to 20th	Day	53% 14% 22% 9% 2	2090
	21st to end	Day	56% 13% 14% 13% 5%	2090
October	1st to 10th	Day	48% 11% 21% 15% 5% 1	3330
	11th to 20th	Day	50% 10% 14% 17% 9% 1	3140
	21st to end	Day	39% 5% 10% 27% 15% 3	3428
November	1st to 10th	Day	39% 7% 12% 18% 19% 6%	1957
	11th to 20th	Day	46% 7% 9% 18% 15% 5%	2000
	21st to end	Day	38% 7% 12% 15% 23% 4	2000
December	1st to 10th	Day	28% 5% 9% 32% 22% 4	3232
	11th to 20th	Day	39% 6% 12% 20% 20% 4	3370
	21st to end	Day	48% 7% 14% 15% 15% 1	4466
January	1st to 10th	Night	33% 6% 13% 27% 17% 3	5889
	11th to 20th	Night	50% 6% 16% 18% 8% 2	5900
	21st to end	Night	40% 6% 8% 28% 14% 4	6370
February	1st to 10th	Night	32% 4 14% 26% 20% 4	2400
	11th to 20th	Night	26% 5% 12% 26% 23% 8%	2212
	21st to end	Night	23% 6% 12% 23% 29% 7%	1815
March	1st to 10th	Night	28% 8% 12% 23% 24% 5%	4918
	11th to 20th	Night	19% 5% 19% 28% 19% 10%	4980
	21st to end	Night	30% 6% 13% 26% 21% 3	5467
April	1st to 10th	Night	20% 7% 18% 35% 18% 3	2200
	11th to 20th	Night	25% 7% 17% 24% 22% 5%	2087
	21st to end	Night	27% 6% 15% 28% 21% 3	2000
May	1st to 10th	Night	14% 6% 24% 40% 14% 2	4520
	11th to 20th	Night	25% 7% 17% 38% 12% 1	4491
	21st to end	Night	27% 9% 28% 28% 8%	4437
June	1st to 10th	Night	50% 12% 22% 14% 2	1800
	11th to 20th	Night	39% 12% 27% 20% 2	1800
	21st to end	Night	38% 15% 36% 10%	1800
July	1st to 10th	Night	34% 12% 37% 17%	2885
	11th to 20th	Night	32% 12% 44% 11%	2876
	21st to end	Night	31% 12% 40% 15% 1	2988
August	1st to 10th	Night	39% 13% 31% 14% 3	2873
	11th to 20th	Night	31% 13% 37% 18% 1	3127
	21st to end	Night	40% 15% 38% 7% 1	3793
September	1st to 10th	Night	39% 16% 28% 14% 2	2090
	11th to 20th	Night	47% 16% 23% 13% 2	2090
	21st to end	Night	48% 14% 19% 13% 6%	2090
October	1st to 10th	Night	48% 12% 19% 17% 4 1	3629
	11th to 20th	Night	49% 8% 18% 14% 10% 1	3779
	21st to end	Night	41% 6% 10% 27% 15% 3	4120
November	1st to 10th	Night	34% 6% 14% 22% 19% 5%	2341
	11th to 20th	Night	42% 8% 9% 24% 15% 4	2400
	21st to end	Night	33% 6% 14% 20% 21% 5%	2400
December	1st to 10th	Night	27% 6% 12% 33% 18% 4	3943
	11th to 20th	Night	41% 5% 11% 22% 17% 5%	4824
	21st to end	Night	48% 7% 14% 16% 13% 1	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	8% 18% 43% 26% 4 1	300
	11th to 20th	Day	17% 16% 42% 21% 3 1	300
	21st to end	Day	11% 17% 51% 18% 2	330
February	1st to 10th	Day	9% 12% 44% 27% 6% 1	300
	11th to 20th	Day	7% 9% 39% 32% 12% 2	426
	21st to end	Day	6% 11% 32% 34% 16% 1	414
March	1st to 10th	Day	5% 15% 40% 30% 8% 2	500
	11th to 20th	Day	7% 14% 36% 29% 13% 2	500
	21st to end	Day	5% 11% 43% 25% 13% 3	550
April	1st to 10th	Day	4 10% 43% 25% 13% 5%	500
	11th to 20th	Day	7% 9% 42% 23% 14% 5%	492
	21st to end	Day	5% 5% 39% 31% 15% 5%	500
May	1st to 10th	Day	3 4 38% 29% 18% 9%	503
	11th to 20th	Day	10% 7% 27% 31% 17% 8%	527
	21st to end	Day	9% 4 15% 36% 23% 13%	573
June	1st to 10th	Day	12% 3 15% 33% 23% 14%	530
	11th to 20th	Day	9% 2 11% 31% 25% 23%	530
	21st to end	Day	6% 2 6% 28% 27% 31%	530
July	1st to 10th	Day	5% 3 5% 27% 29% 31%	525
	11th to 20th	Day	5% 2 5% 28% 32% 27%	524
	21st to end	Day	4 2 5% 26% 29% 34%	532
August	1st to 10th	Day	6% 2 6% 28% 28% 30%	486
	11th to 20th	Day	5% 1 5% 27% 28% 33%	479
	21st to end	Day	8% 3 9% 29% 29% 22%	539
September	1st to 10th	Day	7% 3 5% 28% 28% 29%	490
	11th to 20th	Day	5% 2 6% 23% 34% 30%	490
	21st to end	Day	10% 2 10% 29% 25% 23%	490
October	1st to 10th	Day	10% 4 16% 38% 24% 8%	490
	11th to 20th	Day	11% 4 22% 32% 21% 10%	490
	21st to end	Day	12% 6% 23% 37% 16% 5%	539
November	1st to 10th	Day	7% 4 32% 35% 17% 5%	495
	11th to 20th	Day	11% 5% 31% 33% 17% 3	500
	21st to end	Day	8% 10% 29% 36% 15% 3	500
December	1st to 10th	Day	11% 6% 30% 36% 15% 2	485
	11th to 20th	Day	9% 7% 39% 36% 7% 2	300
	21st to end	Day	13% 12% 36% 31% 8% 1	330
January	1st to 10th	Night	7% 7% 26% 37% 20% 3	800
	11th to 20th	Night	10% 6% 33% 35% 15% 2	800
	21st to end	Night	10% 8% 31% 38% 12% 1	880
February	1st to 10th	Night	9% 5% 25% 39% 20% 2	800
	11th to 20th	Night	7% 5% 29% 39% 18% 2	783
	21st to end	Night	7% 6% 27% 44% 17%	660
March	1st to 10th	Night	6% 6% 34% 38% 15% 2	800
	11th to 20th	Night	6% 6% 28% 41% 18% 2	800
	21st to end	Night	6% 5% 29% 42% 17% 1	880
April	1st to 10th	Night	5% 5% 26% 43% 20% 2	800
	11th to 20th	Night	5% 4 31% 39% 19% 1	791
	21st to end	Night	6% 3 26% 41% 21% 2	800
May	1st to 10th	Night	3 2 23% 41% 28% 3	800
	11th to 20th	Night	9% 3 18% 43% 25% 2	789
	21st to end	Night	8% 1 8% 34% 39% 11%	658
June	1st to 10th	Night	13% 2 7% 28% 37% 13%	600
	11th to 20th	Night	8% 2 4 18% 43% 27%	600
	21st to end	Night	5% 1 2 17% 41% 34%	600
July	1st to 10th	Night	5% 1 2 11% 46% 35%	585
	11th to 20th	Night	3 2 1 14% 47% 33%	582
	21st to end	Night	3 1 1 12% 41% 42%	624
August	1st to 10th	Night	7% 1 3 14% 45% 31%	600
	11th to 20th	Night	4 1 2 18% 41% 34%	750
	21st to end	Night	7% 2 6% 22% 37% 25%	836
September	1st to 10th	Night	7% 2 4 18% 36% 33%	760
	11th to 20th	Night	9% 2 3 19% 35% 32%	760
	21st to end	Night	9% 3 5% 20% 37% 25%	760
October	1st to 10th	Night	11% 3 11% 33% 32% 10%	760
	11th to 20th	Night	10% 2 13% 33% 32% 9%	760
	21st to end	Night	13% 3 21% 35% 23% 6%	836
November	1st to 10th	Night	8% 5% 23% 39% 22% 3	780
	11th to 20th	Night	12% 3 21% 33% 26% 5%	800
	21st to end	Night	9% 4 22% 40% 22% 3	800
December	1st to 10th	Night	7% 7% 18% 42% 25% 2	800
	11th to 20th	Night	10% 4 23% 37% 22% 3	800
	21st to end	Night	11% 6% 24% 36% 21% 3	880

Page generated at 2025-07-18 01:09:58. Version ac07

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