Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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% 8% 3 7% 5% 4 5% 4 6% 8% 38%	131461
	Night	14% 8% 3 7% 5% 4 4 3 5% 7% 39%	131172
Transparency	Day	43% 6% 11% 23% 16% 2	135105
	Night	43% 5% 10% 23% 17% 2	135589
Seeing	Day	17% 11% 26% 23% 15% 7%	17341
	Night	17% 8% 26% 30% 16% 2	27190

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	15% 12% 6% 9% 4 4 4 3 5% 6% 30%	4691
	11th to 20th	Day	12% 10% 4 11% 5% 5% 4 3 4 6% 37%	4710
	21st to end	Day	18% 11% 5% 8% 6% 5% 5% 4 4 6% 27%	5858
February	1st to 10th	Day	17% 10% 4 10% 5% 3 3 2 5% 6% 34%	2100
	11th to 20th	Day	16% 10% 4 8% 5% 4 5% 3 5% 6% 35%	2048
	21st to end	Day	16% 11% 5% 9% 4 4 5% 3 5% 6% 32%	1730
March	1st to 10th	Day	19% 12% 4 7% 4 3 3 2 3 5% 37%	4580
	11th to 20th	Day	11% 9% 3 7% 4 4 4 3 5% 7% 42%	4963
	21st to end	Day	15% 9% 3 7% 5% 5% 5% 4 6% 7% 35%	5760
April	1st to 10th	Day	14% 8% 2 7% 5% 4 5% 3 6% 7% 39%	2520
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 6% 6% 39%	2494
	21st to end	Day	16% 6% 2 7% 6% 4 4 4 5% 7% 38%	2683
May	1st to 10th	Day	19% 9% 3 7% 5% 6% 6% 5% 7% 8% 25%	5875
	11th to 20th	Day	19% 8% 2 7% 5% 4 5% 4 5% 7% 34%	5861
	21st to end	Day	12% 7% 3 8% 6% 6% 5% 5% 8% 9% 32%	6433
June	1st to 10th	Day	11% 5% 2 6% 5% 4 4 4 6% 7% 44%	2712
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 6% 9% 44%	2800
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 7% 11% 42%	2800
July	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 7% 10% 41%	6360
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 5% 8% 11% 39%	6360
	21st to end	Day	8% 7% 3 9% 7% 7% 7% 5% 8% 10% 29%	6453
August	1st to 10th	Day	6% 5% 2 6% 4 5% 5% 4 7% 10% 47%	4234
	11th to 20th	Day	7% 5% 2 6% 4 5% 6% 6% 8% 10% 41%	3945
	21st to end	Day	5% 4 2 4 3 3 5% 4 6% 10% 55%	4312
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 3 5% 7% 48%	2343
	11th to 20th	Day	8% 5% 2 5% 4 5% 4 4 6% 8% 48%	2200
	21st to end	Day	13% 6% 2 5% 5% 4 4 3 4 6% 48%	2200
October	1st to 10th	Day	10% 4 2 5% 4 3 4 4 5% 6% 55%	2200
	11th to 20th	Day	11% 5% 3 6% 4 4 4 3 5% 6% 49%	2099
	21st to end	Day	18% 7% 2 6% 4 4 4 4 4 6% 41%	1980
November	1st to 10th	Day	10% 6% 2 5% 4 3 4 3 5% 6% 50%	1852
	11th to 20th	Day	15% 11% 2 6% 4 3 3 3 5% 6% 41%	1890
	21st to end	Day	18% 9% 3 6% 5% 4 3 3 5% 5% 38%	1890
December	1st to 10th	Day	16% 12% 2 7% 3 3 3 2 4 5% 43%	3020
	11th to 20th	Day	15% 11% 5% 11% 6% 5% 4 3 4 6% 29%	3435
	21st to end	Day	20% 18% 5% 9% 4 5% 4 3 4 6% 22%	4070
January	1st to 10th	Night	14% 12% 5% 11% 5% 4 4 3 5% 6% 30%	8232
	11th to 20th	Night	13% 11% 5% 11% 6% 6% 5% 4 4 5% 31%	8280
	21st to end	Night	17% 11% 3 8% 5% 4 4 3 5% 6% 35%	8318
February	1st to 10th	Night	21% 10% 5% 8% 5% 4 4 3 6% 5% 29%	2520
	11th to 20th	Night	18% 10% 4 9% 5% 4 4 3 4 5% 35%	2458
	21st to end	Night	20% 10% 3 8% 5% 3 4 3 5% 5% 33%	2076
March	1st to 10th	Night	20% 13% 3 6% 4 3 3 3 4 5% 35%	5493
	11th to 20th	Night	12% 6% 3 7% 4 4 4 3 5% 5% 48%	5216
	21st to end	Night	15% 8% 3 8% 5% 5% 4 4 5% 7% 35%	5453
April	1st to 10th	Night	12% 7% 2 7% 4 4 5% 5% 7% 7% 41%	2100
	11th to 20th	Night	14% 8% 2 6% 4 4 5% 3 5% 7% 41%	2075
	21st to end	Night	16% 6% 3 7% 4 4 5% 4 5% 7% 39%	1937
May	1st to 10th	Night	19% 10% 3 8% 5% 5% 4 4 5% 9% 28%	4042
	11th to 20th	Night	16% 7% 3 8% 5% 5% 4 4 6% 8% 34%	4032
	21st to end	Night	11% 7% 3 8% 5% 5% 6% 5% 7% 9% 36%	4421
June	1st to 10th	Night	9% 4 2 6% 4 4 4 4 5% 10% 48%	1682
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 6% 8% 50%	1600
	21st to end	Night	5% 3 1 4 5% 4 4 6% 7% 11% 52%	1600
July	1st to 10th	Night	7% 6% 2 7% 4 4 5% 4 6% 9% 45%	3618
	11th to 20th	Night	6% 5% 2 7% 4 4 5% 5% 6% 11% 44%	3617
	21st to end	Night	7% 6% 3 8% 6% 5% 7% 6% 8% 11% 34%	4383
August	1st to 10th	Night	5% 4 2 6% 4 4 4 3 7% 9% 53%	2915
	11th to 20th	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 47%	3100
	21st to end	Night	3 2 1 3 3 3 3 3 6% 10% 63%	3586
September	1st to 10th	Night	11% 4 2 6% 3 4 4 3 5% 7% 51%	2036
	11th to 20th	Night	9% 5% 2 4 4 3 4 4 5% 9% 53%	2200
	21st to end	Night	13% 5% 2 5% 3 3 3 3 4 7% 52%	2200
October	1st to 10th	Night	8% 4 2 5% 4 3 3 3 4 7% 59%	2200
	11th to 20th	Night	13% 5% 2 6% 3 4 4 3 4 6% 50%	2295
	21st to end	Night	18% 7% 2 5% 4 4 3 2 4 6% 45%	2860
November	1st to 10th	Night	12% 7% 2 6% 4 4 3 2 5% 5% 50%	2666
	11th to 20th	Night	15% 6% 3 6% 5% 3 3 3 5% 7% 44%	2730
	21st to end	Night	17% 10% 3 7% 5% 4 4 3 4 6% 38%	2730
December	1st to 10th	Night	16% 11% 3 7% 4 4 3 2 4 5% 41%	4380
	11th to 20th	Night	19% 10% 3 8% 4 4 4 3 5% 6% 34%	4971
	21st to end	Night	23% 16% 5% 10% 6% 5% 5% 3 4 4 20%	7150

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	55% 3 11% 22% 8% 1	4692
	11th to 20th	Day	49% 3 8% 21% 17% 2	4710
	21st to end	Day	59% 4 8% 18% 7% 5%	5858
February	1st to 10th	Day	52% 3 10% 23% 11% 2	2100
	11th to 20th	Day	51% 4 9% 21% 13% 2	2058
	21st to end	Day	52% 5% 9% 20% 11% 3	1730
March	1st to 10th	Day	52% 2 6% 18% 17% 5%	4580
	11th to 20th	Day	41% 4 9% 26% 17% 2	4963
	21st to end	Day	46% 6% 13% 22% 13% 1	5762
April	1st to 10th	Day	43% 6% 11% 23% 16% 1	2520
	11th to 20th	Day	46% 5% 9% 22% 16% 3	2494
	21st to end	Day	45% 5% 10% 26% 12% 2	2683
May	1st to 10th	Day	51% 9% 13% 16% 10%	5910
	11th to 20th	Day	48% 7% 10% 22% 12%	5865
	21st to end	Day	44% 8% 15% 17% 16% 1	6447
June	1st to 10th	Day	37% 6% 10% 26% 19% 3	2712
	11th to 20th	Day	35% 8% 13% 23% 19% 2	2800
	21st to end	Day	33% 9% 14% 21% 19% 4	2800
July	1st to 10th	Day	37% 6% 11% 24% 20% 2	6291
	11th to 20th	Day	36% 10% 14% 27% 12% 1	6285
	21st to end	Day	46% 9% 14% 19% 10% 1	6269
August	1st to 10th	Day	30% 7% 14% 34% 14% 1	4221
	11th to 20th	Day	34% 7% 12% 29% 16% 1	3984
	21st to end	Day	24% 6% 12% 37% 17% 3	4311
September	1st to 10th	Day	36% 5% 11% 23% 20% 5%	2343
	11th to 20th	Day	33% 6% 10% 26% 22% 3	2200
	21st to end	Day	38% 4 8% 22% 24% 3	2200
October	1st to 10th	Day	20% 4 5% 29% 39% 3	3590
	11th to 20th	Day	26% 4 9% 27% 31% 3	3393
	21st to end	Day	33% 4 8% 27% 23% 5%	3180
November	1st to 10th	Day	35% 3 10% 28% 20% 4	1852
	11th to 20th	Day	44% 3 8% 23% 18% 4	1890
	21st to end	Day	48% 3 8% 23% 16% 1	1890
December	1st to 10th	Day	46% 3 9% 26% 15% 1	3020
	11th to 20th	Day	57% 4 9% 19% 10% 2	3432
	21st to end	Day	64% 5% 11% 13% 6%	4070
January	1st to 10th	Night	54% 5% 11% 21% 9% 1	8233
	11th to 20th	Night	54% 4 7% 17% 15% 3	8280
	21st to end	Night	51% 3 10% 21% 13% 1	8318
February	1st to 10th	Night	56% 4 10% 21% 8% 1	2520
	11th to 20th	Night	52% 4 8% 22% 13% 1	2459
	21st to end	Night	53% 5% 10% 18% 12% 3	2076
March	1st to 10th	Night	52% 3 6% 22% 13% 4	5493
	11th to 20th	Night	38% 5% 9% 26% 20% 2	5216
	21st to end	Night	48% 6% 8% 24% 13% 1	5453
April	1st to 10th	Night	39% 7% 12% 23% 16% 3	2100
	11th to 20th	Night	42% 4 11% 20% 18% 5%	2075
	21st to end	Night	44% 5% 10% 25% 14% 3	1937
May	1st to 10th	Night	52% 8% 11% 20% 8% 1	4069
	11th to 20th	Night	46% 8% 11% 24% 10% 1	4033
	21st to end	Night	41% 8% 15% 21% 13% 2	4429
June	1st to 10th	Night	32% 7% 10% 27% 20% 4	1682
	11th to 20th	Night	30% 7% 15% 22% 22% 5%	1600
	21st to end	Night	24% 9% 16% 24% 20% 7%	1600
July	1st to 10th	Night	34% 8% 11% 25% 20% 2	3580
	11th to 20th	Night	33% 9% 15% 28% 14% 2	3572
	21st to end	Night	40% 10% 14% 21% 13% 1	4254
August	1st to 10th	Night	26% 7% 14% 37% 14% 1	2904
	11th to 20th	Night	29% 7% 14% 31% 18% 1	3127
	21st to end	Night	17% 6% 15% 35% 24% 4	3586
September	1st to 10th	Night	33% 6% 9% 25% 22% 6%	2036
	11th to 20th	Night	29% 5% 9% 27% 25% 5%	2200
	21st to end	Night	34% 5% 8% 23% 26% 5%	2200
October	1st to 10th	Night	20% 3 7% 19% 48% 4	3590
	11th to 20th	Night	25% 3 7% 24% 38% 4	3728
	21st to end	Night	32% 3 7% 22% 30% 6%	4610
November	1st to 10th	Night	37% 3 9% 24% 23% 4	2666
	11th to 20th	Night	40% 4 8% 22% 21% 5%	2730
	21st to end	Night	48% 4 8% 21% 17% 1	2730
December	1st to 10th	Night	48% 3 11% 24% 14%	4380
	11th to 20th	Night	50% 4 13% 20% 12% 1	4973
	21st to end	Night	68% 4 9% 14% 6% 1	7150

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	24% 18% 36% 16% 6%	330
	11th to 20th	Day	27% 25% 33% 12% 3	330
	21st to end	Day	25% 24% 36% 12% 2	363
February	1st to 10th	Day	22% 22% 37% 15% 3	330
	11th to 20th	Day	28% 25% 35% 9% 2 1	330
	21st to end	Day	31% 16% 38% 12% 4	273
March	1st to 10th	Day	28% 21% 33% 15% 3	330
	11th to 20th	Day	23% 18% 39% 12% 6% 1	478
	21st to end	Day	26% 19% 33% 16% 5%	594
April	1st to 10th	Day	21% 19% 30% 22% 7% 1	540
	11th to 20th	Day	21% 20% 27% 20% 9% 3	532
	21st to end	Day	21% 14% 36% 19% 8% 2	540
May	1st to 10th	Day	23% 11% 34% 19% 9% 4	507
	11th to 20th	Day	21% 10% 26% 24% 13% 6%	527
	21st to end	Day	14% 7% 24% 27% 17% 10%	573
June	1st to 10th	Day	13% 6% 26% 29% 20% 8%	642
	11th to 20th	Day	8% 6% 22% 34% 22% 9%	730
	21st to end	Day	9% 5% 26% 29% 22% 11%	730
July	1st to 10th	Day	13% 4 16% 24% 25% 18%	721
	11th to 20th	Day	8% 3 19% 33% 25% 12%	718
	21st to end	Day	9% 3 15% 31% 27% 14%	592
August	1st to 10th	Day	11% 2 16% 27% 25% 18%	526
	11th to 20th	Day	13% 4 17% 29% 25% 12%	519
	21st to end	Day	8% 2 14% 30% 31% 16%	583
September	1st to 10th	Day	12% 3 16% 28% 30% 10%	530
	11th to 20th	Day	10% 2 13% 29% 32% 15%	530
	21st to end	Day	17% 7% 18% 24% 25% 8%	530
October	1st to 10th	Day	10% 5% 22% 35% 21% 7%	530
	11th to 20th	Day	13% 14% 30% 28% 12% 3	507
	21st to end	Day	27% 11% 37% 20% 5%	363
November	1st to 10th	Day	15% 15% 33% 25% 8% 4	330
	11th to 20th	Day	25% 17% 30% 20% 7% 1	330
	21st to end	Day	26% 15% 32% 20% 7%	330
December	1st to 10th	Day	23% 23% 32% 15% 5% 1	330
	11th to 20th	Day	22% 18% 37% 17% 5% 1	330
	21st to end	Day	32% 19% 33% 13% 3	363
January	1st to 10th	Night	20% 17% 33% 26% 3	1050
	11th to 20th	Night	25% 15% 36% 20% 4	1050
	21st to end	Night	22% 16% 35% 26% 2	930
February	1st to 10th	Night	26% 10% 36% 25% 3	840
	11th to 20th	Night	25% 15% 36% 22% 2	823
	21st to end	Night	26% 13% 38% 21% 2	692
March	1st to 10th	Night	22% 15% 32% 27% 4	840
	11th to 20th	Night	22% 10% 37% 23% 7%	840
	21st to end	Night	22% 12% 30% 29% 6% 1	866
April	1st to 10th	Night	16% 10% 36% 30% 9%	630
	11th to 20th	Night	17% 11% 31% 28% 11% 1	623
	21st to end	Night	18% 6% 33% 33% 8% 1	630
May	1st to 10th	Night	18% 5% 32% 32% 13%	603
	11th to 20th	Night	16% 5% 21% 36% 19% 4	591
	21st to end	Night	9% 4 20% 36% 24% 6%	650
June	1st to 10th	Night	10% 2 21% 38% 26% 3	600
	11th to 20th	Night	7% 3 16% 44% 27% 3	600
	21st to end	Night	4 2 18% 41% 31% 4	600
July	1st to 10th	Night	12% 1 11% 31% 36% 10%	585
	11th to 20th	Night	7% 1 11% 34% 40% 7%	582
	21st to end	Night	9% 2 11% 35% 36% 7%	657
August	1st to 10th	Night	9% 1 10% 35% 40% 5%	597
	11th to 20th	Night	6% 1 11% 40% 37% 5%	594
	21st to end	Night	6% 1 8% 34% 43% 7%	660
September	1st to 10th	Night	12% 2 10% 32% 39% 5%	636
	11th to 20th	Night	11% 3 14% 30% 38% 5%	800
	21st to end	Night	15% 5% 18% 29% 30% 4	800
October	1st to 10th	Night	9% 4 19% 35% 28% 5%	800
	11th to 20th	Night	15% 6% 27% 32% 18% 2	800
	21st to end	Night	22% 8% 30% 28% 12%	880
November	1st to 10th	Night	15% 8% 30% 30% 16% 1	820
	11th to 20th	Night	18% 12% 32% 28% 10%	840
	21st to end	Night	23% 12% 27% 28% 10%	840
December	1st to 10th	Night	25% 9% 33% 28% 5%	840
	11th to 20th	Night	25% 10% 32% 26% 7%	846
	21st to end	Night	29% 13% 29% 24% 5%	1155

Page generated at 2026-04-29 02:21:46. Version ac07

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