Dark Star Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Star Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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 4 4 6% 8% 43%	130692
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	130140
Transparency	Day	37% 7% 12% 21% 17% 5%	134298
	Night	37% 6% 10% 21% 20% 6%	134604
Seeing	Day	17% 11% 25% 24% 16% 7%	16907
	Night	16% 9% 26% 30% 17% 2	27232

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	18% 6% 2 4 3 3 3 2 4 5% 51%	5258
	11th to 20th	Day	12% 6% 2 4 2 2 3 3 4 5% 59%	5280
	21st to end	Day	16% 9% 3 5% 3 3 3 2 3 4 47%	5780
February	1st to 10th	Day	13% 6% 2 7% 4 3 5% 4 5% 7% 44%	1890
	11th to 20th	Day	21% 6% 2 5% 3 4 3 3 6% 6% 42%	1826
	21st to end	Day	20% 6% 2 5% 3 4 5% 3 5% 6% 41%	1765
March	1st to 10th	Day	18% 8% 3 6% 4 4 3 3 5% 7% 40%	4935
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 6% 53%	4980
	21st to end	Day	16% 6% 2 6% 4 4 3 3 4 6% 44%	5465
April	1st to 10th	Day	11% 4 2 4 4 3 3 4 6% 8% 51%	2290
	11th to 20th	Day	14% 5% 1 5% 3 4 3 4 5% 7% 49%	2374
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 5% 6% 44%	2473
May	1st to 10th	Day	11% 6% 2 7% 5% 6% 5% 5% 8% 11% 34%	5862
	11th to 20th	Day	21% 7% 3 8% 5% 5% 5% 5% 7% 7% 28%	5861
	21st to end	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 40%	6433
June	1st to 10th	Day	10% 4 2 6% 5% 5% 5% 5% 7% 9% 43%	2600
	11th to 20th	Day	8% 4 2 6% 5% 5% 6% 4 8% 10% 42%	2600
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 8% 10% 42%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 7% 5% 8% 10% 38%	5900
	11th to 20th	Day	7% 5% 2 8% 5% 6% 6% 6% 9% 15% 31%	5900
	21st to end	Day	10% 6% 3 7% 5% 6% 6% 5% 8% 10% 34%	6424
August	1st to 10th	Day	16% 7% 2 7% 5% 5% 6% 5% 9% 10% 28%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 5% 6% 5% 8% 11% 36%	4148
	21st to end	Day	7% 4 2 5% 4 3 4 4 6% 10% 51%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 3 5% 3 6% 8% 49%	2395
	11th to 20th	Day	14% 5% 3 6% 5% 4 4 4 6% 8% 41%	2200
	21st to end	Day	23% 6% 3 5% 4 3 4 3 5% 6% 37%	2194
October	1st to 10th	Day	15% 5% 2 6% 3 3 2 3 4 5% 53%	2000
	11th to 20th	Day	14% 4 2 4 3 3 3 2 4 5% 55%	2000
	21st to end	Day	14% 4 2 5% 3 2 3 2 4 5% 56%	2200
November	1st to 10th	Day	13% 5% 2 4 3 3 4 3 5% 5% 54%	2057
	11th to 20th	Day	14% 5% 1 4 3 3 3 3 5% 6% 53%	1995
	21st to end	Day	13% 5% 1 5% 4 3 4 3 5% 7% 50%	1840
December	1st to 10th	Day	13% 6% 2 5% 3 4 4 3 4 5% 52%	2680
	11th to 20th	Day	19% 6% 2 5% 3 3 4 3 4 6% 46%	3376
	21st to end	Day	20% 5% 2 5% 3 3 4 3 6% 7% 41%	4565
January	1st to 10th	Night	13% 6% 2 4 3 3 3 2 4 5% 55%	7635
	11th to 20th	Night	13% 6% 2 5% 3 2 2 2 3 4 58%	7680
	21st to end	Night	18% 7% 2 4 3 3 3 3 5% 5% 49%	8401
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 4 5% 6% 45%	2730
	11th to 20th	Night	20% 6% 2 5% 3 3 3 3 4 6% 44%	2569
	21st to end	Night	15% 7% 2 5% 4 4 5% 4 5% 7% 44%	1865
March	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 4 4 45%	4918
	11th to 20th	Night	11% 4 2 5% 3 3 4 3 4 6% 55%	4980
	21st to end	Night	15% 6% 2 5% 3 4 4 4 5% 7% 46%	5467
April	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 5% 6% 45%	2110
	11th to 20th	Night	12% 6% 2 4 4 3 4 3 5% 8% 49%	1975
	21st to end	Night	13% 5% 2 5% 4 4 5% 3 5% 6% 47%	1943
May	1st to 10th	Night	14% 6% 3 7% 4 5% 5% 4 6% 9% 38%	4033
	11th to 20th	Night	21% 8% 3 7% 4 4 4 3 6% 7% 34%	4032
	21st to end	Night	16% 5% 2 5% 4 3 4 4 6% 10% 42%	4421
June	1st to 10th	Night	10% 5% 1 5% 5% 5% 5% 5% 8% 10% 42%	1800
	11th to 20th	Night	9% 6% 2 5% 4 4 4 4 7% 11% 43%	1800
	21st to end	Night	7% 6% 1 5% 4 4 5% 4 8% 12% 43%	1800
July	1st to 10th	Night	11% 6% 2 8% 6% 6% 6% 5% 8% 10% 32%	4058
	11th to 20th	Night	10% 7% 3 8% 6% 7% 6% 5% 8% 11% 29%	4057
	21st to end	Night	11% 7% 3 8% 5% 5% 6% 5% 8% 9% 33%	4427
August	1st to 10th	Night	17% 8% 3 7% 5% 6% 7% 4 8% 10% 26%	2915
	11th to 20th	Night	7% 5% 2 7% 5% 6% 6% 4 7% 10% 41%	2875
	21st to end	Night	7% 4 2 7% 4 5% 5% 4 6% 8% 49%	3576
September	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 6% 7% 47%	2000
	11th to 20th	Night	15% 5% 2 8% 5% 4 4 3 6% 6% 42%	2200
	21st to end	Night	22% 8% 2 6% 4 4 4 3 5% 6% 37%	2205
October	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 5% 7% 49%	2400
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 4 5% 54%	2400
	21st to end	Night	14% 4 1 4 3 4 3 2 4 5% 55%	2640
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 5% 53%	2461
	11th to 20th	Night	13% 5% 2 5% 3 4 3 3 5% 5% 53%	2621
	21st to end	Night	14% 4 1 5% 3 4 3 4 5% 6% 52%	2768
December	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 5% 53%	4720
	11th to 20th	Night	20% 8% 3 7% 4 4 3 3 4 4 41%	5025
	21st to end	Night	19% 4 1 4 3 3 4 4 5% 7% 45%	6633

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	36% 3 9% 21% 25% 6%	5259
	11th to 20th	Day	30% 4 4 16% 32% 14%	5280
	21st to end	Day	42% 3 5% 25% 19% 6%	5780
February	1st to 10th	Day	39% 6% 11% 21% 19% 4	1890
	11th to 20th	Day	42% 5% 9% 22% 18% 5%	1835
	21st to end	Day	43% 6% 12% 15% 18% 6%	1765
March	1st to 10th	Day	43% 5% 7% 19% 15% 11%	4935
	11th to 20th	Day	30% 6% 7% 20% 24% 13%	4980
	21st to end	Day	42% 5% 8% 21% 18% 6%	5465
April	1st to 10th	Day	30% 6% 12% 24% 20% 8%	2290
	11th to 20th	Day	35% 4 12% 23% 20% 7%	2374
	21st to end	Day	40% 6% 10% 24% 16% 3	2473
May	1st to 10th	Day	40% 10% 16% 20% 11% 2	5897
	11th to 20th	Day	51% 8% 10% 20% 9% 1	5865
	21st to end	Day	37% 9% 14% 21% 16% 3	6447
June	1st to 10th	Day	33% 10% 16% 26% 13% 2	2600
	11th to 20th	Day	34% 11% 17% 21% 15% 2	2600
	21st to end	Day	33% 11% 16% 24% 15% 1	2600
July	1st to 10th	Day	37% 12% 16% 19% 12% 3	5835
	11th to 20th	Day	39% 13% 25% 19% 5%	5831
	21st to end	Day	42% 10% 16% 21% 10%	6240
August	1st to 10th	Day	47% 10% 17% 21% 5%	4221
	11th to 20th	Day	37% 10% 20% 23% 8% 1	4187
	21st to end	Day	28% 8% 17% 31% 16% 2	4312
September	1st to 10th	Day	33% 8% 15% 20% 20% 4	2395
	11th to 20th	Day	39% 7% 11% 22% 19% 2	2200
	21st to end	Day	48% 6% 10% 17% 17% 3	2194
October	1st to 10th	Day	32% 5% 7% 20% 28% 8%	3260
	11th to 20th	Day	23% 3 5% 22% 29% 18%	3240
	21st to end	Day	25% 4 8% 25% 25% 13%	3538
November	1st to 10th	Day	32% 4 9% 24% 25% 7%	2057
	11th to 20th	Day	32% 4 9% 24% 23% 8%	1995
	21st to end	Day	33% 5% 9% 24% 25% 4	1840
December	1st to 10th	Day	34% 4 7% 24% 23% 7%	2680
	11th to 20th	Day	41% 3 11% 18% 21% 6%	3373
	21st to end	Day	40% 6% 12% 24% 16% 1	4565
January	1st to 10th	Night	32% 3 10% 21% 25% 9%	7636
	11th to 20th	Night	32% 3 4 16% 35% 9%	7680
	21st to end	Night	38% 3 5% 26% 21% 6%	8401
February	1st to 10th	Night	40% 5% 10% 23% 19% 4	2730
	11th to 20th	Night	42% 4 9% 19% 20% 7%	2571
	21st to end	Night	39% 6% 10% 21% 18% 7%	1865
March	1st to 10th	Night	41% 5% 6% 18% 21% 9%	4918
	11th to 20th	Night	30% 4 7% 18% 26% 15%	4980
	21st to end	Night	38% 5% 7% 24% 22% 5%	5467
April	1st to 10th	Night	38% 6% 10% 18% 22% 6%	2110
	11th to 20th	Night	33% 5% 10% 23% 21% 7%	1975
	21st to end	Night	36% 7% 11% 24% 18% 4	1943
May	1st to 10th	Night	43% 9% 15% 19% 13% 2	4060
	11th to 20th	Night	49% 7% 11% 18% 13% 1	4033
	21st to end	Night	36% 8% 15% 26% 12% 3	4429
June	1st to 10th	Night	34% 9% 17% 24% 16% 1	1800
	11th to 20th	Night	33% 10% 19% 21% 15% 2	1800
	21st to end	Night	30% 12% 17% 21% 16% 4	1800
July	1st to 10th	Night	44% 13% 13% 17% 11% 2	4015
	11th to 20th	Night	46% 14% 22% 13% 4 1	4006
	21st to end	Night	45% 9% 15% 21% 9%	4297
August	1st to 10th	Night	50% 9% 16% 17% 6% 1	2904
	11th to 20th	Night	34% 9% 19% 26% 10% 1	2902
	21st to end	Night	31% 8% 12% 33% 14% 2	3576
September	1st to 10th	Night	36% 8% 13% 22% 16% 6%	2000
	11th to 20th	Night	41% 8% 11% 19% 20% 2	2200
	21st to end	Night	49% 6% 10% 17% 15% 3	2205
October	1st to 10th	Night	37% 4 8% 19% 24% 7%	3920
	11th to 20th	Night	24% 4 6% 23% 29% 14%	3899
	21st to end	Night	24% 3 5% 28% 26% 14%	4252
November	1st to 10th	Night	34% 5% 8% 21% 26% 7%	2461
	11th to 20th	Night	33% 4 9% 19% 25% 9%	2621
	21st to end	Night	33% 4 8% 23% 25% 7%	2768
December	1st to 10th	Night	34% 4 7% 25% 27% 4	4720
	11th to 20th	Night	47% 4 8% 18% 16% 8%	5027
	21st to end	Night	37% 6% 14% 23% 18% 2	6633

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	26% 24% 35% 12% 3	330
	11th to 20th	Day	26% 25% 35% 12% 3	330
	21st to end	Day	24% 21% 40% 13% 2	363
February	1st to 10th	Day	19% 24% 39% 14% 5%	330
	11th to 20th	Day	36% 24% 29% 10% 2	315
	21st to end	Day	26% 20% 33% 18% 3	364
March	1st to 10th	Day	22% 15% 35% 20% 7% 1	500
	11th to 20th	Day	21% 17% 37% 19% 5% 1	500
	21st to end	Day	23% 15% 36% 21% 4	550
April	1st to 10th	Day	20% 18% 35% 19% 8% 1	500
	11th to 20th	Day	19% 17% 28% 22% 10% 4	492
	21st to end	Day	19% 12% 30% 26% 11% 2	500
May	1st to 10th	Day	17% 15% 33% 25% 7% 3	503
	11th to 20th	Day	18% 8% 25% 25% 17% 7%	527
	21st to end	Day	18% 5% 20% 28% 20% 8%	573
June	1st to 10th	Day	9% 4 26% 31% 20% 9%	530
	11th to 20th	Day	11% 4 19% 30% 24% 12%	530
	21st to end	Day	10% 3 15% 33% 30% 9%	530
July	1st to 10th	Day	11% 2 10% 31% 34% 12%	525
	11th to 20th	Day	5% 2 12% 28% 35% 18%	524
	21st to end	Day	8% 2 15% 32% 28% 16%	576
August	1st to 10th	Day	15% 2 17% 25% 26% 14%	526
	11th to 20th	Day	11% 4 13% 30% 27% 16%	519
	21st to end	Day	9% 3 12% 29% 27% 19%	583
September	1st to 10th	Day	14% 4 16% 22% 28% 16%	530
	11th to 20th	Day	16% 3 16% 26% 27% 13%	530
	21st to end	Day	27% 8% 21% 25% 16% 4	530
October	1st to 10th	Day	19% 5% 19% 29% 19% 8%	530
	11th to 20th	Day	13% 10% 26% 27% 21% 3	530
	21st to end	Day	17% 11% 26% 29% 14% 3	583
November	1st to 10th	Day	16% 11% 31% 27% 13% 3	535
	11th to 20th	Day	19% 16% 31% 22% 11% 1	435
	21st to end	Day	14% 24% 28% 23% 10% 2	302
December	1st to 10th	Day	22% 17% 42% 14% 5% 1	220
	11th to 20th	Day	21% 16% 35% 21% 6% 1	299
	21st to end	Day	25% 17% 33% 16% 9% 1	363
January	1st to 10th	Night	19% 17% 36% 23% 4	840
	11th to 20th	Night	19% 15% 36% 27% 3	840
	21st to end	Night	20% 13% 40% 25% 2	924
February	1st to 10th	Night	20% 14% 36% 27% 3	840
	11th to 20th	Night	25% 17% 38% 18% 1	804
	21st to end	Night	19% 15% 40% 23% 3	660
March	1st to 10th	Night	18% 16% 37% 25% 4	800
	11th to 20th	Night	20% 13% 36% 24% 7%	800
	21st to end	Night	18% 12% 39% 27% 4	880
April	1st to 10th	Night	17% 16% 35% 27% 5%	800
	11th to 20th	Night	15% 12% 34% 27% 12%	791
	21st to end	Night	16% 8% 34% 35% 9%	743
May	1st to 10th	Night	13% 9% 32% 33% 14%	600
	11th to 20th	Night	18% 5% 25% 32% 18% 1	591
	21st to end	Night	16% 3 14% 35% 29% 3	650
June	1st to 10th	Night	10% 3 17% 41% 28% 2	600
	11th to 20th	Night	11% 2 14% 34% 35% 5%	600
	21st to end	Night	9% 2 14% 38% 34% 3	600
July	1st to 10th	Night	11% 1 10% 34% 39% 5%	585
	11th to 20th	Night	6% 2 6% 36% 44% 6%	582
	21st to end	Night	9% 2 10% 39% 36% 5%	657
August	1st to 10th	Night	14% 2 9% 35% 35% 5%	597
	11th to 20th	Night	12% 2 10% 35% 39% 3	594
	21st to end	Night	10% 2 10% 33% 38% 6%	870
September	1st to 10th	Night	13% 3 12% 32% 35% 6%	800
	11th to 20th	Night	15% 3 13% 31% 34% 5%	800
	21st to end	Night	25% 5% 18% 32% 19% 2	800
October	1st to 10th	Night	17% 5% 20% 27% 26% 5%	800
	11th to 20th	Night	15% 7% 25% 34% 17% 2	800
	21st to end	Night	15% 8% 25% 35% 15% 1	880
November	1st to 10th	Night	16% 8% 26% 35% 13% 2	820
	11th to 20th	Night	15% 12% 30% 32% 10%	840
	21st to end	Night	15% 9% 34% 31% 10% 1	840
December	1st to 10th	Night	19% 12% 36% 27% 6%	840
	11th to 20th	Night	19% 8% 37% 30% 5%	840
	21st to end	Night	23% 11% 31% 30% 5%	924

Page generated at 2026-02-13 01:42:18. Version ac07

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