Starbrite Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starbrite Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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% 6% 2 7% 6% 6% 7% 6% 8% 11% 34%	113608
	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	114683
Transparency	Day	38% 11% 19% 19% 10% 3	117318
	Night	35% 10% 19% 21% 12% 3	119112
Seeing	Day	8% 5% 19% 28% 23% 17%	16575
	Night	8% 3 17% 32% 29% 12%	26761

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% 6% 2 6% 4 4 4 4 7% 10% 48%	4056
	11th to 20th	Day	16% 7% 3 9% 6% 5% 6% 5% 7% 9% 27%	4060
	21st to end	Day	10% 7% 3 7% 6% 5% 4 5% 6% 8% 39%	4534
February	1st to 10th	Day	9% 7% 3 6% 5% 5% 5% 4 6% 9% 41%	2000
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 7% 10% 49%	2073
	21st to end	Day	7% 5% 1 6% 5% 5% 5% 6% 8% 9% 44%	1815
March	1st to 10th	Day	4 4 2 8% 6% 6% 7% 6% 8% 9% 40%	4935
	11th to 20th	Day	3 3 1 3 3 3 4 4 7% 8% 59%	4980
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 9% 42%	5465
April	1st to 10th	Day	4 3 2 5% 5% 5% 6% 6% 9% 13% 44%	2200
	11th to 20th	Day	6% 5% 3 7% 6% 5% 6% 6% 8% 10% 39%	2159
	21st to end	Day	6% 4 2 6% 6% 6% 6% 6% 9% 10% 41%	2280
May	1st to 10th	Day	2 2 1 4 4 4 5% 5% 7% 11% 56%	3765
	11th to 20th	Day	5% 6% 2 7% 5% 7% 7% 6% 9% 11% 35%	3789
	21st to end	Day	6% 5% 2 6% 5% 6% 7% 6% 8% 10% 38%	4476
June	1st to 10th	Day	13% 9% 4 11% 8% 9% 8% 6% 8% 8% 16%	2470
	11th to 20th	Day	8% 8% 4 11% 9% 9% 7% 7% 9% 10% 18%	2470
	21st to end	Day	7% 8% 3 10% 9% 8% 8% 7% 10% 11% 19%	2470
July	1st to 10th	Day	5% 7% 3 8% 7% 8% 7% 7% 9% 14% 26%	4120
	11th to 20th	Day	5% 5% 3 9% 7% 7% 8% 7% 11% 14% 24%	4120
	21st to end	Day	4 4 2 7% 6% 6% 8% 7% 10% 15% 30%	4466
August	1st to 10th	Day	5% 5% 3 11% 8% 8% 10% 8% 12% 13% 19%	4015
	11th to 20th	Day	5% 5% 2 7% 7% 8% 9% 9% 11% 14% 24%	3730
	21st to end	Day	5% 5% 2 8% 7% 8% 10% 8% 12% 17% 19%	3900
September	1st to 10th	Day	7% 7% 3 10% 8% 9% 10% 7% 11% 11% 17%	2090
	11th to 20th	Day	6% 7% 4 11% 9% 10% 9% 7% 8% 11% 19%	2090
	21st to end	Day	11% 10% 3 11% 7% 9% 7% 7% 8% 8% 19%	2090
October	1st to 10th	Day	8% 8% 4 9% 8% 8% 8% 7% 10% 12% 19%	1995
	11th to 20th	Day	10% 5% 3 8% 8% 7% 9% 6% 9% 10% 26%	1900
	21st to end	Day	11% 7% 2 7% 5% 5% 6% 5% 6% 11% 34%	2090
November	1st to 10th	Day	7% 6% 2 6% 5% 5% 7% 5% 8% 10% 37%	1957
	11th to 20th	Day	9% 7% 2 9% 8% 6% 7% 6% 7% 8% 32%	2000
	21st to end	Day	8% 6% 2 6% 6% 6% 5% 5% 8% 10% 39%	2000
December	1st to 10th	Day	5% 4 2 4 5% 4 4 5% 7% 7% 53%	3209
	11th to 20th	Day	8% 6% 2 6% 6% 6% 6% 5% 7% 11% 37%	3373
	21st to end	Day	11% 6% 3 9% 6% 7% 7% 5% 8% 10% 28%	4466
January	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 5% 7% 10% 44%	5889
	11th to 20th	Night	14% 9% 3 9% 5% 6% 5% 4 6% 7% 31%	5900
	21st to end	Night	10% 8% 3 7% 5% 5% 4 3 5% 7% 41%	6418
February	1st to 10th	Night	7% 6% 2 4 4 5% 4 4 7% 9% 47%	2400
	11th to 20th	Night	6% 4 2 4 3 3 5% 4 6% 8% 55%	2211
	21st to end	Night	5% 3 1 4 3 4 5% 4 6% 10% 55%	1815
March	1st to 10th	Night	4 4 2 6% 4 5% 5% 4 6% 8% 51%	4918
	11th to 20th	Night	4 3 1 4 3 4 4 3 6% 9% 60%	4980
	21st to end	Night	6% 4 2 5% 5% 5% 5% 4 7% 8% 49%	5467
April	1st to 10th	Night	3 3 2 4 3 3 4 3 8% 9% 58%	2200
	11th to 20th	Night	4 4 2 4 4 5% 5% 4 7% 10% 50%	1976
	21st to end	Night	6% 5% 2 5% 4 3 4 4 8% 9% 51%	1900
May	1st to 10th	Night	2 2 1 3 2 3 3 3 6% 9% 65%	3130
	11th to 20th	Night	4 4 2 5% 4 5% 5% 5% 8% 11% 47%	3130
	21st to end	Night	5% 5% 2 6% 5% 5% 6% 5% 8% 11% 44%	3079
June	1st to 10th	Night	10% 8% 3 9% 7% 7% 7% 7% 7% 10% 25%	1710
	11th to 20th	Night	7% 6% 2 7% 5% 6% 6% 5% 8% 14% 34%	1710
	21st to end	Night	6% 5% 2 7% 6% 6% 7% 6% 10% 14% 30%	1710
July	1st to 10th	Night	4 5% 2 7% 6% 6% 5% 5% 8% 12% 39%	2840
	11th to 20th	Night	4 5% 2 6% 4 5% 6% 6% 11% 13% 37%	2840
	21st to end	Night	2 4 2 6% 5% 5% 6% 5% 9% 14% 42%	3087
August	1st to 10th	Night	4 4 2 6% 7% 6% 8% 7% 9% 13% 34%	2879
	11th to 20th	Night	3 4 2 6% 5% 6% 7% 6% 10% 15% 37%	3097
	21st to end	Night	4 4 2 7% 6% 6% 8% 7% 13% 16% 26%	3750
September	1st to 10th	Night	6% 4 2 7% 7% 7% 8% 6% 10% 13% 29%	2090
	11th to 20th	Night	7% 7% 3 8% 7% 7% 7% 5% 9% 12% 29%	2090
	21st to end	Night	8% 7% 3 9% 7% 8% 8% 6% 9% 10% 26%	2090
October	1st to 10th	Night	8% 7% 3 9% 7% 8% 6% 8% 9% 10% 27%	2183
	11th to 20th	Night	8% 6% 2 7% 6% 5% 6% 5% 7% 10% 37%	2280
	21st to end	Night	10% 6% 2 7% 5% 5% 5% 5% 6% 9% 40%	2508
November	1st to 10th	Night	8% 4 2 6% 5% 5% 6% 4 6% 9% 46%	2341
	11th to 20th	Night	7% 7% 3 7% 6% 6% 6% 5% 6% 9% 38%	2400
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	2400
December	1st to 10th	Night	5% 5% 2 5% 4 3 4 3 6% 8% 54%	3953
	11th to 20th	Night	12% 7% 3 7% 5% 4 5% 4 7% 9% 37%	4822
	21st to end	Night	8% 8% 3 10% 6% 7% 6% 5% 7% 10% 29%	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	30% 6% 10% 29% 18% 6%	4057
	11th to 20th	Day	51% 7% 14% 19% 7% 2	4060
	21st to end	Day	41% 6% 8% 27% 14% 4	4534
February	1st to 10th	Day	37% 7% 12% 24% 16% 4	2000
	11th to 20th	Day	29% 7% 11% 26% 21% 6%	2083
	21st to end	Day	32% 8% 12% 19% 22% 7%	1815
March	1st to 10th	Day	33% 10% 13% 20% 21% 4	4935
	11th to 20th	Day	19% 7% 16% 28% 19% 11%	4980
	21st to end	Day	33% 8% 12% 28% 15% 5%	5465
April	1st to 10th	Day	27% 9% 18% 28% 16% 4	2200
	11th to 20th	Day	36% 8% 17% 21% 15% 2	2159
	21st to end	Day	31% 9% 15% 23% 19% 3	2280
May	1st to 10th	Day	17% 8% 17% 34% 19% 4	3800
	11th to 20th	Day	35% 12% 27% 22% 5%	3793
	21st to end	Day	32% 12% 18% 29% 9%	4489
June	1st to 10th	Day	61% 12% 16% 10% 2	2470
	11th to 20th	Day	55% 14% 19% 11% 1	2470
	21st to end	Day	52% 15% 25% 8%	2470
July	1st to 10th	Day	38% 15% 32% 15%	4055
	11th to 20th	Day	41% 15% 34% 10%	4051
	21st to end	Day	34% 15% 36% 15%	4324
August	1st to 10th	Day	48% 17% 25% 9% 1	4002
	11th to 20th	Day	36% 18% 31% 14%	3766
	21st to end	Day	42% 17% 35% 5% 1	3900
September	1st to 10th	Day	50% 18% 21% 10% 1	2090
	11th to 20th	Day	51% 18% 21% 8% 2	2090
	21st to end	Day	56% 13% 15% 11% 5%	2090
October	1st to 10th	Day	48% 12% 20% 14% 4 1	3320
	11th to 20th	Day	49% 9% 16% 17% 9% 1	3140
	21st to end	Day	37% 7% 11% 27% 14% 3	3428
November	1st to 10th	Day	38% 9% 13% 17% 17% 6%	1957
	11th to 20th	Day	46% 8% 10% 17% 14% 5%	2000
	21st to end	Day	39% 8% 12% 17% 21% 4	2000
December	1st to 10th	Day	27% 7% 13% 29% 19% 4	3209
	11th to 20th	Day	38% 8% 12% 19% 19% 4	3370
	21st to end	Day	47% 8% 15% 15% 14% 1	4466
January	1st to 10th	Night	34% 8% 13% 25% 17% 3	5889
	11th to 20th	Night	51% 6% 16% 16% 8% 2	5900
	21st to end	Night	40% 7% 9% 27% 12% 4	6418
February	1st to 10th	Night	32% 7% 14% 27% 17% 4	2400
	11th to 20th	Night	25% 7% 12% 27% 22% 7%	2212
	21st to end	Night	25% 8% 11% 24% 25% 6%	1815
March	1st to 10th	Night	28% 8% 13% 22% 24% 4	4918
	11th to 20th	Night	19% 7% 20% 27% 17% 10%	4980
	21st to end	Night	29% 8% 14% 27% 20% 3	5467
April	1st to 10th	Night	20% 7% 19% 33% 17% 4	2200
	11th to 20th	Night	28% 8% 18% 23% 20% 2	1976
	21st to end	Night	26% 8% 16% 27% 19% 4	1900
May	1st to 10th	Night	15% 9% 19% 40% 15% 3	3160
	11th to 20th	Night	26% 11% 25% 29% 9%	3131
	21st to end	Night	27% 14% 21% 30% 8%	3087
June	1st to 10th	Night	50% 13% 22% 13% 2	1710
	11th to 20th	Night	40% 14% 27% 17% 3	1710
	21st to end	Night	38% 17% 32% 11% 1	1710
July	1st to 10th	Night	34% 13% 35% 17%	2795
	11th to 20th	Night	33% 14% 42% 10%	2786
	21st to end	Night	30% 14% 39% 16% 1	2988
August	1st to 10th	Night	36% 16% 30% 15% 2	2868
	11th to 20th	Night	30% 17% 35% 18%	3127
	21st to end	Night	37% 20% 36% 6% 1	3750
September	1st to 10th	Night	40% 19% 27% 12% 1	2090
	11th to 20th	Night	44% 18% 24% 12% 2	2090
	21st to end	Night	50% 15% 16% 13% 6%	2090
October	1st to 10th	Night	48% 14% 18% 16% 3 1	3638
	11th to 20th	Night	47% 10% 18% 14% 10% 1	3779
	21st to end	Night	38% 8% 11% 27% 13% 3	4120
November	1st to 10th	Night	34% 9% 13% 19% 19% 6%	2341
	11th to 20th	Night	42% 9% 9% 22% 13% 5%	2400
	21st to end	Night	35% 8% 14% 19% 18% 5%	2400
December	1st to 10th	Night	28% 8% 13% 30% 18% 3	3953
	11th to 20th	Night	41% 8% 10% 20% 15% 5%	4824
	21st to end	Night	46% 9% 15% 16% 12% 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	8% 11% 41% 28% 11% 1	300
	11th to 20th	Day	16% 12% 34% 28% 10% 1	300
	21st to end	Day	10% 13% 46% 26% 5% 1	330
February	1st to 10th	Day	9% 9% 32% 36% 13% 2	300
	11th to 20th	Day	7% 5% 30% 38% 17% 2	426
	21st to end	Day	6% 6% 26% 38% 22% 2	414
March	1st to 10th	Day	5% 10% 33% 34% 16% 2	500
	11th to 20th	Day	6% 9% 32% 35% 16% 2	500
	21st to end	Day	5% 5% 31% 36% 19% 3	550
April	1st to 10th	Day	2 8% 35% 31% 19% 5%	500
	11th to 20th	Day	8% 7% 30% 30% 19% 6%	452
	21st to end	Day	5% 6% 26% 34% 23% 7%	460
May	1st to 10th	Day	4 1 21% 35% 26% 13%	463
	11th to 20th	Day	9% 4 18% 32% 26% 11%	487
	21st to end	Day	10% 3 12% 29% 26% 20%	529
June	1st to 10th	Day	14% 3 12% 25% 28% 19%	490
	11th to 20th	Day	9% 3 8% 23% 28% 29%	490
	21st to end	Day	5% 2 6% 19% 26% 42%	490
July	1st to 10th	Day	5% 1 3 15% 33% 43%	485
	11th to 20th	Day	5% 5% 17% 34% 39%	484
	21st to end	Day	5% 1 5% 18% 28% 43%	532
August	1st to 10th	Day	6% 2 7% 17% 29% 39%	486
	11th to 20th	Day	5% 1 6% 19% 29% 41%	479
	21st to end	Day	10% 1 7% 22% 29% 30%	539
September	1st to 10th	Day	8% 2 5% 18% 28% 39%	490
	11th to 20th	Day	7% 1 8% 16% 30% 38%	490
	21st to end	Day	13% 2 11% 21% 27% 27%	490
October	1st to 10th	Day	11% 4 12% 35% 26% 12%	490
	11th to 20th	Day	10% 3 19% 30% 26% 11%	490
	21st to end	Day	10% 7% 19% 34% 23% 7%	539
November	1st to 10th	Day	8% 6% 23% 36% 22% 6%	495
	11th to 20th	Day	11% 6% 24% 32% 22% 5%	500
	21st to end	Day	7% 5% 28% 35% 23% 3	500
December	1st to 10th	Day	11% 5% 23% 37% 21% 4	475
	11th to 20th	Day	7% 8% 29% 37% 16% 4	300
	21st to end	Day	10% 8% 32% 34% 12% 4	330
January	1st to 10th	Night	9% 7% 23% 38% 22% 2	800
	11th to 20th	Night	9% 7% 30% 36% 18% 1	800
	21st to end	Night	9% 8% 29% 41% 13% 1	880
February	1st to 10th	Night	10% 6% 23% 40% 19% 2	800
	11th to 20th	Night	6% 6% 26% 41% 20% 1	783
	21st to end	Night	6% 5% 23% 47% 18% 1	660
March	1st to 10th	Night	5% 4 31% 41% 17% 1	800
	11th to 20th	Night	7% 6% 22% 44% 19% 1	800
	21st to end	Night	6% 5% 27% 42% 19% 1	880
April	1st to 10th	Night	4 3 21% 49% 21% 2	800
	11th to 20th	Night	6% 4 24% 45% 20% 1	754
	21st to end	Night	8% 2 20% 45% 22% 3	760
May	1st to 10th	Night	4 1 18% 44% 30% 3	760
	11th to 20th	Night	9% 4 14% 43% 28% 3	749
	21st to end	Night	7% 2 8% 30% 43% 9%	617
June	1st to 10th	Night	12% 2 7% 21% 44% 15%	570
	11th to 20th	Night	7% 1 4 15% 45% 28%	570
	21st to end	Night	5% 1 4 11% 41% 38%	570
July	1st to 10th	Night	6% 3 10% 40% 41%	555
	11th to 20th	Night	6% 3 11% 41% 40%	552
	21st to end	Night	3 1 2 9% 41% 45%	624
August	1st to 10th	Night	8% 3 13% 43% 34%	595
	11th to 20th	Night	3 1 4 13% 41% 36%	750
	21st to end	Night	9% 1 6% 19% 38% 28%	836
September	1st to 10th	Night	7% 1 6% 15% 38% 34%	760
	11th to 20th	Night	10% 1 8% 16% 37% 29%	760
	21st to end	Night	12% 2 9% 18% 37% 23%	760
October	1st to 10th	Night	11% 1 13% 33% 34% 10%	760
	11th to 20th	Night	10% 3 11% 31% 36% 9%	760
	21st to end	Night	13% 2 17% 35% 26% 6%	836
November	1st to 10th	Night	9% 3 24% 33% 27% 3	780
	11th to 20th	Night	10% 3 20% 35% 28% 5%	800
	21st to end	Night	7% 4 23% 41% 22% 3	800
December	1st to 10th	Night	7% 4 19% 38% 29% 2	800
	11th to 20th	Night	9% 6% 21% 37% 24% 3	800
	21st to end	Night	8% 4 24% 37% 24% 2	880

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